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Nutrient modelling and a nutrient budget for Llangorse Lake Final Report

May, L.; Dudley, B.; Spears, B.M.; Hatton-Ellis, T.W.. 2008 Nutrient modelling and a nutrient budget for Llangorse Lake Final Report. Countryside Council for Wales, 78pp. (CCW Contract Science Report No: 831, CEH Project Number: C03202) (Unpublished)

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Abstract/Summary

Llangorse Lake is the largest natural lake in South Wales. There have been concerns about eutrophication problems here for many years. The problem is believed to have been caused by high nutrient loads entering the lake from the surrounding catchment. This study aimed to determine the size and main sources of those loads. The phosphorus (P) and nitrate (NO3-N) load to Llangorse Lake was found to be approximately 2 tonnes P y 1 and 74 tonnes NO3-N y 1 (which is equivalent to 1.5 g P m-2 y-1 and 53 g NO3 N m-2 y-1). Most of the P and NO3-N loads were found to be entering the lake from only two of the inflow streams (i.e. those draining to Sites 2 and 6). These accounted for 85 per cent of the annual P load and 82 per cent of the annual NO3-N load to the lake. The hydrology of the Llangorse catchment appears to be strongly affected by groundwater. This is best demonstrated at Site 6, where the surface water catchment upstream of the site accounts for only 36 per cent of the lake catchment but contributes 67 per cent of annual hydraulic load. This, and the fact that the flow at Site 6 is almost double that which can be accounted for by rainfall, suggests that there is considerable groundwater flow in this area. If the streams that flow into the lake have significant input from groundwater, this will affect their hydrology and chemistry. As this groundwater may enter the drainage system from an area beyond the boundary of the surface water catchment, this has serious implications for catchment management aimed at reducing the nutrient loads to the lake. Some of the inflow streams showed evidence of occasional point source pollution in very wet weather. This suggests that there is a need, in some places at least, to control point sources of pollution that leak or overflow during heavy rainfall. However, the evidence suggests that most sources of nutrients within the catchment are diffuse sources. Most of the published literature concerning the eutrophication and recovery of Llangorse Lake focuses on P. This is probably for historical reasons. This study shows that the water quality of the lake is determined by a delicate balance between P and N availability. So, it is important to consider both of these nutrients in developing a catchment management plan. One final consideration in developing a catchment management plan is the possible impact of occasional flooding of the meadows around the lake in terms of transporting animal waste into the lake. Although flooding was once encouraged by local farmers as a way of carrying plant food to the pastures “both in solution and in very finely divided particles” (Griffiths, 1939), in these days of intensive agriculture, flooding of the meadows is more likely to carry nutrients from the meadows to the lake.

Item Type: Publication - Report (UNSPECIFIED)
Programmes: CEH Programmes pre-2009 publications > Water > WA02 Quantifying processes that link water quality and quantity, biota and physical environment > WA02.2 Hydrochemical and sediment processes
CEH Programmes pre-2009 publications > Water > WA02 Quantifying processes that link water quality and quantity, biota and physical environment > WA02.3 Physico-chemical processes and effects on freshwater biot
CEH Sections: Watt
Funders/Sponsors: Countryside Council for Wales
NORA Subject Terms: Management
Agriculture and Soil Science
Ecology and Environment
Hydrology
Chemistry
Date made live: 03 Aug 2010 14:54
URI: http://nora.nerc.ac.uk/id/eprint/7556

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