A detailed understanding of the formation of the potent neurotoxic methylmercury is needed to explain the large observed variability in methylmercury levels in aquatic systems. While it is known that organic matter interacts strongly with mercury, the role of organic matter composition in the formation of methylmercury in aquatic systems remains poorly understood. Here we show that phytoplankton-derived organic compounds enhance mercury methylation rates in boreal lake sediments through an overall increase of bacterial activity. Accordingly, in situ mercury methylation defines methylmercury levels in lake sediments strongly influenced by planktonic blooms. In contrast, sediments dominated by terrigenous organic matter inputs have far lower methylation rates but higher concentrations of methylmercury, suggesting that methylmercury was formed in the catchment and imported into lakes. Our findings demonstrate that the origin and molecular composition of organic matter are critical parameters to understand and predict methylmercury formation and accumulation in boreal lake sediments.

Organic carbon concentrations have increased in surface waters across parts of Europe and North America during the past decades, but the main drivers causing this phenomenon are still debated. A lack of observations beyond the last few decades inhibits a better mechanistic understanding of this process and thus a reliable prediction of future changes. Here we present past lake-water organic carbon trends inferred from sediment records across central Sweden that allow us to assess the observed increase on a centennial to millennial time scale. Our data show the recent increase in lake-water carbon but also that this increase was preceded by a landscape-wide, long-term decrease beginning already A.D. 1450-1600. Geochemical and biological proxies reveal that these dynamics coincided with an intensification of human catchment disturbance that decreased over the past century. Catchment disturbance was driven by the expansion and later cessation of widespread summer forest grazing and farming across central Scandinavia. Our findings demonstrate that early land use strongly affected past organic carbon dynamics and suggest that the influence of historical landscape utilization on contemporary changes in lake-water carbon levels has thus far been underestimated. We propose that past changes in land use are also a strong contributing factor in ongoing organic carbon trends in other regions that underwent similar comprehensive changes due to early cultivation and grazing over centuries to millennia.lake-water quality | carbon cycling | land use | Holocene | paleoecology O ver the past three decades, monitoring programs have recorded a widespread increase of organic carbon (OC) concentrations in surface waters in parts of Europe and North America (1-4). OC in lakes and rivers plays a major role in the global carbon cycle by transporting carbon from terrestrial to freshwater and marine environments (5), determining drinking water quality and associated treatment costs (6), and affecting aquatic ecosystem functioning. In aquatic ecosystems, OC influences energy mobilization, light conditions (7), water acidity (8), as well as the transport of metals and pollutants (9). The widespread occurrence of this increase in surface water OC, also referred to as browning or brownification due to an associated increase in color, suggests that regional rather than local factors are the drivers behind this phenomenon, but at present the underlying mechanisms are still controversial.Several hypotheses have been proposed to explain this recent OC increase, from climate change to changes in anthropogenic forcing such as declining atmospheric acid deposition or alterations in landscape utilization. A number of climate-sensitive mechanisms have been suggested to cause an increase in OC export from the terrestrial to the aquatic environment; for example, increased temperatures enhance decomposition rates in organic-rich soils (10) and promote vegetation cover (11). Changes in the amount and timing of precipitation potentially lead to alteration...

Photodegradation processes play an important role in releasing elements tied up in biologically refractory forms in the environment, and are increasingly recognized as important contributors to biogeochemical cycles. While complete photooxidation of dissolved organic carbon (to CO 2), and dissolved organic phosphorous (to PO 4 3-) has been documented, the analogous photoproduction of sulfate from dissolved organic sulfur (DOS) has not yet been reported. Recent high-resolution mass spectrometry studies showed a selective loss of organic sulfur during photodegradation of dissolved organic matter, which was hypothesized to result in the production of sulfate. Here, we provide evidence of ubiquitous production of sulfate, methanesulfonic acid (MSA) and methanesulfinic acid (MSIA) during photodegradation of

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