Effects of phytoplankton blooms on fluxes and emissions of greenhouse gases in a eutrophic lake.

Lakes are important sources of greenhouse gases (GHGs) to the atmosphere. Factors controlling CO, CH and NO fluxes include eutrophication and warming, but the integrated influence of climate-warming-driven stratification, oxygen loss and resultant changes in bloom characteristics on GHGs are not well understood. Here we assessed the influence of contrasting meteorological conditions on stratification and phytoplankton bloom composition in a eutrophic lake, and tested for associated changes in GHGs inventories in both the shallow and deep waters, over three seasons (2010-2012). Atmospheric heatwaves had one of the most dramatic effects on GHGs. Indeed, cyanobacterial blooms that developed in response to heatwave events in 2012 enhanced both sedimentary CH concentrations (reaching up to 1mM) and emissions to the atmosphere (up to 8 mmol m d). That summer, CH contributed 52% of the integrated warming potential of GHGs produced in the lake (in CO equivalents) as compared to between 34 and 39% in years without cyanobacterial blooms. High CH accumulation and subsequent emission in 2012 were preceded by CO and NO consumption and under-saturation at the lake surface (uptakes at -30 mmol m d and -1.6 µmol m d, respectively). Fall overturn presented a large efflux of NO and CH, particularly from the littoral zone after the cyanobacterial bloom. We provide evidence that, despite cooling observed at depth during hot summers, CH emissions increased via stronger stratification and surface warming, resulting in enhanced cyanobacterial biomass deposition and intensified bottom water anoxia. Our results, supported by recent literature reports, suggests a novel interplay between climate change effects on lake hydrodynamics that impacts both bloom characteristics and GHGs production in shallow eutrophic lakes. Given global trends of warming and enrichment, these interactive effects should be considered to more accurately predict the future global role of lakes in GHG emissions.