The Taw River Catchment and Estuary: A case study for the effects of NVZ measures Part 1 – The Freshwater Catchment (Draft)

The estuary of the River Taw and its freshwater catchment has been designated as an NVZ on the basis that its estuary is eutrophic. A small part of the catchment drained by the Ashmill Stream has a second designation because it has nitrate concentrations that exceed those set down in the drinking water directive. The Taw estuary catchment covers 1126 km2 and is drained by seven rivers, the Taw, the Caen, the Venn, the Knowle Water, the Bradiford Water, the Langham and the Yeo (Barnstaple) of which the River Taw is by far the largest draining 77% of this area. The aim of this work was to use this catchment as a case study to assess the potential effects of NVZ measures on the eutrophic status of the freshwater streams and the loads of nutrient being delivered to the Estuary. Three main data sources were used for this assessment: 1. Concentrations of nutrients (nitrogen species and ortho-phosphate) and other determinands measured as part of the General Quality Assessment (GQA) programme of the Environment Agency of England and Wales (the Agency). 2. Mean daily flow data provided by the National River Flow Archive (CEH) for four river gauging stations within the freshwater river system of the Taw; 3. Total N loads and total N concentrations derived from the ADAS NEAP-N model run under “prior practice” and under agricultural practice described by the NVZ current action programme measures. These data were used for several analyses which were designed firstly to estimate loads to the estuary under current practice and under NVZ measures for input to the work of the University of Plymouth. And secondly to assess the eutrophic status of the rivers in the Taw catchment and the effect the NVZ measures might have on this status. The analyses were: · An assessment of the spatial distribution of nitrate and ortho-phosphorous concentrations across the catchments. · Estimation of annual and monthly total nitrogen (sum of nitrate, nitrite and ammonium) and ortho-phosphate loads discharged to the estuary through each of the 7 rivers in the catchment. · Estimation of annual loads of total nitrogen in the headwater catchments of the Taw (those not influenced by sewage treatment works effluents). · A comparison between the NEAP-N model output run under “prior practice” and the loads calculated from the observed data. · Estimates of point source loads to the estuary calculated in a previous study were used to estimate the relative importance of point and diffuse source loads. · The estimated change in nitrate concentrations and annual loads in the seven rivers was estimated based on the outputs of the NEAP-N model run under NVZ (“Action Programme”) rules. In making an assessment of the ecological response of the Taw system to the “action programme”, best estimates of limiting nutrient concentrations would be in the order of 5 mg/L N and 0.3 mg/L P; a ratio of 16.67:1 N:P. These are substantially higher than the figures for static waters and are based on the interaction of flow, residence time, nutrient status and ecological variables already in place. DRAFT iv It should be emphasised that, especially in the upper reaches of the Taw, that eutrophication is not obvious from the aquatic macrophyte community. The rapid flow in the very upper catchment even with a mean N of 12.6 mg/L does not permit the development of eutrophic macrophyte species, but a future assessment of the epilithic diatom community may indicate nutrient enrichment. The combination of flow and geology are the dominant factors in determining the plant and diatom community in the lower reaches of the Taw system. Currently, the plant communities observed in the system are not representative of eutrophic conditions. The predicted 10% reduction in N and 5% reduction in P, while not reaching the limiting nutrient values, will contribute to an increase in ecological stability of the system. Systems that operate near the trigger values for eutrophic ecological responses tend to have episodes of excessive plant biomass, occupation of space and hyper-accumulation of nutrients more often than systems with lower nutrient loadings. The consequences of this for the Taw would only be damaging, if flows were to reduce significantly in future. In summary, it is unlikely that AP measures will have a significant impact on existing plant and diatom communities present in the river Taw, as the communities probably do not indicate eutrophic conditions at present. However, reductions in nutrient loading will probably contribute to a reduction in estuarine nutrient loadings, and the ecological response in the estuary may be more significant than that in the river.