The retention or release of nutrients into the water column from sediment, also known as internal loading, is another source of nutrients to the lake water column. Sediment cores collected from Willow Creek Lake in September 2013 and June 2014 were incubated for 7 days to assess potential for sediment to produce or consume water column phosphorus under aerobic (oxic) and anaerobic (anoxic) conditions using techniques similar to Penn and others (2000); the oxic conditions served as a control and the anoxic conditions represented typical lake sediment environments.
Phosphate as phosphorus and total dissolved phosphorus release rates (milligrams phosphorus per square meter per day) were estimated by dividing the change in phosphate concentration by the water residence time in the core chamber (core water volume divided by the pump flow rate) and correcting for the sediment area in each core. Cumulative phosphate as phosphorus and total dissolved phosphorus fluxes were estimated by summing the release rates through time over the course of the experimental incubation.