Role of a productive lake in carbon sequestration within a calcareous catchment

•Terrestrial carbonate weathering is considered a temporary sink for CO2.•Alkaline lakes precipitate calcite reverting chemical weathering reactions.•Algal uptake increased δ13C of dissolved inorganic C while passing through the lake.•40-70% of sediment organic C originated from catchment alkalinity export.•Biological uptake of released CO2 counteracts emissions from reversed weathering.

For a long time, lakes were considered unimportant in the global carbon (C) cycle because of their small total area compared to the ocean. Over the last two decades, a number of studies have highlighted the important role of lakes in both sequestering atmospheric C and modifying the C flux from the catchment by degassing CO2 and methane and burying calcite and organic matter in the sediment. Based on a full C mass balance, high frequency measurements of lake metabolism and stable isotope analysis of a large shallow eutrophic lake in Estonia, we assess the role alkaline lakes play in augmenting the strength of terrestrial carbonate weathering as a temporary CO2 sink. We show that a large part of organic C buried in the sediments in this type of lakes originates from the catchment although a direct uptake from the atmosphere during periods of intensive phytoplankton growth in eutrophic conditions contributes to the carbon sink.

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