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

Winter limnology on the Great Lakes: The role of the U.S. Coast Guard

We describe a unique educational collaboration between the U.S. Coast Guard and Bowling Green State University with the goal of engaging Coast Guard crew members in a monitoring program intended to increase both temporal and spatial resolutions of sampling during winter in Lake Erie, a period where extreme weather conditions and safety considerations challenge our ability to sample. © 2010 International Association for Great Lakes Research

Collecting winter data on U.S. Coast Guard icebreakers

Winter research and monitoring of icebound rivers, lakes, and coastal seas to date has usually involved seagoing civilian scientists leading survey efforts. However, because of poor weather conditions and a lack of safe research platforms, scientists collecting data during winter face some difficult and often insurmountable problems. To solve these problems and to further research and environmental monitoring goals, new partnerships can be formed through integrating efforts of the U.S. Coast Guard (USCG) with citizen science initiatives. USCG and a research group at Ohio\u27s Bowling Green State University are entering the third year of such a partnership, in which icebreaking operations in Lake Erie using USCG Cutter Neah Bay support volunteer data collection. With two additional USCG vessels joining the program this winter season, the partnership serves as a timely and useful model for worldwide environmental research and monitoring through citizen science and government collaboration

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

Evidence against fluvial seeding of recurrent toxic blooms of Microcystis spp. in Lake Erie\u27s western basin

For almost two decades, the western basin of Lake Erie has been plagued with recurring toxic algal blooms dominated by the colonial cyanobacterium, Microcystis spp. Since the Maumee River is a major source of nutrients and sediment inputs into the lake, and Microcystis spp. has been identified as a member of the upstream river algal assemblage, the possibility exists that the river Microcystis species serve as a seed population for the toxic blooms occurring in the lake. Genetic profiling of toxic cyanobacteria using the microcystin synthesis gene, mcyA, clearly indicates that the toxic cyanobacteria of the river are distinct from the toxic Microcystis spp. of Lake Erie. Indeed, mcyA sequences are almost exclusively from toxic Planktothrix spp., similar to what has been documented previously for Sandusky Bay. UniFrac statistical analysis of cyanobacterial community composition by comparison of 16S-23S ITS sequences also show that the Maumee River and Lake Erie communities are distinct. Overall, these data show that despite the importance of nutrient inputs and sediments from the river, the toxic cyanobacterial blooms of Lake Erie do not originate from toxic species endemic to the Maumee River and instead must originate elsewhere, most likely from the lake sediments. © 2011 Elsevier B.V