Nutrient concentrations and planktonic biomass (chlorophyll a) behaviour in the basin of the River Avon, Warwickshire, UK

The 2210 km² basin of the Warwickshire River Avon has experienced increased nutrient concentrations and loadings over the last two decades from both point sources (especially sewage treatment works) and diffuse agricultural runoff. In addition, attached and planktonic biomass appears to have increased as has the incidence of nuisance blue-green algae. This paper presents preliminary data obtained from a major water quality sampling programme initiated in 1994. High chlorophyll a levels are found in three monitored reservoirs and in the lower reaches of the Rivers Alne and Dene, two major tributaries of the Avon which receive no major sewage effluent inputs. Whilst there appears to be an increase in chlorophyll a concentration with distance downstream in the main Avon system, nutrient concentrations do not appear to be the most critical control on the spatial distribution of planktonic biomass. It is suggested that river SiO₂ concentrations limit spring diatom productivity and that river sinuosity may impose a morphological control on residence time and reseeding.