Do benthic macroinvertebrate predators increase mortality of zooplancton in litoral zones?

GONZALEZ SAGRARIO M. A., R. ITUARTE, E. SPIVAK AND E. BALSEIRO

Montevideo , Uruguay

Congreso; VII Jornadas de Zoología del Uruguay; 2003

Sociedad Zoologica de Uruguay, Facultad de Ciencias

Zooplankton seeking refuge among macrophytes in littoral areas of shallow lakes,  may offer an alternative food for predacious macroinvertebrates, which occupy a central position as predator of zooplankton and prey for fishes. We hypothesized that littoral macroinvertebrate predators could depress zooplankton populations and that different predators could have different impacts on pelagic preys. We conducted a field enclosure experiment using plastic bags and complementary laboratory feeding trials. The enclosure experiment consisted in 3 treatments: control (without predators), low diversity (LDT) and high diversity (HDT) of predators. Predators in LDT were ?Oxyagrion rubidum (damselfly) and in HDT were zoeas and adults of Palaemontes argentinus (grass shrimp); Buenoa sp. (backswimmer); Belostoma sp. (bug); Megadytes sp. (diving beettle) and early and late instars of damselfly. Diaphanosoma brachyurum, Bosmina huaronensis and Moina micrura (Cladocera) and males and females  of Notodiaptomus incompositus (Copepoda Calanoida) were negatively impacted by the macroinvertebrate assemblage and their abundance decreased significantly in HDT respect to control or LDT (Anova, p< 0.01). The abundance of the different calanoid copepodite did not differ among treatments. Feeding trials revealed that Buenoa sp., adults of P. argentinus and late instars of damselfly had a significant negative impact on cladocerans (D. brachyurum, B. huaronensis), and copepods (males, females and V+IV stage copepodites) (Anova, p< 0.0001). Submerged macrophytes are considered refuge areas for zooplankton. In contrast, this study showed that littoral predator assemblage negatively impacted on  zooplankton. We emphasize that  the refuge effect offered by macrophytes may be counteracted by the effect of predacious macroinvertebrates that macrophytes host.