ATP, chlorophyll "a", and size structure of phytolpankton in seawater at stations in the Atlantic sector of the Southern Ocean

Chlorophyll "a" and adenozine triphosphate (ATP) concentrations together with size structure of microplankton were investigated in January-April 1989 in the Indian Ocean and in the Weddell Sea. ATP values varied from 11 to 92 ng/l, and chlorophyll "a" concentrations varied from 0.04 to 0.27 µg/l in the Indian Ocean, with prevailing nanoplankton and picoplankton fractions. Both ATP and chlorophyll "a" concentrations increased 2 times to the south of 40°S; in the Weddell Sea they exceeded 400 ng/l and 0.6 µg/l, respectively. Cells of nanophytoplankton and microphytoplankton (mainly diatoms) prevailed in size spectra. Spatial variabilities of the parameters were within one order of magnitude; their values decreased 3-4 times during 1 month. Size structure changed due to increased portion of nanoplankton and picolankton. ATP concentrations in the photic layer (0-200 m) varied from 31.96 mg/m**2 in February to 8.02 mg/m**2 in March to April. ATP concentrations were 61.5 and 98.8 mg/m**2 at depths of 4200 and 4700 m, respectively.

Biomass of nanoprotozooplankton in the Atlantic sector of the Southern Ocean

The dynamic of early spring nanoprotozoa was investigated in three characteristic water masses of the Southern Ocean: the Marginal Ice Zone, the intermediate waters of the Antarctic Circumpolar Current and the Polar Frontal Zone. Biomass and feeding activities of nanoprotozoa were measured, as well as the biomass of their potential prey-bacteria and phototrophic flagellates-on the 6°W meridian in the Southern Ocean along three repetitive transects between 47 and 60° South from October to November 1992. On average, nanoprotozooplankton biomass accounted for 77% of the combined biomass of bacteria and phototrophic flagellates, and was dominated by dinoflagellates and flagellates smaller than 5 µm. As a general trend, low protozoan biomass of 2 mg C/m**3 was typical of the ice covered area, while significantly higher biomasses culminating at 15 mg C/m**3 were recorded at the Polar Front. Biomasses of bacteria and total phytoplankton were distributed accordingly, with larger values at the Polar Front. Phototrophic flagellates did not show any geographical trend. No seasonal trend could be identified in the Marginal Ice Zone and in the intermediate waters of the Antarctic Circumpolar Current. On the other hand, at the Polar Front region a three-fold increase was observed within a 2-month period for nanoprotozooplankton biomass. Such a biomass increase was also detected for bacterioplankton and total phytoplankton biomass. Half-saturation constants and maximum specific ingestion of nanoprotozoan taxons feeding on bacteria and phototrophic flagellates were determined using the technique of fluorescent labelled bacteria (FLB) and algae (FLA) over a large range of prey concentrations. Maximum ingestion rates ranged between 0.002 and 0.015/h for bactivorous nanoprotozoa and heterotrophic flagellates larger than 5 µm feeding on phototrophic flagellates. The markedly high maximum ingestion rates of 0.4/h characterising nanophytoplankton ingestion by dinoflagellates evidenced the strong ability of dinoflagellates for feeding on nanophytoplankton. Daily ingestion rates were calculated from nanoprotozoan grazing parameters and carbon biomass of prey and predators. This indicated that nanoprotozoa ingestion of daily bacterioplankton and phytoplankton production in early spring ranged from 32 to 40%.