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Title: Large differences in potential denitrification and sediment microbial communities across the Laurentian great lakes

item SMALL, GASTON - University Of St Thomas
item BROVOLD, SANDRA - University Of Minnesota
item BULLERJAHN, GEORGE - University Of Minnesota
item FINLAY, JACQUES - University Of Minnesota
item MCKAY, ROBERT - University Of Minnesota
item ROZMARYNOWYCZ, MARK - University Of Minnesota
item Spokas, Kurt
item STERNER, ROBERT - University Of Minnesota

Submitted to: Biogeochemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/17/2016
Publication Date: 4/23/2016
Citation: Small, G.E., Brovold, S.A., Bullerjahn, G.S., Finlay, J.C., McKay, R.M., Rozmarynowycz, M.J., Spokas, K.A., Sterner, R.W. 2016. Large differences in potential denitrification and sediment microbial communities across the Laurentian great lakes. Biogeochemistry. 128(3):353-368.

Interpretive Summary: Freshwater lakes can be important sites for nitrogen removal through denitrification, but little is known about the spatial distribution of this microbial process. This research was aimed at characterizing the factors controlling the rate of denitrification in the Great Lakes and to understand why this process is not always effective at removing nitrate. Nitrate levels in Lake Superior have been increasing over the last 100 years, due to unfavorable conditions for microbial denitrification. Our results show that the rate of denitrification in Lake Superior was occurring at the maximum rate possible. In other words, the amount of nitrate in the water was not limiting this reaction. Therefore, this would place a limit on the ability of Superior to respond to additional nitrate, since it is already denitrifying at its maximum capacity. On the other hand, samples from the other Great Lakes (Lake Erie and Huron) possessed higher capacities for denitrification, and their rates were limited by nitrate aviability. Overall, this data improves the understanding of the microbial processes responsible for denitrification. These results are significant to farmers and policy makers and will assist scientists and engineers in developing improved dentrification reactors (bioreactors) to minimize nitrogen losses.

Technical Abstract: Lakes can be important sites for removal of reactive nitrogen (N) through denitrification, but spatial heterogeneity in denitrification rates can be high, and our understanding of factors controlling the capacity of lakes to remove excess N is incomplete. In oligotrophic Lake Superior, a century-long increase in nitrate concentrations may be due in part to unfavorable conditions for denitrification across much of the lake, in contrast to Lake Erie where sediment conditions are generally more favorable for nitrate removal. We measured rates and controls of denitrification throughout Lake Superior and Lake Erie, with additional observations in Lake Huron and Lake Ontario, between 2009-2011. Potential denitrification rates from offshore sites in Lake Superior were 2-3 orders of magnitude lower than offshore sites in Lake Erie. Variability in potential denitrification rates within Lake Superior was high, with some nearshore sites producing rates as high as those observed throughout Lake Erie. Endogenous denitrification rates were generally equivalent to potential rates in most Lake Superior sites, indicating a limited ability to respond to additional nitrate. In contrast, endogenous denitrification rates in many Lake Erie sites were up to three orders of magnitude lower than potential rates, indicative of nitrate limitation and suggesting that this lake can efficiently remove excess nitrate. The depth of the sediment oxycline was the strongest predictor of potential denitrification rates across this dataset, suggesting that physical control of material flux across the redox gradient is an important regulator of biogeochemical N:P balancing mechanisms.