MODELING PHOSPHORUS CYCLING IN A WELL-MIXED COASTAL-PLAIN ESTUARY

Phosphorus cycling in the Delaware Estuary was examined during 1986-88 using numerical methods to calculate fluxes and estimate net regeneration. In the tidal river, the flux of total phosphorus (TP) increased threefold compared to Delaware River due to municipal inputs, with most of TP flux as dissolved inorganic phosphate (DIP, 56-59%). This observed increase in TP flux in the river, however, is much lower than expected, given known inputs. A mass balance for TP in the river indicated that a large fraction (44-67%) of TP inputs was retained mainly through geochemical processes. Phosphorus cycling in Delaware Bay, in contrast, was dominated by biological uptake and recycling. Within the bay, DIP input from the tidal river was transformed into phytoplankton biomass; the relative flux of particulate phosphorus (PP) increased from 35% of total flux into the bay to 62% at the month due to a 52% increase in mean PP flux. The biological dominance of P cycling in the bay resulted in strong seasonal variations in TP flux; during the annual winter-spring bloom, TP was retained in the bay (53-56%), while the bay was a source (+52%) of TP during the fall. Throughout the year, except during the spring, regenerated DIP was sufficient to supply 102±19, 94±10, and 95±6% of phytoplankton P-demand in the upper, middle, and lower bay respectively. During the spring, there was an imbalance between uptake and regeneration resulting in DIP depletion. A comparison of processes contributing to total regeneration indicated that 71-97% of apparent regeneration occurred within the water column, with sediment DIP release contributing an additional 3-25%. On an annual basis, 84% of TP entering the salinity gradient of the estuary was exported to coastal waters. © 1992 Academic Press Limited.