Uptake and assimilation of ammonium and nitrate by phytoplankton: indices of nutritional status for natural assemblages

Using direct measurements of nitrogen uptake by natural phytoplankton assemblages from the Chesapeake Bay, the Caribbean and Sargasso Seas, and a warm core Gulf Stream ring, we have examined the usefulness of some parameters as potential indices of cellular nutritional status. In different locations, as nutrient and biomass conditions permitted, we applied a variety of approaches. These included the ratio of uptake of $NH4+$ and $NO3-$ at high and low concentrations of these substrates, the rate of assimilation of $NH4+$ or $NO3-$ into protein, and $NH4+$ enhancement of dark carbon fixation. Generalizations concerning the nutritional status of an assemblage were difficult to derive from any single index. However, using a variety of approaches, we were able to demonstrate more severe nutritional stress for Caribbean Sea assemblages when $NH4+$ concentrations were <0.3 μg atom l ⁻¹ than for Sargasso Sea assemblages when $NH4+$ concentrations were lower by at least an order of magnitude. During the decline of phytoplankton blooms in both the Chesapeake Bay and a warm core Gulf Stream ring fairly severe nutritional stress was also demonstrated.