Phytoplankton Forcing by a Record Freshwater Discharge Event into a Large Lagoonal Estuary

We report here the response of the water column and phytoplankton biomass of a large lagoonal estuary to a record freshwater discharge event which followed from extraordinary hurricane activity. In the fall of 1999, three hurricanes passed over eastern North Carolina coast in a 7-wk period: Hurricane Dennis (August 24-September 5), Hurricane Floyd (September 14-17), and Hurricane Irene (October 13-16). The hurricanes delivered record rainfall to the watersheds of the Pamlico Sound, North Carolina, the second largest estuary in North America. Hurricane Floyd was followed by a 500-yr flood that displaced 80% of the volume of the Sound and delivered half the annual nitrogen (N)-nutrient load to this N-limited system. After Hurricane Floyd, buoyancy stratification restricted the mixed layer depth, dissolved inorganic nitrogen (DIN) in surface waters increased, and surface chlorophyll biomass increased up to 4-fold. Chlorophyll biomass did not increase to the potential indicated by residual DIN because of light-limitation attributable to suspended participates, phytoplankton pigments, and colored dissolved organic material (CDOM). The discharge waters created hydrological conditions and supplied materials that we interpret to have both stimulated and restricted phytoplankton blooms. The effects of the discharge event on the hydrology and phytoplankton of the Pamlico Sound persisted about 6 months, after which it returned to its pre-event condition, attesting to the resilience of the system.

Duke Scholars

Author Joseph S. Ramus Marine Science and Conservation