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

Phytoplankton nutrient limitation experiments were performed from 1994 to 1996 at three stations in the Cape Fear River Estuary, a riverine system originating in the North Carolina piedmont. Nutrient addition bioassays were conducted by spiking triplicate cubitainers with various nutrient combinations and determining algal response by analyzing chlorophyll a production and ¹⁴C 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 nutrient additions were rare, with light the likely principal factor limiting phytoplankton production. During summer at a mesohaline station, phytoplankton community displayed significant nitrogen (N) limitation, while both phosphorus (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 turid 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 fall, and P limitation (with strong N+P co-limitation) occurred in winter and spring. The data suggest there are two patterns controlling phytoplankton productivity in the Cape Fear system: 1) a longitudinal pattern of decreasing light limitation and increasing nutrient sensitivity along the salinity gradient, and 2) a seasonal alternation of N limitation, light limitation, and P limitation in the middle-to-lower estuary. Statistical analyses indicated upper watershed precipitation events led to increased flow, turbidity, light attenuation, and nutrient loading, and decreased chlorophyll a and 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.