Temporal and spatial variation in stocks of autotrophic and ...
Deep-Sea Research I 50 (2003) 557–571
Temporal and spatial variation in stocks of autotrophic and heterotrophic microbes in the upper water column of the central Arctic Ocean Evelyn B. Sherr*, Barry F. Sherr, Patricia A. Wheeler, Karen Thompson College of Oceanic and Atmospheric Sciences, Oregon State University, 104 Ocean Admin Bldg, Corvallis, OR 97330-5503, USA Received 12 June 2002; received in revised form 12 November 2002; accepted 4 February 2003
Abstract As part of the SHEBA/JOIS drift experiment, we continually analysed abundance and biomass of autotrophic and heterotrophic microbes in the upper 120 m of the water column of the ice-covered Central Arctic Ocean from November 1997 through August 1998. Microbial biomass was concentrated in the upper 60 m of the water column. There were low but persistent stocks of heterotrophic and autotrophic microbes during the winter months. Phytoplankton biomass began increasing when winter snow melted from the ice-pack in early June, after which there was a progressive decline of nitrate and silicate in the euphotic zone. We observed three distinct blooms over the summer. The initial bloom consisted of diatoms and phytoflagellates, mainly 2 mm-sized Micromonas sp.; the two subsequent blooms were dominated by the flagellated (non-colonial) Phaeocystis sp. The carbon:chlorophyll ratio of the phytoplankton was 31711. Stocks of bacteria and heterotrophic protists approximately doubled during the growing season, increasing in tandem with increase in phytoplankton biomass. Increase in cell abundances of bacteria and of the phytoflagellate Micromonas over 40–50 d periods during the initial bloom period yielded estimates of realised growth rate of 0.025 d1 for bacteria and of 0.11 d1 for Micromonas. Heterotrophic protists included flagellates, ciliates, and dinoflagellates, with biomass divided nearly evenly between nanoplankton (Hnano, 0–20 mm) and microplankton (Hmicro, 20–200 mm) size classes. r 2003 Elsevier Science Ltd. All rights reserved. Keywords: Microplankton; Phytoplankton; Bacteria; Protists; Flagellates; Biomass; Arctic Ocean; Canada Basin
1. Introduction The 1997–1998 SHEBA/JOIS (Surface Heat Budget of the Arctic Ocean/Joint Ocean Ice Study) experiment (Perovich et al., 1999; Melnikov et al., 2002; Macdonald et al., 2002) afforded an unique *Corresponding author. Fax: +1-541-737-2064. E-mail address: [email protected] (E.B. Sherr).
opportunity to investigate temporal and spatial variation in abundance and biomass of pelagic microbes in the permanently ice covered Arctic Ocean. Information regarding the standing stocks of microbial plankton in this major region of the sea is relatively sparse. Past ice camp studies have demonstrated a strong seasonality for phytoplankton, with an initial bloom in late June to early July and one or two peaks during the growing season,
0967-0637/03/$ - see front matter r 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0967-0637(03)00031-1
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which typically ended by early to mid-September (English, 1961; Pautzke, 1979). A more detailed analysis of autotrophic and heterotrophic microbes was carried out during the July–September 1994 Arctic Ocean Section (AOS) (Booth and Horner, 1997; Gosselin et al., 1997; Rich et al., 1997; Sherr et al., 1997). There is very little data on pelagic communities in the Arctic Ocean during winter. The goals of the plankton biology program of the SHEBA/JOIS expedition included determining variation in standing stocks and activity of pelagic microbes (bacteria, phytoplankton, and heterotrophic protists) during the dark winter months as well as the growing season and assessing the timing and magnitude of response of heterotrophic microbes to the spring bloom. Here we report changes in abundance and biomass of autotrophic and heterotrophic microbes in the upper 120 m of the water column during the ice camp drift. We also determined general size ranges and taxonomic categories of autotrophic and heterotrophic protists.
2. Materials and methods 2.1. Sample collection Between October 19, 1997 and September 28, 1998, the SHEBA/JOIS ice camp drifted with the permanent pack ice from its initial position in the southern Canada Basin (75 280 N, 143 400 W) to the Mendeleyev Basin (80 110 N, 166 050 W), traveling over the Northwind Ridge and Chukchi Plateau from February to mid-summer, and drifting into the Mendeleyev Basin in late summer (Perovich et al., 1999; Macdonald et al., 2002; McLaughlin et al., 2003; SHEBA website, http:// sheba.apl.washington.edu). Water samples for determination of nutrient and chlorophyll concentrations, and of microbial abundance and biomass, were collected in the upper 120 m of the water column, with 5-l Niskin bottles deployed on a wire through an ice-hole, on an 8 d schedule throughout the year, with more frequent sampling in the upper 50 m for nutrient and chlorophyll a concentrations during summer (June to September).
2.2. Measurement of inorganic nutrients and chlorophyll a Freshly collected seawater samples were processed for macronutrients nitrate+nitrite, silicate, and phosphate with a Technicon Autoanalyser (Atlas et al., 1971). Water samples for chlorophyll a were filtered onto Whatman GF/F glass fiber filters. Chlorophyll concentrations were determined fluorometrically, after 24-h extraction in 90% methanol at 5 C (Parsons et al., 1984). 2.3. Determination of microbial abundance and biomass Procedures for analysis of microbial biomass were similar to those used during the 1994 AOS (Sherr et al., 1997). Briefly, samples for microbial enumeration were preserved by a three-step procedure: 0.05% final concentration alkaline Lugol solution, followed by the addition of 0.1% final concentration of 3% sodium thiosulfate and 2% final concentration of borate-buffered formalin (Sherr and Sherr, 1993). Samples were allowed to sit for at least 4 h to allow protist cells to harden, and processed within 24 h. Aliquots of the preserved samples were stained with DAPI (25 mg ml1 final concentration) for 7–10 min, and then filtered onto 25 mm diameter black-stained membrane filters (Poretics, Inc.). For each water sample, duplicate 50-ml subsamples were filtered onto 0.8 mm pore size filters for enumeration of o20 mm sized protists, and duplicate 150 ml subsamples were filtered onto 3.0 mm pore size filters for enumeration of >20 mm sized protists. Two additional 10-ml aliquots of each sample were filtered onto 0.2 mm pore size filters for bacterial counts. Filters were removed from the filtration towers under low vacuum, then mounted with Resolvet low viscosity immersion oil onto glass slides with No. 1 coverslips. Samples were stored in slide boxes at 20 C on the ship until returned on dry ice to a 40 C freezer in the laboratory at Oregon State University. All slides were inspected within 1 year of collection. Bacteria and protists were enumerated via direct counts with a Zeiss Universal microscope outfitted for epifluorescence microscopy with a 100 W
E.B. Sherr et al. / Deep-Sea Research I 50 (2003) 557–571
mercury lamp and Zeiss filter set 47 77 02 (G 365 excitation filter/LP 420 barrier filter) for UV light excitation of DAPI-stained cells, and Zeiss filter set 47 77 09 (BP 450–490 excitation filter/LP 520 barrier filter) for blue light excitation of chlorophyll autofluorescence. Bacteria and o20 mm sized protists were enumerated at 1000 , and >20 mm protists at 160–400 (Sherr et al., 1993). During inspection of both bacterial and nanoplankton samples, it was noted that coccoid cyanobacteria were virtually absent from all samples. Each protist cell counted was sized for biovolume calculations with a calibrated ocular micrometer and grouped into general taxonomic categories: diatoms, mixed species nanoflagellates, choanoflagellates, dinoflagellates, and ciliates. Two taxa of phytoflagellates, Micromonas sp. and Phaeocystis sp. (probably Phaeocystis poucheii), were identified to genus level based on past experience with phytoplankton in this region of the Arctic Ocean (Booth and Horner, 1997; Sherr et al., 1997). Bacterial biomass was estimated from a value of 20 fg C cell1, from the prior results of Sherr et al. (1997). Protistan carbon biomass was estimated from empirically determined biovolumes using equations suggested by Menden-Deuer and Lessard (2000). Based on a comprehensive review of the literature, Menden-Deuer and Lessard derived C: volume relationships for marine protists, one for protists excluding diatoms (pg C cell1= 0.216 volume in cubic microns raised to the power 0.939 [C ¼ 0:216 V 0:939 ]) and one for diatoms (pg C cell1=0.288 volume in cubic microns raised to the power 0.811 [C ¼ 0:288 V 0:811 ]). These equations result in C: volume factors of 0.20 pg C mm3 for 2 mm diameter protists, 0.15 pg C mm3 for 10 mm diameter protists, 0.13 pg C mm3 for 20 mm diameter protists, and 0.060 pg C mm3 for 20 mm diameter diatoms. Stocks of nutrients and of chlorophyll a were integrated for the upper 50 m of the water column, since the more frequent summer profiles only included depths to 50 m. In contrast, microbial biomass data were based on the routine, 8-d interval profiles that sampled depths to 120 m, including depths of 40 and 60 m. In these profiles,
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higher summer microbial biomass extended to 60 m depth; thus we integrated biomass over the upper 60 m. Chlorophyll a concentrations at depths >50 m were o0.2 mg l1, so there would be little difference in phytoplankton biomass for a 0–50 m versus a 0–60 m integration depth. We also calculated winter and summer average cell abundances of the various microbial stocks for two depth intervals: 0–40 m (summer euphotic zone), and 80–120 m (depths beneath the euphotic zone).
3. Results 3.1. Variation in nutrient and chlorophyll a concentrations During the SHEBA/JOIS experiment, the icestation passed from the deep, oligotrophic Canada Basin to relatively shallower regions of the Northwind Ridge and Chukchi Plateau, and then finally over the deep Mendeleyev Basin (Fig. 1C). This path crossed a hydrographically dynamic part of the Arctic Ocean, with three major identified regimes: (1) the south-central Canada Basin from Fall, 1997 to early winter, 1998; (2) the Northwind Ridge and Chukchi Plateau from February to midsummer; (3) the Chukchi Plateau and Mendeleyev Basin from June to September (Macdonald et al., 2002; McLaughlin et al., submitted) (Fig. 1C). Nutrient distributions in the upper 120 m encountered during the ice camp drift were influenced by change in water mass characteristics, especially by variation in relative contribution of Pacific and Atlantic-origin water masses (McLaughlin et al., submitted). In the fall and early winter, nitrate+nitrite stocks were o0.5 mM in the upper 50 m, but increased to 2–7 mM after the ice camp traversed the Northwind Ridge (Fig. 1A). Silicate stocks increased similarly. Higher nitrate and silicate content of water over the Northwind Ridge and Chukchi Plateau was likely due to water of Pacific origin (McLaughlin et al., submitted). Starting in mid-June, nutrient stocks began to decrease in the upper 50 m, coincident with an increase in chlorophyll a stocks as the spring bloom began (Fig. 1A and B). Stocks of both nitrate+nitrite and of silicate integrated over 0–50 m showed a
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Fig. 1. Variation in concentrations of (A) nitrate+nitrate (mM) and (B) chlorophyll a (mg liter1) in the upper 120 m of the water column, and in (C) bathymetry during the SHEBA/JOIS experiment. Dots indicate profile sampling depths. The approximate demarcations of the three hydrographic regimes encountered during the ice camp drift are indicated. Regime 1—south-central Canada Basin from fall, 1997 to early winter, 1998; Regime 2—Northwind Ridge and Chukchi Plateau from February to June; Regime 3— Chukchi Plateau and Mendeleyev Basin from June to September.
progressive decline from May 29 to September 25, while stocks of phosphate showed little seasonality (Fig. 2). In the upper 20 m, nitrate+nitrite decreased from an average of 2.8 mM to 0.01 mM, and silicate from an average of 8.7 mM to 1.7 mM, from June 15 to August 30.
3.2. Variation in microbial stocks All groups of pelagic microbes appeared to exhibit a strong response to the large annual amplitude in solar insolation, with low biomass in spring and higher biomass during the short
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autotrophs varied dramatically in summer because of blooms. Heterotrophic protists made up, on average, 86% of total protistan biomass in winter, and 44% in summer.
silicate
0-50 m nutrient stock, mM
561
600
400
3.3. Distribution of protist biomass
nitrate + nitrite 200
phosphate 0
347
32
82
133
182
232
Day of year
Fig. 2. Change in integrated stocks (mM) of nitrate+nitrite, silicate, and phosphate in the upper 50 m of the water column during the SHEBA/JOIS experiment.
summer growing season (June–September) (Fig. 3). Microbial biomass was concentrated in the upper 40–60 m of the water column (Figs. 1B and 3). There was a rapid increase in phytoplankton stocks in June (beginning about day 140), after snow cover melted from the ice surface (Fig. 4). Seasonal increases in stocks of bacteria and of heterotrophic protists (Fig. 3A and C) were nearly simultaneous with increase in autotrophic protists (phytoplankton) (Figs. 1B and 3B). Stocks of bacteria and of heterotrophic protists remained relatively high in the upper 60 m throughout the summer, even after phytoplankton biomass had declined (Fig. 3A and C). Table 1 presents average winter (November– May) and summer (June–September) values of abundance (cells ml1) and volumetric biomass (mg C m3) for bacteria and for autotrophic and heterotrophic protists in nanoplankton and microplankton size categories. To clearly distinguish seasonal effects, we averaged values for two depth zones: the summer euphotic zone (upper 40 m), and subeuphotic depths (80–120 m). Stocks of bacteria and of heterotrophic protists approximately doubled from winter to summer in the euphotic zone. Bacterial stocks showed no increase at the lower depth interval. Autotrophic protists exhibited an approximately 20-fold increase in numbers and biomass during summer compared to winter stocks. Cell numbers and biomasses of
Variations in depth-integrated (0–60 m) protistan biomass from January to September 1998 are shown in Fig. 5. Three discrete peaks in biomass of autotrophic protists were observed, the first from the end of June to the first week of July, and the other two during the last two weeks of July (Fig. 5A and B). These peaks mirrored the three peaks in chlorophyll concentration found during summer (Fig. 1B and 4). Autotrophic protist biomass was much higher than biomass of heterotrophic protists during these blooms (Fig. 5A). Since microplanktonic diatoms have a lower C: volume ratio compared to nanoplanktonic cells (Menden-Deuer and Lessard, 2000), in terms of carbon biomass all three blooms were dominated by o20 mm sized cells (Fig. 5B). Over the entire growing season, diatoms averaged 24711% of the total biomass of autotrophic cells. Heterotrophic protist biomass was fairly evenly divided between nanoplankton and micro-plankton size classes during the winter, but during the summer nanoplanktonic heterotrophic protists (Hnano) showed a sharp peak in biomass coincident with the initial spring bloom (Fig. 5C). A striking result was the small size of most protistan cells, both heterotrophic and autotrophic. From 72% to 95% (average 87%) of heterotrophic flagellates were o5 mm in size. Picoflagellates (1.5–2 mm preserved size) dominated the Hnano assemblage throughout the year, ranging in abundance from 100 to 1600 cells ml1, and averaging 52% of total Hnano numbers. Autotrophic protists were also numerically dominated by o5 mm sized cells, which made up 44– 99% (average 95%) of cells in the phytoplankton assemblage during the growing season. A large proportion of small phytoplankton cells were 2 mm sized Micromonas sp., which had a maximum abundance of 28,000 cells ml1 during the initial spring bloom, and were present at abundances of
E.B. Sherr et al. / Deep-Sea Research I 50 (2003) 557–571
Day of Year
A. Bacteria 30
80
130
180
230
20
Depth, m
40
4
11
4
6
345
6
562
4
4
4
8
4
60
4
5
4
80
2
100
2
120
B. Autotrophic protists 345 30
80
130
180
230 80
10
10
20
30 20
Depth, m
40 60
60
40
80 20
100 0
120
C. Heterotrophic protists 345
30
80
130
180
230 20
Depth, m
10
20 40
15
60 10
80 5
100 120
Dec 11
Jan 30
Mar 21
May 10
Jun 30
Aug 20
3
Fig. 3. Variation in carbon biomass (mg C m ) of planktonic microbes in the upper 120 m of the water column during the SHEBA/ JOIS experiment: (A) Heterotrophic bacteria, (B) Autotrophic protists (phytoplankton), and (C) Heterotrophic protists. Dots indicate profile sampling depths.
1000–10,000 cells ml1 in the upper 40 m during the rest of the growing season. 3.4. Carbon:chlorophyll ratio Analysis of the biomass of autotrophic protists in water samples for which chlorophyll a concen-
tration was also determined during the SHEBA/ JOIS experiment allowed calculation of empirical carbon:chlorophyll (C:Chl) ratios for phytoplankton in the central Arctic Ocean. C:Chl ratios were calculated for 36 discrete water samples collected in the upper 60 m from 19 May to 22 September (Table 2). Phytoplankton biomass varied from 1.3
E.B. Sherr et al. / Deep-Sea Research I 50 (2003) 557–571 snow cover melt [------]
1.5
300 1.0
200
0.5 100
0
Phytoplankton biomass, gC m-2
Short wave radiation, Watts m-2
400
0.0 296
346
31
81
131
181
233
281
Day of Year
Fig. 4. Variation in short wave radiation (dotted line, W m2, measured with an Eppley Precision Spectral Pyranometer, range of 295–2500 nm), and in 0–50 m integrated phytoplankton biomass estimated from chlorophyll a concentrations and an average C:chl a ratio of 31 (solid line), during the SHEBA/ JOIS experiment. The period during which winter snow cover melted from the ice surface is indicated. Data on radiation and ice melt was provided by the SHEBA Project Office, Applied Physics Laboratory, University of Washington.
to 112 mg C m3, and chlorophyll a concentration from 0.07 to 4.3 mg m3, in these samples. The average C:Chl ratio determined from this sample set was 31711. The C:Chl ratio was biased upward by values greater than 50 calculated for July 7 and July 31 data. Without these high values, the average C:Chl ratio was 2878. The lowest ratio was 13, found for phytoplankton at 20 m on May 31 and at 40 m on July 23 (Table 2). 3.5. Taxonomic composition of protists Autotrophic protists o20 mm in size were mainly phytoflagellates, including Micromonas sp, the flagellated form of Phaeocystis pouchetii, other haptophytes, and occasional cryptomonads and small autotrophic dinoflagellates. Diatoms occurred, but were not abundant, in the nanoplankton fraction. Microplanktonic autotrophs were mostly diverse species of diatoms, including centric and pennate species previously identified by Booth and Horner (1997) in this region. Abundance of autotrophic dinoflagellates was typically less than 1 cell ml1, but did show a peak of
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25 cells ml1 at a depth of 20 m during the initial spring bloom. Autotrophic ciliates, Mesodinium sp., were rare, 0–3 cells ml1. Phagotrophic ciliates with sequestered chloroplasts were common in summer, but were included with the heterotrophic protists. During winter, phytoplankton were present as nanoflagellates, mainly Micromonas sp. and unidentified haptophytes, at abundances of hundreds of cells ml1, plus >20 mm diatoms and pigmented dinoflagellates at abundances of about 1 cell ml1. Phytoplankton composition changed dramatically during the growing season (Fig. 6). Phytoplankton biovolume during the initial bloom was composed about equally of >20 mm diatoms and o20 mm phytoflagellates, primarily 2 mm Micromonas. The two later blooms were dominated by flagellated Phaeocystis, 4–6 mm in size, at abundances of 5000–18,000 cells ml1. Microplanktonsized flagellates, mostly autotrophic dinoflagellates, were not an important component of the phytoplankton assemblage during the blooms (Fig. 6). Nanoplankton-sized heterotrophic protists were a mixed species assemblage of non-pigmented flagellates. Most of these flagellates, especially the very small, 2 mm sized species could not be identified. Choanoflagellates were present, but generally rare, with abundances ranging from 0 to 150 cells ml1 (average 13 cells ml1). Non-armored heterotrophic dinoflagellates 10–20 mm in size, including spherical gymnodinoid forms and spindle-shaped Katodinium-like species, were found at an average abundance of 17 cells ml1 (range 1– 60 cells ml1). Non-pigmented cryptomonad-like flagellates similar to the species Leucocryptos marina (Vors, 1992) were occasionally observed, and o20 mm ciliates occurred in the nanoplankton fraction at an average abundance of 1 cell ml1. Heterotrophic protists in the microplankton size class were dominated by non-pigmented dinoflagellates, which made up, on average, 83% of the numerical abundance and 63% of the biomass of microzooplanktonic protists. Heterotrophic dinoflagellates >20 mm in size were present throughout the year at abundances of 1–12 cells ml1, and included both armored Protoperidium-like species as well as spindle-shaped non-armored forms. Ciliates occurred at abundances of 0.1–2 cells ml1, and were mostly spherical or elongate
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Table 1 Comparison of winter (Nov.–May) and summer (June–Sept.) cell abundances (cells ml1) and biomasses (mg C m3) in 0–40 m and 80– 120 m depth zones, of bacteria, autotrophic protists, and heterotrophic protists (ranges of values in parentheses) Abundance (cells ml1)
Bacteria 0–40 m 80–120 m Autotrophic protists 0–40 m 2–20 mm 20–200 mm
80–120 m 2–20 mm 20–200 mm
Heterotrophic protists 0–40 m 2–20 mm 20–200 mm
80–120 m 2–20 mm 20–200 mm
Biomass (mg C m3)
Winter
Summer
Winter
Summer
1.7670.28 105 (1.32–2.86) 1.2770.26 105 (0.72–2.07)
3.2370.70 105 (1.9–6.7) 1.1170.19 105 (0.72–1.78)
3.570.6 (2.6–5.9) 2.670.5 (1.4–4.1)
6.471.4 (3.8–13.3) 2.270.3 (1.4–3.6)
2307230 (3–1500) 0.970.4 (0.05–2.6)
560075100 (100–28,000) 23716 (0.7–85)
0.4670.36 (0.02–2.1) 0.3470.34 (0.0–3.4)
10.9711 (1.0–112) 2.172.1 (0.4–15)
ND
1778 (3–42) 2.371.5 (0.8–5.6)
ND
0.1170.07 (0.01–0.21) 0.1870.18 (0.2–0.48)
ND
ND
260780 (90–490) 6.1–1.3 (3.0–9.7)
5907310 (210–2300) 8.072.3 (2.5–18)
1.970.6 (0.7–4.6) 1.871.0 (0.6–9.2)
4.572.7 (1.0–18) 3.371.6 (10–11)
ND
150727 (110–200) 3.471.5 (1.4–6.0)
ND
1.370.4 (0.9–2.2) 0.8370.3 (0.4–1.1)
ND
ND
ND=no data.
spirotrichs. During the summer, mixotrophic ciliates with sequestered chloroplasts were frequently observed. Tintinnids were rare. Other microplanktonic protists included unidentified heterotrophic flagellates at an average abundance of 0.2 cells ml1. 3.6. Contents of protistan food vacuoles During summer, ingested phytoplankton cells could be identified as chlorophyll autofluorescence in the food vacuoles of phagotrophic protists. During the initial spring bloom heterotrophic nanoflagellates, including abundant 2 mm sized
cells and choanoflagellates, commonly ingested Micromonas. In the later Phaeocystis-dominated blooms, heterotrophic nanoflagellates were also observed with ingested phytoplankton prey. Ciliates and non-armored dinoflagellates were frequently observed with ingested phytoflagellates and smaller-sized diatoms. Thecate dinoflagellates were rarely found with recognizable food vacuole contents. 3.7. Microbial growth rates We were able to calculate microbial growth rates from the abundance data in two cases: for
E.B. Sherr et al. / Deep-Sea Research I 50 (2003) 557–571 5 autotrophic
gC m
-2
4 3 2 heterotrophic
1
565
These estimates should be considered realized growth rates (r) for the prevailing in situ conditions of environmental parameters and mortality processes; intrinsic growth rates (m) of bacteria and Micromonas at these temperatures, substrate concentrations, and light conditions would be expected to be higher.
0
(a) 2.5
Anano < 20 µm
gC m
-2
2.0 1.5 1.0
Amicro > 20 µm
0.5 0.0
(b) 1.0
Hnano < 20 µm
gC m
-2
0.8 0.6 Hmicro > 20 µm
0.4 0.2 0.0 0
(c)
50
100 150 Day of year
200
250
Fig. 5. Variation in integrated carbon biomass (g C m2) of protists in the upper 60 m of the water column from January to September, 1998: (A) Comparison of biomass of heterotrophic (dashed line) and autotrophic (solid line) protists, (B) Comparison of biomass of autotrophic protists in the nanoplanktonic (Anano, 2–20 mm, solid line) and microplanktonic (Amicro, 20– 200 mm, dashed line) size classes, (C) Comparison of biomass of heterotrophic protists in the nanoplanktonic (Hnano, solid line) and microplanktonic (Hmicro, dashed line) size classes.
bacterioplankton and for Micromonas sp. in the upper 20 m of the water column over periods spanning the initial spring bloom. The abundance of bacterioplankton increased exponentially from May 7 to June 24, as did Micromonas cell abundance from May 22 to June 23. Regressions of natural logarithm of cell abundance with time during these periods (Fig. 7) yielded an estimate for bacterial growth rate of 0.025 d1 (doubling time of 28 d) and an estimate for Micromonas growth rate of 0.11 d1 (doubling time of 6.1 d).
4. Discussion 4.1. Variation in abundance and biomass of microbial plankton Stocks of all components of the microplankton were concentrated in the upper 40–60 m of the water column, and showed a strong seasonal response to the short growing season in the high Arctic (Figs. 1B, 3 and 4). Autotrophic protists (phytoplankton) had the largest amplitude in seasonal abundance and biomass. The phytoplankton included diatoms and Phaeocystis, which constitute a large fraction of planktonic autotrophs in both northern and southern polar systems (El-Sayed and Fryxell, 1993; Smith, 1994; Bigidare et al., 1996; Dennett et al., 2001). We also found that 2 mm phytoflagellates, identified as the prasinophyte alga Micromonas sp., were a large fraction of the initial spring bloom. Micromonas-like prasinophytes have been previously found to be abundant in the Northeast Water polynya off Greenland (Booth and Smith, 1977) and in the central Arctic Ocean (Booth and Horner, 1997). Phytoplankton in the picoplankton fraction, typically prasinophytes and prymnesiophytes, can also be important in the Southern Ocean (El-Sayed and Fryxell, 1993; Agawin et al., 2002). During summer blooms, peak abundances of Micromonas were 28,000 cells ml1, and of Phaeocystis were 18,000 cells ml1. These are high values for eukaryotic phytoplankton in oligotrophic ocean environments. For example, in the NE Pacific, abundances of autotrophic nanoflagellates were on the order of 2000 cells ml1 year-round (Boyd et al., 1995). During the 1994 AOS expedition across the polar cap, Booth and Horner (1997) reported maximum abundance values of
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Table 2 Calculated carbon:chlorophyll a ratios for phytoplankton during the 1998 growing season in the central Arctic Ocean Date
Day of year
Sample depth (m)
Chl a (mg m3)
Biomass (mg m3)
C:Chl a
May 19
139
May 22
142
May 31 June 8 June 17
151 159 168
June 23 July 7
174 188
July 15 July 23
196 204
July 31
212
Aug. 14
226
Aug. 20
232
Aug. 28
240
Sept. 7
250
Sept. 14
257
Sept. 22
265
10 20 4 10 20 20 10 10 20 10 10 20 40 4 10 20 40 10 40 10 20 40 10 40 10 20 40 60 10 40 10 40 4 10 30 50 60
0.23 0.21 0.18 0.18 0.16 0.21 0.33 0.59 0.44 1.41 0.51 0.64 0.12 0.35 0.44 4.3 0.16 0.23 0.21 0.11 1.6 0.21 0.09 0.38 0.11 0.12 0.35 0.07 0.09 0.20 0.07 0.15 0.12 0.13 0.08 0.11 0.08
3.6 3.8 4.8 4.7 3.6 2.6 11 25 17 48 36 34 4.3 7.9 12 112 2.0 14 11 3.3 58 4.1 3.4 6.4 2.4 4.1 12 1.3 3.8 4.1 2.9 3.3 2.2 3.2 3.1 4.6 1.7
15 18 26 26 23 13 33 42 38 34 70 53 36 23 27 26 13 62 52 31 36 19 36 17 21 33 33 19 40 21 40 23 18 25 40 41 21
10,000 cells ml1 for 2 mm phytoflagellates (tentatively identified as Micromonas pusilla) and of 470 cells ml1 for flagellated Phaeocystis pouchetii. Agawin et al. (2002) reported pico-phytoflagellate abundances from o1000 to B10,000 cells ml1, with abundances of >20,000 cells ml1 found at one station, in the Bransfield Strait, Antarctia. Our intensive sampling program through the growing season allowed us to observe bloom peaks that might have been missed by previous shorter-term sampling efforts.
Our empirically calculated carbon:chlorophyll ratios, which ranged from 13 to 70 and averaged 31711, were similar to the C:Chl ratios, range of 13 to 59, average of 28710 (excluding one very low value of 4), estimated by Booth and Horner (1997) for central Arctic phytoplankton in summer. These relatively low C:Chl ratios are typical for phytoplankton adapted to low irradiances (Smith and Sakshaug, 1990). Heterotrophic bacteria approximately doubled in abundance in the euphotic zone from a winter
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Fig. 6. Proportional taxonomic composition of phytoplankton during the three major bloom periods of the growing season, in terms of biovolume: D=diatom biovolume (clear bars); NF=nanoflagellate (cells o20 mm) biovolume (striped bars); MF=microflagellate (cells >20 mm) biovolume (solid bars).
Ln bacterial cells ml -1
13.5 13.0
r = 0.025 d -1 Td = 28 d
12.5 12.0 11.5 125
135
145
155
165
175
Ln Micromonas cells ml -1
(a)
(b)
11 10 9
r = 0.11 d -1 Td = 6.1 d
8
low of 1.8 105 cells ml1 to a summer high of 3.2 105 cells ml1, with a maximum abundance of 6.7 105 cells ml1 (Table 1). Bacterioplankton abundances during the summer of 1998 were lower than the bacterial abundances (3.8–11.7 105 cells ml1, average 4.871.3 105 cells ml1) we had previously reported for the upper 50 m of the water column in the central Arctic Ocean during the growing season (Sherr et al., 1997). Our summer 1998 bacterial abundance values are similar to the range of abundances of 2–5 105 cells ml1 found by Cota et al. (1996) in water samples taken over the continental shelf and slope of the Chukchi Sea and Canada Basin in August 1993. Heterotrophic protists showed an early spring increase in biomass, before the initial spring bloom, perhaps due to differences in water mass history during the SHEBA/JOIS drift (Fig. 3). The summer stocks of heterotrophic protists (Hnan, 210–2300 cells ml1; Hmicro, 2.5–18 cells ml1) were similar to protistan abundances found during the summer 1994 AOS expedition (Hnan 160– 1900 cells ml1, Hmicro 2–15 cells ml1) (Sherr et al., 1997). The abundances of protists in this region are comparable to those reported for other oligotrophic regions of the world ocean, for example the Sargasso Sea (Caron et al., 1995) and the Ross Sea, Antarctica (Dennett et al., 2001). The general taxonomic composition of heterotrophic protists was similar to that found in the central Arctic Ocean during the summer of 1994, and to the protistan community described for Antarctic waters (Garrison and Gowing, 1993; Dennett et al., 2001). These results confirm the importance of 2 mm-sized heterotrophic flagellates in the Hnano, and heterotrophic dinoflagellates in the Hmicro, size categories. 4.2. Survival of planktonic microbes during the Arctic winter
7 6 135
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145
155
165
175
Day of Year
Fig. 7. Regressions of natural logarithm of cell abundance versus time in the upper 20 m during the initial spring bloom period, from which growth rates were determined for (A) bacteria and (B) Micromonas.
Virtually all identifiable groups of microbial cells in the plankton were present in the upper 120 m during winter sampling. Bacterioplankton maintained relatively high winter cell abundance (Table 1), although the cell-specific activity assessed via rate of uptake of tritiated leucine
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was much lower than in summer (Sherr and Sherr, 2003). Heterotrophic bacteria are able to enter a state of low metabolism (starvation survival) when substrates are scarce (Morita, 1997). In contrast, phytoplankton stocks were low during the winter, with average cell abundances of only several hundred per ml, about an order of magnitude lower than winter cell abundances of phytoflagellates in the NE subarctic Pacific (Boyd et al., 1995). All components of the phytoplankton assemblage were present during winter, although identifiable cells of Phaeocystis, which was an important component of summer blooms, were rare. The biomass stocks of heterotrophic protists were higher than those of either bacteria or autotrophic protists during winter (Table 1), and included all categories of identified cells. Phagotrophic protists should be able to continue feeding during the winter. Some phytoflagellates, such as the prasinophyte Micromonas sp., which was abundant during summer blooms, may also be capable of phagotrophy (Gonzalez et al., 1993; Caron, 2000) and thus have a survival advantage over non-phagotrophic phytoplankton. These results are likely generally applicable to ice-covered Arctic systems during winter, although the fact that the SHEBA camp was in a different hydrographic regime during summer compared to winter is a complication for seasonal interpretation of our data set. 4.3. Response of planktonic microbes during the spring bloom Despite the cold water temperatures (1.0 C to 1.8 C) that prevailed year-round in the central Arctic, both bacteria and heterotrophic protists increased in biomass over the same time period as did the phytoplankton during the spring bloom, without any significant lag (Fig. 3). Bacterial cellspecific activity also increased rapidly during the spring bloom (Sherr and Sherr, 2003). During the initial bloom period, the autotrophic picoflagellate Micromonas sp. grew at a rate of 0.11 d1, equivalent to a doubling time of about 6 d. This rate is at the low end of the growth rates that Smith (1994) estimated for Phaeocystis pouchetii blooms in the Greenland Sea (0.2570.20 d1
(range of 0.13–1.05 d1, doubling times of 5.3– 0.7 d). Bacterioplankton abundance increased at a slower rate, 0.025 d1, equivalent to a doubling time of about 28 d. Bacterial growth rates during June, independently estimated from rates of incorporation of tritiated leucine, were as high as 0.02–0.04 d1 for specific depths in the euphotic zone (Sherr and Sherr, 2003). Rich et al. (1997) previously estimated bacterial growth rates ranging from 0.05 to 0.5 d1 (doubling times of 14– 1.4 d) in the central Arctic Ocean during summer, also based on rates of leucine incorporation. However, the rates of leucine incorporation by planktonic bacteria during the 1994 AOS expedition (Rich et al., 1997) were about an order of magnitude higher compared to rates measured during summer of the SHEBA year (Sherr and Sherr, 2003). A comparison of data collected in 1994 (Rich et al., 1997) and in 1998 (this paper, and Sherr and Sherr, 2003) suggests there may be significant inter-annual variability both in stocks and in growth rates of bacterioplankton in the central Arctic Ocean. Our calculated in situ growth rates based on observed increases in cell number over time should be less than the intrinsic rate of increase at in situ conditions, as heterotrophic nanoflagellates were abundant during the spring bloom and were capable of ingesting both bacteria and Micromonas (Sherr et al., 1997). Viral lysis could also have represented a source of mortality for bacterioplankton and for phytoplankton (Fuhrman and Suttle, 1993; Suttle, 1994; Steward et al., 1996). 4.4. Inferred food web relationships Inspection of food vacuole contents during the growing season showed that all size classes of heterotrophic protists, from the smallest nanoflagellates to the largest ciliates and dinoflagellates, consumed phytoplankton. In particular, the abundant 2 mm-sized heterotrophic flagellates, as well as choanoflagellates, were routinely observed with ingested Micromonas. These groups of flagellates have been thought to prey primarily on heterotrophic or autotrophic bacteria rather than on
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eukaryotic phytoplankton (Caron, 2000). We had previously noted phytoplankton ingestion by o5 mm heterotrophic flagellates as well as by dinoflagellates and ciliates in the central Arctic Ocean during the summer of 1994 (Sherr et al., 1997). It is clear that the entire spectrum of heterotrophic protists can be significant consumers of phytoplankton in the Arctic Ocean; this trophic pathway needs to be quantified. Microplanktonic protists are known to ingest heterotrophic as well as autotrophic nanoflagellates (Verity, 1991; Solic and Krstulovic, 1994; Sherr and Sherr, 2000), and grazing by microzooplankton may have been at least partly responsible for the cycling of heterotrophic nanoflagellate biomass during summer (Fig. 3C). Because of their larger cell sizes, microplankton protists are, in turn, more likely to be subject to grazing by mesozooplankton than are heterotrophic nanoflagellates (Stoecker and Capuzzo, 1990). We speculate, as for the 1994 AOS results (Sherr et al., 1997; Thibault et al., 1999), that copepod grazing could have affected summer biomass fluctuations observed for the microzooplankton. 4.5. Major conclusions (1) Microbial abundance and biomass in the central Arctic Ocean showed a strong response to the large annual amplitude in solar radiation. Phytoflagellates were the most important component of phytoplankton blooms, especially after the initial spring bloom in which diatoms were also abundant in terms of biovolume. Availability both of light (spring) and of macronutrients (late summer) appeared to be limiting to phytoplankton growth in the ice-covered central Arctic Ocean. (2) Heterotrophic microbes: bacteria and protists, were present in the upper water column throughout the winter, had B2-fold higher biomass during the summer, and increased in concert with phytoplankton during the initial spring bloom. Stocks of both bacteria and heterotrophic protists remained high throughout summer and into early fall.
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(3) Heterotrophic protists appeared to be feeding primarily on phytoplankton of all size categories, but were likely also significant grazers of bacteria, and probably represented an important food resource for zooplankton.
Acknowledgements We are indebted to the captains and crews of the Canadian Coast Guard research vessel Des Grosielliers, the SHEBA support staff of the Applied Physics Laboratory, University of Washington, and our Canadian colleague Dr. Harold (Buster) Welch for their invaluable logistics support of the ocean biology program during the SHEBA year. We thank Julie Arrington and Andy Ross for their dedicated technical assistance at the SHEBA drift camp, and Julie Arrington for assistance with microscopic analysis and data work-up of plankton samples. Funding was provided by NSF Grant OCE 9708088 to P. Wheeler, B. Sherr, and E. Sherr.
References Agawin, N.S.R., Agusti, S., Duarte, C.M., 2002. Abundance of Antarctic picophytoplankton and their response to light and nutrient manipulation. Aquatic Microbial Ecology 29, 161–172. Atlas, E.L., Hager, S.W., Gordon, L.I., Park, P.K., 1971. A practical manual for use of the Technicon Autoanalyser in seawater nutrient analyses. Department of Oceanography, Oregon State University, Corvallis, OR. Revised OSU Technical Report 215, Ref. No. 71–22, 48pp. Bigidare, R.R., Iriate, J.L., Kang, S.-H., Karentz, D., Ondrusek, M.E., Fryxell, G.A., 1996. Phytoplankton: quantitative and qualitative assessments. In: Ross, R.M., Hofmass, E.E., Quentin, L.B. (Eds.), Foundations for Ecological Research West of the Antarctic Peninsula. Antarctic Research Series, Vol. 70. American Geophysical Union, Washington, DC, pp. 173–198. Booth, B.C., Horner, R.A., 1997. Microalgae on the Arctic Ocean Section, 1994: species abundance and biomass. DeepSea Research II 44, 1607–1622. Booth, B.C., Smith, W.O., 1977. Autotrophic flagellates and diatoms in the Northeast Water Polynya, Greenland: summer 1993. Journal of Marine Systems 10, 241–261. Boyd, P.W., Strom, S., Whitney, F.A., Doherty, S., Wen, M.E., Harrison, P.J., Wong, C.S., Varela, D.E., 1995. The NE
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Pacific in winter: I. Biological standing stocks. Marine Ecology Progress Series 128, 11–24. Caron, D.A., 2000. Symbiosis and mixotrophy among pelagic organisms. In: Kirchman, D.L. (Ed.), Microbial Ecology of the Oceans. Wiley–Liss, New York, pp. 495–523. Caron, D.A., Dam, H.G., Kremer, P., Lessard, E.J., Madin, L.P., Malone, T.C., Napp, J.M., Peele, E.R., Roman, M.R., Youngbluth, M.J., 1995. The contribution of microorganisms to particulate carbon and nitrogen in surface waters of the Sargasso Sea near Bermuda. Deep-Sea Research I 42, 943–972. Cota, G.F., Pomeroy, L.R., Harrison, W.G, Jones, E.P., Peters, F., Sheldon Jr., W.M., Weingartner, T.R., 1996. Nutrients, primary production and microbial heterotrophs in the southeastern Chukchi Sea: Arctic summer nutrient depletion and heterotrophy. Marine Ecology Progress Series 135, 247–258. Dennett, M.R., Malthot, S., Caron, D.A., Smith Jr., W.O., Lonsdale, D., 2001. Abundance and distribution of phototrophic and heterotrophic nano- and microplankton in the southern Ross Sea. Deep-Sea Research II 48, 4019–4037. El-Sayed, S.Z., Fryxell, G.A., 1993. Phytoplankton. In: Friedmann, E.M. (Ed.), Antarctic Microbiology. Wiley–Liss, New York, pp. 65–122. English, T.S., 1961. Some biological oceanographic observations in the central North Polar Sea, Drift Station Alpha 1957–1958. Arctic Institute of North America, Research Paper 13. Fuhrman, J.A., Suttle, C.A., 1993. Viruses in marine planktonic systems. Oceanography 6, 51–63. Garrison, D.L., Gowing, M.M., 1993. Protozooplankton. In: Friedmann, E.M. (Ed.), Antarctic Microbiology. WileyLiss, New York, pp. 123–165. Gonzalez, J.M., Sherr, B.F., Sherr, E.B., 1993. Digestive enzyme activity as a quantitative measure of protistan grazing: The acid lysozyme assay for bacterivory. Marine Ecology-Progress Series 100, 197–206. Gosselin, M., Levasseur, M., Wheeler, P.A., Horner, R., Booth, B.C., 1997. New measurements of phytoplankton and ice algal production in the Arctic Ocean. Deep-Sea Research II 44, 1623–1644. Macdonald, R.W., McLaughlin, F.A., Carmack, E.C., 2002. Freshwater and its sources during the SHEBA drift in the Canada Basin of the Arctic Ocean. Deep-Sea Research I 49, 1769–1785. McLaughlin, F., Carmack, E., Macdonald, R., Melling, H., Swift, J., Sherr, B., Sherr. E., 2003. On the juxtaposition of Atlantic and Pacific-origin waters in the Canada Basin, 1997–1998. Deep-Sea Research I, submitted for publication. Melnikov, I.A., Kolosova, E.G., Welch, H.A., Zhitina, L., 2002. Sea ice biological communities and nutrient dynamics in the Canada Basin of the Arctic Ocean. Deep-Sea Research I 49, 1623–1649. Menden-Deuer, S., Lessard, .E.J., 2000. Carbon to volume relationships for dinoflagellates, diatoms, and other protist plankton. Limnology and Oceanography 45, 569–579.
Morita, R.Y., 1997. Bacteria in Oligotrophic Environments. Chapman & Hall, New York, 529pp. Pautzke, C.G., 1979. Phytoplankton primary production below the Arctic ice pack: an ecosystems analysis. Ph.D. Thesis, University of Washington, 180pp. Parsons, T.R., Maita, Y., Lalli, C.M., 1984. A manual of chemical and biological methods for seawater analysis. Pergamon Press, Toronto. Perovich, D.K., Andreas, E.L., Curry, J.A., Eiken, H., Fairrall, C.W., Grenfell, T.C., Guest, P.S., Intrieri, J., Kadko, D., Lindsay, R.W., McPhee, M.G., Morrison, J., Moritz, R.E., Paulson, C.A., Pegau, W.S., Persson, P.O.G., Pinkel, R., Richter-Menge, J.A., Stanton, T., Stern, H.M., Tucker III, W.B., Uttal, T., 1999. Year on ice gives climate insights. Eos, Transactions, American Geophysical Union 80 (481), 485–486. Rich, J., Kirchman, D.L., Gosselin, M., Sherr, E., Sherr, B., 1997. High bacterial production, uptake and concentration of dissolved organic matter in the central Arctic Ocean. Deep-Sea Research II 44, 1645–1663. Sherr, E.B., Sherr, B.F., 1993. Preservation and storage of samples for enumeration of heterotrophic protists. In: Kemp, P., Sherr, B., Sherr, E., Cole, J. (Eds.), Current Methods in Aquatic Microbial Ecology. Lewis Publ., NY, pp. 207–212. Sherr, E.B., Sherr, B.F., 2000. Phagotrophy in aquatic microbial food webs. In: Newell, S. (Ed.), Manual of Environmental Microbiology II. ASM Press, Washington, DC, pp. 409–418. Sherr, E.B., Sherr, B.F., 2003. Community respiration/production and bacterial activity in the upper water column of the central Arctic Ocean. Deep-Sea Research I, submitted for publication. Sherr, E.B., Caron, D.A., Sherr, B.F., 1993. Staining of heterotrophic protists for visualization via epifluorescence microscopy. In: Kemp, P., Sherr, B., Sherr, E., Cole, J. (Eds.), Current Methods in Aquatic Microbial Ecology. Lewis Publishers, Boca Raton, FL, pp. 213–228. Sherr, E.B., Sherr, B.F., Fessenden, L., 1997. Heterotrophic protists in the central Arctic Ocean. Deep-Sea Research II 44, 1665–1682. Smith, W.O., 1994. Primary productivity of a Phaeocystis bloom in the Greenland Sea during spring, 1989. In: Johannessen, O.M., Munch, R.D., Overland, J.E. (Eds.), The Polar Oceans and their Role in Shaping the Global Environment. American Geophysical Union, Washington, DC, pp. 263–272. Smith, W.O., Sakshaug, E., 1990. Polar phytoplankton. In: Smith, W.O. (Ed.), Polar Oceanography, Part B. Academic, San Diego, CA, pp. 477–525. Solic, M., Krstulovic, N., 1994. Role of predation in controlling bacterial and heterotrophic nanoflagellate standing stocks in the coastal Adriatic Sea: seasonal patterns. Marine Ecology Progress Series 114, 219–235. Steward, G.F., Smith, D.C., Azam, F., 1996. Abundance and production of bacteria and viruses in the Bering and Chukchi Seas. Marine Ecology Progress Series 131, 287–300.
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Verity, P.G., 1991. Measurement and simulation of prey uptake by marine planktonic ciliates fed plastidic and aplastidic nanoplankton. Limnology and Oceanography 36, 729–750. Vors, N., 1992. Ultrastructure and autecology of the marine, heterotrophic flagellate Leucocryptos marina (Braarud) Butcher 1967 (Katablepharidaceae/Kathabelpharidae) with a discussion of the genera Leucocryptos and Katablepharis/ Kathablepharis. European Journal of Protistology 28, 369–389.
Temporal and spatial variation in stocks of autotrophic and heterotrophic microbes in the upper water column of the central Arctic Ocean Evelyn B. Sherr*, Barry F. Sherr, Patricia A. Wheeler, Karen Thompson College of Oceanic and Atmospheric Sciences, Oregon State University, 104 Ocean Admin Bldg, Corvallis, OR 97330-5503, USA Received 12 June 2002; received in revised form 12 November 2002; accepted 4 February 2003
Abstract As part of the SHEBA/JOIS drift experiment, we continually analysed abundance and biomass of autotrophic and heterotrophic microbes in the upper 120 m of the water column of the ice-covered Central Arctic Ocean from November 1997 through August 1998. Microbial biomass was concentrated in the upper 60 m of the water column. There were low but persistent stocks of heterotrophic and autotrophic microbes during the winter months. Phytoplankton biomass began increasing when winter snow melted from the ice-pack in early June, after which there was a progressive decline of nitrate and silicate in the euphotic zone. We observed three distinct blooms over the summer. The initial bloom consisted of diatoms and phytoflagellates, mainly 2 mm-sized Micromonas sp.; the two subsequent blooms were dominated by the flagellated (non-colonial) Phaeocystis sp. The carbon:chlorophyll ratio of the phytoplankton was 31711. Stocks of bacteria and heterotrophic protists approximately doubled during the growing season, increasing in tandem with increase in phytoplankton biomass. Increase in cell abundances of bacteria and of the phytoflagellate Micromonas over 40–50 d periods during the initial bloom period yielded estimates of realised growth rate of 0.025 d1 for bacteria and of 0.11 d1 for Micromonas. Heterotrophic protists included flagellates, ciliates, and dinoflagellates, with biomass divided nearly evenly between nanoplankton (Hnano, 0–20 mm) and microplankton (Hmicro, 20–200 mm) size classes. r 2003 Elsevier Science Ltd. All rights reserved. Keywords: Microplankton; Phytoplankton; Bacteria; Protists; Flagellates; Biomass; Arctic Ocean; Canada Basin
1. Introduction The 1997–1998 SHEBA/JOIS (Surface Heat Budget of the Arctic Ocean/Joint Ocean Ice Study) experiment (Perovich et al., 1999; Melnikov et al., 2002; Macdonald et al., 2002) afforded an unique *Corresponding author. Fax: +1-541-737-2064. E-mail address: [email protected] (E.B. Sherr).
opportunity to investigate temporal and spatial variation in abundance and biomass of pelagic microbes in the permanently ice covered Arctic Ocean. Information regarding the standing stocks of microbial plankton in this major region of the sea is relatively sparse. Past ice camp studies have demonstrated a strong seasonality for phytoplankton, with an initial bloom in late June to early July and one or two peaks during the growing season,
0967-0637/03/$ - see front matter r 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0967-0637(03)00031-1
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which typically ended by early to mid-September (English, 1961; Pautzke, 1979). A more detailed analysis of autotrophic and heterotrophic microbes was carried out during the July–September 1994 Arctic Ocean Section (AOS) (Booth and Horner, 1997; Gosselin et al., 1997; Rich et al., 1997; Sherr et al., 1997). There is very little data on pelagic communities in the Arctic Ocean during winter. The goals of the plankton biology program of the SHEBA/JOIS expedition included determining variation in standing stocks and activity of pelagic microbes (bacteria, phytoplankton, and heterotrophic protists) during the dark winter months as well as the growing season and assessing the timing and magnitude of response of heterotrophic microbes to the spring bloom. Here we report changes in abundance and biomass of autotrophic and heterotrophic microbes in the upper 120 m of the water column during the ice camp drift. We also determined general size ranges and taxonomic categories of autotrophic and heterotrophic protists.
2. Materials and methods 2.1. Sample collection Between October 19, 1997 and September 28, 1998, the SHEBA/JOIS ice camp drifted with the permanent pack ice from its initial position in the southern Canada Basin (75 280 N, 143 400 W) to the Mendeleyev Basin (80 110 N, 166 050 W), traveling over the Northwind Ridge and Chukchi Plateau from February to mid-summer, and drifting into the Mendeleyev Basin in late summer (Perovich et al., 1999; Macdonald et al., 2002; McLaughlin et al., 2003; SHEBA website, http:// sheba.apl.washington.edu). Water samples for determination of nutrient and chlorophyll concentrations, and of microbial abundance and biomass, were collected in the upper 120 m of the water column, with 5-l Niskin bottles deployed on a wire through an ice-hole, on an 8 d schedule throughout the year, with more frequent sampling in the upper 50 m for nutrient and chlorophyll a concentrations during summer (June to September).
2.2. Measurement of inorganic nutrients and chlorophyll a Freshly collected seawater samples were processed for macronutrients nitrate+nitrite, silicate, and phosphate with a Technicon Autoanalyser (Atlas et al., 1971). Water samples for chlorophyll a were filtered onto Whatman GF/F glass fiber filters. Chlorophyll concentrations were determined fluorometrically, after 24-h extraction in 90% methanol at 5 C (Parsons et al., 1984). 2.3. Determination of microbial abundance and biomass Procedures for analysis of microbial biomass were similar to those used during the 1994 AOS (Sherr et al., 1997). Briefly, samples for microbial enumeration were preserved by a three-step procedure: 0.05% final concentration alkaline Lugol solution, followed by the addition of 0.1% final concentration of 3% sodium thiosulfate and 2% final concentration of borate-buffered formalin (Sherr and Sherr, 1993). Samples were allowed to sit for at least 4 h to allow protist cells to harden, and processed within 24 h. Aliquots of the preserved samples were stained with DAPI (25 mg ml1 final concentration) for 7–10 min, and then filtered onto 25 mm diameter black-stained membrane filters (Poretics, Inc.). For each water sample, duplicate 50-ml subsamples were filtered onto 0.8 mm pore size filters for enumeration of o20 mm sized protists, and duplicate 150 ml subsamples were filtered onto 3.0 mm pore size filters for enumeration of >20 mm sized protists. Two additional 10-ml aliquots of each sample were filtered onto 0.2 mm pore size filters for bacterial counts. Filters were removed from the filtration towers under low vacuum, then mounted with Resolvet low viscosity immersion oil onto glass slides with No. 1 coverslips. Samples were stored in slide boxes at 20 C on the ship until returned on dry ice to a 40 C freezer in the laboratory at Oregon State University. All slides were inspected within 1 year of collection. Bacteria and protists were enumerated via direct counts with a Zeiss Universal microscope outfitted for epifluorescence microscopy with a 100 W
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mercury lamp and Zeiss filter set 47 77 02 (G 365 excitation filter/LP 420 barrier filter) for UV light excitation of DAPI-stained cells, and Zeiss filter set 47 77 09 (BP 450–490 excitation filter/LP 520 barrier filter) for blue light excitation of chlorophyll autofluorescence. Bacteria and o20 mm sized protists were enumerated at 1000 , and >20 mm protists at 160–400 (Sherr et al., 1993). During inspection of both bacterial and nanoplankton samples, it was noted that coccoid cyanobacteria were virtually absent from all samples. Each protist cell counted was sized for biovolume calculations with a calibrated ocular micrometer and grouped into general taxonomic categories: diatoms, mixed species nanoflagellates, choanoflagellates, dinoflagellates, and ciliates. Two taxa of phytoflagellates, Micromonas sp. and Phaeocystis sp. (probably Phaeocystis poucheii), were identified to genus level based on past experience with phytoplankton in this region of the Arctic Ocean (Booth and Horner, 1997; Sherr et al., 1997). Bacterial biomass was estimated from a value of 20 fg C cell1, from the prior results of Sherr et al. (1997). Protistan carbon biomass was estimated from empirically determined biovolumes using equations suggested by Menden-Deuer and Lessard (2000). Based on a comprehensive review of the literature, Menden-Deuer and Lessard derived C: volume relationships for marine protists, one for protists excluding diatoms (pg C cell1= 0.216 volume in cubic microns raised to the power 0.939 [C ¼ 0:216 V 0:939 ]) and one for diatoms (pg C cell1=0.288 volume in cubic microns raised to the power 0.811 [C ¼ 0:288 V 0:811 ]). These equations result in C: volume factors of 0.20 pg C mm3 for 2 mm diameter protists, 0.15 pg C mm3 for 10 mm diameter protists, 0.13 pg C mm3 for 20 mm diameter protists, and 0.060 pg C mm3 for 20 mm diameter diatoms. Stocks of nutrients and of chlorophyll a were integrated for the upper 50 m of the water column, since the more frequent summer profiles only included depths to 50 m. In contrast, microbial biomass data were based on the routine, 8-d interval profiles that sampled depths to 120 m, including depths of 40 and 60 m. In these profiles,
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higher summer microbial biomass extended to 60 m depth; thus we integrated biomass over the upper 60 m. Chlorophyll a concentrations at depths >50 m were o0.2 mg l1, so there would be little difference in phytoplankton biomass for a 0–50 m versus a 0–60 m integration depth. We also calculated winter and summer average cell abundances of the various microbial stocks for two depth intervals: 0–40 m (summer euphotic zone), and 80–120 m (depths beneath the euphotic zone).
3. Results 3.1. Variation in nutrient and chlorophyll a concentrations During the SHEBA/JOIS experiment, the icestation passed from the deep, oligotrophic Canada Basin to relatively shallower regions of the Northwind Ridge and Chukchi Plateau, and then finally over the deep Mendeleyev Basin (Fig. 1C). This path crossed a hydrographically dynamic part of the Arctic Ocean, with three major identified regimes: (1) the south-central Canada Basin from Fall, 1997 to early winter, 1998; (2) the Northwind Ridge and Chukchi Plateau from February to midsummer; (3) the Chukchi Plateau and Mendeleyev Basin from June to September (Macdonald et al., 2002; McLaughlin et al., submitted) (Fig. 1C). Nutrient distributions in the upper 120 m encountered during the ice camp drift were influenced by change in water mass characteristics, especially by variation in relative contribution of Pacific and Atlantic-origin water masses (McLaughlin et al., submitted). In the fall and early winter, nitrate+nitrite stocks were o0.5 mM in the upper 50 m, but increased to 2–7 mM after the ice camp traversed the Northwind Ridge (Fig. 1A). Silicate stocks increased similarly. Higher nitrate and silicate content of water over the Northwind Ridge and Chukchi Plateau was likely due to water of Pacific origin (McLaughlin et al., submitted). Starting in mid-June, nutrient stocks began to decrease in the upper 50 m, coincident with an increase in chlorophyll a stocks as the spring bloom began (Fig. 1A and B). Stocks of both nitrate+nitrite and of silicate integrated over 0–50 m showed a
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Fig. 1. Variation in concentrations of (A) nitrate+nitrate (mM) and (B) chlorophyll a (mg liter1) in the upper 120 m of the water column, and in (C) bathymetry during the SHEBA/JOIS experiment. Dots indicate profile sampling depths. The approximate demarcations of the three hydrographic regimes encountered during the ice camp drift are indicated. Regime 1—south-central Canada Basin from fall, 1997 to early winter, 1998; Regime 2—Northwind Ridge and Chukchi Plateau from February to June; Regime 3— Chukchi Plateau and Mendeleyev Basin from June to September.
progressive decline from May 29 to September 25, while stocks of phosphate showed little seasonality (Fig. 2). In the upper 20 m, nitrate+nitrite decreased from an average of 2.8 mM to 0.01 mM, and silicate from an average of 8.7 mM to 1.7 mM, from June 15 to August 30.
3.2. Variation in microbial stocks All groups of pelagic microbes appeared to exhibit a strong response to the large annual amplitude in solar insolation, with low biomass in spring and higher biomass during the short
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autotrophs varied dramatically in summer because of blooms. Heterotrophic protists made up, on average, 86% of total protistan biomass in winter, and 44% in summer.
silicate
0-50 m nutrient stock, mM
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600
400
3.3. Distribution of protist biomass
nitrate + nitrite 200
phosphate 0
347
32
82
133
182
232
Day of year
Fig. 2. Change in integrated stocks (mM) of nitrate+nitrite, silicate, and phosphate in the upper 50 m of the water column during the SHEBA/JOIS experiment.
summer growing season (June–September) (Fig. 3). Microbial biomass was concentrated in the upper 40–60 m of the water column (Figs. 1B and 3). There was a rapid increase in phytoplankton stocks in June (beginning about day 140), after snow cover melted from the ice surface (Fig. 4). Seasonal increases in stocks of bacteria and of heterotrophic protists (Fig. 3A and C) were nearly simultaneous with increase in autotrophic protists (phytoplankton) (Figs. 1B and 3B). Stocks of bacteria and of heterotrophic protists remained relatively high in the upper 60 m throughout the summer, even after phytoplankton biomass had declined (Fig. 3A and C). Table 1 presents average winter (November– May) and summer (June–September) values of abundance (cells ml1) and volumetric biomass (mg C m3) for bacteria and for autotrophic and heterotrophic protists in nanoplankton and microplankton size categories. To clearly distinguish seasonal effects, we averaged values for two depth zones: the summer euphotic zone (upper 40 m), and subeuphotic depths (80–120 m). Stocks of bacteria and of heterotrophic protists approximately doubled from winter to summer in the euphotic zone. Bacterial stocks showed no increase at the lower depth interval. Autotrophic protists exhibited an approximately 20-fold increase in numbers and biomass during summer compared to winter stocks. Cell numbers and biomasses of
Variations in depth-integrated (0–60 m) protistan biomass from January to September 1998 are shown in Fig. 5. Three discrete peaks in biomass of autotrophic protists were observed, the first from the end of June to the first week of July, and the other two during the last two weeks of July (Fig. 5A and B). These peaks mirrored the three peaks in chlorophyll concentration found during summer (Fig. 1B and 4). Autotrophic protist biomass was much higher than biomass of heterotrophic protists during these blooms (Fig. 5A). Since microplanktonic diatoms have a lower C: volume ratio compared to nanoplanktonic cells (Menden-Deuer and Lessard, 2000), in terms of carbon biomass all three blooms were dominated by o20 mm sized cells (Fig. 5B). Over the entire growing season, diatoms averaged 24711% of the total biomass of autotrophic cells. Heterotrophic protist biomass was fairly evenly divided between nanoplankton and micro-plankton size classes during the winter, but during the summer nanoplanktonic heterotrophic protists (Hnano) showed a sharp peak in biomass coincident with the initial spring bloom (Fig. 5C). A striking result was the small size of most protistan cells, both heterotrophic and autotrophic. From 72% to 95% (average 87%) of heterotrophic flagellates were o5 mm in size. Picoflagellates (1.5–2 mm preserved size) dominated the Hnano assemblage throughout the year, ranging in abundance from 100 to 1600 cells ml1, and averaging 52% of total Hnano numbers. Autotrophic protists were also numerically dominated by o5 mm sized cells, which made up 44– 99% (average 95%) of cells in the phytoplankton assemblage during the growing season. A large proportion of small phytoplankton cells were 2 mm sized Micromonas sp., which had a maximum abundance of 28,000 cells ml1 during the initial spring bloom, and were present at abundances of
E.B. Sherr et al. / Deep-Sea Research I 50 (2003) 557–571
Day of Year
A. Bacteria 30
80
130
180
230
20
Depth, m
40
4
11
4
6
345
6
562
4
4
4
8
4
60
4
5
4
80
2
100
2
120
B. Autotrophic protists 345 30
80
130
180
230 80
10
10
20
30 20
Depth, m
40 60
60
40
80 20
100 0
120
C. Heterotrophic protists 345
30
80
130
180
230 20
Depth, m
10
20 40
15
60 10
80 5
100 120
Dec 11
Jan 30
Mar 21
May 10
Jun 30
Aug 20
3
Fig. 3. Variation in carbon biomass (mg C m ) of planktonic microbes in the upper 120 m of the water column during the SHEBA/ JOIS experiment: (A) Heterotrophic bacteria, (B) Autotrophic protists (phytoplankton), and (C) Heterotrophic protists. Dots indicate profile sampling depths.
1000–10,000 cells ml1 in the upper 40 m during the rest of the growing season. 3.4. Carbon:chlorophyll ratio Analysis of the biomass of autotrophic protists in water samples for which chlorophyll a concen-
tration was also determined during the SHEBA/ JOIS experiment allowed calculation of empirical carbon:chlorophyll (C:Chl) ratios for phytoplankton in the central Arctic Ocean. C:Chl ratios were calculated for 36 discrete water samples collected in the upper 60 m from 19 May to 22 September (Table 2). Phytoplankton biomass varied from 1.3
E.B. Sherr et al. / Deep-Sea Research I 50 (2003) 557–571 snow cover melt [------]
1.5
300 1.0
200
0.5 100
0
Phytoplankton biomass, gC m-2
Short wave radiation, Watts m-2
400
0.0 296
346
31
81
131
181
233
281
Day of Year
Fig. 4. Variation in short wave radiation (dotted line, W m2, measured with an Eppley Precision Spectral Pyranometer, range of 295–2500 nm), and in 0–50 m integrated phytoplankton biomass estimated from chlorophyll a concentrations and an average C:chl a ratio of 31 (solid line), during the SHEBA/ JOIS experiment. The period during which winter snow cover melted from the ice surface is indicated. Data on radiation and ice melt was provided by the SHEBA Project Office, Applied Physics Laboratory, University of Washington.
to 112 mg C m3, and chlorophyll a concentration from 0.07 to 4.3 mg m3, in these samples. The average C:Chl ratio determined from this sample set was 31711. The C:Chl ratio was biased upward by values greater than 50 calculated for July 7 and July 31 data. Without these high values, the average C:Chl ratio was 2878. The lowest ratio was 13, found for phytoplankton at 20 m on May 31 and at 40 m on July 23 (Table 2). 3.5. Taxonomic composition of protists Autotrophic protists o20 mm in size were mainly phytoflagellates, including Micromonas sp, the flagellated form of Phaeocystis pouchetii, other haptophytes, and occasional cryptomonads and small autotrophic dinoflagellates. Diatoms occurred, but were not abundant, in the nanoplankton fraction. Microplanktonic autotrophs were mostly diverse species of diatoms, including centric and pennate species previously identified by Booth and Horner (1997) in this region. Abundance of autotrophic dinoflagellates was typically less than 1 cell ml1, but did show a peak of
563
25 cells ml1 at a depth of 20 m during the initial spring bloom. Autotrophic ciliates, Mesodinium sp., were rare, 0–3 cells ml1. Phagotrophic ciliates with sequestered chloroplasts were common in summer, but were included with the heterotrophic protists. During winter, phytoplankton were present as nanoflagellates, mainly Micromonas sp. and unidentified haptophytes, at abundances of hundreds of cells ml1, plus >20 mm diatoms and pigmented dinoflagellates at abundances of about 1 cell ml1. Phytoplankton composition changed dramatically during the growing season (Fig. 6). Phytoplankton biovolume during the initial bloom was composed about equally of >20 mm diatoms and o20 mm phytoflagellates, primarily 2 mm Micromonas. The two later blooms were dominated by flagellated Phaeocystis, 4–6 mm in size, at abundances of 5000–18,000 cells ml1. Microplanktonsized flagellates, mostly autotrophic dinoflagellates, were not an important component of the phytoplankton assemblage during the blooms (Fig. 6). Nanoplankton-sized heterotrophic protists were a mixed species assemblage of non-pigmented flagellates. Most of these flagellates, especially the very small, 2 mm sized species could not be identified. Choanoflagellates were present, but generally rare, with abundances ranging from 0 to 150 cells ml1 (average 13 cells ml1). Non-armored heterotrophic dinoflagellates 10–20 mm in size, including spherical gymnodinoid forms and spindle-shaped Katodinium-like species, were found at an average abundance of 17 cells ml1 (range 1– 60 cells ml1). Non-pigmented cryptomonad-like flagellates similar to the species Leucocryptos marina (Vors, 1992) were occasionally observed, and o20 mm ciliates occurred in the nanoplankton fraction at an average abundance of 1 cell ml1. Heterotrophic protists in the microplankton size class were dominated by non-pigmented dinoflagellates, which made up, on average, 83% of the numerical abundance and 63% of the biomass of microzooplanktonic protists. Heterotrophic dinoflagellates >20 mm in size were present throughout the year at abundances of 1–12 cells ml1, and included both armored Protoperidium-like species as well as spindle-shaped non-armored forms. Ciliates occurred at abundances of 0.1–2 cells ml1, and were mostly spherical or elongate
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Table 1 Comparison of winter (Nov.–May) and summer (June–Sept.) cell abundances (cells ml1) and biomasses (mg C m3) in 0–40 m and 80– 120 m depth zones, of bacteria, autotrophic protists, and heterotrophic protists (ranges of values in parentheses) Abundance (cells ml1)
Bacteria 0–40 m 80–120 m Autotrophic protists 0–40 m 2–20 mm 20–200 mm
80–120 m 2–20 mm 20–200 mm
Heterotrophic protists 0–40 m 2–20 mm 20–200 mm
80–120 m 2–20 mm 20–200 mm
Biomass (mg C m3)
Winter
Summer
Winter
Summer
1.7670.28 105 (1.32–2.86) 1.2770.26 105 (0.72–2.07)
3.2370.70 105 (1.9–6.7) 1.1170.19 105 (0.72–1.78)
3.570.6 (2.6–5.9) 2.670.5 (1.4–4.1)
6.471.4 (3.8–13.3) 2.270.3 (1.4–3.6)
2307230 (3–1500) 0.970.4 (0.05–2.6)
560075100 (100–28,000) 23716 (0.7–85)
0.4670.36 (0.02–2.1) 0.3470.34 (0.0–3.4)
10.9711 (1.0–112) 2.172.1 (0.4–15)
ND
1778 (3–42) 2.371.5 (0.8–5.6)
ND
0.1170.07 (0.01–0.21) 0.1870.18 (0.2–0.48)
ND
ND
260780 (90–490) 6.1–1.3 (3.0–9.7)
5907310 (210–2300) 8.072.3 (2.5–18)
1.970.6 (0.7–4.6) 1.871.0 (0.6–9.2)
4.572.7 (1.0–18) 3.371.6 (10–11)
ND
150727 (110–200) 3.471.5 (1.4–6.0)
ND
1.370.4 (0.9–2.2) 0.8370.3 (0.4–1.1)
ND
ND
ND=no data.
spirotrichs. During the summer, mixotrophic ciliates with sequestered chloroplasts were frequently observed. Tintinnids were rare. Other microplanktonic protists included unidentified heterotrophic flagellates at an average abundance of 0.2 cells ml1. 3.6. Contents of protistan food vacuoles During summer, ingested phytoplankton cells could be identified as chlorophyll autofluorescence in the food vacuoles of phagotrophic protists. During the initial spring bloom heterotrophic nanoflagellates, including abundant 2 mm sized
cells and choanoflagellates, commonly ingested Micromonas. In the later Phaeocystis-dominated blooms, heterotrophic nanoflagellates were also observed with ingested phytoplankton prey. Ciliates and non-armored dinoflagellates were frequently observed with ingested phytoflagellates and smaller-sized diatoms. Thecate dinoflagellates were rarely found with recognizable food vacuole contents. 3.7. Microbial growth rates We were able to calculate microbial growth rates from the abundance data in two cases: for
E.B. Sherr et al. / Deep-Sea Research I 50 (2003) 557–571 5 autotrophic
gC m
-2
4 3 2 heterotrophic
1
565
These estimates should be considered realized growth rates (r) for the prevailing in situ conditions of environmental parameters and mortality processes; intrinsic growth rates (m) of bacteria and Micromonas at these temperatures, substrate concentrations, and light conditions would be expected to be higher.
0
(a) 2.5
Anano < 20 µm
gC m
-2
2.0 1.5 1.0
Amicro > 20 µm
0.5 0.0
(b) 1.0
Hnano < 20 µm
gC m
-2
0.8 0.6 Hmicro > 20 µm
0.4 0.2 0.0 0
(c)
50
100 150 Day of year
200
250
Fig. 5. Variation in integrated carbon biomass (g C m2) of protists in the upper 60 m of the water column from January to September, 1998: (A) Comparison of biomass of heterotrophic (dashed line) and autotrophic (solid line) protists, (B) Comparison of biomass of autotrophic protists in the nanoplanktonic (Anano, 2–20 mm, solid line) and microplanktonic (Amicro, 20– 200 mm, dashed line) size classes, (C) Comparison of biomass of heterotrophic protists in the nanoplanktonic (Hnano, solid line) and microplanktonic (Hmicro, dashed line) size classes.
bacterioplankton and for Micromonas sp. in the upper 20 m of the water column over periods spanning the initial spring bloom. The abundance of bacterioplankton increased exponentially from May 7 to June 24, as did Micromonas cell abundance from May 22 to June 23. Regressions of natural logarithm of cell abundance with time during these periods (Fig. 7) yielded an estimate for bacterial growth rate of 0.025 d1 (doubling time of 28 d) and an estimate for Micromonas growth rate of 0.11 d1 (doubling time of 6.1 d).
4. Discussion 4.1. Variation in abundance and biomass of microbial plankton Stocks of all components of the microplankton were concentrated in the upper 40–60 m of the water column, and showed a strong seasonal response to the short growing season in the high Arctic (Figs. 1B, 3 and 4). Autotrophic protists (phytoplankton) had the largest amplitude in seasonal abundance and biomass. The phytoplankton included diatoms and Phaeocystis, which constitute a large fraction of planktonic autotrophs in both northern and southern polar systems (El-Sayed and Fryxell, 1993; Smith, 1994; Bigidare et al., 1996; Dennett et al., 2001). We also found that 2 mm phytoflagellates, identified as the prasinophyte alga Micromonas sp., were a large fraction of the initial spring bloom. Micromonas-like prasinophytes have been previously found to be abundant in the Northeast Water polynya off Greenland (Booth and Smith, 1977) and in the central Arctic Ocean (Booth and Horner, 1997). Phytoplankton in the picoplankton fraction, typically prasinophytes and prymnesiophytes, can also be important in the Southern Ocean (El-Sayed and Fryxell, 1993; Agawin et al., 2002). During summer blooms, peak abundances of Micromonas were 28,000 cells ml1, and of Phaeocystis were 18,000 cells ml1. These are high values for eukaryotic phytoplankton in oligotrophic ocean environments. For example, in the NE Pacific, abundances of autotrophic nanoflagellates were on the order of 2000 cells ml1 year-round (Boyd et al., 1995). During the 1994 AOS expedition across the polar cap, Booth and Horner (1997) reported maximum abundance values of
E.B. Sherr et al. / Deep-Sea Research I 50 (2003) 557–571
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Table 2 Calculated carbon:chlorophyll a ratios for phytoplankton during the 1998 growing season in the central Arctic Ocean Date
Day of year
Sample depth (m)
Chl a (mg m3)
Biomass (mg m3)
C:Chl a
May 19
139
May 22
142
May 31 June 8 June 17
151 159 168
June 23 July 7
174 188
July 15 July 23
196 204
July 31
212
Aug. 14
226
Aug. 20
232
Aug. 28
240
Sept. 7
250
Sept. 14
257
Sept. 22
265
10 20 4 10 20 20 10 10 20 10 10 20 40 4 10 20 40 10 40 10 20 40 10 40 10 20 40 60 10 40 10 40 4 10 30 50 60
0.23 0.21 0.18 0.18 0.16 0.21 0.33 0.59 0.44 1.41 0.51 0.64 0.12 0.35 0.44 4.3 0.16 0.23 0.21 0.11 1.6 0.21 0.09 0.38 0.11 0.12 0.35 0.07 0.09 0.20 0.07 0.15 0.12 0.13 0.08 0.11 0.08
3.6 3.8 4.8 4.7 3.6 2.6 11 25 17 48 36 34 4.3 7.9 12 112 2.0 14 11 3.3 58 4.1 3.4 6.4 2.4 4.1 12 1.3 3.8 4.1 2.9 3.3 2.2 3.2 3.1 4.6 1.7
15 18 26 26 23 13 33 42 38 34 70 53 36 23 27 26 13 62 52 31 36 19 36 17 21 33 33 19 40 21 40 23 18 25 40 41 21
10,000 cells ml1 for 2 mm phytoflagellates (tentatively identified as Micromonas pusilla) and of 470 cells ml1 for flagellated Phaeocystis pouchetii. Agawin et al. (2002) reported pico-phytoflagellate abundances from o1000 to B10,000 cells ml1, with abundances of >20,000 cells ml1 found at one station, in the Bransfield Strait, Antarctia. Our intensive sampling program through the growing season allowed us to observe bloom peaks that might have been missed by previous shorter-term sampling efforts.
Our empirically calculated carbon:chlorophyll ratios, which ranged from 13 to 70 and averaged 31711, were similar to the C:Chl ratios, range of 13 to 59, average of 28710 (excluding one very low value of 4), estimated by Booth and Horner (1997) for central Arctic phytoplankton in summer. These relatively low C:Chl ratios are typical for phytoplankton adapted to low irradiances (Smith and Sakshaug, 1990). Heterotrophic bacteria approximately doubled in abundance in the euphotic zone from a winter
E.B. Sherr et al. / Deep-Sea Research I 50 (2003) 557–571
Fig. 6. Proportional taxonomic composition of phytoplankton during the three major bloom periods of the growing season, in terms of biovolume: D=diatom biovolume (clear bars); NF=nanoflagellate (cells o20 mm) biovolume (striped bars); MF=microflagellate (cells >20 mm) biovolume (solid bars).
Ln bacterial cells ml -1
13.5 13.0
r = 0.025 d -1 Td = 28 d
12.5 12.0 11.5 125
135
145
155
165
175
Ln Micromonas cells ml -1
(a)
(b)
11 10 9
r = 0.11 d -1 Td = 6.1 d
8
low of 1.8 105 cells ml1 to a summer high of 3.2 105 cells ml1, with a maximum abundance of 6.7 105 cells ml1 (Table 1). Bacterioplankton abundances during the summer of 1998 were lower than the bacterial abundances (3.8–11.7 105 cells ml1, average 4.871.3 105 cells ml1) we had previously reported for the upper 50 m of the water column in the central Arctic Ocean during the growing season (Sherr et al., 1997). Our summer 1998 bacterial abundance values are similar to the range of abundances of 2–5 105 cells ml1 found by Cota et al. (1996) in water samples taken over the continental shelf and slope of the Chukchi Sea and Canada Basin in August 1993. Heterotrophic protists showed an early spring increase in biomass, before the initial spring bloom, perhaps due to differences in water mass history during the SHEBA/JOIS drift (Fig. 3). The summer stocks of heterotrophic protists (Hnan, 210–2300 cells ml1; Hmicro, 2.5–18 cells ml1) were similar to protistan abundances found during the summer 1994 AOS expedition (Hnan 160– 1900 cells ml1, Hmicro 2–15 cells ml1) (Sherr et al., 1997). The abundances of protists in this region are comparable to those reported for other oligotrophic regions of the world ocean, for example the Sargasso Sea (Caron et al., 1995) and the Ross Sea, Antarctica (Dennett et al., 2001). The general taxonomic composition of heterotrophic protists was similar to that found in the central Arctic Ocean during the summer of 1994, and to the protistan community described for Antarctic waters (Garrison and Gowing, 1993; Dennett et al., 2001). These results confirm the importance of 2 mm-sized heterotrophic flagellates in the Hnano, and heterotrophic dinoflagellates in the Hmicro, size categories. 4.2. Survival of planktonic microbes during the Arctic winter
7 6 135
567
145
155
165
175
Day of Year
Fig. 7. Regressions of natural logarithm of cell abundance versus time in the upper 20 m during the initial spring bloom period, from which growth rates were determined for (A) bacteria and (B) Micromonas.
Virtually all identifiable groups of microbial cells in the plankton were present in the upper 120 m during winter sampling. Bacterioplankton maintained relatively high winter cell abundance (Table 1), although the cell-specific activity assessed via rate of uptake of tritiated leucine
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was much lower than in summer (Sherr and Sherr, 2003). Heterotrophic bacteria are able to enter a state of low metabolism (starvation survival) when substrates are scarce (Morita, 1997). In contrast, phytoplankton stocks were low during the winter, with average cell abundances of only several hundred per ml, about an order of magnitude lower than winter cell abundances of phytoflagellates in the NE subarctic Pacific (Boyd et al., 1995). All components of the phytoplankton assemblage were present during winter, although identifiable cells of Phaeocystis, which was an important component of summer blooms, were rare. The biomass stocks of heterotrophic protists were higher than those of either bacteria or autotrophic protists during winter (Table 1), and included all categories of identified cells. Phagotrophic protists should be able to continue feeding during the winter. Some phytoflagellates, such as the prasinophyte Micromonas sp., which was abundant during summer blooms, may also be capable of phagotrophy (Gonzalez et al., 1993; Caron, 2000) and thus have a survival advantage over non-phagotrophic phytoplankton. These results are likely generally applicable to ice-covered Arctic systems during winter, although the fact that the SHEBA camp was in a different hydrographic regime during summer compared to winter is a complication for seasonal interpretation of our data set. 4.3. Response of planktonic microbes during the spring bloom Despite the cold water temperatures (1.0 C to 1.8 C) that prevailed year-round in the central Arctic, both bacteria and heterotrophic protists increased in biomass over the same time period as did the phytoplankton during the spring bloom, without any significant lag (Fig. 3). Bacterial cellspecific activity also increased rapidly during the spring bloom (Sherr and Sherr, 2003). During the initial bloom period, the autotrophic picoflagellate Micromonas sp. grew at a rate of 0.11 d1, equivalent to a doubling time of about 6 d. This rate is at the low end of the growth rates that Smith (1994) estimated for Phaeocystis pouchetii blooms in the Greenland Sea (0.2570.20 d1
(range of 0.13–1.05 d1, doubling times of 5.3– 0.7 d). Bacterioplankton abundance increased at a slower rate, 0.025 d1, equivalent to a doubling time of about 28 d. Bacterial growth rates during June, independently estimated from rates of incorporation of tritiated leucine, were as high as 0.02–0.04 d1 for specific depths in the euphotic zone (Sherr and Sherr, 2003). Rich et al. (1997) previously estimated bacterial growth rates ranging from 0.05 to 0.5 d1 (doubling times of 14– 1.4 d) in the central Arctic Ocean during summer, also based on rates of leucine incorporation. However, the rates of leucine incorporation by planktonic bacteria during the 1994 AOS expedition (Rich et al., 1997) were about an order of magnitude higher compared to rates measured during summer of the SHEBA year (Sherr and Sherr, 2003). A comparison of data collected in 1994 (Rich et al., 1997) and in 1998 (this paper, and Sherr and Sherr, 2003) suggests there may be significant inter-annual variability both in stocks and in growth rates of bacterioplankton in the central Arctic Ocean. Our calculated in situ growth rates based on observed increases in cell number over time should be less than the intrinsic rate of increase at in situ conditions, as heterotrophic nanoflagellates were abundant during the spring bloom and were capable of ingesting both bacteria and Micromonas (Sherr et al., 1997). Viral lysis could also have represented a source of mortality for bacterioplankton and for phytoplankton (Fuhrman and Suttle, 1993; Suttle, 1994; Steward et al., 1996). 4.4. Inferred food web relationships Inspection of food vacuole contents during the growing season showed that all size classes of heterotrophic protists, from the smallest nanoflagellates to the largest ciliates and dinoflagellates, consumed phytoplankton. In particular, the abundant 2 mm-sized heterotrophic flagellates, as well as choanoflagellates, were routinely observed with ingested Micromonas. These groups of flagellates have been thought to prey primarily on heterotrophic or autotrophic bacteria rather than on
E.B. Sherr et al. / Deep-Sea Research I 50 (2003) 557–571
eukaryotic phytoplankton (Caron, 2000). We had previously noted phytoplankton ingestion by o5 mm heterotrophic flagellates as well as by dinoflagellates and ciliates in the central Arctic Ocean during the summer of 1994 (Sherr et al., 1997). It is clear that the entire spectrum of heterotrophic protists can be significant consumers of phytoplankton in the Arctic Ocean; this trophic pathway needs to be quantified. Microplanktonic protists are known to ingest heterotrophic as well as autotrophic nanoflagellates (Verity, 1991; Solic and Krstulovic, 1994; Sherr and Sherr, 2000), and grazing by microzooplankton may have been at least partly responsible for the cycling of heterotrophic nanoflagellate biomass during summer (Fig. 3C). Because of their larger cell sizes, microplankton protists are, in turn, more likely to be subject to grazing by mesozooplankton than are heterotrophic nanoflagellates (Stoecker and Capuzzo, 1990). We speculate, as for the 1994 AOS results (Sherr et al., 1997; Thibault et al., 1999), that copepod grazing could have affected summer biomass fluctuations observed for the microzooplankton. 4.5. Major conclusions (1) Microbial abundance and biomass in the central Arctic Ocean showed a strong response to the large annual amplitude in solar radiation. Phytoflagellates were the most important component of phytoplankton blooms, especially after the initial spring bloom in which diatoms were also abundant in terms of biovolume. Availability both of light (spring) and of macronutrients (late summer) appeared to be limiting to phytoplankton growth in the ice-covered central Arctic Ocean. (2) Heterotrophic microbes: bacteria and protists, were present in the upper water column throughout the winter, had B2-fold higher biomass during the summer, and increased in concert with phytoplankton during the initial spring bloom. Stocks of both bacteria and heterotrophic protists remained high throughout summer and into early fall.
569
(3) Heterotrophic protists appeared to be feeding primarily on phytoplankton of all size categories, but were likely also significant grazers of bacteria, and probably represented an important food resource for zooplankton.
Acknowledgements We are indebted to the captains and crews of the Canadian Coast Guard research vessel Des Grosielliers, the SHEBA support staff of the Applied Physics Laboratory, University of Washington, and our Canadian colleague Dr. Harold (Buster) Welch for their invaluable logistics support of the ocean biology program during the SHEBA year. We thank Julie Arrington and Andy Ross for their dedicated technical assistance at the SHEBA drift camp, and Julie Arrington for assistance with microscopic analysis and data work-up of plankton samples. Funding was provided by NSF Grant OCE 9708088 to P. Wheeler, B. Sherr, and E. Sherr.
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