Phosphate uptake along a coastal plain estuary

Biotic PO 4 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 PO 4 3− uptake were measured in the tidal river and the lower bay (salinity > 15‰) where primary productivity is high. Between the river and the lower bay, the concentration of suspended sediment is high, and PO 4 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 PO 4 3− throughout the estuary. The proportion of uptake by bacteria, however, changed spatially. In the tidal river where anthropogenic inputs are high, bacteria dominated PO 4 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 PO 4 3− uptake. The competition between bacteria and phytoplankton for PO 4 3− was affected by anthropogenic inputs into the tidal river.