Primary production, algal biomass, light conditions and dissolved inorganic nutrient concentrations were monitored simultaneously along the Colne Estuary, UK for a period of 15 mo. Phytoplanktonic primary production and chlorophyll a (chl a) concentration ranged from 0.64 x 10(-3) to 17 mug C l(-1) h(-1) and 0.5 to 37.5 mug chl a l(-1), respectively. Nutrient concentrations decreased along a downstream gradient, with high concentrations of nitrate (564 muM), ammonium (291 muM) and phosphate (52 muM) present at the head of the estuary. Despite high light-attenuation coefficients (mean K = 3.2 m(-1)), the greatest phytoplankton biomass and primary production were measured at the head of the estuary, where nutrient concentrations were high, and phytoplankton biomass and primary production decreased down the estuarine gradient. Despite year-round high nutrient concentrations, chlorophyll a concentrations and rates of primary production remained below levels reported for bloom formation in similar systems. Comparison of field data with measurements derived from phytoplanktonic P vs E curves showed that simulated in situ rates of surface-water biomass-normalised production (P-B) were usually significantly lower than the P-max(B) values of the phytoplankton. Therefore phytoplankton primary production in the Colne Estuary was light-limited for most of the year, and increased algal biomass and production towards the head of the estuary (despite high attenuation coefficients) was due to a shallow, well-mixed water column (low Z(mix):Z(eu) [critical mixing ratio]).