Palaeoecological responses to multiple anthropogenic stressors in a UK lowland lake, and the implications for freshwater-diversity loss

Global biodiversity is decreasing due to anthropogenic activity, especially in freshwater ecosystems and within insect communities. Freshwater insect-diversity loss impacts all trophic levels and ecosystem services within and beyond the freshwater environment. Insect diversity monitoring typically covers short timescales relative to the onset of anthropogenic activity. Therefore, the ability to identify the dominant drivers of biodiversity loss is limited, restricting effective ecosystem management. Palaeoecological studies provide a long-term perspective on changing ecosystems, especially with regard to insect communities. Loe Pool, a lake in the south-west of England, has a known history of varied anthropogenic activity: mining, a water treatment works, agriculture, and flow regulation have impacted the lake and its catchment since the eighteenth century. Two sediment cores were retrieved from Loe Pool and were analysed using chronological, sedimentological, and palaeoecological
techniques. This created a centennial- scale record of the diversity dynamics of lower trophic levels. Our results show the impacts of both mining waste and agriculture-associated nutrient pollution on chironomid and diatom assemblages over time. An intensification of mining (1900–1938) was reflected by chironomid and diatom taxa tolerant to disturbance and pollution, especially mining effluent and heavy metals. Some post-mining recovery away from metal pollution-tolerant taxa was interrupted by the canalisation of the River Cober (1947). Following an intensification of agriculture in the catchment from the 1960s, both chironomids and diatoms shifted towards eutrophy-indicating taxa. While some reduction in eutrophic taxa post-1995 is seen in the chironomid record, this was not apparent in the diatom record, suggesting that systemic recovery has not taken place. Combined, we show that the chironomid and diatom assemblage and diversity changes clearly respond to anthropogenic activity and land use in the Loe Pool catchment. Despite some potential biodiversity recovery, the original composition and diversity have not been restored. This lack of success may be due to insufficient management efforts, or the need for more time for restoration efforts to make a positive impact.