Seasonal development of the cladoceran assemblage in a turbid lake: the role of emergent macrophytes

Abstract

In eutrophic Lake Hiidenvesi, the water is turbid and the biomass of submerged macrophytes consequently low. In such lakes, due to low refuge availability, the highest zooplankton biomass usually occurs in the beginning of the growing season. In L. Hiidenvesi, however, the highest biomasses are found in late July-early August. To clarify the role of emergent macrophytes, we investigated the seasonal development in cladoceran community structure and biomass in different zones of an emergent macrophyte bed (Typha angustifolia). Total cladoceran biomass in the different zones did not differ, but the biomass of free-swimming filter feeders was significantly higher within the emergent stand than at the edge or outside the stand. This was mostly due to the higher biomass of Bosmina longirostris in the inner zone. Mean size of Ceriodaphnia quadrangula was also highest within the stand, The biomass of free-swimming filler feeders peaked on 7 July at 40 μg C/l and declined rapidly thereafter, whereas at the edge and outside the stand the biomass remained below 5 μg C/l throughout the study period. The observed seasonal development of the cladoceran community was probably a consequence of concomitant changes in the predation pressure and refuge availability. At the edge and ouside the stand, the estimated consumption by free-swimming filter feeders remained below 10 % of phytoplankton biomass, while in the inner zone it reached 70 %. Outside the emergent stand, the estimated grazing pressure by the plant-associated Sida crystallina was higher than total consumption of the free-swimming species. The result demonstrated that also nymphaeids (which were found outside the emergent stand) may be of great importance for herbivorous zooplankton, especially plant-associated species.