Effect of zooplankton predation on phytoplankton communities in a small reservoir in a Tabuleiro Forest, southeastern Brazil

Abstract

Phytoplankton is the name given to a community of photoautotrophic organisms that live most of their life cycle in the pelagic zone of oceans, lakes, ponds and reservoirs. There are many factors affecting the development and control of the phytoplankton, so that the most adapted species to the environmental conditions are able to remain in the environment. In freshwater environments, the zooplankton community is characterized by high species richness, being composed by different groups of invertebrate predators of algae such as protozoa, rotifers, cladocerans and copepods. Among the biological factors controlling phytoplankton populations, predation by zooplankton is a major one. The Vale Nature Reserve is located in the north of the Espírito Santo, between the municipalities of Linhares and Jaguaré, and presents an area of 22,000 ha, altitudes between 28-65 m and warm and humid climate with rainy summer and dry winter. The objective of this study was to evaluate the effect of predation of zooplankton on the phytoplankton communities. Eighteen litters of water were collected on the surface of the reservoir. After collection, six microcosms were set in the laboratory in transparent plastic chambers containing 3 liters of water each. Three of them served as controls and in the other three (experiment), zooplankton organisms were removed by filtration of the water on 250 mm mesh. The phytoplankton community of each microcosm was analyzed qualitatively and the population density was estimated by the sedimentation method. The counting procedure used was the random fields, where 6 slides of 0.5 ml were analyzed for each microcosm, totaling 18 slides per treatment. The results were expressed as density. The Student t-test looks for significant biological differences among different groups of phytoplankton (Classes) between the two treatments. The phytoplankton communities were represented by the classes Cyanophyceae, Chlorophyceae, Zygnemaphyceae, Bacillariophyceae and Euglenophyceae were found. The phytoplankton density showed significant differences among the treatments studied for the class Chlorophyceae. The cyanobacteria have benefited at the expense of green algae in the absence of herbivores. The Chlorophyceae represent preferred food source for zooplankton, since they have thin cell walls, resulting in a high ratio organic carbon: dry weight ratio.