Nutrient release from reservoir sediments impacted by a legacy of historical agriculture practices
Old, small eutrophic reservoirs accumulate and store a substantial quantity of nutrients in their sediments. The main determinant of whether these nutrients become permanently buried in sediments or whether they are recycled back into the water column is the dissolved oxygen (DO) concentration in the reservoir’s hypolimnion. Low DO conditions trigger internal recycling from the sediments, thereby altering the biogeochemical cycling in the reservoir. Here, we report the results of a whole-ecosystem experiment in which we manipulated hypoxia by sequentially injecting supersaturated concentrations of oxygen into the hypolimnion of Falling Creek Reservoir (FCR) (Vinton, Virginia, USA). FCR is a small, eutrophic drinking water reservoir with a legacy of agriculture in its catchment. We analyzed hypolimnetic DO concentrations using a high-frequency (15 minute resolution) sensor located 1 m above the reservoir sediments. Throughout the experiment, we collected weekly water samples from the reservoir and in the reservoir’s only upstream tributary to calculate internal and external loads of nutrients. We also collected sediment cores during oxic and hypoxic conditions and manipulated core redox conditions in laboratory incubations to measure potential loading rates of nutrients from the reservoir’s 117-year-old sediments.
Throughout our experiment, we were able to successfully manipulate oxic (~12 mg/L) and hypoxic (<2 mg/L) hypolimnetic conditions sequentially for ~4 week periods. During the hypoxic conditions, we observed diel oscillations of DO concentrations up to 2 mg/L within a day 1 m above the sediments, which exhibited low rates of respiration. In contrast, during oxic conditions, the large addition of oxygen (25 kg/day) stopped the diel fluctuations in DO concentration, and resulted in a substantial increase in respiration. Oxic conditions in the hypolimnion successfully suppressed the release of ammonium, phosphorus, and iron from the sediments for a majority of the stratified period. The sediment core experiments were used to determine the maximum potential nutrient release rates from the reservoir’s sediments and followed similar trends as observed in the whole-ecosystem experiment. From both the whole-ecosystem experiment and laboratory sediment core incubations, we observed that internal loading, not external loading, dominated nutrient fluxes in the reservoir, suggesting that historical agriculture practices from the early 1900s still have a profound effect on today’s reservoir water quality.