Bottom-up and top-down controls of the microbial food web in the Southern Ocean: experiments with manipulated microcosms

Abstract

We have studied bottom-up and top-down control of the Southern Ocean microbial food web by microcosm experiments. Water from the Weddell Sea and Weddell Scotia Confluence were used for the experiments. Microcosms were manipulated by nutrients and light, and by size-selective screening. Incubation at the higher light level doubled phytoplankton growth rates from 0.12 to 0.24 day-1 in the Weddell experiment and from 0.15 to 0.30 day-1 in the Confluence experiment. Nutrient enrichment had no significant effect on growth rates in either of the experiments, indicating that phytoplankton growth was not nutrient-limited. In the microcosms where dinoflagellate growth rate was different, high dinoflagellate numbers were reflected as depressed nanoflagellate growth as well as depressed growth of phytoplankton, suggesting that dinoflagellates controlled both heterotrophic nanoflagellates and autotrophic nanoplankton. Only during short periods, when dinoflagellate numbers were low, could exponential growth of nanoflagellates be demonstrated. Bacterioplankton growth rates were, on average, 0.26 day-1 in the Weddell experiment and 0.22 day-1 in the Confluence experiment. Bacteria were controlled by heterotrophic nanoflagellates. Potential growth rates up to 0.75 day-1 were measured from batch cultures without predators. With the microcosm experiments, we could demonstrate a strong top-down control by dinoflagellates on phytoplankton and on heterotrophic nanoflagellates, and a control by heterotrophic nanoflagellates on bacteria. We could also demonstrate weak nutrient limitation on autotrophs and substrate limitation on heterotrophic bacteria. In the two study areas, biomass production and carbon flow were mediated mainly by organisms that passed through a 20 μm net and had growth rates in the order of 0.20 to 0.30 day-1.