The influence of nutrients on algal biomass and primary production was investigated in the estuary of the River Colne, a turbid hypernutrified estuary on the east coast of England. Different approaches were used to examine nutrient regulation of both algal biomass formation and algal primary production in the Colne Estuary. Stoichiometric nutrient ratios indicated that silicate was always potentially limiting to biomass relative to nitrogen and phosphorus, and there were only small numbers of diatoms present in the phytoplankton community. Addition of silicate stimulated diatom growth. At the freshwater end of the estuary, low N:P ratios resulting from the input of effluent from sewage treatment works with high P levels created a potential for N limitation of algal biomass production. The N:P ratios at the seaward end of the estuary were much higher, suggesting greater potential for P limitation of algal biomass formation. This is the reverse of the usual assumption for P limitation in freshwater and N limitation in seawater. Nutrient depletion experiments corroborated the indication of greater N limitation of phytoplankton in the upper estuary. Ammonium rather than nitrate was used first by the algae despite being at much lower concentrations than nitrate. F-ratios measured with N-15 ammonium or nitrate showed that, except on 1 occasion in the upper estuary, > 95 % of the N uptake was from ammonium. Despite the high nutrient concentrations in the estuary, algal productivity was strongly limited by light availability. This, combined with the freshwater flushing time for the Colne Estuary (14 d), probably meant that depletion of nutrients and imposition of nutrient limitation on algal growth would only occur outside the estuary in the coastal zone.