Phosphate uptake along a coastal plain estuary

Biotic PO43− 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 PO43− 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 PO43− 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 PO43− throughout the estuary. The proportion of uptake by bacteria, however, changed spatially. In the tidal river where anthropogenic inputs are high, bacteria dominated PO43− 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 PO43− uptake. The competition between bacteria and phytoplankton for PO43− was affected by anthropogenic inputs into the tidal river.