Wide-spread microbial sulfate reduction (MSR) in northern European freshwater systems: Drivers, magnitudes and seasonality.

Microbial sulfate reduction (MSR), which transforms sulfate into sulfide through the consumption of organic matter, is an integral part of sulfur and carbon cycling. Yet, the knowledge on MSR magnitudes is limited and mostly restricted to snap-shot conditions in specific surface water bodies. Potential impacts of MSR have consequently been unaccounted for, e.g., in regional or global weathering budgets. Here, we synthesize results from previous studies on sulfur isotope dynamics in stream water samples and apply a sulfur isotopic fractionation and mixing scheme combined with Monte Carlo simulations to derive MSR in entire hydrological catchments. This allowed comparison of magnitudes both within and between five study areas located between southern Sweden and the Kola Peninsula, Russia. Our results showed that the freshwater MSR ranged from 0 to 79 % (interquartile range of 19 percentage units) locally within the catchments, with average values from 2 to 28 % between the catchments, displaying a non-negligible catchment-average value of 13 %. The combined abundance or deficiency of several landscape elements (e.g., the areal percentage of forest and lakes/wetlands) were found to indicate relatively well whether or not catchment-scale MSR would be high. A regression analysis showed specifically that average slope was the individual element that best reflected the MSR magnitude, both at sub-catchment scale and between the different study areas. However, the regression results of individual parameters were generally weak. The MSR-values additionally showed differences between seasons, in particular in wetland/lake dominated catchments. Here MSR was high during the spring flood, which is consistent with the mobilization of water that under low-flow winter periods have developed the needed anoxic conditions for sulfate-reducing microorganisms. This study presents for the first time compelling evidence from multiple catchments of wide-spread MSR at levels slightly above 10 %, implying that the terrestrial pyrite oxidation may be underestimated in global weathering budgets.