Causes of low oxygen in a lowland, regulated eutrophic river in Eastern England

In the River Brett, Eastern England, over the period 1955-1998 there was a significant long-term decline in dissolved oxygen (DO), as well as increases in TON (total oxidised nitrogen) and SRP (soluble reactive phosphorus). Flow decreased from 1963 to 1998. Field studies in 1998-2000 showed increased pH and a gradient of DO beneath the filamentous alga Cladophora glomerata. DO decreased through the summer. Macrophytes accounted for 45% of community respiration at the study site, while sediment accounted for 36%. In container studies, muddy sediments had the highest maximum sediment oxygen demand (SOD), but canopies of C. glomerata and Lemna minor together increased the SOD by up to 90% over control samples. During periods of high temperature, abundant growths of C. glomerata and/or L. minor would increase the SOD of organic mud in river areas with shallow, ponded water, eventually leading to anoxic conditions and the release of nutrients from the sediment. If a river had large areas of mud, these processes could dramatically affect the river's oxygen budget, and hence its ecology.