Redox gradients at the low oxygen boundary of lakes

Abstract

The distribution of oxygen (O2) at the oxic/anoxic interface in the water column of two Swiss lakes was measured with sub-micromolar sensitivity, high precision, and high spatial resolution. The O2 distribution was found to be highly variable and it is shown that N-cycling and the redox gradients of Mn, Fe and CH4 are controlled by O2 distributions down to the nanomolar concentration range. The profiles reveal that apparent gaps between the oxic zone and the sites of CH4 and Mn oxidation are bridged by zones with 0.01−1 µmol L–1 O2 concentrations and thus CH4 and Mn oxidation clearly occur at oxic conditions. Directly below the steep oxycline of Lake Rot a broad low O2 zone in the depth range of 6−7.5 m was now detectable. The O2 increase during daylight in this zone was comparable to the O2 flux along the oxycline. Here photosynthesis could be responsible for a substantial part of the chemotrophic oxidation processes. An even broader zone (0.8−3.8 m) with sub-micromolar O2 and evidence for methanotrophic and lithotrophic activities found at 160 m depth in the deep, dark hypolimnion of Lake Zug was maintained by transport, reaction- and mixing processes. The submicromolar zones could not have been resolved with traditional CTD-profiles. Their existence expands the oxic zone downwards and implies that substantial parts of “suboxic zones” characterized by the absence of both O2 and H2S may actually belong to the realm of oxic processes if more sensitive measurement techniques are used for their characterization.