Influence of nutrient reduction, light and light-nutrient interactions on phytoplankton biomass, primary production and community composition in the Neuse River, USA

The impact of nutrient reduction, light and light-nutrient interactions on phytoplankton biomass (chlorophyll-a), primary production and community composition was evaluated in laboratory mesocosms for the Neuse River, USA. Dissolved inorganic nitrogen (DIN) and soluble reactive phosphorus (SRP) for the seven experiments averaged 36.2 and 1.0 mu mol/L, while chlorophyll-a averaged 2.7 mu g/L. The average photosynthetic photon flux density (PPFD) in the mixed water column for the two weeks prior to sample collection for each experiment varied from 1.9 to 3.3E m(-2) d(-1). Dilution bioassays conducted at PPFDs similar to in situ river values showed no differences in chlorophyll-a, primary production or nutrient consumption relative to in situ values over 5 or 6 d for 15, 30 or 45 % dilutions of SRP, DIN or SRP plus DIN. An increase in PPFD by a factor of 7, however, gave significant increases in chlorophyll-a and nutrient consumption over the ensuing 2 d. Dilution bioassays conducted at PPFDs elevated (6.3 to 7.9 E m(-2) d(-1)) relative to in situ levels showed continuous decreases in nutrients and increases in chlorophyll-a and primary production of as much as two orders of magnitude over 5 to 7d. DIN-diluted mesocosms at the 30 and 45 % levels showed nutrient limitation, suggesting that N availability would limit phytomass development under light-saturated conditions. Samples incubated along a light gradient shifted from light to nutrient control of phytoplankton biomass at > 5.5E m(-2) d(-1). The phytoplankton was dominated by Chlorophytes and diatoms plus Chrysophytes. Nutrient dilution had no impact on the broad taxonomic composition of the phytoplankton, but increased light favored diatoms plus Chrysophytes over Chlorophytes. Overall, our results suggest that light is the primary control over the phytoplankton in the Neuse River.