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1. Historical nutrient changes in Grasmere were investigated using a 300-year record derived from six sediment cores. One core was investigated at high resolution for diatoms, total sedimentary phosphorus, and loss-on-ignition (LOI), and was dated using 210 Pb and 137 Cs. Six other cores were scanned for magnetic susceptibility, diatoms and LOI to confirm the stratigraphic integrity of the primary record. 2. A rise in nutrient levels occurred after 1855 AD. This event was marked by a shift away from benthic diatom assemblages and a rise in Asterionella formosa. The onset of eutrophication from 1855 corresponds to the expansion of the local and tourist population in the area. 3. The replacement of A. formosa with Cyclotella spp. ca 1945-65 indicates reduced nutrient loads, possibly because of enhanced flushing brought about by the seasonal rainfall distribution. 4. After 1965 a step-wise increase in both absolute and relative amounts of Asterionella was found. High sedimentary P and diatom inferred TP confirmed the high nutrient loading of the lake. Nutrient increase is attributable to problems with the Grasmere village sewage system and the installation of a wastewater treatment works (WwTW) on the River Rothay in 1971. Modifications to the WwTW in 1982 caused an initial improvement, but have not led to a full recovery to pre-1965 ecological conditions. 5. The diatom record indicates a further improvement after 1990 by a return toward Achnanthes minutissima. 6. The sedimentary archive of sensitive sites provides important benchmarks against which to judge the attainment of water quality targets.