Large differences in potential denitrification and sediment microbial communities across the Laurentian great lakes
Large lakes can efficiently remove reactive nitrogen (N) through denitrification, but nitrate levels in some large oligotrophic lakes are increasing, indicating that denitrification in these lakes is not capable of removing excess N. To better understand how lake trophic status and sediment redox conditions affect the capacity of the microbial community to remove excess N, we measured potential denitrification rates at 86 different stations across Lakes Superior, Huron, Erie, and Ontario. We also relate sediment microbial communities to potential denitrification rates and sediment characteristics for a subset of these sites. In eutrophic/mesotrophic Lake Erie, characterized by sediment with minimal oxygen penetration and relatively high sediment carbon (C) and N, potential denitrification rates were relatively high and increased by 2–3 orders of magnitude in response to additional nitrate and organic C. In contrast, in oligotrophic Lakes Superior and Ontario, and mesotrophic Lake Huron, where oxygen can penetrate several cm into sediment, potential denitrification rates were generally low and did not respond to additional nitrate and organic carbon. Sediment microbial communities showed a similar pattern across this gradient, correlated with potential denitrification rates, sediment %C, and bottom-water nitrate concentrations. This observed relationships between sediment redox conditions, potential denitrification rates, and microbial diversity suggest that sediment microbial communities in these and other oligotrophic large lakes may already be operating at or near their maximum denitrification rates. Unlike mesotrophic Lake Erie, microbial communities in oligotrophic lake sediments may lack the ability to mitigate increases in N loading through denitrification.