David L. Garrison

Organism losses during ice melting: A serious bias in sea ice community studies

SummaryWhen ice samples are melted, microorganisms living within the brine inclusions are subjected to rapid and extreme changes in salinities. This procedure results in substantial losses of flagellates and ciliates. Most of these losses can be prevented if ice samples are melted in larger volumes of sterile sea water to buffer salinity and osmotic changes. Since most studies on the ice biota have ignored, or have been unable to avoid this bias, current views of the composition and activity of sea ice communities are based on assemblages over-representing organisms with rigid cell material.

ALGAL ASSEMBLAGES IN ANTARCTIC PACK ICE AND IN ICE-EDGE PLANKTON1

A significant amount of the primary production in the Southern Ocean and other ice‐covered oceans takes place in localized ice edge plankton blooms. The dynamics of these blooms appear to be closely related to seasonal melting of sea ice. Algal cells released from the ice are a possible source of ice edge planktonic assemblages, but evidence for this “seeding” has been equivocal. We compared algal assemblages in ice and water in the Weddell Sea during the austral spring of 1983 at a receding ice edge with a well‐developed ice edge bloom. The high degree of similarity between ice and water column assemblages, the spatial and temporal patterns in the distribution and abundances of species, and preliminary evidence for the viability and growth of ice‐associated species provide evidence for seeding from sea ice of some species in Antarctica.

Sea Ice Microbial Communities in AntarcticaThese communities may provide an important food resource in deep-water pelagic systems

ccounts of early naturalists first suggested the richness of Antarctic waters. Along with observations of abundant stocks of great whales, seals, and seabirds came reports of sea ice floes stained and discolored by algae. Although sea ice microbial communities (SIMCOs) have been observed and studied for several decades, fundamental questions about their role remain. Do these communities represent a significant food source for benthic or pelagic food webs in ice-covered oceans? If so, what factors contribute to their productivity? Studies of SIMCOs in the landfast ice at McMurdo Sound and in the pack ice region of the Weddell Sea are part of an ongoing effort to understand the role and importance of sea ice communities in the Antarctic marine ecosystem and the physical and chemical attributes of their habitat.

The biota of Antarctic pack ice in the Weddell sea and Antarctic Peninsula regions

SummaryPack ice surrounding Antarctica supports rich and varied populations of microbial organisms. As part of the Antarctic Marine Ecosystem Research in the Ice Edge Zone (AMERIEZ) studies, we have examined this community during the late spring, autumn, and winter. Although organisms are found throughout the ice, the richest concentrations often occur in the surface layer. The ice flora consists of diatoms and flagellates. Chrysophyte cysts (archaeomonads) of unknown affinity and dinoflagellate cysts are abundant and may serve as overwintering stages in ice. The ice fauna includes a variety of heterotrophic flagellates, ciliates, and micrometazoa. The abundance of heterotrophs indicates an active food web within the ice community. Ice may serve as a temporary habitat or refuge for many of the microbial forms and some of these appear to provide an inoculum for planktonic populations when ice melts. Larger consumers, such as copepods and the Antarctic krill, Euphausia superba are often found on the underside of ice floes and within weathered floes. The importance of the ice biota as a food resource for these pelagic consumers is unknown.

CONFIRMATION OF DOMOIC ACID PRODUCTION BY PSEUDONITZSCHIA AUSTRALIS (BACILLARIOPHYCEAE) CULTURES1

Single done isolates of Pseudonitzschia australis Frenguelli (= Nitzschia pseudoseriata Hasle) isolated from a toxic bloom in Monterey Bay, California produced domoic acid in culture. Although long‐term historical records do not indicate previous blooms of this species on the Pacific coast, this is probably because it has been often misidentified as Nitzschia seriata Hasle; previous evidence for toxicity is lacking. Hydrographic data suggest that areas such as Monterey Bay might be “hot spots” for domoic acid‐producing blooms.

Protists from the ice-edge region of the Weddell Sea

Abstract We present some of the first records and quantitative data on protists from the ice-edge region of the Weddell Sea. Major groups were diatoms (33 species), dinoflagellates (4 species), prymnesiophytes (2 species), cryptomonads (2 species), prasinophytes (1 species), chrysophytes (3 species), and choanoflagellates (11 species). The prymnesiophyte, Phaeocystis pouchetii , diatoms (predominantly Nitzschia of the Fragilariopsis group), and choanogflagellates (Acanthoecidae) were numerically dominant, although other protists sometimes contributed significantly to plankton populations. Most groups were more abundant at the ice edge than under the pack ice or in the open water. The abundance of choanoflagellates suggests that food chains based on bacterial production must be important, as well as the traditional diatom-krill-vertebrate food chain.

Protozooplankton in the Weddell Sea, Antarctica: Abundance and distribution in the ice-edge zone

SummaryProtozooplankton were sampled in the iceedge zone of the Weddell Sea during the austral spring of 1983 and the austral autumn of 1986. Protozooplankton biomass was dominated by flagellates and ciliates. Other protozoa and micrometazoa contributed a relatively small fraction to the heterotrophic biomass. During both cruises protozoan biomass, chlorophyll a concentrations, phytoplankton production and bacterial biomass and production were low at ice covered stations. During the spring cruise, protozooplankton, phytoplankton, and bacterioplankton reached high concentrations in a welldeveloped ice edge bloom ∼ 100 km north of the receding ice edge. During the autumn cruise, the highest concentrations of biomass were in open water well-separated from the ice edge. Integrated protozoan biomass was <12% of the biomass of phytoplankton during the spring cruise and in the autumn the percentages at some stations were >20%. Bacterial biomass exceeded protozooplankton biomass at ice covered stations but in open water stations during the fall cruise, protozooplankton biomass reached twice that of bacteria in the upper 100m of the water column. The biomass of different protozoan groups was positively correlated with primary production, chlorophyll a concentrations and bacterial production and biomass, suggesting that the protozoan abundances were largely controlled by prey availability and productivity. Population grazing rates calculated from clearance rates in the literature indicated that protozooplankton were capable of consuming significant portions of the daily phyto- and bacterioplankton production.

Nano- and microplankton in the northern Arabian Sea during the Southwest Monsoon, August–September 1995 A US–JGOFS study

As part of the US Joint Global Ocean Flux Studies (JGOFS) Arabian Sea Program, we determined the abundance and biomass of autotrophic and heterotrophic nano- and microplankton in the upper 100 m at 10 stations in the northern Indian Ocean during the late Southwest Monsoon from 17 August through 15 September 1995. Autotrophic nano- and microplankton biomass ranged from 0.2 to 68.0 μg C l-1, with most of the biomass in the upper 20–60 m. Phytoplankton assemblages varied markedly in composition along a transect from onshore to about 1500 km offshore. Larger forms, such as diatoms and colonies of the prymnesiophyte Phaeocystis, dominated stations inshore of about 1000 km, whereas picoplankton dominated offshore. Heterotrophic nano- and microplankton biomass varied from ∼1 to 12 μg C l-1, and nanoflagellates, dinoflagellates, and ciliates reached maximum biomass at different locations and depths. Heterotrophs comprised 18–27% of the biomass over most of the transect. Biomass of all groups of organisms was strongly negatively correlated with depth and positively correlated with each other, suggesting a dynamic food web. Size structure of organisms among stations suggested that larger consumers occurred where phytoplankton cells were large. Sediment trap data indicate high organic carbon and biogenic silica flux at the time of our study. Our findings of abundant diatoms over much of the study area and their apparent transition from healthy-looking cells nearshore to senescent ones offshore suggest that populations could have sunk as a bloom terminated, in addition to being available for mesozooplankton grazers.

CARBON PARTITIONING WITHIN PHAEOCYSTIS ANTARCTICA (PRYMNESIOPHYCEAE) COLONIES IN THE ROSS SEA, ANTARCTICA

The haptophyte Phaeocystis antarctica Karsten is a dominant species within the seasonal bloom in the Ross Sea. One of the unique characteristics of this form is that carbon is partitioned between the cells and the colonial matrix, a relationship that is poorly documented for this region. We combined particulate organic carbon measurements and microscopic analysis of P. antarctica‐dominated samples to assess the contribution of single cells, colony‐associated cells, and mucilage to the carbon concentrations of waters with P. antarctica. Two cruises to the Ross Sea were completed, one in austral spring 1994 and one in summer 1995–1996. In 1994 the bloom was dominated by colonial P. antarctica that contributed up to 96% of the total autotrophic carbon, whereas in 1995–1996 a mixture of P. antarctica and diatoms occurred. P. antarctica colony volume (V ) was related to colonial cell number (NC) by the relationship V = 417 ×NC1.67. Total colony carbon (CCOL) was calculated as the sum of cell carbon (CCC) and mucus‐related carbon (CM). We found the contribution of mucus carbon to be 213 ng C mm−3 of colony volume. For P. antarctica‐dominated assemblages sampled at the peak of the bloom, CM represented a minor fraction (14 ± 4%) of colony carbon, and during early summer conditions CM was at most 33% of CCOL. This organism plays a cardinal role in the carbon cycle of many regions. These results constrain the partitioning of carbon between cellular material and the colony matrix, information that is necessary to accurately describe the biogeochemical cycles influenced by this species.

An overview of the abundance and role of protozooplankton in Antarctic waters

Abstract The classic view of the Antarctic pelagic system has suggested that food web dynamics are dominated by the diatom-krill food web link. Recent observations, however, have indicated that this is an oversimplification and that the antarctic food web has a complexity similar to that found in lower latitude systems. More specifically, small particulate feeding protozoans appear to have a much greater importance than was previously assumed. Only a few studies have been sufficiently extensive to characterize the Antarctic pelagic protozoan assemblage. These indicate that heterotrophic flagellates (dinoflagellates and other heterotrophic nanoplankton) and ciliates (mostly non-loricate oligotrichs) dominate the protozooplankton assemblages in surface waters. The combined biomass of protozooplankton has been reported to comprise from 75% of the total nano- and microplankton biomass depending on season and location. Protozoans are also found in sea ice communities where their abundances exceed those typically found in the plankton. Several protozoan species occupy both ice and water habitats, suggesting that seasonally melting sea ice may be the source of ice-edge protozooplankton assemblages. The feeding rates of protozooplankton in Antarctic waters are poorly documented. Consumption estimates based on clearance rates and some preliminary grazing experiments, however, indicate that the protozooplankton should be capable of utilizing a significant proportion of the daily primary and bacterioplankton production. Protozoans may contribute to vertical flux, but present evidence suggests that their contribution will be lower than from other sources.