AN ASSESSMENT OF RECENT TROPHIC CHANGES IN WINDERMERE SOUTH BASIN (ENGLAND) BASED ON DIATOM REMAINS AND FOSSIL PIGMENTS

Cyanobacterial carotenoids and diatom remains have been analyzed in recent sediments from the Windermere South Basin (WSB) to study the trophic evolution experienced by the lake. Dates in the top 30 cm were specifically established through radionuclide (Pb-210 and Cs-137) analyses. Diatom stratigraphy shows dominance of the centric diatoms Cyclotella comensis and C. radiosa and several benthic taxa in the early postglacial. This indicates oligotrophy in the WSB during that period. This assemblage was replaced by another dominated by the diatom Asterionella formosa in the 1870's, as has been established from the Pb-210 dating. From that date onwards, the lake underwent a progression towards eutrophy, indicated by the progressive increase in Aulacoseira subarctica (c. 1930's), Fragilaria crotonensis (c. 1943), and more recently, of the centrics Stephanodiscus parvus (c. 1971) and Cyclotella meneghiniana (1988). Carotenoid stratigraphy reveals the differences between different sections of the core. Oscillaxanthin and myxoxanthophyll had very low records in the early and medium parts of the core, but increased from c. 1950's, showing peaks at c. 1967, 1979 and 1987. Some of these peaks indicated a differential abundance of Oscillatoria, and are matched to those observed directly during the ongoing monitoring of the phytoplankton of the lake. The coincidence between the historic appearance of diatoms associated with nutrient-rich waters and the enhanced carotenoid occurrence suggest a common response to phosphorus enrichment, and that the progressive change towards eutrophy has been accentuated during the last twenty-five years.