Nitrogen Isotope Signatures in Lake Huron's Middle Island Sinkhole Sediments

Global climate change is causing declining oxygen levels in lakes, impacting nutrient availability and cycling. Changes in nitrogen content and composition within lakes are of particular interest as this element is crucial for biomass production, metabolic processes, and strongly influences carbon cycling. Nitrogen isotopic signatures observed in lake sediments are informative of autochthonous (i.e. primary production) or allochthonous (i.e. anthropogenic inputs) sources of organic matter. Middle Island Sinkhole (MIS), a 23-m-deep karst feature in Lake Huron, is a modern low oxygen environment with geochemical and microbial qualities potentially resembling future oxygen depleted lakes. Anoxic groundwater venting into MIS promotes organic matter preservation within underlying sediments in addition to the formation of benthic microbial mat communities dominated by cyanobacteria and sulfate-reducing bacteria. Microbial fractionation of source C is only observed in the mats themselves, and sediment δ13Corg values otherwise closely match local water column sources. While groundwater, water column, and sedimenting particulate organic matter δ15N are all >1 ‰, sediments at MIS vary between -3.2 and 1.8‰, isotope values well below other lakes (δ15N -3.4 to 13‰). There is no relationship between δ15N and %N or δ13Corg, suggesting that the offset in sediment δ15N from water column δ15N is not related to microbial remineralization at the sediment-water interface, and is instead occurring in situ within the sediments. Microbial nitrogen cycling patterns may be linked to minimal nitrate (NO3-) and increasing ammonium (NH4+) with depth observed in sediment porewater. These findings suggest that shifting lake oxygen levels as a result of climate change may prompt shifts in nitrogen speciation thus affecting local and global nitrogen cycling.