Microbial biomass and productivity in the western Bransfield Strait, Antarctica during the 1986–87 austral summer

Abstract

Studies from December 1986 to March 1987 show that waters of Gerlache Strait and the southwestern Bransfield Strait support a rich seasonal bloom of micro-organisms, consisting primarily of phytoplankton. Chlorophyll a concentrations achieved maximum values (>20 mg Chl a m −3) in December, began to decline in January and exhibited a rapid decrease in February and March to concentrations ≤0.3 mg Chl a m −3. The mean cell size of the planktonic assemblage did change, however, from a predominately microplankton (≥20 μm) crop in December to one dominated by nanoplankton (≤20 μm) later in the season. Total microbial biomass as estimated by measurement of ATP, however, increased between December and January, followed by a rapid decline in February and March. However, the putative microheterotrophs, bacteria, showed no significant increases either during or immediately following the December to January bloom of phytoplankton, in apparent contrast to studies reported from temperature waters. Bacterial cell numbers did not vary systematically during the 4-month period observation and thus appeared to be “uncoupled” from phytoplankton dynamics. Rates of assimilation tritiated organic compounds (glutamate, thymidine, adenine) paralleled the total microbial biomass estimates. All available data (e.g. patterns and rates of organic matter assimilation, Chl/ATP ratios, Chl a concentrations and bacterial cell numbers) suggest that much of the measured heterotrophic activity may be due to the activity of phytoplankton cells. The nature of the microbial food web during the seasonal bloom in Antarctic coastal waters is believed to be a complex function of temperature, photobiological effects related to seasonal deepening of the upper mixed layer, grazing and nutrient regeneration/replenishment. Together these processes determine the composition and total biomass of the microbial assemblage that affects other ecological properties such as the magnitude of new production and the rate of particle flux to deeper waters.