Light‐Nutrient Influences on Biomass, Photosynthetic Potential and Composition of Suspended Algal Assemblages in the Middle Cape Fear River, USA

Dilution bioassays were conducted to evaluate the impact of nutrients on biomass (chlorophyll a), photosynthetic potential, and class composition of suspended algal assemblages in the middle Cape Fear River (USA). Ambient concentrations of dissolved inorganic nitrogen (DIN) and soluble reactive phosphorus (SRP) varied from 37.2 to 83.8 and from 1.0 to 7.1 μmol L–1. Experiments conducted at photosynthetic photon flux densities (PPFDs) similar to the mixed water column (< 1 to 5.3 E m–2d–1) showed no change in chlorophyll a, photosynthetic potential, or algal class composition at 15, 30, and 45% dilutions of DIN, SRP, or both. However, chlorophyll a and photosynthetic potential increased significantly in nutrient dilution bioassays conducted at PPFDs four-fold above ambient, while the abundance of diatoms and Chrysophytes increased under elevated PPFD and ambient or elevated nutrients. Overall, the data indicate that irradiance is more important than nutrients in determining the size and class composition of suspended algal assemblages. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)