DAVID A. CARON, Biology Department, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543
DARCY J. LONSDALE, Marine Sciences Research Center, State University of New York at Stony Brook, Stony Brook, New York 11794-5000
MARK R. DENNETT, Biology Department, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543
The Ross Sea polynya is a site of an extensive, albeit seasonally restricted, phytoplankton bloom (Smith and Gordon 1997). It is also presently an active study site for the Antarctic Environment Southern Ocean Process Study (AESOPS), a research program within the U.S. Joint Global Ocean Flux Study (JGOFS). A major goal of this initiative is to characterize the production and fate of primary productivity in this coastal ecosystem and the factors affecting these processes. Within the context of this multi-investigator JGOFS program, we have been examining the trophic activities of herbivorous and bacterivorous microbial assemblages during a series of four cruises spanning October 1996 to December 1997.
Heterotrophic microbial processes play a pivotal role in determining the fate of pelagic production (figure 1). Microbial consumers, predominantly protozoa, are important consumers of primary producers in ocean plankton communities as well as bacterial assemblages that are supported by the utilization of dissolved and particulate organic material (Caron and Finlay 1994; Sherr and Sherr 1994). Conventional wisdom suggests that much of the organic material consumed by protozoa is remineralized and recycled in the water column because of the small size of these consumers and the waste material that they release. In contrast, consumption by metazoa can result in the production of relatively large, rapidly sinking particles. Therefore, metazoan grazing may contribute significantly to the removal of compounds of considerable biogeochemical significance (e.g., carbon dioxide) from the water column, whereas microbial grazing in surface waters may promote the retention of these materials in the upper ocean and atmosphere (Michaels and Silver 1988).
Relatively few investigators report the grazing activities of protozoa in antarctic ecosystems (Burkill, Edwards, and Sleigh 1995; Archer et al. 1996; Froneman and Perissinotto 1996). Nevertheless, heterotrophic protists have been reported to constitute a significant, and sometimes dominant, component of the total protistan abundance and biomass in the southern oceans, their coastal seas, and the sea-ice microbial communities associated with these water masses (Gowing and Garrison 1992; Stoecker, Buck, and Putt 1993). Quantification of the rates of growth and trophic activity of these microbial consumers is, therefore, imperative to understanding how these food webs function.
Our component of the JGOFS process study is designed to obtain information on the abundances and trophic activity of microbial assemblages within the water column of the Ross Sea proper, mostly along a transect line at 76°30'S 168°E to 178°W (Anderson 1993). We have completed three cruises at this time (October and November 1996; January and February 1997; April and May 1997) with one remaining (November and December 1997) to provide seasonal coverage from austral spring through fall. Our ongoing research program entails
We observed standing stocks of chlorophyll during mid-to-late austral summer (January and February 1997) that were nearly three orders of magnitude greater than chlorophyll concentrations observed during cruises conducted in the spring or fall. Surprisingly, rates of herbivory during all three cruises were exceedingly low, even during periods of high chlorophyll concentrations (table). The validity of these experimental results was corroborated by independent methods of examining herbivory. These ancillary measurements indicated that microorganisms smaller than 200 micrometers had only a minor impact on the standing stock of phytoplankton. Herbivory by larger zooplankton was not demonstrable. Herbivorous microplankton were present during austral summer (as evidenced by microscopical examination), but their grazing activities did not affect significantly the phytoplankton assemblages that were composed largely of colonies of Phaeocystis spp. in the central polynya and diatoms at the periphery of the sea.
Samples from bacterivory experiments are being analyzed at this time, but preliminary conclusions based on available data indicate that bacterivores were abundant and active during the period of the phytoplankton bloom and subsequent decrease during the austral summer cruise. Up to 30 percent of the bacterial assemblages were removed daily by bacterivores at specific depths and locales during that cruise. High abundances of colonial choanoflagellates in surface waters at that time accounted for much of the grazing pressure on bacteria (figure 2). Our preliminary analyses of microbial grazing during the AESOPS cruises revealed the surprising finding that microbial trophic activities did not result in high rates of mortality of phytoplankton even during periods of peak phytoplankton biomass. We speculate that other processes (e.g., sinking) must be responsible for the removal of much of this primary production. On the other hand, a bacterivorous protozoan fauna was abundant and active during austral summer 1997, and bacteria were consumed at significant rates in some of our experiments. We speculate that much of the energy entering the "microbial loop" in this coastal sea at that time may be a consequence of release of dissolved organic material by the abundant phytoplankton assemblage with subsequent uptake and growth by the heterotrophic bacterial assemblage.
This research is supported by National Science Foundation grant OPP 96-33703.
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