NUTRIENT CYCLING IN THE GREAT OUSE ESTUARY AND ITS IMPACT ON NUTRIENTFLUXES TO THE WASH, ENGLAND

A programme of surveys of the Great Ouse estuary (England) was conduct ed to investigate the cycling of nutrients during mixing and to quanti fy nutrient budgets for the estuary. The Great Ouse estuary is shallow , well mixed and relatively turbid, and the dominant source of fresh w ater to The Wash. Surveys were conducted once or twice a month between February 1992 and January 1994 at high tide to yield seasonal and int erannual information. High levels of chlorophyll a (>100 mu g l(-1)), oxygen supersaturation (>120%) and non-conservative nutrient distribut ions during spring and summer periods of low freshwater flow strongly suggest that primary production in the low-salinity reaches of the est uary may significantly modify nutrient fluxes to The Wash, despite the relatively high turbidity. Zn support, calculated nutrient budgets in dicate that biological removal in the river and estuary during the gro wing season results in depleted nutrient fluxes and higher N:Si and P: Si ratios which may affect primary production in coastal waters by con tributing to the shift in species dominance from diatoms to flagellate s and affecting the likelihood of bloom conditions occurring during th e summer months. However, the influence of estuarine biological proces ses on riverine nutrient fluxes is interrupted by periods of high fres hwater flow, more characteristic of the winter months, which result in high flushing rates. Under such conditions, nutrient distributions re vert to a more conservatively mixed regime. This fundamental control e xerted by freshwater flow generally limits the significance of estuari ne processes on an annual basis, the exception being the inorganic rem oval of phosphate in the low-salinity reaches of the estuary which acc ounts for one-third of the annual input to the head of the salinity gr adient. Given the character of this agriculturally impacted estuary, w hich permits the relatively straightforward identification and quantif ication of nutrient cycling processes, results from this study may hav e wider application. (C) 1997 Academic Press Limited.