Phosphorus cycling and water quality in an agriculural watershed

Abstract

Excess rural and urban nutrient inputs have led to downstream water quality degradation. Landowners in a small watershed in south central Manitoba, Canada have installed small dams as flood control mechanisms. Previous work has shown these dams and reservoirs are effective at decreasing total phosphorus (P) export, however questions of permanence, daily P fluctuation, and mechanisms influencing P retention still remain. Sediment nutrient dynamics can exert an important control on water quality on daily, monthly, and yearly timescales. To help better understand spatial and temporal patterns of P retention, P sorption assays were constructed (equilibrium P concentration or EPC0) and compared monthly measurements of EPC0 in small dammed reservoirs with their natural analog, stream pools. Dammed reservoirs and stream pools both showed a strong capacity to sorb P from the water column and as such, sediment processes represent a P sink across much of the catchment. In situ high frequency P sensors were deployed to assess short-term changes in P concentrations in four dammed reservoirs. Diel changes were only apparent later in the summer (August) but what drives these changes is unknown. Dam design to optimize nutrient retention should consider factors affecting P retention, including sediment geochemistry, but also residence time, and water chemistry as potential controls on P sorption. Diel sampling results suggest that water quality monitoring regimes that rely on singular grab samples should aim to sample in the mid-morning, especially later in the summer, so as to not over or underestimate P concentrations in water bodies.