Sediment Phosphorus Release at Beaver Reservoir, Northwest Arkansas, USA, 2002-2003: A Preliminary Investigation

Phosphorus (P) release from bottom sediments can be a significant source to the overlying water column, potentially maintaining and enhancing algal growth and eutrophic conditions in lakes and reservoirs. Thus, the objectives of this study were to: (1) measure P flux under aerobic and anaerobic conditions from intact sediment cores collected at Beaver Reservoir, northwest Arkansas, (2) evaluate the spatial variability in measured sediment P flux under aerobic and anaerobic conditions along the reservoir, and (3) compare external and internal P loads to Beaver Reservoir. Six intact sediment cores were collected at three sites representing the lacustrine, transitional, and riverine zones during June 2003, September 2003 and February 2004 and incubated for 21 days in the dark at ~22°C. Three cores from each site were incubated under aerobic conditions and anaerobic conditions. Water samples were collected from the overlying water column in each core daily for the first five days and every other day thereafter and analyzed for soluble reactive phosphorus (SRP). Water removed from the core was replaced with filtered lake water, maintaining a constant overlying water volume of 1 l. Sediment P flux under anaerobic conditions (<0.01-1.77 mg m-² day-¹) was generally greater than that measured under aerobic conditions (<0.01-0.89 mg m-² day-¹). Some spatial variability existed in sediment P flux where P flux was generally greatest at the sites in the riverine and transitional zones. Maximum sediment P flux was observed under anaerobic conditions in cores collected from the transitional zone during September 2003. Average sediment P flux under aerobic conditions (0.09 mg m-² day-¹) and anaerobic conditions (0.31 mg m-² day-¹) was greater than the external P flux (0.05 mg m-² day-¹) estimated from the Beaver Reservoir tributaries. Results showed that the annual internal P load (7 Mg year-¹) from bottom sediments in Beaver Reservoir was less than 10% of the annual external P load (~81 Mg P year-¹). The internal P load was significant, but it would not currently be cost effective to manage this P source given the large surface area of Beaver Reservoir.