Temporal and geographical variation in lake trophic status in the English Lake District: evidence from (sub)fossil diatoms and aquatic macrophytes

1. A sediment core (representing 250-300 years) was taken from each of thre e lakes of conservation interest and contrasting trophic status in the Engl ish Lake District: Wastwater, Bassenthwaite Lake and Esthwaite Water. Litho stratigraphic analyses, radiometric dating and analysis of fossil diatoms w ere carried out. 2. Transfer functions, based on the diatoms, were used to reconstruct total phosphorus (TP) and, thus, eutrophication at the study lakes. In Wastwater , changes in lake pH were also reconstructed. 3. The lakes were also classified according to their present macrophyte flo ra, the latter being compared with previous records. 4. The fossil diatoms of Wastwater were continuously dominated by taxa typi cal of oligotrophic, circumneutral waters, indicating that the lake has not been enriched or acidified in the last 250 years. The aquatic macrophyte f lora has probably remained unchanged since before the Industrial Revolution . 5. The diatom assemblages of both Bassenthwaite Lake and Esthwaite Water be gan to change in the mid-1800s. Further change occurred from the 1960s, at the onset of a recent period of eutrophication. These two lakes have experi enced continued nutrient enrichment throughout the 1970s, 80s and 90s, larg ely associated with increasing phosphorus inputs from sewage effluent. Ther e is no evidence of any recovery in response to recent reductions in extern al nutrient loads. 6. Only in Esthwaite Water has the change in aquatic macrophytes been prono unced. 7. Palaeolimnological reconstruction is useful in determining background co nditions and natural variation in lake ecosystems.