INVESTIGATING CONTROLS ON CARBON AND NITROGEN CYCLING IN A MODERN ANOXIC LAKE ENVIRONMENT

Anoxia, once prevalent in ancient bodies of water, is predicted to affect modern lakes due to oxygen depletion resulting from rising atmospheric temperatures. With nitrogen being a limiting nutrient influencing lake productivity and thus organic matter (OM) processing, it is imperative to understand how anoxic freshwater conditions will impact regional and global nutrient cycling. This study investigates nutrient source to sink dynamics in a modern low oxygen system within Lake Huron, Middle Island Sinkhole (MIS), using carbon and nitrogen isotope signatures of sediments. The average δ¹⁵N composition of MIS sediments (-0.5 ± 0.8 ‰) is isotopically heavier than settling particulate organic matter from the overlying water column (-1.6 ± 0.2 ‰), likely indicating microbial transformations of OM sources upon their deposition into the sediments. Microbial discrimination against heavier ¹⁵N sources in the presence of high percent nitrogen results in more variable and lighter δ¹⁵N signatures in surface sediments (0-1 cm, average -0.7 ± 0.9 ‰) compared to deeper layers (6–15 cm, average -0.3 ± 0.6 ‰). Microbial influences on organic carbon preservation are also observed with carbon isotopes of chlorophyll (δ¹³Cchl) showing an enrichment downcore from surface (average -26.6 ± 1.8 ‰) to deep sediments (average -24.3 ± 0.5 ‰). Though sediment isotopic signatures in MIS reveal microbial controls on organic matter burial and preservation, these may not be reflective of other freshwater regions. MIS exhibits distinct clustering in δ¹³C (range -28.8 to -20.9 ‰) and δ¹⁵N (range -3.2 to 1.6 ‰) data space compared to lakes of varying oxygen regimes (n = 137, δ¹³C range -33.4 to -18.32‰, δ¹⁵N range -4.25 to 12.62‰). Anoxic conditions within MIS may foster unique biogeochemical dynamics that highlight the variability and unpredictability of nutrient cycling transformations amidst increasing freshwater deoxygenation resulting from global climate change.