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1. Mesotrophic lakes are a threatened habitat in the United Kingdom (UK), and are specified within the UK Biodiversity Action Plan (UKBAP) as requiring protection. Lough Melvin is a large mesotrophic lake that is of particular interest owing to the genetic diversity of its brown trout, but it is currently showing signs of nutrient enrichment. 2. In 1990, average lake total phosphorus (TP) was less than 19 µg P L-1 but increased to 29.5 µg P L-1 by July 2001. Inflow TP also increased from 34 µg P L-1 to 41 µg P L-1 over the same period. Neither phosphorus nor nitrogen appeared to be limiting. 3. Despite higher lake TP, annual chlorophyll a did not increase, remaining less than 5 µg L-1. The phytoplankton was dominated by cyanobacteria and seems to have remained unchanged since the 1950s. Rotifer numbers increased significantly after 2001 but the macro-zooplankton did not. The absence of a phytoplankton response to P enrichment is attributed to light limitation caused by peat staining and thorough mixing. 4. In the catchment, an accelerated programme of clear-felling began in 1999. Recent changes in the lake are consistent with the known impacts of clear-felling conifers on peat soils, namely larger P and dissolved organic carbon (DOC) losses. The latter was reflected in a lower Secchi depth and an enhanced microbial food-web supporting a larger rotifer population. 5. It is suggested that, for the purposes of the Water Framework Directive (WFD), Lough Melvin and other large, alkaline but peat-stained lakes may be treated as a distinct lake type as they do not fit easily into the conventional classifications of dystrophic or mesotrophic lakes. [ABSTRACT FROM AUTHOR]