Dissolved nutrients in the Tweed Estuary, UK: inputs, distributions and effects of residence time

Abstract

Results are presented from a campaign in which dissolved inorganic nutrients were measured in the Tweed Estuary, UK. The data utilised here were derived from surveys undertaken on a monthly basis from January to August 1997. There was consistency between the nutrient concentrations observed in the freshwater reaches of the tidal estuary and earlier measurements made several km upstream, in the freshwater river. Typically, nitrate was of the order of 100 μM and silicate approximately half this, whereas phosphate and nitrite were typically approximately 1 μM. Seasonal fluctuations in freshwater nitrate and silicate were very large, with concentrations exceeding (respectively) 300 and 100 μM in winter and less than 30 and 10 μM in summer. The estuarine nutrients surveyed followed this general seasonal pattern. A simple box model was used to investigate the temporal behaviour of a hypothetical estuarine nutrient. The model demonstrated that large deviations from the ‘broad-brush’ seasonal pattern for nitrate and silicate could be expected to occur within the estuary as a result of runoff and residence-time variations. The model also demonstrated that mid-estuarine peaks observed in the longitudinal distributions of phosphate could only have arisen as a result of non-freshwater, wastewater phosphate inputs to the estuary. Without this mid-estuarine input, rapid flushing rates would have ensured that transients due to changing freshwater phosphate concentrations, runoff and tides (via the residence time) would have had relatively little impact on the classic, linear ‘conservative’ mixing diagram for the estuary. Conservative mixing was largely applicable to the other nutrients.