Re-evaluating coring sites in paleolimnological studies of a large, deep lake based on chironomid assemblage representativeness

Abstract

In paleolimnology, ecological trajectories of biological communities are typically reconstructed using a sediment core from the deepest part of a lake. However, the representativeness of chironomid assemblages from the profundal zone and its influence on ecological response to environmental change in large, deep lakes remain poorly understood. To address this knowledge gap, we conducted a study on subfossil chironomids in Fuxian Lake, including thirty-five surface sediment samples and two sediment cores from different water depths. Our findings reveal a distinct bathymetric distribution pattern of subfossil chironomid assemblages attributed to limited homogenization in the large, deep lake. The profundal assemblages exhibited the lowest richness representativeness, while nearshore assemblages showed the least similarity to the entire-lake assemblage. Consequently, the response pattern of chironomid assemblage to the environmental change varied with depth. Two notable shifts in the mid-depth core, occurring in 1979 CE and 2005 CE, were primarily attributed to eutrophication processes. However, the chironomid assemblage from the deepest zone exhibited only one significant change in 2005 CE, likely indicating exacerbated hypoxia in the profundal zone. Our combined analysis of surface samples and sediment cores underscores that the profundal zone, where bottom water is characterized by low temperature and anoxia, is unsuitable as a coring site in large, deep lakes due to its extremely simplified species composition. Instead, mid-depth zones should be prioritized as candidate coring sites in large, deep lakes, given their relatively high richness representativeness, head capsule concentration, and stable sedimentary conditions. Specifically, we recommended retrieving sedimentary cores around the water depth of 50 m when conducting subfossil chironomid analysis in Fuxian Lake.