Dynamics and control of nutrient, metal and oxygen fluxes at the profundal sediment-water interface of lakes and reservoirs.

摘要

Deep, productive lakes commonly exhibit summertime hypolimnetic anoxia, resulting in a flux of problem-causing compounds from profundal sediment to overlaying water. Using experimental 1.8 L chambers, I examined nutrient, metal and DO flux dynamics at the sediment-water interface, and evaluated hypolimnetic oxygenation as a potential control strategy. Study sites included six municipal water supply reservoirs and two natural lakes of varying size and trophic status. Anoxic release rates of soluble reactive phosphorus (SRP) and ammonia from anoxic sediments ranged from 0.25–9 mgP m−2 d−1 and 0.5–14 mg-N m−2 d−1, and P and N release rates covaried across study sites (r2 = 0,78, p 0.001). Anoxic nutrient flux also strongly correlated with sediment Fe:Mn ratio (r2 = 0.92, p 0.001). SRP release rate correlated with sediment total phosphorus (TP) (r2 = 0.48, p 0.05), but did not correlate with the ORP-sensitive fraction of sediment TP. Anoxic release of iron and manganese ranged from 0.25–28 and 1–12 mg m−2 d−1 , respectively. Low rates of iron release were observed in sulfide-rich chambers, suggesting the formation of iron sulfides. Under oxic conditions, SRP and metal releases were almost completely inhibited, while ammonia release was inhibited only in sediments from lakes of low to moderate trophic status. Observed patterns of SRP release support the Einsele-Mortimer model of sediment P release. The source of ammonia release under anoxic conditions appears to be a loss of heterotrophic assimilation potential. Sediment oxygen demand (SOD) measured under quiescent conditions ranged from 0.05–0.4 g m −2 d−1. Moderate mixing at the sediment-water interface (velocities of 4–5 cm s−1) consistently increased SOD by a factor of three to four. The SOD component of total hypolimnetic oxygen demand in study sites significantly increased with decreasing mean depth (r2 = 0.81, p 0.05). Nutrient release rates and SOD were found to significantly correlate with a Trophic Index developed for the study sites (r2 > 0.75, p 0.02) based on a principal component analysis of chlorophyll-a, secchi depth and anoxic factor. Calculated rates of in situ nutrient release and SOD were in general agreement with rates determined experimentally. Hypolimnetic oxygenation, an emerging lake management technology, shows promise in reducing undesired chemical fluxes from sediments. However, design oxygen capacity must be carefully assessed since mixing after system startup can increase hypolimnetic oxygen demand, particularly in stratified lakes of moderate depth.