The maintenance of intermediate redox states in deep waters of Lake Zug

In the absence of O₂, anaerobic biogeochemical cycling is driven by a host of electron acceptors (Mn(IV), NO₃^-, Fe(III), SO₄^2-), with decreasing energy yield for equivalent reactions. The anoxic environments in which these chemical species drive biogeochemical cycling have a disproportionately high impact on global processes through their roles in climate-active gas fluxes, regulating nutrient availability, and sequestration of trace elements. In modern surface waters, such environments can be found year-round in marine basins with limited deep-water renewal (e.g., the Black and Baltic Seas) and in lakes where vertical mixing is restricted, for example, by strong salinity-driven density gradients (e.g., Lake Cadagno). Despite a range of potential electron acceptors, in most systems these are quickly exhausted, resulting in more strongly reducing anoxic environments, characterized by either high dissolved Fe (ferruginous) or sulfide (euxinic) concentrations, reflecting respiration driven by reduction of Fe(III) and SO₄^2-, respectively.

 

Lake Zug (Switzerland) is an exception to this, with the maintenance of an intermediate anoxic redox state. The south basin of Lake Zug (198 m deep) is anoxic for approximately the lower 60-70 m, with some seasonal and interannual variation. Deep waters have been regularly anoxic since monitoring began in the 1950s; however, despite the stability of anoxia, the typical strongly reducing ferruginous or euxinic conditions found in other such basins are not reached. Instead, NO₃^- concentrations remain moderate in anoxic waters, with minimal NO₂^- and with NH₄⁺ accumulating only well below the oxic-anoxic interface. Similarly, Mn is reactive across the oxic-anoxic interface, with the reduction of manganese oxides and the accumulation of dissolved Mn(II). However, both Fe and SO₄^2- show low reactivity across the oxic-anoxic interface, with minimal net Fe(III) and SO₄^2- reduction apparent throughout almost the entire anoxic zone. This strongly contrasts other seasonally (or permanently anoxic systems (e.g., the Black & Baltic Seas, Saanich Inlet, Lake Pavin, Lake Matano). This high abundance of electron acceptors in anoxic Lake Zug water, in contrast to other anoxic basins, has implications for the biogeochemical cycling of nutrients and climate active gasses. Potential mechanisms for maintaining this state, as well as influences on redox-sensitive major and minor elements, will be discussed.