Phosphorus availability in sediments from a tidal river receiving runoff water from agricultural fields

Eutrophication of surface water is a worldwide concern. Sediments may play an important role in buffering phosphorus (P) concentration in the overlying water column. However, information on the spatial variation of sediment P availability as affected by agricultural water discharge and hydrological conditions is limited. In this study river sediments were sampled in spring, summer, fall and winter, respectively from seven locations along a main tributary (Ten Mile Creek, TMC), which receives surface runoff water from agricultural lands and discharges into the Indian River Lagoon, south Florida, USA, and analyzed for P availability. Simultaneously, hydrological variables were measured on the spot and river water samples were collected for analyses of water quality. The results demonstrated that available P in the sediments of TMC as measured by several commonly used extraction procedures had a large spatial variation. The downstream locations had a greater amount of available P in the sediments than the upstream locations, which is attributable to the settlement of finer particles due to slower water flow and increased influence from salt water in the downstream locations where the fresh river water gradually mixed with salt water from the Indian River Lagoon. Phosphorus availability in the sediments appeared to be related to P sorption by iron and aluminum oxides and the competition for adsorbing sites between SO42- and PO43-. This spatial variation of P availability agrees with the elevation of chlorophyll a (Chla) in overlying water body in the downstream locations of TMC, indicating that the internal P source plays an important role in triggering an algal boom in surface water systems.