Nutrient Cycling in the Great Ouse Estuary and its Impact on Nutrient Fluxes to The Wash, England

Abstract

A programme of surveys of the Great Ouse estuary (England) was conducted to investigate the cycling of nutrients during mixing and to quantify nutrient budgets for the estuary. The Great Ouse estuary is shallow, well mixed and relatively turbid, and the dominant source of fresh water to The Wash. Surveys were conducted once or twice a month between February 1992 and January 1994 at high tide to yield seasonal and interannual information. High levels of chlorophylla(>100μgl−1), oxygen supersaturation (>120%) and non-conservative nutrient distributions during spring and summer periods of low freshwater flow strongly suggest that primary production in the low-salinity reaches of the estuary may significantly modify nutrient fluxes to The Wash, despite the relatively high turbidity. In support, calculated nutrient budgets indicate that biological removal in the river and estuary during the growing season results in depleted nutrient fluxes and higher N:Si and P:Si ratios which may affect primary production in coastal waters by contributing to the shift in species dominance from diatoms to flagellates and affecting the likelihood of bloom conditions occurring during the summer months. However, the influence of estuarine biological processes on riverine nutrient fluxes is interrupted by periods of high freshwater flow, more characteristic of the winter months, which result in high flushing rates. Under such conditions, nutrient distributions revert to a more conservatively mixed regime. This fundamental control exerted by freshwater flow generally limits the significance of estuarine processes on an annual basis, the exception being the inorganic removal of phosphate in the low-salinity reaches of the estuary which accounts for one-third of the annual input to the head of the salinity gradient.Given the character of this agriculturally impacted estuary, which permits the relatively straightforward identification and quantification of nutrient cycling processes, results from this study may have wider application.