Maximum growing depth of macrophytes in Loch Leven, Scotland, United Kingdom, in relation to historical changes in estimated phosphorus loading

Eutrophication is a common problem of shallow lakes situated in lowland areas. In their natural state, most shallow lakes would have clear water and a thriving aquatic plant community. However, eutrophication often causes turbid water, high algal productivity, and low species diversity and abundance of submerged macrophytes. In severe cases, these impacts can have serious economic consequences on local communities. A key indicator of the ecological health of lake ecosystems is the maximum growing depth (MGD) of aquatic plants. However, few existing studies have yet quantified the relationship between changes in external phosphorus (P) input to a lake and associated variation in MGD. This study examines the relationship between these parameters in Loch Leven, a shallow, eutrophic loch in Scotland, UK. A baseline MGD value from 1905 and a series of more recent MGD values collected between 1972 and 2006 are compared to estimated phosphorus loads to the lake over a period of eutrophication and recovery. The main factors that affect MGD within the loch are explored and the effectiveness of this parameter as an indicator of eutrophication and recovery in shallow lakes is considered. The results suggest a close relationship between changes in MGD of macrophytes and changes in the external P load to the loch over the study period. Variation in MGD macrophytes also seemed to reflect the “light history” that submerged macrophytes had been exposed to over the 5 year period prior to sampling, rather than responding to short term, within year, variations in water clarity. This suggests that changes in macrophyte MGD may be a good indicator of overall, long term, changes in water quality that occur during the eutrophication and restoration of shallow lakes.