The effects of early spring stocking in an agricultural lake: a trophic cascade hypothesis

Lake Josava (Croatia) is a shallow reservoir surrounded by agricultural land. In the present study, the trophic cascade was tested by examining the effects of stocking with common carp on plankton and periphytic microphytes. Before stocking, the phytoplankton community was dominated by the chrysophyte Synura uvella. In the epilithon and epiphyton, the predominant diatoms were prostrate, stalk-forming, and motile taxa representing an important food source for adult copepods. After stocking, phytoplankton biomass declined and the community shifted towards small centric diatoms, allowing the small-bodied zooplankton to exploit them. The lower biomass of adult copepods allowed rotifers to proliferate and exploit phytoplankton, while small cladocerans and nauplii fed primarily on epilithon. One month after stocking, phytoplankton was dominated by cryptophytes, small centric diatoms and chlorophytes, which were an important food for rotifers, while none of the zooplankton groups showed a significant relationship with the epilithic and epiphytic communities. By the end of the experiment, food was scarce due to reduced biomass of autotrophs, and zooplankton possibly began to feed on other sources. Our results add to the knowledge about the trophic cascade hypothesis in small shallow reservoirs.

Automated summary

In this study, the effects of stocking common carp on plankton and periphytic microphytes in Lake Josava were examined. The results showed that after stocking, phytoplankton biomass declined and the community shifted towards small centric diatoms, allowing small-bodied zooplankton to exploit them. This led to reduced biomass of autotrophs and the zooplankton possibly began to feed on other sources.