Nutrient Inputs to Estuaries from Nine Scottish East Coast Rivers; Influence of Estuarine Processes on Inputs to the North Sea

Abstract

Nutrient distributions (nitrate, ammonia, phosphate and silicate) have been determined in the surface waters of nine North Sea estuaries: Tweed, Forth, Tay, Dee, Don, Ythan, Beauly/Inverness Firth, Cromarty Firth and Dornoch Firth. Seasonal variability has been examined by conducting surveys in April, July and September 1991 and February 1992. On each occasion, surveys of all nine estuaries were normally completed in 3-5 days of each other, around high water on spring tides. This intensive and strictly controlled sampling regime ensures a realistic comparison between nutrient concentrations in individual estuaries. Nutrient concentrations in individual rivers and estuaries are demonstrated to be related to land use. River catchments with intensive agriculture and low freshwater input, such as the Don and Ythan, have enhanced nitrate (up to 600 μM) and phosphate (up to 5 μM) concentrations in their estuaries. By contrast, Highland river catchments with mineral-poor soils, low populations and low agricultural intensity (Inverness, Cromarty and Dornoch Firths) generally lead to nutrient concentrations being lower in river water than in coastal seawater. Conservative mixing of dissolved nutrients is demonstrated to be a function of estuarine flushing time which controls the extent to which internal processes (biological and abiological) can modify nutrient inputs. Nutrients tend to behave conservatively in short rapidly flushed estuaries such as the Tweed, Don and Ythan. In contrast, internal processes are shown to be important when estimating riverine nutrient fluxes to the coastal zone from large slowly flushed estuaries such as the Forth, Tay and Dornoch Firth. For these systems, estimates of riverine inputs to the estuary do not provide a good estimate of the load entering the coastal zone. This is primarily due to the cycling of nutrient elements between dissolved and particulate (including sediment) phases. On a regional basis, gross nutrient inputs are dominated by the pattern of freshwater flow rather than by high concentrations in individual rivers. In consequence the Tay is quantitatively the most important riverine source of nutrients to Scottish North Sea coastal waters. For most of the estuaries examined the N:P ratio at high salinity is close to that observed in coastal waters. For the Don and Ythan, however, (and to a lesser extent the Tweed) the ratio is higher, probably reflecting the influence of agricultural nitrogen inputs; possible implications to phytoplankton growth in coastal waters are discussed. In the northern estuaries (Beauly/Inverness, Cromarty and Dornoch Firths), a source of silicate is evident at low salinity, this is attributed to desorption of silicate from estuarine sediments to nutrient-poor river water.