Nutrients and fish effects on plankton community in freshwater mesocosms

Eutrophication affects many lakes in the world. Two important mechanisms to solve this problem are nutrient and fish control. The main aim of the present work was to study nutrient and fish effects on plankton community in freshwater shallow lakes. Two mesocosm experiments were performed, one in spring and the other in winter to test differences in response of planktonic organisms to nutrient and fish addition in different seasons. The results would be of extremely importance for the management of these ecosystems. Mesocosms were polyethylene bags filled with lake water and placed into the lake. Three different nutrient additions and three different fish densities were considered. All the possible combinations between these two factors were tested. Each mesocosm had a replicate, for a total of eighteen mesocosms in both seasons. Lake water was also sampled to test any differences between mesocosms and the natural ecosystem. Both experiments lasted six weeks. In the first week mesocosms were filled with lake water and nutrient and fishes were added. Each week, after sampling nutrients (nitrate and orthophosphate) were added. Fish used was Gambusia holbrooki mainly because its largely recognized planktivory. After the last week, benthic macroinvertebrates were sampled and stomach contents of fishes were analyses. On the basis of the many studies demonstrating the significance to take into account functional groups of zooplankton, instead of consider total zooplankton density, in the present work four groups (rotifers, cladocerans, copepods and nauplii) were considered with regards to the zooplankton community. Nutrients addition was associated to increase in phytoplankton biomass both in spring and winter although this increase was higher during spring. Nutrients did not affect zooplankton density neither in spring nor in winter. High fish density was associated to increase in phytoplankton biomass in both seasons. On zooplankton community, fish had a composite effect. They did not increase total zooplankton density neither in spring nor in winter. Fish changed zooplankton community. In spring, with increasing fish density rotifers increased while cladocerans and copepods decreased. In winter, zooplankton community was composed mainly by rotifers. Fish did not affect rotifers, but decreased cladocerans, copepods and nauplii. In spring, fishes could prey also on benthic macroinvertebrates. During winter, planktivory by fish was more intense due to the lack of benthic macroinvertebrates. Results show different response of plankton community between seasons. Results show the importance of considering as many variables as possible.