LEGACY PHOSPHOROUS IN AGRICULTURAL DRAINAGE DITCHES OF THE LAKE ERIE WATERSHED

Agricultural best management practices have reduced the total phosphorous (P) being delivered to Lake Erie for almost 40 years, yet the proportion of bioavailable P (soluble reactive phosphorous, SRP) has steadily increased, contributing to harmful algal blooms. A possible continuous source of P to Lake Erie, despite efforts of farmers and policy-makers, is sediment in agricultural drainage ditches, which contains sorbed P that is chronically retained and released due to changes in shallow redox zonation. Here we performed a sequential P extraction for two consecutive seasons on ten cores from two agricultural drainage ditches in the Lake Erie watershed to estimate the potential pool of legacy P in agricultural ditches and the fractions that are dissolved in pore water or associated with redox sensitive minerals. Stream water quality and organic matter content were also determined. Preliminary data suggest that loosely bound P contributes to <1% of the total SRP, whereas P associated with redox-sensitive Fe and Mn minerals constitutes ~ 15% of the total SRP. The rest of the SRP originates mostly from P bound to redox insensitive Fe and Al mineral oxides. On average late summer measurements presented higher P concentrations than late spring measurements. These results are attributed to the late summer reducing conditions created by stagnant, suboxic water, which can mobilize P. Dissolved oxygen concentrations were less than 3 mg/L in stream water during late summer in both ditches. One of the ditches had chronically higher P concentrations (up to three times more), despite the implementation of best management practices such as grass buffers. This site also had low dissolved oxygen concentrations (~3 mg/L) and significantly higher organic matter content in the late summer (p < 0.05). Overall, these results suggest that variations in hydrology, dissolved oxygen demand, and other factors could influence hot spots of P retention and release throughout the watershed.