Oxygen and dissolved organic carbon control release of N, P and Fe from the sediments of a shallow, polymictic lake

We examined potential causes of nutrient release from the sediments of a shallow polymictic reservoir that, unlike many other shallow lakes, had not received large amounts of nutrient loading. It was hypothesised that the sediments released greater amounts of nutrients under anoxic rather than oxic conditions and that nitrogen (N) and phosphorus (P) release was limited by the availability of different types of organic substrate.
Materials and methods: Sediment incubation experiments under oxic and anoxic conditions were conducted in summer and in winter. One anoxic incubation experiment with the addition of different types of dissolved organic substrate (glucose, acetate, formate and propionate) was carried out. Concentrations of nitrite and nitrate (NOx −-N), ammonium (NH4 +-N), soluble reactive P (srP) and iron (Fe(II)) were monitored in the supernatant of the incubation chambers over 28 days. Thermal stratification was monitored in situ for 17 months and sediment composition was analysed.
Results and discussion: NOx −-N concentration increased by 5.67 ± 0.33 mg N l−1 in anoxic conditions but was much lower in oxic treatments. Release of srP was low in anoxic and oxic treatments (0.004 ± 0.001 mg P l−1) but was stimulated by 0.027 ± 0.015 mg P l−1 under additions of formate. Fe(II) release was mostly stimulated by glucose additions (25.59 ± 4.28 mg Fe l−1). P release was much lower than observed in eutrophic lakes. P release does not appear to be as a result of the reduction of Fe minerals and concurrent dissolution of orthophosphate, often reported as the most prevalent pathway for P release from anaerobic lake sediments.
Conclusions: Iron and P flux from lake sediments can be independent from each other and can be controlled by dissolved organic C. Further, the study confirms the role of oxygen availability in controlling nutrient release from the sediments.