Patchy distribution of phyto- and zooplankton in large and shallow lagoon under ice cover and resulting trophic interactions

The plankton of coastal lagoons during ice-cover are rarely studied. We hypothesise that under ice: i.) the lack of wind mixing leads to strong diversification in the spatial distribution of both phyto- and zooplankton communities, and ii.) top-down regulation is small, and does not significantly impact the development of phytoplankton. We tested these hypotheses in the Vistula Lagoon (South Baltic), a few weeks after ice thickness had developed to 10 cm. Considerable spatial differences in some physical (turbidity, transparency) and chemical water properties (Cl-, Ntot. and Ptot.) occurred. According to a two-dimensional CCA, these differences were related to the heterogeneous distribution of several plant and animal taxa, supporting the first stated hypothesis. The ratios between total biomass of non-predatory zooplankton (BZp) and biomass of edible phytoplankton ≤ 35 µm (BPh) were low (mean = 0.08, range = 0.02–017), and differed across the lagoon. The significant correlation between the values of BZp and BPh (Spearman test, r = 0.70; p ≤ 0.05), however, suggests a considerable role of zooplankton filter feeders in controlling phytoplankton development. Therefore, top down control (hypothesis ii) may be quite strong.