Abstract
Abstract
External input of nutrients through point and nonpoint discharges is considered critical to supporting eutrophication of surface waters. This study investigated internal recycling of nutrients, measured the ambient sediment oxygen demand (SOD) and nutrient fluxes, in a fresh water shallow alkaline lake. The lake has been experiencing frequent Harmful algal blooms. Provo Bay in the lake, which receives treated point discharges and flows from tributaries, was found to have the highest SOD (4.61 g O2/m2/day) and water column respiration (−6.66 g O2/m3/day) among seven sampled sites. A linear correlation was plotted between SOD and percentage of volatile solids (%VS) in sediments (r = 1.59, R2 = 0.97). Lake sediments (excluding Provo Bay, site 1) were continuously adding nutrients to the water column at an average rate of 0.057 ± 0.057 g N/m2/day and 0.016 ± 0.027 g P/m2/day. The isolated Provo Bay has a sediment nitrogen flux of 1.44 g N/m2/day and phosphorus flux of 0.01 g P/m2/day. Most nutrients released were bioaccumulated by phytoplankton to compensate for water column nutrient limitations. It was also concluded that ammonia, rather than ortho-P, had significant flux rates under ambient conditions. Internal P-cycling was supported by P-rich sediment (up to 1,767 mg P/kg dry mass) and continuous external loading. This accounted for 1,500 tons P per year, which was 5 times higher than the external P loading. Inorganic P was the dominant P species in this carbonate-rich sediment. Ambient sediment P concentration was rich in the order of Ca-P > Fe/Mn-P > residual-P > Al-P > loosely-sorbed P for most sites. After changing pH and dissolved oxygen conditions, results showed that loosely sorbed-P and Fe/Mn-P were reactive and possibly released ortho-P at decreased pH values under anaerobic condition in the sediments.
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