Effects of reduced water level on the dynamics of zooplankton and their predators in two shallow lakes in a semi-arid tropical region
Extreme drought;Trophic cascade;Eutrophication;Biomanipulation;|Trophic interaction;
The reduction in the volume of lakes and reservoirs due to the increase in water deficit during periods of drought should affect the structure and trophic dynamics of these ecosystems, increasing the concentration of organisms, the rates of encounter between predators and prey and, consequently, the pressure of predation. This work aimed to investigate the effects of water level reduction caused by extreme drought events on the densities of zooplanktonic organisms and their predators in two reservoirs in the Brazilian semi-arid tropical region. The tested hypothesis is that the densities of zooplankton predators increase with the reduction in the volume of water caused by drought, favoring zooplankton prey less vulnerable to predation over the most vulnerable. To test this hypothesis, the densities of zooplanktonic organisms (rotifers, cladocerans and copepods) and their main predators (fish and shrimp larvae and juveniles, hydracarina mite, chaoboridae larvae and other aquatic insects) were monitore for 24 months in two lakes in the Brazilian semi-arid region during a drought period (2012-2014). In one of the lakes, about 5.8 tonnes of adult fish were remove to prevent mass mortality from fish when the volume of water and oxygen reached a critical level. Zooplanktonic organisms were collected monthly by means of vertical trawls with a plankton net with 68 µm opening and their predators were collected monthly by means of horizontal trawls with ichthyoplankton mesh with 500 µm mesh opening and a flow meter to measure the volume filtered. The organism densities were analyze over the study period and Spearman correlations were performed to assess the possible interactions between zooplanktonic organisms and their predators and the effects of reduced water volume on the studied organisms. In the lake where there was no removal of adult fish, the densities of fish larvae increase, the densities of chaoboridae larvae decreased and the densities of hemiptera and copepods increased with the reduction of the water level. On the other hand, in the lake where adult fish were remove, the densities of fish and shrimp larvae increased while the densities of chaoboridae, hemiptera and rotifers larvae decreased with the reduction of the water level. These results suggest that the reduction in water level caused by drought tends to increase the abundance of fish larvae and reduce the abundance of Chaoboridae larvae, modifying the relative importance of predation by vertebrates and invertebrates and causing changes in the structure of zooplankton communities.