Sediment Phosphorus Release in a Shallow Eutrophic Reservoir Cove

Abstract

Abstract. Internal phosphorus (P) loading is a leading contributor to eutrophication in reservoirs and can cause harmful algal blooms as well as treatment issues for drinking water reservoirs. Coves are an area of reservoirs that have not received adequate attention, even though they experience higher nutrient and sediment deposition and primary production per unit area when compared to the pelagic zone of the reservoir. This study investigates a shallow eutrophic cove in a northwest Arkansas reservoir called Beaver Lake to better understand the cove’s potential to contribute to P loading and eutrophication within the reservoir. The study period was 3 to 16 July 2018. Water column profiles of depth, temperature, and dissolved oxygen were measured with a floating sensor platform that also contained a weather station. Cove bed sediment samples were collected at three locations in the cove and analyzed for chemical composition through Mehlich III extraction and P, nitrate + nitrite (N+N), and ammonia release rates with aerobic and anaerobic sediment core incubations. Bathymetry data were collected using a depth sonar system. Sensor platform profiles indicated dynamic bottom temperature and dissolved oxygen conditions with transient influxes of hypoxic waters that occurred several times for less than 24 h. The P release rates from bed sediment incubations were as high as 2.02 mg m-2 d-1 under aerobic conditions and 4.45 mg m-2 d-1 under anaerobic conditions. Upon initiation of nitrogen gas bubbling in the sediment cores, anaerobic conditions were delayed by the presence of N+N. Phosphorus release did not occur until denitrification decreased the N+N concentrations enough for reducing conditions to be present. For the study period, a P flux into the water of roughly 1 kg was determined using cove profiles, bathymetry, and P release rates. When compared to whole-lake P release averages for Beaver Lake, eutrophic coves are a disproportionate source of P per unit area within the reservoir. This may offer opportunities for more efficient use of internal loading remediation techniques, such as alum application. The results of this study also suggest that we should not continue to overlook shallow-area bed sediment P flux when considering the P mass balance of a reservoir. Keywords: Eutrophication, Sediment core incubation, Stratification.

Magnitude of External Phosphorus Loading Likely Reduces Effectiveness of Aluminum Sulfate Treatments for Management of Sediment Phosphorus Flux

“…Therefore, no effect of alum treatment was observed for P fluxes under aerobic conditions throughout the duration of this study. Additionally, P fluxes were relatively low throughout the study (<0.00 to 1.47 mg m -2 day -1 ), and aerobic rates were on the lower end of P fluxes reported in North America for eutrophic and hypereutrophic reservoirs (<0.00 to 5.00 mg m -2 day -1 ) (Doig et al, 2017;Haggard et al, 2005;Holdren and Armstrong, 1980;James et al, 1995;Lasater and Haggard, 2017;McCarty, 2019;Moore et al, 1998;Steinman et al, 2004). It should be noted that core collection in the summer months, when temperatures are higher and there is a greater chance of stratification in Lake Wister, may result in underestimated P fluxes because sediment P has already been released into the water column.…”

Sediment phosphorus composition controls hot spots and hot moments of internal loading in a temperate reservoir

“…While the highest rates of sediment P release occurred under anoxic conditions at the deep site, the other elevated flux rates at the shallow and intermediate sites happened when dissolved oxygen was available at the sediment–water interface. Other studies of hypereutrophic reservoirs have also observed aerobic sediment P release, which was associated with intense algal production and P mobilization during the decomposition of sediment organic matter (McCarty, 2019; Song & Burgin, 2017). As a hypereutrophic waterbody, GVL also experiences severe algal blooms throughout the summer months, so it is likely that microbial decomposition of algal detritus contributed to the observed aerobic P release.…”