Horizontal Heterogeneities of Functioning of Phyto- and Zooplankton in a Lake With Wind Currents

Abstract

A hypothesis about the formation of horizontal heterogeneities of zooplankton and phytoplankton for the lake subjected to regular daily changes in wind currents has been tested. Formation of horizontal heterogeneities is based on a combination of low-amplitude vertical migration of zooplankton and epilimnion wind currents: surface currents, which bring water depleted in zooplankton to the downwind shore (in the direction in which the wind is blowing), and compensatory above-thermocline ones, which bring zooplankton-enriched water to the upwind shore (against the wind). The spatial separation of phytoplankton and zooplankton may result in the weakening of trophic links between these trophic levels. The hypothesis was tested in 2020 in the pelagic zone of Lake Shira (Khakassia, Russia), a brackish meromictic water body with simple bathymetry and a simple food web. The epilimnion horizontal heterogeneities were assessed using a survey across the lake by measuring biological and physical parameters with a submersible fluorimeter probe and a plankton net at 11 stations and recording the dynamics of wind speed and direction. Differences in the values of primary production, plankton destruction, and intensity of phytoplankton grazing by zooplankton near the downwind and upwind shores were estimated using the bottle method in 3 experiments. The experiments confirmed the expected differences in the functioning of the trophic cascade near the northeastern (more often upwind during the day and downwind at night) and south-southwestern (downwind during the day and upwind at night) shores. Namely, the concentration of chlorophyll a, the gross and net primary production of phytoplankton (estimated by bottle and fluorescent methods), and the daily intensity of zooplankton feeding (based on chlorophyll) were higher near the southern coast, while the biomass of net zooplankton and the respiration rate of the plankton community were higher near the northeastern shore, which coincided with the pattern of phyto- and zooplankton distribution over the lake according to the sampling data under similar weather conditions. The hypothesis was confirmed and supplemented by the data on the evening-night vertical migrations of zooplankton.