Dissolved organic phosphorus in the Mississippi River plume during spring and fall 2002

Dissolved organic phosphorus (DOP) may play an important role in controlling primary productivity in coastal systems. In an attempt to understand the controls on seasonal and spatial variability in the Mississippi River plume, DOP samples were collected during spring and fall 2002. DOP concentrations were determined using an autoanalyzer with in-line thermal/UV oxidation. For both seasons, DOP concentrations were highest in the river and decreased with distance from the river. Salinity, chlorophyll a, dissolved organic carbon and nitrogen are parameters which have been shown to correlate with DOP in other systems. Within the Mississippi River plume, no correlation was found between DOP and any of these parameters, and less than 5% of the total dissolved phosphorus was high molecular weight, as separated by tangential flow ultrafiltration. Our results suggest that DOP may cycle quickly in the plume and be a source of inorganic phosphorus in the late summer, leading to seasonal changes in the inorganic N/P ratio and potentially a shift in the planktonic community.