Interações entre invertebrados planctônicos de lagoas impactadas por peixes não-nativos (médio rio Doce – MG): uma abordagem experimental e paleolimnológica

Abstract

Currently, the trophic webs of several middle Doce River lakes (MG) are strongly impacted by non-native piscivorous fish. In such communities, the native planktivorous ichthyofauna became scarce and the predatory Chaoborus larvae (Diptera) may have formed dense populations (“ecological release”). As a result, small-sized cladocerans were possibly suppressed or even became extinct, which may have stimulated the growth of phytoplankton biomass. This research had as general objective to study the ecology of Chaoborus larvae from two middle Doce River lakes (Lakes Carioca and Jacaré). Specifically, it sought to investigate how the local Chaoborus larvae interact with each other and with their zooplanktonic prey. For this, we used different research approaches: in situ experiments (mesocosms), larval diet description based on the crop analysis, traditional population monitoring techniques, larval biometry, paleolimnological evaluation of sediments, visual observations of live zooplankters, and laboratory predation experiments. According to our mesocosm experiments in Lake Carioca, the Chaoborus larvae does not strongly control (top-down) the dominant rotifers and cyclopoid copepods. Locally, the larvae of the three sympatric Chaoborus species are omnivorous ingesting a wide variety of prey items throughout their development. There, heterospecific individuals of the same instar exhibit distinct morphometric characteristics and relatively low diet overlap which, in theory, may be contributing to their coexistence. The microfossil analysis of invertebrates indicated a significant numerical decline of cladoceran caparaces (genus Bosmina) in the sediment samples corresponding to the period after the invasion of non-native piscivorous fish. Thus, in fact, they may have 22 negatively affected not only their direct prey species (native planktivorous fish) but also the organisms of other trophic levels (herbivorous cladocerans). Additionally, our visual observations of live zooplankters from Lake Jacaré showed that an anti-predation behavioral reaction (akinesis) does not fully explain the low consumption of Physocypria schubarti ostracods by local Chaoborus larvae. Laboratory predation experiments also suggested that high densities of this ostracod species in the water column negatively affect the predatory performance of Chaoborus larvae on Ceriodaphinia silvestrii neonates. In general, our data demonstrated how complex and variable can be the ecological interactions established between freshwater planktonic invertebrates even in poor communities highly impacted by non-native piscivorous fish such as those from the middle Doce River lakes.