PHOSPHATE-UPTAKE ALONG A COASTAL-PLAIN ESTUARY

Biotic PO4(3)-uptake rates were measured along the Delaware estuary to examine seasonal and spatial patterns of variation. Uptake was highest (40-90 nmol liter-1 h-1) in summer and lowest ( <10 nmol liter-1 h-1) in winter. The highest rates of PO4(3-) uptake were measured in the tidal river and the lower bay (salinity > 15-parts per thousand) where primary productivity is high. Between the river and the lower bay, the concentration of suspended sediment is high, and PO4(3-) uptake was reduced ( < 5 nmol liter-1 h-1) at all times of the year. Partitioning of uptake by size revealed that both bacteria and phytoplankton are important in the uptake of PO4(3-) throughout the estuary. The proportion of uptake by bacteria, however, changed spatially. In the tidal river where anthropogenic inputs are high, bacteria dominated PO4(3-) uptake and accounted for up to 70% of the total. Within the salinity gradient of the estuary, the proportion of bacterial uptake was much lower, with bacteria accounting for only 15-25% of PO4(3-) uptake. The competition between bacteria and phytoplankton for PO4(3-) was affected by anthropogenic inputs into the tidal river.