Cladoceran community responses to eutrophication, fish introduction and macrophyte degradation over the past century in Lake Erhai

Abstract

Obvious environment changes have occurred in some lakes of Yunnan Province due to intensifying human impacts over the past decades, especially in large lakes. Lake Erhai is the second largest lake in the Yunnan by surface area, which experienced the pressures of multiple environmental stressors (e.g. eutrophication, fish invasion and macrophyte degradation). The application of lake sediment records enables us to reconstruct the long-term changes of lake environment and ecosystem dynamics in situation of less monitoring data. Here multi-proxy analyses of lake sediments were conducted from a short sediment core in the south and shallow basin of Lake Erhai, which are currently eutrophic. We aimed to recognize the long-term variations and driving factors of cladoceran community, and to test for whether spatial heterogeneity exists in community responses through a comparison with monitoring data and previous results from the deep basin of Lake Erhai. Our results showed that cladoceran production showed an overall increasing trend in the past century, and since 1990s bosminid production increased significantly with a general trend of the oligotrophic taxon of Bosmina longispina replaced by B. longirostris. In addition, bosminid production and the body size (i.e. carapace and antennule length) displayed a decrease during early 1990s, were in good correspondence with the period when the population of the exotic noodlefish was well established. Based on the constrained cluster analysis, the historical changes of cladoceran community were characterized by three distinct phases: littoral cladocerans were dominant before the 1990s and the relative abundance of pelagic cladocerans increased rapidly from the 1990s before the pelagic cladocerans became predominant during early 2000s. The principal component analysis showed that the environmental gradient associated with macrophyte coverage accounted for 61.64% of the total variance in the cladoceran community. Compared with previous studies, we observed that the increased production of cladocerans during the 1970s and an abrupt decrease during the 1980s recorded in shallow basin of Lake Erhai were not reflected in the core from the deep basin, indicating that spatial heterogeneity exists in ecosystem changes of Lake Erhai. We also found the obvious shift of bosminid body sizes in response to fish introduction. Overall, our results revealed the long-term changes of cladoceran production and community structure mainly as a consequence of lake eutrophication and macrophyte degradation, while fish introduction also played a role in structuring zooplankton community through changing predation pressure. Therefore the environmental stressors which could be identified through sedimentary records should be taken into account for sustainable management and ecological restoration of Lake Erhai through facilitating lake ecosystem assessment.