Sedimentary pigments reveal complex ecosystem responses of primary producers to mid-Holocene summer anoxia in a small Greenland lake

Freshwater ecosystems in the Arctic are particularly sensitive to climate change, and current anthropogenic warming has demonstrably influenced Arctic lake productivity. Paleolimnological records reveal how primary producers have responded to past environmental changes and thus provide insight on their future shifts. Sedimentary pigments are molecular biomarkers that can record detailed information about soft-bodied algae and cyanobacteria, which are not well represented by more conventional proxies. We analyzed diverse sedimentary pigments (echinenone, okenone, alloxanthin, canthaxanthin, β -carotene, and chlorophyll- a and its degradation products) from two cores using HPLC-MS, as well as bulk sediment organic and inorganic geochemistry, to reconstruct Holocene changes in primary production in a small, subarctic lake in South Greenland. We find multi-proxy evidence for a multi-millennial, mid-Holocene period of hypolimnetic anoxia from 6650 to 3500 cal yr BP within part of the lake. We suggest that summer thermal stratification and high productivity driven by warmer-than-present temperatures and increased catchment-derived nutrient influx drove changes in summer lakewater oxygenation. Our reconstructions reveal two distinct steady ecosystem states associated with oxygen status: a eukaryotic algae-dominated community during oxic conditions, versus cyanobacterial dominance during periods with anoxic bottom waters. Notably, we find that markers of anoxia and associated ecosystem shifts are stronger and probably longer-lived at one coring site, secondarily revealing considerable spatial (in addition to temporal) heterogeneity in oxygenation. As such, we propose that multi-core comparisons in paleolimnology can capture biogeochemical changes across both space and time, even in small lakes, and that spatial variations can provide clues about potentially localized drivers of past limnological change. Overall, our work broadly indicates that primary producer communities in some subarctic lakes were highly sensitive to warmer-than-present temperatures in the mid-Holocene, with landscape processes and thermal stratification playing secondary roles in driving productivity shifts. This implies that future warming could prompt widespread transformations in Arctic lake ecosystems, biogeochemistry (including carbon cycling and oxygenation), and water quality. • Sedimentary pigments inform records of primary productivity and lake oxygenation. • Muli-proxy evidence reveals period of mid-Holocene anoxia in a South Greenland lake. • Cyanobacteria dominated the lake ecosystem in reduced oxygen conditions. • Anoxia driven by summer thermal stratification during the Holocene Thermal Maximum. • Other drivers of anoxia include high primary productivity and terrigenous input.