Regulating mechanisms of phytoplankton growth and biomass in a shallow estuary

Abstract

Abstract Regulating mechanisms of phytoplankton growth rate and biomass were studied during two periods in March and June in a shallow (< 5 m), semi-enclosed basin in the Roskilde Fjord, Denmark. Phytoplankton community net growth rate (i.e. phytoplankton growth rate - pelagic grazing rate) was quantified by following changes in chlorophyll-a concentrations in unfiltered water enclosed in 10 liter transparent bags deployed at different depths in the water column. During spring, at high nutrient concentrations phytoplankton community growth rate was positive at photon flux densities above 0.2 mol m-2 d-1 and community growth approached 0.4 d-1 at ca. 3 mol m-2 d-1 (≈ surface light). During summer, the minimum photon flux density required for positive community growth rate was 2-4 mol m 2 d1, while maximum community growth rate at ca. 15 mol m-2 d-1 varied between 0.1 and 0.6 d-1 according to the availability of inorganic nitrogen. Phytoplankton biomass in the water column increased during periods of densit...