The Thames plume is a moderately turbid (suspended load up to 80 mg dm-3), high nutrient (summer NO3- concentrations >10 µM, summer PO43- concentrations >2.5 µM) and well-mixed aquatic ecosystem which connects the Thames estuary to the southern North Sea. Six cruises were undertaken to investigate the response of an inshore site in this system to these high nutrient levels via a comparison with a site in the seasonally nutrient-depleted southern North Sea. The seasonal cycle of chlorophyll concentrations in both environments was similar, consisting of higher chlorophyll a levels in spring than in summer. A spring bloom of diatoms occurs in both environments; in the plume it is succeeded by low chlorophyll levels and a diatom-dominated community at low silicate levels and at the offshore site by a non-siliceous community at similar silicate levels. The low summer chlorophyll levels and diatom dominance, which occur at the inshore site despite an abundance of nitrate and phosphate may be due to a simultaneous inhibition of non-siliceous growth and silicate limitation of diatom growth. We hypothesise that this is caused by the high turbidity at the inshore site reducing water column light levels such that they become adequate for diatom growth but inadequate for non-siliceous growth; however, we have inadequate data to confirm this suggestion. The benthic silicate flux cannot support the inferred diatom silicate requirement at the inshore site suggesting that silicate mineralisation in the water column may occur and control diatom growth. Nutrient/salinity plots suggest that the net effect of this complex biogeochemistry is a semi-conservative transport of NO3- and PO43- through the plume to the offshore region.