Top-down control on plankton components in an Antarctic pond: experimental approach to the study of low-complexity food webs

In order to address the top-down effect on the different phytoplankton size-fractions and ciliates, a survey at microcosm scale was conducted in a hypertrophic Antarctic pond, testing the hypotheses that (1) the picophytoplankton is regulated by a top-down control exerted by organisms of the bigger size-fractions, and (2) the nanoplankton fraction (algae and ciliates) is not regulated by a top-down control exerted by the microplankton. The treatments enclosed pond water that was filtered to obtain the different plankton sizes: (a) through 55 μm, (b) 20 μm, and (c) 3 μm pore size filters. The variation in the net growth rate (k′) of the phytoplankton size-fractions and ciliates was analysed after 4 days. The results determined a significant difference (P<0.011) in the k′ value of the picophytoplankton when nano and micro-sized fractions where removed. Conversely, nanophytoplankton and nanociliates were not affected by the removal of bigger size-fractions. We suggest that in this pond the top-down control of the picophytoplankton is relevant, and that the grazing impact is not a key factor in the regulation of the nano-sized (algae and ciliates) plankton components.