Llangorse Lake is the largest natural lake in South Wales, UK, and is of European conservation importance. The site has a long history of eutrophication problems and, in recent years, significant efforts have been made to meet water quality restoration targets at this site by reducing the input of phosphorus (P) from external sources. Although the lake has improved substantially in quality since the late 1970s, it is still not meeting its ecological targets. Phosphorus concentrations have remained high and there has been little reduction in algal biomass. Management decisions to reduce P input were originally based on the widely held assumption that shallow lakes are P-limited in summer. However, this study clearly shows that this is not always the case; Llangorse Lake, at least, is strongly nitrogen (N) limited over the summer months. As a result, bio-available P released from the sediments cannot be used by the phytoplankton population. So, it accumulates in the water column, causing very high concentrations to occur in late summer. This puts the lake at very high risk of developing algal blooms when N availability increases, usually in early autumn. The study also found that the hydrology of the lake was strongly affected by sub-surface flow. This suggested that nutrients and water could be delivered to the lake from areas beyond the topographically defined surface water catchment. These findings have widespread implications for the successful management of external inputs to lakes, which currently tends to focus on management of the surface water catchment only. The results are discussed in relation to the restoration and management of nitrogen-limited lakes, and of those that are significantly affected by sub-surface flow.
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