Ammonium uptake and growth limitation in marine phytoplankton

We used continuous culture techniques and a sensitive fluorescence method to quantify relationships among ammonium (NHz4) concentration, cellular NH z 4 uptake rates, and growth rate limitation for five marine algal species ranging in diameter from 3 to 11 mm. The growth rate of two high–nutrient-requiring coastal diatoms, Thalassiosira weissflogii and Thalassiosira pseudonana, were limited at NHz4 concentrations,90–100 nmol L 21, while those of low-nutrient–adapted species, the oceanic coccolithophore Emiliania huxleyi, the coastal pelagophyte Aureoumbra lagunensis, and the coastal trebouxiophyte Nannochloris atomus, were limited at concentrations,30–40 nmol L21. In the nitrogen-limited cyclostats, the residence times of dissolved ammonium were extremely short (4–12 min at chlorophyll a levels of 20–30 mg L21), and these short times had to be taken into account to obtain accurate NHz4 measurements. Ammonium and nitrate concentrations in stratified surface ocean waters (3–15 nmol L21) fall within the range that substantially limited algal growth rates in our experiments, providing evidence for nitrogen limitation in these waters. Ammonium uptake rates and associate