Combined effects of nutrients and trace metals on chironomid composition and morphology in a heavily polluted lake in central China since the early 20th century

Eutrophication and trace metal pollution seriously threaten the health of lake ecosystems; however, little is known about the response of zoobenthos to their combined impacts. In order to detect their effects on the biotic community of a lake, subfossil chironomids were analyzed in a sediment core from Sanliqi Lake, a hypereutrophic and severely metal-polluted lake located in the middle reaches of the Yangtze River in central China. The sediment core provides a record of environmental changes since the 1930s. Increases in pollutant concentrations began before the 1990s, and increases in total P and Pb began from the 1950s. Significant increases in nutrient and metal concentrations in the 1990s document the acceleration of eutrophication and metals pollution. As a consequence, macrophyte-related chironomid taxa (e.g., Cricotopus sylvestris -type and Dicrotendipes sp.) which dominated the subfossil assemblages prior to the 1990s were replaced by pollution-tolerant species (i.e., Tanypus chinensis -type and Procladius choreus -type) thereafter. Chironomid diversity gradually decreased from the 1950s with an abrupt change occurring in 1995. Multivariate statistical analyses reveal that subfossil chironomid assemblages are significantly correlated with total N, Pb, and Cd, highlighting the combined impact of nutrients and trace metals on the chironomid communities. In addition, the relative abundance of Procladius choreus -type with mouthpart deformities increased over time and is significantly positively correlated with trace metals and nutrients. Nevertheless, further laboratory studies to assess the linkage between sediment contamination and mouthpart deformities are needed in order to enhance the utility of the latter as an indicator of environmental health.