NUTRIENTS AND EUTROPHICATION IN THE TAW ESTUARY
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Increased inputs of nutrients to estuaries and coastal waters can cause undesirable effects associated with eutrophication, including nuisance and toxic algal blooms, reduced amenity value, changes in species composition, bottom anoxia and fish kills. The main sources of nutrients to estuaries are river runoff, sewage discharges, atmospheric inputs and possibly submarine groundwater discharges. For the UK, estuarine eutrophication has been shown to occur in at least 16 estuaries (including the Taw in North Devon). Consequently, these systems have been designated as '(Potential) Problem Area' under the OSPAR Common Procedure for the Identification of Eutrophication and also as a 'Eutrophic Water' under the EU's Nitrates Directive and I or a 'Sensitive Area (Eutrophic)' under the Urban Wastewater Treatment Directive or both. Significant reductions in N and P inputs have been realized following application of the EU's Urban Waste Water Treatment Directive. Atmospheric NOx and NHx emissions have also decreased and are expected to decrease further as implementation of existing legislation continues and new controls are introduced for activities such as shipping. The Nitrates Directive was introduced to tackle N discharges from agriculture but little change in N loads to estuaries has been recorded. Using the Taw Estuary as an example, data routinely collected by the Environment Agency for England and Wales (EA) over the period 1990-2004 were interrogated to identify the drivers of excessive algal growth. The estuary was highly productive with chlorophyll a concentrations regularly exceeding 100 µg Lˉ¹ , mostly during periods of low freshwater input from the River Taw when estuarine water residence times were longest. The reported approach demonstrates the value of applying conventional statistical analyses in a structured way to existing monitoring data and is recommended as a useful tool for the rapid assessment of estuarine eutrophication. However, understanding of primary production dynamics in the Taw was constrained because of the low temporal resolution, heterogeneity and gaps in the EA data. Therefore, a temporal high resolution monitoring campaign was conducted in summer 2008 to document the development and decline of three algal blooms. The significance of long water residence times following low freshwater inflow and neap tides was confirmed. During peaks in chlorophyll a concentration (max. 226 µg Lˉ¹ ) , nutrient limitation switched from P to Si and persisted for more than 2 weeks in the outer estuary. Signs of ammonium and phosphate ( <0.2 µM) and silicate ( <2 µM) depletion were also observed. Using multivariate statistics, five distinct sets of environmental conditions present in the Taw at different stages of algal growth were identified and directly linked to freshwater inflow. UK Climate Impacts Programme scenarios predict a 30-50% decrease in Q95 flows (the flow which is exceeded 95% of the time) of rivers in south Britain by 2050. Under the current nutrient regime, this is likely to severely increase the severity and duration of symptoms of eutrophication in the Taw and favour potentially hazardous phytoplankton groups instead of diatoms. To mitigate future eutrophication events in the Taw, it is recommended to further reduce N and P inputs. It is also crucial to perform a detailed assessment of potential climate change consequences for the Taw Estuary and similar systems.
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