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 re cent sediments from the Windermere South Basin (WSB) to study the trop hic evolution experienced by the lake. Dates in the top 30 cm were spe cifically established through radionuclide (Pb-210 and Cs-137) analyse s. Diatom stratigraphy shows dominance of the centric diatoms Cyclotel la comensis and C. radiosa and several benthic taxa in the early postg lacial. This indicates oligotrophy in the WSB during that period. This assemblage was replaced by another dominated by the diatom Asterionel la formosa in the 1870's, as has been established from the Pb-210 dati ng. From that date onwards, the lake underwent a progression towards e utrophy, indicated by the progressive increase in Aulacoseira subarcti ca (c. 1930's), Fragilaria crotonensis (c. 1943), and more recently, o f the centrics Stephanodiscus parvus (c. 1971) and Cyclotella meneghin iana (1988). Carotenoid stratigraphy reveals the differences between d ifferent sections of the core. Oscillaxanthin and myxoxanthophyll had very low records in the early and medium parts of the core, but increa sed from c. 1950's, showing peaks at c. 1967, 1979 and 1987. Some of t hese peaks indicated a differential abundance of Oscillatoria, and are matched to those observed directly during the ongoing monitoring of t he phytoplankton of the lake. The coincidence between the historic app earance of diatoms associated with nutrient-rich waters and the enhanc ed carotenoid occurrence suggest a common response to phosphorus enric hment, and that the progressive change towards eutrophy has been accen tuated during the last twenty-five years.