RACER: Distribution of nitrite in the Gerlache Strait
References
Kishino, M., M. Takahashi, N. Okami, and S. Ichimura. 1985. Estimation of spectral absorption coefficients of phytoplankton in the sea. Bulletin of Marine Science, 37:634-642.
RACER: Distribution of nitrite in the Gerlache Strait J. E. DORE AND D. M. KARL School of Ocean and Earth Science and Technology University of Hawaii Honolulu, Hawaii 96822
The surface waters of the southern oceans are characterized by high concentrations of inorganic nutrients (nitrate, phosphate, silicate; Gordon et al. 1981) which could potentially be used for the photosynthetic production of organic matter and removal of dissolved carbon dioxide from the ocean's surface (Martin 1990). A thorough understanding of nutrient dynamics in antarctic marine ecosystems is crucial to our understanding of food web dynamics and global carbon dioxide fluxes. Of the major bioelements, nitrogen is generally considered to be an important
Mitchell, B. G. 1990. Algorithms for determining the absorption coefficient of aquatic particulates using the quantitative filter technique (QFT). Ocean Optics X, SPIE, 1301:137-148. Mitchell, B. G. and 0. Holm-Hansen. 1991. Bio-optical properties of Antarctic Peninsula waters: Differentiation from temperature ocean models. Deep-Sea Research, 38:1,009-1,028.
limiting nutrient in seawater. To date, most studies of nitrogen in the southern ocean have focused either on the most oxidized form, nitrate, or on the most reduced form, ammonium. By comparison, few data exist on the distribution or turnover rate of the redox intermediate nitrite. Accumulations of nitrite in natural waters indicate zones in which an uncoupling exists between the oxidative and reductive reactions affecting nitrate and ammonium (Rakestraw 1936). We anticipated that an evaluation of the dynamics of nitrite distributions during the spring phytoplankton bloom would provide us with additional information on the nutritional state and regenerative capacity of the microbial community. The horizontal distribution of dissolved nitrite was examined during the three quasi-synoptic fast sampling grids performed in the Gerlache Strait during the Research on Antarctic Coastal Ecosystem Rates 3 (RACER3) cruise to the Antarctic Peninsula region (1991-1992 austral summer). In addition, depth profiles of nitrite (0 to 200 meters) were generated at the time-series station A (equivalent to fast grid station 33) during each of the four occupations. Cruise dates, station locations, and sampling protocols are described elsewhere (Holm-Hansen and Huntley this issue). Fast grid samples were bucket-collected from the surface, screened through al ow
M
(n
M
*r
0
0
P tZr
CY
/?
•e¼
-
L Figure 1. Contour map of surface nitrite concentrations across the RACER fast grid study area from 15 to 18 December 1991 (FA). Concentrations are in units of micromoies per liter; contour spacing Is 0.05 micromoiar. Estimated analytical precision is 0.01 micromoiar. Solid symbols indicate stations where nitrite was sampled.
164
Figure 2. As In figure 1, except data are from 27 to 30 Decembe 1991 (FC).
I
ANTARCTIC JOURNAL
a 20 micrometers mesh and stored in the dark in polyethylene bottles at 4 °C prior to shipboard analysis by a standard colonmetric method (Strickland and Parsons 1972). Station A samples were collected in Niskin bottles with a conductivity-temperature-depth (CTD)/rosette system at ten depths from surface to 200 meters, then stored and analyzed as above. Every effort was made to analyze samples promptly, and little change was observed in replicate samples analyzed fresh and after storage several hours under these conditions. All shipboard incubations of seawater samples were performed in acid-washed polycarbonate or polyethylene bottles in a flowing surface seawater bath. Surface nitrite concentrations within the fast grids ranged from 0.09 to 0.35 micromolar; high levels were generally found in the coastal embayments, while the lowest level was found near the southwest inlet of the strait. Contour plots of surface nitrite concentrations during fast grids FA (15 to 18 December) and FC (27 to 30 December) are shown in figures 1 and 2. Examination of the contoured data reveals a trend toward increasing nitrite concentrations, particularly within the coastal embayments. Only near the strait's northeast sector do we see an overall decrease of surface nitrite from grids FA to FC. The depth profiles of nitrite from the four station A occupations are shown in figure 3. All show near-surface maxima with depth-decreasing concentrations. Depth-integrated (0 to 200 meters) values increased from 11 millimoles per square meter (12 December) to 19 (19 December) to 30 (25 December), then dipped slightly to 29 millimoles per square meter (30 December). Note that while the total water column (0 to 200 meters) content of nitrite decreased slightly from 25 to 30 December, nitrite in the upper 10 meters continued to rise. A subsurface nitrite maximum, omnipresent in oligotrophic open oceans, was not observed at station A. We believe that the shape and development of the station A trite profiles indicate an early to mid-bloom situation (type MB; Dore et al. this issue). The rapidly accumulating nitrite at shallow depths is most likely a product of phytoplankton nitrite excretion during nitrate assimilation (Kiefer et al. 1976). This view is supported by the strong near-surface (0 to 20 meters) negative correlation of nitrite with nitrate (r -0.82, n = 20), and by the lack of a positive correlation with ammonium (r = -0.33,n = 20). Nitrite production near the surface by a nitrification mechanism (Olson 1981) is unlikely; the high light at shallow depths would be expected to strongly inhibit microbial nitrification. Tupas et al. (1990) in their nitrogen-15 isotope dilution experiments found no evidence of near-surface nitrification at the same site during a previous summer cruise. Further evidence for a phytoplankton nitrite source comes from our bottle incubations; light incubations of unscreened near-surface samples (24-hour duration, or more) yielded substantial nitrite increases, while dark incubations yielded little or no change. The general increase of surface nitrite during December is, therefore, probably indicative of rapid nitrate assimilation by phytoplankton during the formation of the annual bloom across most of the study area. Part of the nitrite increase observed in deeper waters (50 to 200 meters) may be from downward diffusion but nitrification (ammonium oxidation) is probably the primary source at these depths. The accumulation of nitrite indicates that nitrite-oxidizing bacteria are not very active, which is somewhat puzzling because at the low light levels associated with these depths neither of these two autotrophic processes should be photoinhibited. We believe the RACER study area to be in a mid-bloom stage, with substantial regenerative processes only beginning to develop. It is therefore
1992 REVIEW
NO,pM III
0 0.1 0.2 0.3 0.4 0.5
50
E CL 100 a) a
150
200 Figure 3. Concentration vs. depth profiles for nitrite during the four station A occupations. Concentration is in units of micromoles per liter, depth is in meters. Stations Indicated in legend. Estimated analytical precision is 0.01 micromolar.
possible that nitrite oxidizers lag heterotrophs and ammonium oxidizers, in a successional sense, based upon the order of availability of substrates (dissolved organics -> ammonium -> nitrite). This hypothesis should be tested in future nutrient regeneration studies of the coastal antarctic ecosystem. The project S-046 field team consisted of C. Tien, R. Letelier, C. Parrish, I. Szyper, and J . Burgett, in addition to the authors. We thank the crew of the R/V Polar Duke and the entire RACER scientific party for their assistance in sample collection. This research was supported by National Science Foundation grant DPP 88-18899. SOEST contribution #3131. References
Dore, J. E., C. Tien, R. Letelier, C. Parrish, J . Szyper, J. Burgett, and D. M. Karl. 1992. RACER: Distributions of nitrogenous nutrients near receding pack ice in Marguerite Bay. Antarctic Journal of the U.S., this issue. Gordon, A. L., E. Molinelli, and T. Baker. 1981. Southern Ocean Atlas. New York: Columbia University Press. Holm-Hansen, 0. and M. Huntley. 1992. Research on Antarctic Coastal Ecosystem Rates (RACER): 1991-1992 field season. Antarctic Journal of the U.S., this issue. Kiefer, D. A., R. J. Olson, and 0. Holm-Hansen. 1976. Another look at the nitrite and chlorophyll maxima in the central north Pacific. Deep-Sea Research, 23:1,199-1,208.
165
Martin, J. H. 1990. Glacial-interglacial CO2 change: The iron hypothesis. Paleoceanography, 5:1-13.
Olson, R. 1981. Differential photoinhibition of marine nitrifying bacteria: A possible mechanism for the formation of the primary nitrite maximum. Journal of Marine Research, 39:227-238. Rakestraw, H. W. 1936. The occurrence and significance of nitrite in the sea. Biological Bulletin, 71:133-167.
Seasonal variability in microbial biomass in the Gerlache Strait: A feast-or-famine existence G. TIEN, J. BURGEIT, J. Doiu, M. GEREN, T. HOULIHAN, R. LETELIER, U. MAGAARD, C. PARRISH, J. SZYPER, AND D. KARL School of Ocean and Earth Science and Technology University of Hawaii Honolulu, Hawaii 96822
Previous Research on Antarctic Coastal Ecosystem Rates (RACER) field experiments conducted in the northern Gerlache Strait during 1986-1987, 1989, and 1990-1991 have revealed significant regional variability in the distribution and abundance of micro- and nanoplankton, often with order-of-magnitude con-
Strickland, J., D. H. Parsons, and T. R. Parsons. 1972. A practical handbook of seawater analysis, second edition. Bulletin of Fishing Research Board of Canada, 167:310. Tupas, L. M., I. Koike, and 0. Holm-Hansen. 1990. RACER: Microbial uptake and regeneration of ammonium during the austral spring bloom. Antarctic Journal of the U.S., 25(5):154-155.
changes over horizontal scales of 100 kilometers or less (e.g., Bird and Karl 1991; Karl et al. 1991; Tien et al. 1990). Superimposed on these spatially diverse habitats is an intense temporal variability, perhaps the most extreme seasonality observed anywhere in the world ocean. To date, measurements of microbial processes in the southern ocean are predominantly from the Antarctic Peninsula region during summer. This could result in a serious spatial and temporal bias of the extant data base. While the standing stocks of micro-organisms are expected to be low during the austral winter, direct observations are almost non-existent. Furthermore, we know of no previous study that has occupied the same set of hydrographic stations during both the spring-summer bloom and in mid-winter. Comparisons of this type are crucial for a complete understanding of food web dynamics, particle fluxes, and ocean-atmosphere exchange of biogenic gases. During the RACER4 expedition (July and August 1992), we had an opportunity to return to a region of the Antarctic Peninsula where we previously had measured summer microbial biomasses that were among the highest ever reported for the neritic portion of the world ocean. The following initial results, reported underway from the R/VNathaniel B. Palmer in Gerlache Strait, indicate that microbial biomasses in this same region centration
.1 V^14 64
64%
•0
al 64' 30
63W
62'W
61'W
Figure 1. P-ATP concentration contours for the surface waters of the RACER study area for the period 16 to 19 November 1989. Solid circles indicate the locations of the hydrostations sampled during each "fast grid" survey. Concentrations of P-ATP are in micrograms per liter. Contour interval is 0.5 micrograms per liter.
166
63'W
62'W
61'W
Figure 2. P-ATP concentration contours (as in figure 1)for the period 201026 July 1992. Concentrations of P-ATP in nanograms per liter. Contour interval is 2 nanograms per liter.
ANTARCTIC JOURNA
Kishino, M., M. Takahashi, N. Okami, and S. Ichimura. 1985. Estimation of spectral absorption coefficients of phytoplankton in the sea. Bulletin of Marine Science, 37:634-642.
RACER: Distribution of nitrite in the Gerlache Strait J. E. DORE AND D. M. KARL School of Ocean and Earth Science and Technology University of Hawaii Honolulu, Hawaii 96822
The surface waters of the southern oceans are characterized by high concentrations of inorganic nutrients (nitrate, phosphate, silicate; Gordon et al. 1981) which could potentially be used for the photosynthetic production of organic matter and removal of dissolved carbon dioxide from the ocean's surface (Martin 1990). A thorough understanding of nutrient dynamics in antarctic marine ecosystems is crucial to our understanding of food web dynamics and global carbon dioxide fluxes. Of the major bioelements, nitrogen is generally considered to be an important
Mitchell, B. G. 1990. Algorithms for determining the absorption coefficient of aquatic particulates using the quantitative filter technique (QFT). Ocean Optics X, SPIE, 1301:137-148. Mitchell, B. G. and 0. Holm-Hansen. 1991. Bio-optical properties of Antarctic Peninsula waters: Differentiation from temperature ocean models. Deep-Sea Research, 38:1,009-1,028.
limiting nutrient in seawater. To date, most studies of nitrogen in the southern ocean have focused either on the most oxidized form, nitrate, or on the most reduced form, ammonium. By comparison, few data exist on the distribution or turnover rate of the redox intermediate nitrite. Accumulations of nitrite in natural waters indicate zones in which an uncoupling exists between the oxidative and reductive reactions affecting nitrate and ammonium (Rakestraw 1936). We anticipated that an evaluation of the dynamics of nitrite distributions during the spring phytoplankton bloom would provide us with additional information on the nutritional state and regenerative capacity of the microbial community. The horizontal distribution of dissolved nitrite was examined during the three quasi-synoptic fast sampling grids performed in the Gerlache Strait during the Research on Antarctic Coastal Ecosystem Rates 3 (RACER3) cruise to the Antarctic Peninsula region (1991-1992 austral summer). In addition, depth profiles of nitrite (0 to 200 meters) were generated at the time-series station A (equivalent to fast grid station 33) during each of the four occupations. Cruise dates, station locations, and sampling protocols are described elsewhere (Holm-Hansen and Huntley this issue). Fast grid samples were bucket-collected from the surface, screened through al ow
M
(n
M
*r
0
0
P tZr
CY
/?
•e¼
-
L Figure 1. Contour map of surface nitrite concentrations across the RACER fast grid study area from 15 to 18 December 1991 (FA). Concentrations are in units of micromoies per liter; contour spacing Is 0.05 micromoiar. Estimated analytical precision is 0.01 micromoiar. Solid symbols indicate stations where nitrite was sampled.
164
Figure 2. As In figure 1, except data are from 27 to 30 Decembe 1991 (FC).
I
ANTARCTIC JOURNAL
a 20 micrometers mesh and stored in the dark in polyethylene bottles at 4 °C prior to shipboard analysis by a standard colonmetric method (Strickland and Parsons 1972). Station A samples were collected in Niskin bottles with a conductivity-temperature-depth (CTD)/rosette system at ten depths from surface to 200 meters, then stored and analyzed as above. Every effort was made to analyze samples promptly, and little change was observed in replicate samples analyzed fresh and after storage several hours under these conditions. All shipboard incubations of seawater samples were performed in acid-washed polycarbonate or polyethylene bottles in a flowing surface seawater bath. Surface nitrite concentrations within the fast grids ranged from 0.09 to 0.35 micromolar; high levels were generally found in the coastal embayments, while the lowest level was found near the southwest inlet of the strait. Contour plots of surface nitrite concentrations during fast grids FA (15 to 18 December) and FC (27 to 30 December) are shown in figures 1 and 2. Examination of the contoured data reveals a trend toward increasing nitrite concentrations, particularly within the coastal embayments. Only near the strait's northeast sector do we see an overall decrease of surface nitrite from grids FA to FC. The depth profiles of nitrite from the four station A occupations are shown in figure 3. All show near-surface maxima with depth-decreasing concentrations. Depth-integrated (0 to 200 meters) values increased from 11 millimoles per square meter (12 December) to 19 (19 December) to 30 (25 December), then dipped slightly to 29 millimoles per square meter (30 December). Note that while the total water column (0 to 200 meters) content of nitrite decreased slightly from 25 to 30 December, nitrite in the upper 10 meters continued to rise. A subsurface nitrite maximum, omnipresent in oligotrophic open oceans, was not observed at station A. We believe that the shape and development of the station A trite profiles indicate an early to mid-bloom situation (type MB; Dore et al. this issue). The rapidly accumulating nitrite at shallow depths is most likely a product of phytoplankton nitrite excretion during nitrate assimilation (Kiefer et al. 1976). This view is supported by the strong near-surface (0 to 20 meters) negative correlation of nitrite with nitrate (r -0.82, n = 20), and by the lack of a positive correlation with ammonium (r = -0.33,n = 20). Nitrite production near the surface by a nitrification mechanism (Olson 1981) is unlikely; the high light at shallow depths would be expected to strongly inhibit microbial nitrification. Tupas et al. (1990) in their nitrogen-15 isotope dilution experiments found no evidence of near-surface nitrification at the same site during a previous summer cruise. Further evidence for a phytoplankton nitrite source comes from our bottle incubations; light incubations of unscreened near-surface samples (24-hour duration, or more) yielded substantial nitrite increases, while dark incubations yielded little or no change. The general increase of surface nitrite during December is, therefore, probably indicative of rapid nitrate assimilation by phytoplankton during the formation of the annual bloom across most of the study area. Part of the nitrite increase observed in deeper waters (50 to 200 meters) may be from downward diffusion but nitrification (ammonium oxidation) is probably the primary source at these depths. The accumulation of nitrite indicates that nitrite-oxidizing bacteria are not very active, which is somewhat puzzling because at the low light levels associated with these depths neither of these two autotrophic processes should be photoinhibited. We believe the RACER study area to be in a mid-bloom stage, with substantial regenerative processes only beginning to develop. It is therefore
1992 REVIEW
NO,pM III
0 0.1 0.2 0.3 0.4 0.5
50
E CL 100 a) a
150
200 Figure 3. Concentration vs. depth profiles for nitrite during the four station A occupations. Concentration is in units of micromoles per liter, depth is in meters. Stations Indicated in legend. Estimated analytical precision is 0.01 micromolar.
possible that nitrite oxidizers lag heterotrophs and ammonium oxidizers, in a successional sense, based upon the order of availability of substrates (dissolved organics -> ammonium -> nitrite). This hypothesis should be tested in future nutrient regeneration studies of the coastal antarctic ecosystem. The project S-046 field team consisted of C. Tien, R. Letelier, C. Parrish, I. Szyper, and J . Burgett, in addition to the authors. We thank the crew of the R/V Polar Duke and the entire RACER scientific party for their assistance in sample collection. This research was supported by National Science Foundation grant DPP 88-18899. SOEST contribution #3131. References
Dore, J. E., C. Tien, R. Letelier, C. Parrish, J . Szyper, J. Burgett, and D. M. Karl. 1992. RACER: Distributions of nitrogenous nutrients near receding pack ice in Marguerite Bay. Antarctic Journal of the U.S., this issue. Gordon, A. L., E. Molinelli, and T. Baker. 1981. Southern Ocean Atlas. New York: Columbia University Press. Holm-Hansen, 0. and M. Huntley. 1992. Research on Antarctic Coastal Ecosystem Rates (RACER): 1991-1992 field season. Antarctic Journal of the U.S., this issue. Kiefer, D. A., R. J. Olson, and 0. Holm-Hansen. 1976. Another look at the nitrite and chlorophyll maxima in the central north Pacific. Deep-Sea Research, 23:1,199-1,208.
165
Martin, J. H. 1990. Glacial-interglacial CO2 change: The iron hypothesis. Paleoceanography, 5:1-13.
Olson, R. 1981. Differential photoinhibition of marine nitrifying bacteria: A possible mechanism for the formation of the primary nitrite maximum. Journal of Marine Research, 39:227-238. Rakestraw, H. W. 1936. The occurrence and significance of nitrite in the sea. Biological Bulletin, 71:133-167.
Seasonal variability in microbial biomass in the Gerlache Strait: A feast-or-famine existence G. TIEN, J. BURGEIT, J. Doiu, M. GEREN, T. HOULIHAN, R. LETELIER, U. MAGAARD, C. PARRISH, J. SZYPER, AND D. KARL School of Ocean and Earth Science and Technology University of Hawaii Honolulu, Hawaii 96822
Previous Research on Antarctic Coastal Ecosystem Rates (RACER) field experiments conducted in the northern Gerlache Strait during 1986-1987, 1989, and 1990-1991 have revealed significant regional variability in the distribution and abundance of micro- and nanoplankton, often with order-of-magnitude con-
Strickland, J., D. H. Parsons, and T. R. Parsons. 1972. A practical handbook of seawater analysis, second edition. Bulletin of Fishing Research Board of Canada, 167:310. Tupas, L. M., I. Koike, and 0. Holm-Hansen. 1990. RACER: Microbial uptake and regeneration of ammonium during the austral spring bloom. Antarctic Journal of the U.S., 25(5):154-155.
changes over horizontal scales of 100 kilometers or less (e.g., Bird and Karl 1991; Karl et al. 1991; Tien et al. 1990). Superimposed on these spatially diverse habitats is an intense temporal variability, perhaps the most extreme seasonality observed anywhere in the world ocean. To date, measurements of microbial processes in the southern ocean are predominantly from the Antarctic Peninsula region during summer. This could result in a serious spatial and temporal bias of the extant data base. While the standing stocks of micro-organisms are expected to be low during the austral winter, direct observations are almost non-existent. Furthermore, we know of no previous study that has occupied the same set of hydrographic stations during both the spring-summer bloom and in mid-winter. Comparisons of this type are crucial for a complete understanding of food web dynamics, particle fluxes, and ocean-atmosphere exchange of biogenic gases. During the RACER4 expedition (July and August 1992), we had an opportunity to return to a region of the Antarctic Peninsula where we previously had measured summer microbial biomasses that were among the highest ever reported for the neritic portion of the world ocean. The following initial results, reported underway from the R/VNathaniel B. Palmer in Gerlache Strait, indicate that microbial biomasses in this same region centration
.1 V^14 64
64%
•0
al 64' 30
63W
62'W
61'W
Figure 1. P-ATP concentration contours for the surface waters of the RACER study area for the period 16 to 19 November 1989. Solid circles indicate the locations of the hydrostations sampled during each "fast grid" survey. Concentrations of P-ATP are in micrograms per liter. Contour interval is 0.5 micrograms per liter.
166
63'W
62'W
61'W
Figure 2. P-ATP concentration contours (as in figure 1)for the period 201026 July 1992. Concentrations of P-ATP in nanograms per liter. Contour interval is 2 nanograms per liter.
ANTARCTIC JOURNA
