Plankton assemblages in the ice edge zone of the Weddell Sea during the austral winter

Abstract

Plankton studies in Antarctic waters have emphasized the importance of diatoms. The species composition, abundances and contribution to biomass of the other planktonic groups are stil poorly documented. This is particularly true for the heterotrophic members of the nano- and microplankton assemblage. As part of the Antarctic Marine Ecosystem Research in the Ice age Zone (AMERIEZ) program, we sampled nano- and microplankton across the ice edge zone during the austral winter. A variety of microscopial techniques allowed us to the composition, trophic mode and biomass of the spectrum of organisms that make up the winter plankton assemblage. Total nano- and microplankton biomass in the upper 100 meters of the water column ranged from 0.3 to 0.6 gC m −2. The biomass composition of plankton assemblages among the stations was relatively uniform throughout the ice edge zone; however, the autotrophic flagellates and dinoflagellates showed significantly higher biomass at the ice edge or in open water relative to ice covered stations. The heterotrophic biomass (protozooplankton) exceeded the biomass of phytoplankton at most stations. Among the autotrophic forms, dinoflagellates made up 38% of the biomass, followed by other autotrophic flagellates (35%) and diatoms (27%). The phytoplankton biomass was dominated by nanoplankton (< 20 μm). Dinoflagellates also dominated among the heterotrophs, making up as much as 73% of the total heterotrophic biomass. Most of the heterotrophic dinoflagellates were < 20 μm. The ciliates present were mostly non-sheathed oligotrichs. Tintinnids were present but in densities of usually < 71 −1. Many of the larger oligotrichous ciliates examined contained sequestered chloroplasts, which have been reported to retain photosynthetic activity. Larger protozoans (e.g., Radiolaria, Acantharia and Foraminifera) were a minor constituent of the plankton biomass in surface waters. This study suggests that winter plankton assemblages were dominated by nanoplankton for both algae and protozoans. The abundance of athecate dinoflagellates, both autotrophic and heterotrophic forms, was unusual and warrants further investigation of the role of these organisms in the Antarctic plankton community. Generally low production and the high abundance of heterotrophic organisms relative to the phytoplankton, suggests the protozooplankton will be important in utilizing winter primary and bacterial production and transferring this production to higher levels of the food web.