Impacts of inorganic nutrient enrichment on phytoplankton community structure and function in Pamlico Sound, NC, USA

Human activities in the watersheds of the tributaries of Pamlico Sound (PS) in North Carolina have resulted in increased riverine loading of nutrients. Pamlico Sound is a regionally important aquatic resource, and provides crucial foraging and nursery habitats for Southeast Atlantic fisheries. Changes in phytoplankton community composition that may result from increased frequency and quantity of inorganic nutrient inputs could have negative ecological effects on PS. In this study we conducted a series of nutrient bioassays to assess the relationship between increased inorganic nutrient concentration and phytoplankton community structure and function. Experiments were conducted on the native phytoplankton community of the southwest basin of PS. We utilized nutrient addition treatments and all-but-one nutrient treatments in bioassays. This allowed the comparison of the effect of adding one potentially growth-limiting nutrient (e.g. nitrogen) to adding all potentially limiting nutrients except one (e.g. all except nitrogen). Data from these bioassays indicated that the phytoplankton community in PS is primarily nitrogen (N) limited. Dissolved inorganic N concentrations in PS were relatively stable during this study. The biology of its estuarine tributaries, as has been shown for the Neuse River Estuary (NRE), acted as an effective filter for most of the nutrients transported from upstream. We found stoichiometric predictors of phytoplankton community nutrient limitations to be reliable in some instances, but inaccurate in others. Some taxa-specific responses to nutrient additions were observed, however there were no consistent patterns throughout the experiments. Results indicated that changes in the PS phytoplankton community could result from changes in nutrient regime, and changes may not be consistent across phytoplankton taxonomic groups. Unlike the NRE where pulses of riverine N have significant effects on phytoplankton community structure, the PS phytoplankton community did not appear to be subjected to these periodic N enrichments.