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

Abstract

The plankton of coastal lagoons while they are covered by ice is rarely studied. We hypothesised that, under ice: (1) the lack of wind mixing would lead to strong diversification in the spatial distribution of both phyto- and zooplankton communities across the lagoon; and (2) top-down regulation would be small, and would not significantly affect the development of phytoplankton. We tested these hypotheses in Vistula Lagoon (southern Baltic Sea) a few weeks after the thickness of the ice had reached 10cm. Considerable horizontal and vertical spatial variation in some physical (turbidity, transparency) and chemical (Cl–, total N and total P) water properties were observed. According to a two-dimensional canonical correspondence analysis, these differences were related to the heterogeneous distribution of several plant and animal taxa, supporting our first hypothesis. The ratio of the total biomass of non-predatory zooplankton (BZp) to the biomass of edible phytoplankton ≤35µm (BPh) was low across the different sampling sites (mean 0.08; range 0.02–017), and differed across the lagoon. However, the significant correlation between BZp and BPh values (Spearman test, r=0.70; P≤0.05) suggests a considerable role of zooplankton filter feeders in controlling the development of phytoplankton. Therefore, top-down control may be quite strong, refuting Hypothesis 2.