Ecological processes in macrophyte- and phytoplankton-dominated shallow lakes

Abstract

Stratified lakes often receive more attention from limnologists although most of the world's lakes are indeed shallow.This study describes ecological processes in three shallow lakes, one dominated by macrophytes, one by phytoplankton, and one with both of them.Investigations focused on abiotic (water chemistry) as well as biotic factors such as phytoplankton, bacteria, zooplankton, and macrophytes.The macrophyte-and plankton-dominated lakes differed clearly in water transparency, which was far greater in macrophyte-dominated lakes due to phytoplankton setback in summer.Similar nutrient and zooplankton dynamics occurred in both types of lakes, suggesting that an alternative mechanism may account for the observed phytoplankton decline in macrophyte lakes.In contrast, herbivorous ciliates dominated only in the macrophyte lake.More intensive ciliate grazing could be one factor causing a decrease in phytoplankton numbers.In the plankton-dominated lake bacteria were mostly related to phytoplankton production but that was not the case in the macrophytedominated lake.In the plankton-dominated lake the grazing rate of ciliates on bacteria was higher than in the macrophyte-dominated one.However, when grazing on small algae was considered, the highest grazing rates were found in the macrophyte-dominated lake.This suggests that the microbial loop was weaker in the macrophyte-dominated lake than in the plankton-dominated lake.