RACER: Vertical migration and bioenergetics of «Metridia gerlachei
RACER: Vertical migration and bioenergetics of Metridia gerlachei
during spring 1991-1992 MARK E. HUNTLEY AND SANDOR KAUPP Scripps Institution of Oceanography La Jolla, California 92093
MAI D. G. LOPEZ Marine Science Institute University of the Philippines Diliman, Quezon City 1101, Philippines
The copepod, Metridia gerlachei Geisbrecht, dominates copepod biomass in the southern ocean (Zmijewska 1985; Hopkins 1985b; Hopkins and Torres 1988), especially in coastal regions (Huntley and Escritor 1992). It is a strong, vertical migrator, residing between 200 and 500 meters during the day and ascending to the upper 50 meters at night (Hardy and Gunther 1935; Vervoort 1965; Schnack et al. 1985; Huntley and Escritor 1992). Although M.gerlachei is capable of carnivory (Hopkins 1985b), it can meet its metabolic requirements on a diet of phytoplankton (Schnack et al. 1985). Here, we report on the herbivorous feeding and the diel vertical-migration cycles of M.gerlachei. This research was conducted as part of the RACER (Research on Antarctic Coastal Ecosystem Rates) program, designed to study ecosystem dynamics during the spring bloom. Samples were collected at 64'12'S 6r20'W(station A) during the 37-hour period, beginning at 2400, 20 December 1991 and a 24-hour period beginning at 1800, 25 December 1991. We used a multiple opening-closing net and environmental sensing system (MOCNESS), with333-meter mesh nets, set to sample 8 depth intervals with depth horizons generally at 5,15,50,90,130,170,210, and 290 meters. During the day, when the population was at greater depth, we sometimes eliminated the 5-meter depth horizon and exchanged it for one at 250 meters. All tows were completed within 45 minutes, surface to surface. Sample preservation and resetting of the net system generally required an additional 15 minutes, so that at best, we were able to sample the population throughout the water column about once every hour. A fraction of the catch from each cod end was filtered onto a GF/D filter and immediately frozen at -70 C. Groups of ten adult female M.gerlachei were picked from the frozen, filtered samples for each fluorometric measurement of gut pigment content (Mackas and Bohrer 1976). These were passively extracted for a minimum of 2 hours in 7 milliliters of absolute methanol (Huntley et al. 1987). At the depths where M.gerlachei was most abundant, we were usually able to obtain ten such measurements; only one measurement was usually possible for other depths. A second fraction was frozen in filtered seawater for determination of RNA and DNA content, and of the activity of the following metabolic enzymes: citrate synthase, pyruvate kinase, lactate dehydrogenase and malate dehydrogenase, for each individual. A third fraction was preserved in 5
172
percent borate-buffered formalin for enumeration. Females in the formalin-preserved sample were counted, and their abundance calculated in quantity per cubic meter, based on the measurement of volume flowing through each net. A few individuals were reserved for measurements of respiration rate, using an oxygen microelectrode system. Photosynthetically active radiation and the vertical distribution of chlorophyll a were also measured during the same period (Holm-Hansen et al. this issue). Water samples for the measurement of phytoplankton pigments were collected at standard depths, with a conductivity-temperature-depth/rosette system deployed at 6-hour intervals. In late December females dominated the population of M.gerlachei. These exhibited pronounced diel vertical-migration behavior, which imposed a similar rhythm on herbivorous grazing. The population was essentially absent from the surfacei during daylight hours, most of it concentrated well below 200 meters (bottom depth at station A is approximately 325 meters). Abundance at the depths where the female population was centered was usually in the range of 25 to 100 individuals per cubic meter; most of the population was concentrated in two of
LU W Cr
Metrkh gellachel, adult CVI DEPTH OF MAXIMUM ABUNDANCE
I B
160 :1001
I
W a IL -
00
7663
07 06 96 90 N N N
I
1111111 1111111
)0 08:00 1200 1600 20:00 2400 04:00 06:00 12:00 16:00 a Metrda gerlachel AT DEPTH OF MAXIMUM ABUNDANCE
w Z W
CCL
I.-
C C2 TIME
Diel vertical migration and herbivorous feeding by adult female Metridia gerlachei at station A, 25-26 December 1991: (A) Photosynthetically active radiation in relative units during the 37-hour sampling period; (B) Modal depth interval occupied by the adult female M.gerlachel during the sampling period (numbers are the percentage of females in the 0 to 290 meters water column found within the modal depth Interval); (C) Mean gut pigment content of the adult female M. gerlachel found In the modal depth interval during the sampling period.
ANTARCTIC JOURNM
the eight nets (i.e., over a 80-meter depth interval). At 2000, about hours before the onset of maximum darkness, the population began to rise perceptibly above 200 meters and generally reached he upper portion of its nighttime excursion at 0100 (local time). Almost immediately thereafter, the downward migration began, eturning the population to its daytime depth range by the time urface-light intensity had increased to about 10 percent of its aaily maximum (figure la,b). The upper limit of the vertical xcursion of the bulk of the population could not have been haliower than 15 meters, and sometimes was deeper. Based on hese observations, we calculate a minimum migration velocity pf 25 meters per hour. Grazing activity, as indicated by the level of gut fluorescence, was strongly tied to the nightly vertical migration event (figure 1c). Mean gut pigment content during the day was less than 0.15 ianogram chlorophyll equivalents for each individual. As the animals migrated toward the surface, the gut contents began to increase, even at depths well below the chlorophyll maximum. This could have been due to feeding on pigment-rich fecal material, to sporadic occurrences of phytoplankton, or to individuals whose diel migration was slightly out of phase with the majority of the population. Maximum gut pigment content—almost two orders of magnitude greater than the daytime gut content—was attained simultaneously with arrival near the surface and persisted for some time after the population began its descent. The evacuation rate of the in situ population was not significantly different than that of individuals removed from the population and maintained in filtered seawater on deck for two hours. The absolute concentration of pigments in deck-incubated fauna was reduced to near zero about two hours before that of the field population, suggesting that the field population continues to feed during descent. This research was conducted as part of the RACER (Research on Antarctic Coastal Ecosystem Rates) program, designed to
RACER: Composition and vertical distribution of larval fishes at a time. series station in Gerlache Strait, November 1989 .
VALERIE J LOEB
Moss Landing Marine Laboratories Moss Landing, California 95039
Among objectives of the 1989 Research on Antarctic Coastal Ecosystem Rates (RACER) Program were studies to determine whether the relatively high zooplankton abundance within Gerlache Strait is due to accumulation from physical processes or whether it originates there through high rates of local reproduction, development, and survival (Huntley et al. 1990). To address this question, an intensive program of vertically and horizontally stratified zooplankton sampling was undertaken by using a multiple-opening-closing-net-and-environmentalsensing system (MOCNESS) at grid stations within Gerlache
1992 REVIEW
study ecosystem dynamics during the spring bloom. Our continning study will assess the relationship between diel feeding and vertical migration behavior to the bioenergetics of individual M.gerlachei. This will be done by comparing some of the results discussed here with cycles of activity as reflected by the RNA and DNA content, the activity of metabolic enzymes, and the respiration measurements made throughout the daily cycle. This research was supported by National Science Foundation grant DPP 90-17779. The authors thank E. Brinton, J . Lovett, W. Nordhausen, and E. Venrick for their superhuman "MOCNESSing" efforts; 0. Holm-Hansen and M. Vernet for providing data on light and pigments; and the RACER science crew as well as the crew of the R/V Polar Duke for their help. References Hardy, A. C. and E. R. Gunther. 1935. The plankton of the South Georgia whaling grounds and adjacent waters, 1926-27. Discovery Reports, 11:1-456. Hopkins, T. L. 1985a. Food web of antarctic midwater ecosystem. Marine Biology, 89:197-212. Hopkins, T. L. 1985b. The zooplankton community of Croker Passage, Antarctica. Polar Biology, 4:161-170. Hopkins, T. L. and J. J . Torres. 1988. The zooplankton community in the vicinity of the ice edge, western Weddell Sea, March 1986. Polar Biology, 9:79-87. Huntley, M. E. and F. Escritor. 1992. Ecology of Metridiagerlachei Geisbrecht in the western Bransfield Strait, Antarctica. Deep-Sea Research, in press. Schnack, S. B., V. Smetacek, B. von Bodungen, and P. Stegmann. 1985. Utilization of phytoplankton by copepods in antarctic waters during spring. In J . S. Gray and M. E. Christiansen (Eds.), Marine biology of polar regions and effects of stress on marine organisms. New York: John Wiley & Sons, 65-81. Vervoort, W. 1965. Note on the biogeography and ecology of freeliving marine copepods. InJ.V. Miehgem, P.V.Oye, and J . Schell (Eds.), Biogeography and ecology of Antarctica. The Hague: D. W. Junk, 381400.
Strait and southwest Bransfield Strait and at a time-series station (station A) located in the eastern Gerlache Strait (figure 1). About 1,200 MOCNESS samples were collected and they are being analyzed to provide information on the early life stages of fish in addition to the dominant euphausiid and copepod species. Little is presently known about the planktonic fish assemblages in Gerlache Strait. Given the high primary productivity and zooplankton biomass (Huntley et al. 1990) and the prevalence of inshore habitats and coastal eddies (Niiler et al. 1990), Gerlache Strait has the potential for supporting a high abundance of larvae—some from species that are rarely encountered in offshore waters. Presented here are the preliminary results from analysis of the fish collected during time-series sampling at station A. Drifter buoy studies indicate that the residence time of water near this station is in the order of two months (Niiler et al. 1990). It is of interest to consider whether this area may retain the pelagic larvae of species spawning in coastal waters, thereby reducing their advection offshore. Data on larval fish at station A were obtained from vertically stratified MOCNESS samples collected over a 48-hour period during 20-21 November 1989 (Huntley et al. 1990). Each MOCNESS tow sampled 9 depth strata between 0 to 290 meters (0 to 5, 5 to 15, 15 to 50, 50 to 90, 90 to 130, 130 to 170, 170 to
173
during spring 1991-1992 MARK E. HUNTLEY AND SANDOR KAUPP Scripps Institution of Oceanography La Jolla, California 92093
MAI D. G. LOPEZ Marine Science Institute University of the Philippines Diliman, Quezon City 1101, Philippines
The copepod, Metridia gerlachei Geisbrecht, dominates copepod biomass in the southern ocean (Zmijewska 1985; Hopkins 1985b; Hopkins and Torres 1988), especially in coastal regions (Huntley and Escritor 1992). It is a strong, vertical migrator, residing between 200 and 500 meters during the day and ascending to the upper 50 meters at night (Hardy and Gunther 1935; Vervoort 1965; Schnack et al. 1985; Huntley and Escritor 1992). Although M.gerlachei is capable of carnivory (Hopkins 1985b), it can meet its metabolic requirements on a diet of phytoplankton (Schnack et al. 1985). Here, we report on the herbivorous feeding and the diel vertical-migration cycles of M.gerlachei. This research was conducted as part of the RACER (Research on Antarctic Coastal Ecosystem Rates) program, designed to study ecosystem dynamics during the spring bloom. Samples were collected at 64'12'S 6r20'W(station A) during the 37-hour period, beginning at 2400, 20 December 1991 and a 24-hour period beginning at 1800, 25 December 1991. We used a multiple opening-closing net and environmental sensing system (MOCNESS), with333-meter mesh nets, set to sample 8 depth intervals with depth horizons generally at 5,15,50,90,130,170,210, and 290 meters. During the day, when the population was at greater depth, we sometimes eliminated the 5-meter depth horizon and exchanged it for one at 250 meters. All tows were completed within 45 minutes, surface to surface. Sample preservation and resetting of the net system generally required an additional 15 minutes, so that at best, we were able to sample the population throughout the water column about once every hour. A fraction of the catch from each cod end was filtered onto a GF/D filter and immediately frozen at -70 C. Groups of ten adult female M.gerlachei were picked from the frozen, filtered samples for each fluorometric measurement of gut pigment content (Mackas and Bohrer 1976). These were passively extracted for a minimum of 2 hours in 7 milliliters of absolute methanol (Huntley et al. 1987). At the depths where M.gerlachei was most abundant, we were usually able to obtain ten such measurements; only one measurement was usually possible for other depths. A second fraction was frozen in filtered seawater for determination of RNA and DNA content, and of the activity of the following metabolic enzymes: citrate synthase, pyruvate kinase, lactate dehydrogenase and malate dehydrogenase, for each individual. A third fraction was preserved in 5
172
percent borate-buffered formalin for enumeration. Females in the formalin-preserved sample were counted, and their abundance calculated in quantity per cubic meter, based on the measurement of volume flowing through each net. A few individuals were reserved for measurements of respiration rate, using an oxygen microelectrode system. Photosynthetically active radiation and the vertical distribution of chlorophyll a were also measured during the same period (Holm-Hansen et al. this issue). Water samples for the measurement of phytoplankton pigments were collected at standard depths, with a conductivity-temperature-depth/rosette system deployed at 6-hour intervals. In late December females dominated the population of M.gerlachei. These exhibited pronounced diel vertical-migration behavior, which imposed a similar rhythm on herbivorous grazing. The population was essentially absent from the surfacei during daylight hours, most of it concentrated well below 200 meters (bottom depth at station A is approximately 325 meters). Abundance at the depths where the female population was centered was usually in the range of 25 to 100 individuals per cubic meter; most of the population was concentrated in two of
LU W Cr
Metrkh gellachel, adult CVI DEPTH OF MAXIMUM ABUNDANCE
I B
160 :1001
I
W a IL -
00
7663
07 06 96 90 N N N
I
1111111 1111111
)0 08:00 1200 1600 20:00 2400 04:00 06:00 12:00 16:00 a Metrda gerlachel AT DEPTH OF MAXIMUM ABUNDANCE
w Z W
CCL
I.-
C C2 TIME
Diel vertical migration and herbivorous feeding by adult female Metridia gerlachei at station A, 25-26 December 1991: (A) Photosynthetically active radiation in relative units during the 37-hour sampling period; (B) Modal depth interval occupied by the adult female M.gerlachel during the sampling period (numbers are the percentage of females in the 0 to 290 meters water column found within the modal depth Interval); (C) Mean gut pigment content of the adult female M. gerlachel found In the modal depth interval during the sampling period.
ANTARCTIC JOURNM
the eight nets (i.e., over a 80-meter depth interval). At 2000, about hours before the onset of maximum darkness, the population began to rise perceptibly above 200 meters and generally reached he upper portion of its nighttime excursion at 0100 (local time). Almost immediately thereafter, the downward migration began, eturning the population to its daytime depth range by the time urface-light intensity had increased to about 10 percent of its aaily maximum (figure la,b). The upper limit of the vertical xcursion of the bulk of the population could not have been haliower than 15 meters, and sometimes was deeper. Based on hese observations, we calculate a minimum migration velocity pf 25 meters per hour. Grazing activity, as indicated by the level of gut fluorescence, was strongly tied to the nightly vertical migration event (figure 1c). Mean gut pigment content during the day was less than 0.15 ianogram chlorophyll equivalents for each individual. As the animals migrated toward the surface, the gut contents began to increase, even at depths well below the chlorophyll maximum. This could have been due to feeding on pigment-rich fecal material, to sporadic occurrences of phytoplankton, or to individuals whose diel migration was slightly out of phase with the majority of the population. Maximum gut pigment content—almost two orders of magnitude greater than the daytime gut content—was attained simultaneously with arrival near the surface and persisted for some time after the population began its descent. The evacuation rate of the in situ population was not significantly different than that of individuals removed from the population and maintained in filtered seawater on deck for two hours. The absolute concentration of pigments in deck-incubated fauna was reduced to near zero about two hours before that of the field population, suggesting that the field population continues to feed during descent. This research was conducted as part of the RACER (Research on Antarctic Coastal Ecosystem Rates) program, designed to
RACER: Composition and vertical distribution of larval fishes at a time. series station in Gerlache Strait, November 1989 .
VALERIE J LOEB
Moss Landing Marine Laboratories Moss Landing, California 95039
Among objectives of the 1989 Research on Antarctic Coastal Ecosystem Rates (RACER) Program were studies to determine whether the relatively high zooplankton abundance within Gerlache Strait is due to accumulation from physical processes or whether it originates there through high rates of local reproduction, development, and survival (Huntley et al. 1990). To address this question, an intensive program of vertically and horizontally stratified zooplankton sampling was undertaken by using a multiple-opening-closing-net-and-environmentalsensing system (MOCNESS) at grid stations within Gerlache
1992 REVIEW
study ecosystem dynamics during the spring bloom. Our continning study will assess the relationship between diel feeding and vertical migration behavior to the bioenergetics of individual M.gerlachei. This will be done by comparing some of the results discussed here with cycles of activity as reflected by the RNA and DNA content, the activity of metabolic enzymes, and the respiration measurements made throughout the daily cycle. This research was supported by National Science Foundation grant DPP 90-17779. The authors thank E. Brinton, J . Lovett, W. Nordhausen, and E. Venrick for their superhuman "MOCNESSing" efforts; 0. Holm-Hansen and M. Vernet for providing data on light and pigments; and the RACER science crew as well as the crew of the R/V Polar Duke for their help. References Hardy, A. C. and E. R. Gunther. 1935. The plankton of the South Georgia whaling grounds and adjacent waters, 1926-27. Discovery Reports, 11:1-456. Hopkins, T. L. 1985a. Food web of antarctic midwater ecosystem. Marine Biology, 89:197-212. Hopkins, T. L. 1985b. The zooplankton community of Croker Passage, Antarctica. Polar Biology, 4:161-170. Hopkins, T. L. and J. J . Torres. 1988. The zooplankton community in the vicinity of the ice edge, western Weddell Sea, March 1986. Polar Biology, 9:79-87. Huntley, M. E. and F. Escritor. 1992. Ecology of Metridiagerlachei Geisbrecht in the western Bransfield Strait, Antarctica. Deep-Sea Research, in press. Schnack, S. B., V. Smetacek, B. von Bodungen, and P. Stegmann. 1985. Utilization of phytoplankton by copepods in antarctic waters during spring. In J . S. Gray and M. E. Christiansen (Eds.), Marine biology of polar regions and effects of stress on marine organisms. New York: John Wiley & Sons, 65-81. Vervoort, W. 1965. Note on the biogeography and ecology of freeliving marine copepods. InJ.V. Miehgem, P.V.Oye, and J . Schell (Eds.), Biogeography and ecology of Antarctica. The Hague: D. W. Junk, 381400.
Strait and southwest Bransfield Strait and at a time-series station (station A) located in the eastern Gerlache Strait (figure 1). About 1,200 MOCNESS samples were collected and they are being analyzed to provide information on the early life stages of fish in addition to the dominant euphausiid and copepod species. Little is presently known about the planktonic fish assemblages in Gerlache Strait. Given the high primary productivity and zooplankton biomass (Huntley et al. 1990) and the prevalence of inshore habitats and coastal eddies (Niiler et al. 1990), Gerlache Strait has the potential for supporting a high abundance of larvae—some from species that are rarely encountered in offshore waters. Presented here are the preliminary results from analysis of the fish collected during time-series sampling at station A. Drifter buoy studies indicate that the residence time of water near this station is in the order of two months (Niiler et al. 1990). It is of interest to consider whether this area may retain the pelagic larvae of species spawning in coastal waters, thereby reducing their advection offshore. Data on larval fish at station A were obtained from vertically stratified MOCNESS samples collected over a 48-hour period during 20-21 November 1989 (Huntley et al. 1990). Each MOCNESS tow sampled 9 depth strata between 0 to 290 meters (0 to 5, 5 to 15, 15 to 50, 50 to 90, 90 to 130, 130 to 170, 170 to
173
