14-9 Sedimentary pigments reveal complex ecosystem responses of primary producers to mid-Holocene anoxia in a small low-Arctic Greenland lake

Session: Lake Sedimentary Records of Past Climate and Environment

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The Arctic is warming four times faster than any region on our planet. Enhanced primary productivity, changes in lake stratification, and other radiative effects have been observed in Arctic lakes, but it is unclear how these will influence cyanobacteria in lakes that remote northern communities rely on for drinking water. A paleolimnological approach focusing on previous warm periods such as the Holocene Thermal Maximum provides foundational insight for potential future changes in primary productivity, especially in minimally monitored regions such as the Arctic. We used sedimentary photosynthetic pigments, alongside numerous other organic and inorganic proxies, to investigate how temperature influenced oxygenation in a small sub-arctic Greenland lake over the Holocene, and how these changes impacted or were facilitated by primary production. We find evidence for a multi-millennial period of anoxia in the lake (5.8-3.5 kcal yr BP), and suggest that a combination of warmer-than-present temperatures that drove thermal stratification and high productivity resulted in an anoxic hypolimnion. Our sedimentary pigment analyses reveal that potentially colonial or bloom-forming cyanobacteria dominated during the anoxic period, while diatoms, green algae, and zooplankton experienced significant declines. Finally, our comparison between sediment cores from different coring sites suggests that oxygenation may have been spatially heterogenous across the lake basin, with proximity to the lake inflow where terrigenous input is highest playing a secondary role in enhancing anoxia. Overall, our study has important implications for the role of temperature in driving lake oxygenation changes (both directly and indirectly), and the impacts anoxia may have on primary producers. We also illustrate the power of sedimentary pigments as molecular biomarkers capable of capturing diverse primary producers and revealing past anoxic periods (through pigments indicative of anoxic photosynthetic bacteria). Reconstructions of how cyanobacteria respond to warm periods like ours today provide insight on the future of toxic or blooming species in untreated lake reservoirs that Northern communities rely on.

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