A geochemical investigation into the drivers of recent algal community change in small English lakes

This thesis presents an investigation into the drivers of algal community change in small English lakes, with a focus on 20th century climate warming. Sediment cores were extracted from three small lakes across northern England (Lake Gormire, Blea Tarn and Sunbiggin Tarn) and radiometrically dated. Sedimentary photosynthetic pigments were quantified using high-performance liquid chromatography and changes in the source of sedimentary organic matter were established using stable isotopes (δ13C, δ15N). Sedimentary trace elemental data were obtained to elucidate any additional drivers of change. The impacts of 20th century climate warming were investigated through regression analysis, comparing the reconstructed changes in algal biomass and community composition to meteorological data. The marl lake of Sunbiggin Tarn was shown to have been heavily impacted by a rapid increase in the resident gull population during the 1980s which resulted in an enhanced flux of lutein, zeaxanthin and diatoxanthin pigments. However, an even more dramatic change in the algal community occurred at the start of the 1990s with increases in chlorophyll-a and a rapid shift from echinenone to canthaxanthin, both cyanobacterial pigments. This shift occurred due to the large gull populations degrading extensive areas of the surrounding fenland which further enhanced nutrient loading (increased δ15N) to the lake and obscured any direct links to temperature change. In contrast, temperature had a significant impact on the algal community of the two non-marl lakes (Blea Tarn and Lake Gormire). In Lake Gormire warming stimulated all algal groups from the 1960s where in Blea Tarn only chlorophytes and filamentous cyanobacteria had a relationship to temperature. The greater change in Lake Gormire is predominantly due to its closed hydrological basin and enhanced by its relatively small catchment which makes it particularly sensitive to environmental changes. While closed basin lakes may be particularly at risk of future climate change, the findings of this research also indicate their potential to be used as ‘early warning systems’ for other lakes, both regionally and nationally.