Oceanographic influences on marine benthic
Oceanographic influences on marine benthic communities in McMurdo Sound, Antarctica J.P. BARRY and
Sound, and indicate that benthic standing stock and productivity are related to both large- and small-scale current patterns a0
P.K. DAYTON
Scripps Institution of Oceanography La Jolla, California 92093
During the 1983-1984 and 1984-1985 field seasons in McMurdo Sound, Antarctica, we collected data concerning several aspects of the dynamics of sub-ice benthic communities. Photographic transects were collected with a deep submersible camera at several locations within the sound including White Island, Strand Moraine, two locations under the Koettlitz Glacier outflow near Heald Island, and along the Ross Ice Shelf edge near Scott Base. Current measurements were made at three month-long stations across the sound and at seven shortterm stations (1 week or less). Water-column samples were collected at 19 stations from which analyses of temperature, salinity, density, oxygen, chlorophyll a, nitrate, nitrite, phosphate, and silicate were completed. In addition, several plankton samples tows were collected from the upper water column. The diving program was able to resurvey monitoring projects and manipulative experiments begun during the 1970's as well as establish new experiments measuring recruitment, growth, and survivorship of benthic fauna in the eastern and western sound. Photographic data show diverse faunal communities throughout the Sound, even in deep water in the western
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Figure 1. McMurdo Sound, Antarctica. Stars indicate station locations for current measurements. Solid arrows indicate mean surface flow. Dashed arrows indicate mean flow below 100 meters. Dots are locations of hydrographic sampling stations.
180
Figure 2. Cross sound section of selected hydrographic parameters. These sections represent data collected from stations located between Strand Moraine and Cape Armitage, as indicated in figure 1. A. Density (sigma-tin kilograms per cubic meter). B. Water temperature (°C). C. Chlorophyll a (in milligrams per cubic meter). ("m" denotes "meter.") ANTARCTIC JOURNAL
(Dayton, Barry, and Kooyman in preparation). Deep-water photographs from beneath the Ross and McMurdo Ice Shelves show the highest faunal biomass in the eastern sound and lowest in the west, similar to the shallow benthic community (Dayton and Oliver 1977), and apparently related to differences in the concentration of phytoplankton advected into each area. Interestingly, these data also indicate that local variability in current speed may also be important in local benthic production and standing stock. Sites which appear to have more intense currents have higher faunal standing stocks than sites with apparently sluggish flow. Analyses of current-meter data documented the large-scale flow pattern in McMurdo Sound during 1984-1985 with strong southerly flow in the eastern Sound and an opposing northerly flow in the western Sound (figure 1; Barry and Dayton). Semidiurnal tidal motion dominated all current records. These data were similar to previous current observations (Heath 1977; Lewis and Perkin 1985), though there appears to be considerable interannual variability in the intensity of the southerly flow in the eastern Sound which may be related to variability in atmospheric forcing functions. a
b NITRATE
OXYGEN 01
Hydrographic section maps compiled from nutrient and physical water-column data show that nutrient levels were high and nearly homogeneous throughout the Sound during spring 1984. Water temperature was very low and salinity and density were high though somewhat lower in the western Sound (figure 2). Oxygen was near 6.8 milliliters per liter and chlorophyll a was low at most stations and was highest in the eastern Sound (figure 2). During summer 1984, conditions below 200 meters were very similar to springtime conditions, though in shallower water, nutrients levels were depressed, salinity and density were lower, and temperatures were higher. Near surface salinity had decreased, due to sea-ice and glacial meltwater. Nitrate and phosphate levels were dramatically reduced and near 0 at some locations, while nitrite and oxygen concentrations increased somewhat during summer (figure 3; Barry in preparation). These investigations allowed considerable advances in our understanding of the dynamics of local benthic communities, particularly their relationship to oceanographic processes. Subice faunal communities, especially those under thick ice and snow cover depend on phytoplankton growth from well-lit areas and the subsequent advection of this food under the ice to
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Figure 3. Spring vs. summer composite nutrient profiles from McMurdo Sound, Antarctica. A. Oxygen. B. Nitrate. C. Phosphate. D. Nitrite. ("mg at/I" denotes "milligram atoms per liter:' moles/I" denotes "micromoles per liter:' "m" denotes "meter:')
1986 REVIEW
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benthic filter feeders and grazers which in turn are consumed by other community members. Thus, the large-scale current pattern is obviously important to the McMurdo Sound benthos. Moreover, it appears that local variability in current intensity may be responsible in part for the structure of benthic communities. Those areas with high current speeds are expected to have higher particle fluxes and lower sedimentation rates than nearby sluggish current sites, resulting in different faunal assemblages. These physical processes may exert primary control over community productivity and are superimposed on the biotic interactions which have also been shown to be of importance in regulating benthic community structure (Dayton et al. 1974). We intend to continue to investigate the relationship between physical processes and sub-ice benthic community structure. The dramatic north-south and east-west gradients of productivity and species composition in McMurdo Sound appear directly related to ice cover and oceanographic features. As such, McMurdo Sound benthos are particularly amenable to testing hypotheses regarding the roles of in situ vs. advected primary production, and/or the importance of biotic (competition, predation) vs. abiotic (current patterns, etc.) control of community dynamics. We hope to investigate the importance of large- and local-scale current intensity on community production by coupling current observations with growth rate and survivorship studies. In addition, we will continue and expand our benthic productivity studies (Dayton et al. in press) and hope to evaluate the roles of advected vs. in situ primary production on
Contrasting tempos of reproduction by shallow-water animals in McMurdo Sound, Antarctica J.S. PEARSE, I. BOSCH, J.B. MCCLINTOCK, B. MARINOVIC, and R. BRITTON Institute of Marine Sciences University of California Santa Cruz, California 95064
Recent work by our group has revealed that a wide range of reproductive modes prevails in McMurdo Sound, Antarctica, as in other parts of the world ocean (Pearse, Bosch, and McClintock 1986). The production of pelagic larvae is common, although a large proportion of the larvae are lecithotrophic. Among those species that have planktotrophic larvae, there is little evidence of starvation even when phytoplankton levels are very low (Olson, Bosch, and Pearse in press), probably because such larvae are able to use dissolved organic material and to feed on bacteria (Rivkin et al. 1986). Larval energy requirements are probably relatively low; energy stores in lecithotrophic larvae, at least, function mainly to produce large juveniles and are little used during larval development (McClintock and Pearse 1986). 182
microbial and meiofaunal activity, general benthic trophic relationships, and recolonization. This work was supported by National Science Foundation grant DPP 81-00189. References
Barry, J.P. In preparation. Oceanographic patterns in McMurdo Sound, Antarctica. Barry, JR. and P.K. Dayton. Current patterns in McMurdo Sound, Antarctica and their relationship to local benthic communitites. Lw,nology and Oceanography.
Dayton, P.K., J.P. Barry, and C. Kooyman. In preparation. Deep water benthic faunal patterns under the Ross and McMurdo Ice Shelves, Antarctica. Dayton, P.K., and J.S. Oliver. 1977. Antarctic soft bottom benthos in oligotrophic and eutrophic environments. Science, 197, 55-58. Dayton, P.K., G.A. Robilliard, R.T. Paine, and L.B. Dayton. 1974. Biological accomodation in the benthic community at McMurdo Sound, Antarctica. Ecological Monographs, 44(1), 105-128. Dayton, P.K., D. Watson, A. Palmisano, J.P. Barry, J.S. Oliver, and D. Rivera. In press. Distribution patterns of benthic microalgal standing stock at McMurdo Sound, Antarctica. Polar Biology. Heath, R.A. 1977. Circulation across the ice shelf edge in McMurdo Sound, Antarctica. In M.J. Dunbar (Ed.), Polar oceans. (Proceedings of the Polar Ocean Conference.) Lewis, EL., and R.G. Perkin. 1985. The winter oceanography of McMurdo Sound, Antarctica. In S. Jacobs (Ed.), Oceanology of the Antarctic Continental Shelf. (Antarctic Research Series vol. 43.) Washington, D.C.: American Geophysical Union.
The timing of reproduction may be related closely to mode of reproduction. Animals with planktotrophic larvae, for example, need to synchronize the production of larvae with periods when adequate food supply is available in the plankton. In contrast, species that have lecithotrophic larvae or that bypass larval production altogether by brooding or encapsulating embryos and releasing juveniles may be uncoupled from periods of phytoplankton production; reproductive periods of such forms may be extended over much or all of the year. During our field work at McMurdo Station, from August 1984 to January 1986, we were able to collect data on both the mode and timing of many of the common, shallow-water invertebrates there. From preliminary analyses completed SO far, we can estimate the temporal pattern of reproduction of 15 species of common invertebrates in McMurdo Sound. Temporal reproductive patterns have been described by other workers for six additional species of animals in the area, bringing the total number to 21 (table). Seven of the species have planktotrophic larvae. Most of these species have discrete reproductive periods, spawning in late winter, spring, or early summer. Only the nemertean Parbolasia corrugatus spawns with little or no seasonal pattern, and pilidium larvae were collected from the plankton throughout the year. These larvae, as well as those of the asteroids and echinoid, are able to at least supplement their diet with bacteria and thereby not be directly dependent on the midsummer phytoplankton bloom (Rivkin et al. 1986). The larvae of Euphausia crystallorophias, on the other hand, probably feed nearly exclusively on phytoplankton, and they are produced in synANTARCTIC JOURNAL
Sound, and indicate that benthic standing stock and productivity are related to both large- and small-scale current patterns a0
P.K. DAYTON
Scripps Institution of Oceanography La Jolla, California 92093
During the 1983-1984 and 1984-1985 field seasons in McMurdo Sound, Antarctica, we collected data concerning several aspects of the dynamics of sub-ice benthic communities. Photographic transects were collected with a deep submersible camera at several locations within the sound including White Island, Strand Moraine, two locations under the Koettlitz Glacier outflow near Heald Island, and along the Ross Ice Shelf edge near Scott Base. Current measurements were made at three month-long stations across the sound and at seven shortterm stations (1 week or less). Water-column samples were collected at 19 stations from which analyses of temperature, salinity, density, oxygen, chlorophyll a, nitrate, nitrite, phosphate, and silicate were completed. In addition, several plankton samples tows were collected from the upper water column. The diving program was able to resurvey monitoring projects and manipulative experiments begun during the 1970's as well as establish new experiments measuring recruitment, growth, and survivorship of benthic fauna in the eastern and western sound. Photographic data show diverse faunal communities throughout the Sound, even in deep water in the western
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b0
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164'E 165 166' 167' 600 KILOMETERS 77'1 5'S
Co 77,3(
200 E I 77,,
Ia. W
400
78' 600 KILOMETERS
Figure 1. McMurdo Sound, Antarctica. Stars indicate station locations for current measurements. Solid arrows indicate mean surface flow. Dashed arrows indicate mean flow below 100 meters. Dots are locations of hydrographic sampling stations.
180
Figure 2. Cross sound section of selected hydrographic parameters. These sections represent data collected from stations located between Strand Moraine and Cape Armitage, as indicated in figure 1. A. Density (sigma-tin kilograms per cubic meter). B. Water temperature (°C). C. Chlorophyll a (in milligrams per cubic meter). ("m" denotes "meter.") ANTARCTIC JOURNAL
(Dayton, Barry, and Kooyman in preparation). Deep-water photographs from beneath the Ross and McMurdo Ice Shelves show the highest faunal biomass in the eastern sound and lowest in the west, similar to the shallow benthic community (Dayton and Oliver 1977), and apparently related to differences in the concentration of phytoplankton advected into each area. Interestingly, these data also indicate that local variability in current speed may also be important in local benthic production and standing stock. Sites which appear to have more intense currents have higher faunal standing stocks than sites with apparently sluggish flow. Analyses of current-meter data documented the large-scale flow pattern in McMurdo Sound during 1984-1985 with strong southerly flow in the eastern Sound and an opposing northerly flow in the western Sound (figure 1; Barry and Dayton). Semidiurnal tidal motion dominated all current records. These data were similar to previous current observations (Heath 1977; Lewis and Perkin 1985), though there appears to be considerable interannual variability in the intensity of the southerly flow in the eastern Sound which may be related to variability in atmospheric forcing functions. a
b NITRATE
OXYGEN 01
Hydrographic section maps compiled from nutrient and physical water-column data show that nutrient levels were high and nearly homogeneous throughout the Sound during spring 1984. Water temperature was very low and salinity and density were high though somewhat lower in the western Sound (figure 2). Oxygen was near 6.8 milliliters per liter and chlorophyll a was low at most stations and was highest in the eastern Sound (figure 2). During summer 1984, conditions below 200 meters were very similar to springtime conditions, though in shallower water, nutrients levels were depressed, salinity and density were lower, and temperatures were higher. Near surface salinity had decreased, due to sea-ice and glacial meltwater. Nitrate and phosphate levels were dramatically reduced and near 0 at some locations, while nitrite and oxygen concentrations increased somewhat during summer (figure 3; Barry in preparation). These investigations allowed considerable advances in our understanding of the dynamics of local benthic communities, particularly their relationship to oceanographic processes. Subice faunal communities, especially those under thick ice and snow cover depend on phytoplankton growth from well-lit areas and the subsequent advection of this food under the ice to
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Figure 3. Spring vs. summer composite nutrient profiles from McMurdo Sound, Antarctica. A. Oxygen. B. Nitrate. C. Phosphate. D. Nitrite. ("mg at/I" denotes "milligram atoms per liter:' moles/I" denotes "micromoles per liter:' "m" denotes "meter:')
1986 REVIEW
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benthic filter feeders and grazers which in turn are consumed by other community members. Thus, the large-scale current pattern is obviously important to the McMurdo Sound benthos. Moreover, it appears that local variability in current intensity may be responsible in part for the structure of benthic communities. Those areas with high current speeds are expected to have higher particle fluxes and lower sedimentation rates than nearby sluggish current sites, resulting in different faunal assemblages. These physical processes may exert primary control over community productivity and are superimposed on the biotic interactions which have also been shown to be of importance in regulating benthic community structure (Dayton et al. 1974). We intend to continue to investigate the relationship between physical processes and sub-ice benthic community structure. The dramatic north-south and east-west gradients of productivity and species composition in McMurdo Sound appear directly related to ice cover and oceanographic features. As such, McMurdo Sound benthos are particularly amenable to testing hypotheses regarding the roles of in situ vs. advected primary production, and/or the importance of biotic (competition, predation) vs. abiotic (current patterns, etc.) control of community dynamics. We hope to investigate the importance of large- and local-scale current intensity on community production by coupling current observations with growth rate and survivorship studies. In addition, we will continue and expand our benthic productivity studies (Dayton et al. in press) and hope to evaluate the roles of advected vs. in situ primary production on
Contrasting tempos of reproduction by shallow-water animals in McMurdo Sound, Antarctica J.S. PEARSE, I. BOSCH, J.B. MCCLINTOCK, B. MARINOVIC, and R. BRITTON Institute of Marine Sciences University of California Santa Cruz, California 95064
Recent work by our group has revealed that a wide range of reproductive modes prevails in McMurdo Sound, Antarctica, as in other parts of the world ocean (Pearse, Bosch, and McClintock 1986). The production of pelagic larvae is common, although a large proportion of the larvae are lecithotrophic. Among those species that have planktotrophic larvae, there is little evidence of starvation even when phytoplankton levels are very low (Olson, Bosch, and Pearse in press), probably because such larvae are able to use dissolved organic material and to feed on bacteria (Rivkin et al. 1986). Larval energy requirements are probably relatively low; energy stores in lecithotrophic larvae, at least, function mainly to produce large juveniles and are little used during larval development (McClintock and Pearse 1986). 182
microbial and meiofaunal activity, general benthic trophic relationships, and recolonization. This work was supported by National Science Foundation grant DPP 81-00189. References
Barry, J.P. In preparation. Oceanographic patterns in McMurdo Sound, Antarctica. Barry, JR. and P.K. Dayton. Current patterns in McMurdo Sound, Antarctica and their relationship to local benthic communitites. Lw,nology and Oceanography.
Dayton, P.K., J.P. Barry, and C. Kooyman. In preparation. Deep water benthic faunal patterns under the Ross and McMurdo Ice Shelves, Antarctica. Dayton, P.K., and J.S. Oliver. 1977. Antarctic soft bottom benthos in oligotrophic and eutrophic environments. Science, 197, 55-58. Dayton, P.K., G.A. Robilliard, R.T. Paine, and L.B. Dayton. 1974. Biological accomodation in the benthic community at McMurdo Sound, Antarctica. Ecological Monographs, 44(1), 105-128. Dayton, P.K., D. Watson, A. Palmisano, J.P. Barry, J.S. Oliver, and D. Rivera. In press. Distribution patterns of benthic microalgal standing stock at McMurdo Sound, Antarctica. Polar Biology. Heath, R.A. 1977. Circulation across the ice shelf edge in McMurdo Sound, Antarctica. In M.J. Dunbar (Ed.), Polar oceans. (Proceedings of the Polar Ocean Conference.) Lewis, EL., and R.G. Perkin. 1985. The winter oceanography of McMurdo Sound, Antarctica. In S. Jacobs (Ed.), Oceanology of the Antarctic Continental Shelf. (Antarctic Research Series vol. 43.) Washington, D.C.: American Geophysical Union.
The timing of reproduction may be related closely to mode of reproduction. Animals with planktotrophic larvae, for example, need to synchronize the production of larvae with periods when adequate food supply is available in the plankton. In contrast, species that have lecithotrophic larvae or that bypass larval production altogether by brooding or encapsulating embryos and releasing juveniles may be uncoupled from periods of phytoplankton production; reproductive periods of such forms may be extended over much or all of the year. During our field work at McMurdo Station, from August 1984 to January 1986, we were able to collect data on both the mode and timing of many of the common, shallow-water invertebrates there. From preliminary analyses completed SO far, we can estimate the temporal pattern of reproduction of 15 species of common invertebrates in McMurdo Sound. Temporal reproductive patterns have been described by other workers for six additional species of animals in the area, bringing the total number to 21 (table). Seven of the species have planktotrophic larvae. Most of these species have discrete reproductive periods, spawning in late winter, spring, or early summer. Only the nemertean Parbolasia corrugatus spawns with little or no seasonal pattern, and pilidium larvae were collected from the plankton throughout the year. These larvae, as well as those of the asteroids and echinoid, are able to at least supplement their diet with bacteria and thereby not be directly dependent on the midsummer phytoplankton bloom (Rivkin et al. 1986). The larvae of Euphausia crystallorophias, on the other hand, probably feed nearly exclusively on phytoplankton, and they are produced in synANTARCTIC JOURNAL
