Nutrients and chlorophyll at two sites in the Thames plume and southern North Sea

The Thames plume is a moderately turbid (suspended load up to 80 mg dm(-3)) , high nutrient (summer NO3- concentrations >10 muM, summer PO43- concentra tions >2.5 muM) and well-mixed aquatic ecosystem which connects the Thames estuary to the southern North Sea. Six cruises were undertaken to investiga te the response of an inshore site in this system to these high nutrient le vels via a comparison with a site in the seasonally nutrient-depleted south ern North Sea. The seasonal cycle of chlorophyll concentrations in both env ironments was similar, consisting of higher chlorophyll a levels in spring than in summer. A spring bloom of diatoms occurs in both environments; in t he plume it is succeeded by low chlorophyll levels and a diatom-dominated c ommunity at low silicate levels and at the offshore site by a non-siliceous community at similar silicate levels. The low summer chlorophyll levels an d diatom dominance, which occur at the inshore site despite an abundance of nitrate and phosphate may be due to a simultaneous inhibition of non-silic eous growth and silicate limitation of diatom growth. We hypothesise that t his is caused by the high turbidity at the inshore site reducing water colu mn light levels such that they become adequate for diatom growth but inadeq uate for non-siliceous growth; however, we have inadequate data to confirm this suggestion. The benthic silicate flux cannot support the inferred diat om silicate requirement at the inshore site suggesting that silicate minera lisation 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 pO(4)(3-) through the plume to the offshore region. (C) 2001 Elsevier Science B.V. All rights reserved.