Alternation of factors limiting phytoplankton production in the Cape Fear River Estuary

Phytoplankton nutrient limitation experiments were performed from 1994 to 1 996 at three stations in the Cape Fear River Estuary, a riverine system ori ginating in the North Carolina piedmont. Nutrient addition bioassays were c onducted by spiking triplicate cubitainers with various nutrient combinatio ns and determining algal response by analyzing chlorophyll a production and C-14 uptake daily for 3 d. Ambient chlorophyll a, nutrient concentration, and associated physical data were collected throughout the estuary as well. At a turbid, nutrient rich oligohaline station, significant responses to n utrient additions were rare, with light the likely principal factor limitin g phytoplankton production. During summer at a mesohaline station, phytopla nkton community displayed significant nitrogen (N) limitation, while both p hosphorus (P) and N were occasionally limiting in spring with some N + P co -limitation. Light was apparently limiting during fall and winter when the water was turbid and nutrient-rich, as well as during other months of heavy rainfall and runoff. A polyhaline station in the lower estuary had clearer water and displayed significant responses to nutrient additions during all enrichment experiments. At this site N limitation occurred in summer and f all, and P limitation (with strong N + P co-limitation) occurred in winter and spring. The data suggest there are two patterns controlling phytoplankt on productivity in the Cape Fear system: 1) a longitudinal pattern of decre asing light limitation and increasing nutrient sensitivity along the salini ty gradient, and 2) a seasonal alternation of N limitation, light limitatio n, and P limitation in the middle-to-lower estuary. Statistical analyses in dicated upper watershed precipitation events led to increased flow, turbidi ty, light attenuation, and nutrient loading, and decreased chlorophyll a an d nutrient limitation potential in the estuary. Periods of low rainfall and river flow led to reduced estuarine turbidity, higher chlorophyll a, lower ambient nutrients, and more pronounced nutrient limitation.