LAUSR

ABSTRACT The worldwide release of excessive nutrients (nitrogen [N], silicon [Si] and phosphorus [P]) into coastal waters is impacting phytoplankton growth and community structure. The coastal Bay of Bengal (BoB) is predicted to be 90% more eutrophic in the future; the impacts on phytoplankton are largely unknown. We present the first experimental results concerning the responses of coastal phytoplankton to changing nutrient stoichiometry (N:Si:P) in the southwest BoB. A gradual increase in Si, N and P supply resulted in exponential growth and significant nutrient depletion (Δ). The nutrients reaching the Bay are likely to possess a short residence time. We also show that any alteration in nutrient loading ratio may impact nutrient removal dynamics of N, Si and P differently. Growth-saturating concentrations for Si, N and P were ~55, 65 and 10 µM respectively, whereas luxury uptake was seen up to 20 µM for P and 65 µM for N and Si. Net consumption of N (ΔN) and P (ΔP) was decoupled from increasing Si concentrations. Gradual N enrichment under P- and Si-saturated conditions linearly increased ΔSi as well as ΔP, and diatoms dominated over cyanobacteria. The diatoms Thalassiosira spp. seemed to respond positively to N enrichment, probably due to their high nitrate uptake and storage capacity. Nitzschia spp, a pennate diatom, showed the opposite trend. Such modifications may have direct consequences on the food chain. The rates of N, Si and P removal under saturated concentrations were 1.03 ± 0.27, 1.35 ± 0.14 and 0.14 ± 0.02 µmol chlorophyll a –1 d–1, respectively and yielded a ratio of 7.5:9.8:1 (N:Si:P). The departure from the Redfield ratio (indicating eutrophication) in the Bay may potentially alter nutrient removal patterns, phytoplankton community composition and autochthonous organic matter stoichiometry.