| This study compared the effects of changes in dissolved inorganic nitrogen (DIN) and phosphorus (DIP) concentrations and DIN:DIP ratios on the growth and community composition of phytoplankton assemblages from Delaware's eutrophic Inland Bays. Batch and semi-continuous cultures used natural algal communities and individual species from the bays. Nutrients were added to bay water or experimental media to yield varying DIN:DIP ratios at either one or two nutrient concentrations. Phytoplankton response was monitored through measurements of phytoplankton biomass, productivity, nutrient content, nutrient uptake, and taxonomic composition. Different initial phytoplankton communities in the three bays responded differently to the nutrient modifications. In batch cultures, increases in the DIN:DIP ratio generally increased phytoplankton biomass, C and N content, and nutrient uptake rates, decreased C:N and C:chl a ratios, and had no effect on primary productivity and diagnostic pigment concentrations. Diatom species from all three bays generally increased in abundance with increases in the DIN:DIP ratio more than other taxa. Semi-continuous cultures used the same DIN:DIP ratios, but at two concentration levels. Results suggested that increases in DIN and DIP concentration increased the extent to which DIN:DIP ratios affected phytoplankton growth and composition. Different phytoplankton groups were observed between the two concentration levels at equivalent DIN:DIP ratios, so DIN and DIP concentrations seem to play a major role in determining community structure in these estuaries. In experiments using batch cultures of individual species, DIN:DIP ratios only had an effect on the growth of all three species studied when DIN and DIP concentrations were increased five-fold. The dinoflagellate Prorocentrum minimum was able to outcompete a diatom and a prasinophyte at very low DIN:DIP ratios but high DIN and DIP concentrations. Both DIN:DIP ratios, and to a greater extent DIN and DIP concentrations, play a role in regulating phytoplankton growth and community structure. Management strategies for the Inland Bays should consider the simultaneous reduction of both nitrogen and phosphorus to avoid changes in the DIN:DIP ratio, and potentially undesirable changes in phytoplankton community structure. |
