Hydrological change in lakes inferred from midge assemblages through use of an intralake calibration set

Abstract

Midge (Insecta: Diptera: Nematocera) assemblages were examined in surface sediment samples from Lake Pieni‐Kauro and the Saavanjoki River, eastern Finland, using their sedimentary remains. The aim was first to identify the forcing factors behind midge distribution within an aquatic ecosystem and second to develop new midge‐based calibration models for the reconstruction of past environmental conditions, with a special focus on paleohydrology. The third objective was to apply the models to a midge stratigraphy. The effect of sampling‐point selection in paleolimnological studies and its possible influence on chironomid‐inferred temperature reconstructions was also assessed. The species assemblages in Lake Pieni‐Kauro showed high levels of heterogeneity within the surface sediment samples, signifying that the assemblages predominantly incorporate locally dwelling fauna, instead of integrating remains from a larger area. Stream flow and water depth were the most important factors explaining the midge distributions in the lake. In addition, sampling distance from littoral vegetation (DLV), proximity of wetland, distance from shore, and organic content of the sediment were significantly correlated statistically with the midge assemblages. In the lacustrine samples, absolute midge abundance and taxon richness were highest at intermediate water depths. Because hydrological variables were the most important factors controlling the species' distributions, paleohydrological calibration models for water depth and stream flow were developed. Additionally, a transfer function for the sampling DLV was developed that can be used to track changes in the past location of littoral vegetation stands, which is often closely related to hydrological factors. The models' test use on the subfossil midge assemblages of the previously available downcore sediment sequence showed coherent and expected results, compared with previous studies and the modern observed environmental conditions of the lake. Based on the results of this study, these models can provide reliable estimates of past hydrological variability that is closely related to climatic changes and thus may provide a valuable tool for environmental management and global change assessments. This study also indicates that care should be taken in choosing the sampling point in chironomid‐based calibration studies because the influence of depth may distort the temperature estimations.