FRACTIONATION OF P SPECIES IN SUSPENDED SEDIMENTS OF IRONDEQUOIT CREEK, NY

The Irondequoit Creek watershed drains an area of approximately 388 km². Land use is mixed, but dominated by urbanized (43.6%) and forested (25%). The watershed receives 79 cm of liquid equivalent precipitation annually. It discharges into Irondequoit Bay, a 672 hectare embayment on the southern shoreline of Lake Ontario. Irondequoit Bay has historic problems associated with excess phosphorus, being hypereutrophic prior to about 1980, and requires continued management to maintain adequate water quality.

In this study, we investigate the fractionation of P within suspended sediment, the primary mode of P transport through the system. Approximately 10 to 12 liters of water were collected at 4 locations within the watershed during stormflow events. Suspended sediment was collected by centrifugation. Stream bed sediments were collected from approximately the top 5 cm in a small core. Suspended sediment and stream bed sediment samples were fractionated following the Psenner method into exchangeable, Fe-association, Al-associated, organic matter (OM) associated and Ca-associated P. Suspended and stream bed samples have similar percentages of exchangeable and Al-associated P with approximately 4% exchangeable P, and 26 to 31% Al-associated P. Suspended sediment samples are, on average, significantly higher in Fe and OM associated P, and significantly lower in Ca-associated P. Ca-associated P, which represents the most stable and least bioavailable form of P, averages 21% of the P in suspended sediment but 48% in stream bed sediment. These results suggest that Ca-associated P increases with depth so that sediment from the surface, which may be remobilized during stormflow, has the lower concentrations in this fraction. Furthermore, with higher amounts of OM-associated P in suspended sediment, it suggests that P may be more bioavailable from this source.