How is eutrophication playing a role in the methane cycle of freshwater lakes?Agricultural runoff of nitrogen and phosphorous limit the growth of phytoplankton in aquatic ecosystems. As a result of increased nutrient runoff and the corresponding growth of phytoplankton are likely increasing CO2 absorption by lakes. However, phytoplankton sink into the anaerobic sediments of lakes, where the labile phytoplankton carbon is rapidly converted by microbes into useable substrates for CH4 production. What portion of the CH4 produced in lake sediments that escapes to the surface of the lake is dependent upon microbial consumption of CH4, diffusion properties, lake mixing, and ebullition (bubbling) from wind shear and abrupt hydrostatic pressure changes in shallow waters. We have evidence that shallow eutrophic lakes are likely emitting greater quantities of CH4 than large deep lakes and are presumably offsetting the carbon balance (C released = CH4 and CO2 emitted or absorbed).
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What key roles do microbes play in the production of methane in lakes?The flux of methane from or to lakes is regulated by two major microbial groups. Methanogenic Archaea, present in the sediments of lakes, are responsible methane production. Methane oxidizing bacteria or methanotrophs are responsible for the consumption of methane in lakes and other ecosystems.The activity of these microbes are regualted by ecological interactions with their environment and other lake inhabitants.
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Fluorescent microscopy of cultured freshwater lake methanogens
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What is the greenhouse effect and how does methane from lakes contribute to climate warming?Although carbon dioxide is most often discussed in the global warming conversation, methane (CH4) is a less abundant but more potent greenhouse gas (21x more effective at trapping heat). Lakes represent a key component of the global carbon cycle, and CH4 emissions are estimated to contribute up to 12% of current atmospheric methane concentrations. Most of the CH4 emitted from lakes is from the shallow areas in the form of bubbles (West et al. 2015) and it appears that shallow eutrophic lakes and wetlands may be the greatest natural source of CH4 to the atmosphere.
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