Rates and controls of nitrification in a large oligotrophic lake

Recent discoveries have altered prevailing paradigms concerning the conditions under which nitrification takes place and the organisms responsible for nitrification in aquatic ecosystems. In Lake Superior, nitrate (NO-3) concentrations have increased fivefold in the past century. Although previous evidence indicated that most NO-3 is generated by nitrification within the lake, important questions remain concerning the magnitude and controls of nitrification, and which microbial groups are primarily responsible for this process. We measured water-column nitrification rates in the western basin of Lake Superior during five research cruises from November 2009 to March 2011. Using in situ bottle incubations at 10 depths, we quantified nitrification rates using both the oxidation of 15N-labeled ammonium (NH+4) and the uptake of 14C associated with nitrification. Average rates of NH+4 oxidation ranged from 18-34 nmol N L-1 d-1 across the five cruises, similar to values reported for the coastal ocean, and two orders of magnitude lower than values reported from other lakes. Low nitrification rates observed in the epilimnion corresponded to the absence of ammonium-oxidizing archaea and nitrite-oxidizing bacteria. The measured rates of nitrification are \u3e 50-fold greater than the long-term NO-3 rise in the lake, indicating that N is actively cycling and that long-term change in this ecosystem is mediated by internal dynamics. © 2013, by the Association for the Sciences of Limnology and Oceanography, Inc

Transitions in microbial communities along a 1600 km freshwater trophic gradient

This study examined vertically-resolved patterns in microbial community structure across a freshwater trophic gradient extending 1600 km from the oligotrophic waters of Lake Superior to the eutrophic waters of Lake Erie, the most anthropogenically influenced of the Laurentian Great Lakes system. Planktonic bacterial communities clustered by Principal Coordinates Analysis (PCoA) on UniFrac distance matrices into four groups representing the epilimnion and hypolimnion of the upper Great Lakes (Lakes Superior and Huron), Lake Superior\u27s northern bays (Nipigon and Black bays), and Lake Erie. The microbes within the upper Great Lakes hypolimnion were the most divergent of these groups with elevated abundance of Planctomycetes and Chloroflexi compared to the surface mixed layer. Statistical tests of the correlation between distance matrices identified temperature and sample depth as the most influential community structuring parameters, reflecting the strong UniFrac clustering separating mixed-layer and hypolimnetic samples. Analyzing mixed-layer samples alone showed clustering patterns were correlated with nutrient concentrations. Operational taxonomic units (OTU) which were differentially distributed among these conditions often accounted for a large portion of the reads returned. While limited in coverage of temporal variability, this study contributes a detailed description of community variability that can be related to other large freshwater systems characterized by changing trophic state

Increasing stoichiometric imbalance in North America’s largest lake: Nitrification in Lake Superior

Lake Superior has exhibited a continuous, century-long increase in nitrate whereas phosphate remains at very low levels. Increasing nitrate and low phosphate has led to a present-day severe stoichiometric imbalance; Lake Superior’s deepwater NO3 :PO43 molar ratio is 10,000, more than 600 times the mean requirement ratio for primary producers. We examine the rate of [NO3?] increase relative to budgets for NO3 and fixed N. Nitrate in Lake Superior has continued to rise since 1980, though possibly at a reduced rate. We constructed whole-lake NO3 and N budgets and found that NO3 must be generated in the lake at significant rates. Stable O isotope results indicate that most NO3 in the lake originated by in-lake oxidation. Nitrate in the lake is responding not just to NO3 loading but also to oxidation of reduced forms of nitrogen delivered to the lake. The increasing [NO3]:[PO43] stoichiometric imbalance in this large lake is largely determined by these in-situ processes

Rates and controls of nitrification in a large oligotrophic lake

Recent discoveries have altered prevailing paradigms concerning the conditions under which nitrification takes place and the organisms responsible for nitrification in aquatic ecosystems. In Lake Superior, nitrate (NO-3) concentrations have increased fivefold in the past century. Although previous evidence indicated that most NO-3 is generated by nitrification within the lake, important questions remain concerning the magnitude and controls of nitrification, and which microbial groups are primarily responsible for this process. We measured water-column nitrification rates in the western basin of Lake Superior during five research cruises from November 2009 to March 2011. Using in situ bottle incubations at 10 depths, we quantified nitrification rates using both the oxidation of 15N-labeled ammonium (NH+4) and the uptake of 14C associated with nitrification. Average rates of NH+4 oxidation ranged from 18-34 nmol N L-1 d-1 across the five cruises, similar to values reported for the coastal ocean, and two orders of magnitude lower than values reported from other lakes. Low nitrification rates observed in the epilimnion corresponded to the absence of ammonium-oxidizing archaea and nitrite-oxidizing bacteria. The measured rates of nitrification are \u3e 50-fold greater than the long-term NO-3 rise in the lake, indicating that N is actively cycling and that long-term change in this ecosystem is mediated by internal dynamics. © 2013, by the Association for the Sciences of Limnology and Oceanography, Inc

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