Ecosystem responses to internal and watershed organic matter loading: consequences for hypoxia in the eutrophying Neuse river estuary, North Carolina, USA

The contrasting impacts of externally supplied (runoff) and internally generated (nutrient-stimulated phytoplankton blooms) organic matter on oxygen (O-2) depletion were examined and evaluated in the eutrophic, salinity-stratified Neuse River Estuary, North Carolina, USA. This nitrogen (N)-limited estuary is experiencing increasing anthropogenic N loading from expanding urban, agricultural and industrial development in its watershed. Resultant algal blooms, which provided organic matter loads capable of causing extensive low O-2 (hypoxic) and depleted O-2 (anoxic) conditions, have induced widespread mortality of resident fin-and shellfish. Phytoplankton blooms followed periods of elevated N loading, except during extremely high runoff periods (e.g. hurricanes), when high rates of flushing and reduced water residence times did not allow sufficient time for bloom development. During these periods, hypoxia and anoxia were dominated by watershed-derived organic matter loading. Externally vs internally generated organic matter loading scenarios were examined in sequential years (1994 to 1996) to compare the differential impacts of an average discharge year (10 yr mean hydrological conditions) (1994), N-stimulated summer algal blooms (1995), and a major hurricane (Fran; September 1996). The responses of primary production, hypoxia, and anoxia to these hydrologically contrasting years and resultant organic matter loadings help distinguish watershed from internal forcing of O-2 dynamics and fish kills.