Temporal variability in nutrient concentrations and loads in the River Tamar and its catchment (SW England) between 1974 and 2004

Abstract

This study reports the results from the analyses of a 30-year (1974-2004) river water quality monitoring dataset for NO x -N (NO₃-N + NO2-N), NH₄-N, PO₄-P and SiO₂-Si at the tidal limit of the River Tamar (SW England), an agriculturally dominated and sparsely populated catchment. Annual mean concentrations of NH4-N, PO₄-P and SiO₂-Si were similar to other rural UK rivers, while annual mean concentrations of NO x -N were clearly lower. Estimated values for the 1940s were much lower than for those of post-1974, at least for NO₃-N and PO₄-P. Flow-weighted mean concentrations of PO₄-P decreased by approximately 60 % between 1974 and 2004, although this change cannot be unequivocally ascribed to either PO₄-P stripping from sewage treatment work effluents or reductions in phosphate fertiliser applications. Lower-resolution sampling (to once per month) in the late 1990s may also have led to the apparent decline; a similar trend was also seen for NH4-N. There were no temporal trends in the mean concentrations of NO x -N, emphasising the continuing difficulty in controlling diffuse pollution from agriculture. Concentrations of SiO₂-Si and NO x -N were significantly and positively correlated with river flows ≤15 m(3) s(-1), showing that diffuse inputs from the catchment were important, particularly during the wet winter periods. In contrast, concentrations of PO₄-P and NH4-N did not correlate across any flow window, despite the apparent importance of diffuse inputs for these constituents. This observation, coupled with the absence of a seasonal (monthly) cycle for these nutrients, indicates that, for PO₄-P and NH4-N, there were no dominant sources and/or both undergo extensive within-catchment processing. Analyses of nutrient fluxes reveal net losses for NO₃-N and SiO₂-Si during the non-winter months; for NO3-N, this may be due to denitrification. Areal fluxes of NO x -N from the catchment were towards the higher end of the range for the UK, while NH₄-N and PO₄-P were closer to the lower end of the ranges for these nutrients. These data, taken together with information on sestonic chlorophyll a, suggest that water quality in the lower River Tamar is satisfactory with respect to nutrients. Analyses of these monitoring data, which were collected at considerable logistical and monetary cost, have revealed unique insights into the environmental behaviour of key nutrients within the Tamar catchment over a 30-year period.

Orthophosphate-P in the nutrient impacted River Taw and its catchment (SW England) between 1990 and 2013

“…46 The diffuse catchment concentrations estimated here are similar to those observed for the rural R. Tamar catchment (southwest England) (range 0.025-0.118 mg P L À1 ) but lower than those calculated for the urbanised R. Thames and R. Thame catchments (southeast England) (range 0.097-0.298 mg P L À1 ). 11,36 The approximately conservative behaviour of orthophosphate-P evidenced by the positive and signicant coefficients of determination in the load vs. ow plots in ESI 4(c) † is perhaps counter-intuitive given the multitude of orthophosphate-P sources across the catchment, the downward temporal trend in river orthophosphate-P concentrations and known in situ orthophosphate-P reactivity in rivers. 3,43 With respect to in situ reactivity, exchange of phosphorus between river bed sediments, pore waters and overlying waters in the upper Taw river and tributaries has been reported to be limited.…”

Temperature response of denitrification rate and greenhouse gas production in agricultural river marginal wetland soils

“…The following day, the GHG cores were flooded with 400 mL of water spiked with 5 mg L −1 (1.13 mg ‐N L −1 ) resulting in 0.452 mg ‐N per core. Mean annual ‐N within the River Tamer ranged from 2.06 to 3.58 mg N L −1 between 1974 and 2004 (Tappin et al ., ). Thus, concentrations applied were slightly below average river concentrations.…”

Plymouth — A World Harbour through the ages

“…Much of the upper estuary is nutrient-rich, with the majority of nutrients from diffuse sources including agricultural run-off and sewerage discharge, although annual mean concentrations of NO x -N, NH 4 -N, PO 4 -P and SiO 2 -Si are similar to other UK rural rivers (Tappin et al, 2013 (Uncles et al, 2002;Tappin et al, 2013) and surface water concentrations greatly exceed those required under current water quality legislation (Howden and Burt, 2009;Burt et al, 2011a).…”