Nutrient Fluxes From Profundal Sediment of Ultra‐Oligotrophic Lake Tahoe, California/Nevada: Implications for Water Quality and Management in a Changing Climate. Issue 3 (5th March 2018) - Beutel, Marc W.
Abstract: A warming climate is expected to lead to stronger thermal stratification, less frequent deep mixing, and greater potential for bottom water anoxia in deep, temperate oligotrophic lakes. As a result, there is growing interest in understanding nutrient cycling at the profundal sediment‐water interface of these rare ecosystems. This paper assessed nutrient content and nutrient flux rates from profundal sediment at Lake Tahoe, California/Nevada, USA. Sediment is a large reservoir of nutrients, with the upper 5 cm containing reduced nitrogen (∼6, 300 metric tons) and redox‐sensitive phosphorus (∼710 metric tons) equivalent to ∼15 times the annual external load. Experimental results indicate that if deep water in Lake Tahoe goes anoxic, profundal sediment will release appreciable amounts of phosphate (0.13–0.29 mg P/m 2 ·d), ammonia (0.49 mg N/m 2 ·d), and iron to overlaying water. Assuming a 10 year duration of bottom water anoxia followed by a deep‐water mixing event, water column phosphate, and ammonia concentrations would increase by an estimated 1.6 µg P/L and 2.9 µg N/L, nearly doubling ambient concentrations. Based on historic nutrient enrichment assays this could lead to a ∼40% increase in algal growth. Iron release could have the dual effect of alleviating nitrate limitation on algal growth while promoting the formation of fine iron oxyhydroxide particles that degrade water clarity. If the depth and frequency of lake mixing decrease in the future as hydrodynamic models suggest, large‐scale in‐lake management strategies that impede internal nutrient loading in Lake Tahoe, such as bottom water oxygen addition or aluminum salt addition, may need to be considered. Key Points: A warming climate will lead to less deep‐water mixing, deoxygenation of bottom waters, and enhanced internal nutrient loading in deep lakes Profundal surficial sediment contains an enormous pool of bioavailable nitrogen and phosphorus that can be released under anoxic condition Bottom water anoxia followed by deep‐water mixing will result in meaningful increases in water column phosphorus, ammonia, and iron