The paradox of increasing long-term carbon sequestration in lake ecosystems despite reoligotrophication : the case of four large French perialpine lakes

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English. <p>Although lakes only represent a small fraction of the surface of the earth, a growing number of studies have shown that they play a critical role in the global carbon cycle (<sup>[i],[ii],[iii]</sup>), mediating carbon transfer from land to the atmosphere, and burying organic carbon in their sediments. The magnitude and temporal variability of carbon burial is, however, poorly constrained, and the degree to which lake productivity has influenced lake carbon cycling has not been systematically assessed (<sup>[iv]</sup>). Here, trends in total organic carbon (TOC) sequestration and primary production are reconstructed from sediment records for the last 300 years in four perialpine deep lakes. We rely on High Performance Liquid Chromatography (HPLC) and geochemical proxies to investigate changes in algal communities. Then, we evaluate the temporal contribution of algal assemblages to the variability of lake primary production, as well as the potential effects on carbon sequestration magnitude. Other contributors to carbon sequestration derived from the IPER RETRO project (2009-2013) are also investigated, such as past oxygen conditions, lake thermal structure or allochthonous supplies of carbon. Our results suggest that despite reoligotrophication of all lakes (e.g., decrease in dissolved phosphorus concentration in water column and relative restoration of diatoms communities (<sup>[v]</sup>)) over the last 3 decades, TOC in lakes sediments is still increasing in the sediment. The study of algal pigments suggests that changes in algal assemblages and oxygen conditions could be responsible of this persistent increase in carbon burial. Future development (e.g., DNA analysis) should provide more detail on algal communities to validate these results.</p><div><br><div> <p>[i] J. J. Cole et al., ‘Plumbing the Global Carbon Cycle: Integrating Inland Waters into the Terrestrial Carbon Budget’, Ecosystems 10, no. 1 (May 2007): 172–85, https://doi.org/10.1007/s10021-006-9013-8.</p> </div> <div> <p>[ii] Tom J. Battin et al., ‘Biophysical Controls on Organic Carbon Fluxes in Fluvial Networks’, Nature Geoscience 1, no. 2 (February 2008): 95–100, https://doi.org/10.1038/ngeo101.</p> </div> <div> <p>[iii] Lars J. Tranvik et al., ‘Lakes and Reservoirs as Regulators of Carbon Cycling and Climate’, Limnology and Oceanography 54, no. 6part2 (November 2009): 2298–2314, https://doi.org/10.4319/lo.2009.54.6_part_2.2298.</p> </div> <div> <p>[iv] N. J. Anderson et al., ‘Anthropogenic Alteration of Nutrient Supply Increases the Global Freshwater Carbon Sink’, Science Advances 6, no. 16 (April 2020): eaaw2145, https://doi.org/10.1126/sciadv.aaw2145.</p> </div> <div> <p>[v] Vincent Berthon et al., ‘Trophic History of French Sub-Alpine Lakes over the Last 150 Years: Phosphorus Reconstruction and Assessment of Taphonomic Biases’, Journal of Limnology 72, no. 3 (September 2013): 34, https://doi.org/10.4081/jlimnol.2013.e34.</p> </div> </div>