Collaborative Research: Sources and Sinks of Stoichiometrically Imbalanced Nitrate in the Laurentian Great Lakes

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This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5). Over large scales encompassing heterogeneous conditions, biogeochemical mechanisms act to achieve a stoichiometric balance between nitrogen and phosphorus. Locally, however, imbalances can develop. The Laurentian Great Lakes are a vast freshwater system where nitrate has been steadily accumulating for decades. Previous work has shown that in Lake Superior, the headwaters of the system, nitrate enters the lake water primarily due to in-lake biogeochemical processes, not due to passive accumulation of nitrate as a conservative substance as previously believed. An extreme stoichiometric imbalance of nitrate/phosphate ratios (~ 10,000 by moles) is present and is apparently growing. This set of prior findings opens up two major questions. First, what are the principal biogeochemical control points that tip the N cycle toward buildup of excess nitrate? And second, how does the extreme stoichiometric imbalance affect the ecology and evolution of Lake Superior's biota? In this project, researchers at the University of Minnesota - Twin Cities and the Bowling Green State University, who previously documented the nitrate buildup in Lake Superior, will continue their research program and address these two questions. The project is organized around making comparative measurements of N assimilation, nitrification, denitrification, anammox, and microbial community structure in Lake Superior and in the central basin of Lake Erie. These two environments differ greatly in many ways including redox state and organic carbon production rates. From the standpoint of N balancing mechanisms, they can be considered end members within the Laurentian Great Lakes. Lake Superior contains an endemic assemblage of heterotrophic and autotrophic microbes, whereas the comparable organisms in Lake Erie and other Great Lakes represent cosmopolitan clades. Connections between the extreme stoichiometric imbalance in Lake Superior and the unique microbial assemblage are unknown but will be examined in this study. Additional data will be collected across a larger region of the Upper Great Lakes including Lake Huron and northern Lake Michigan. Up-to-date mass balance budgets of nitrogen of the most of the Great Lakes (Lake Superior is already done) will be constructed and linked with hydrologic fluxes to gain insights into the dynamics of N across the entire Laurentian Great Lakes System. Observations of water chemistry will be made with ship-board sampling together with field-deployed nitrate sensors in shallow and deep waters. Process studies will be performed in the water column and at the sediment-water interface and will involve sensitive stable isotope techniques. These will include measurements of NO3 and NH4+ uptake into different size fractions, exchanges of different forms of N and C between the water column and sediments, nitrification, denitrification, and anammox. The diversity and abundance of ammonia oxidizing Archea (AOA) and bacteria (AOB) will be studied using quantitative real time PCR and DGGE. Similarly, the genetic composition of denitrifyers and anammox bacteria will be studied to see if they too are represented by novel clades in Lake Superior. Cultured nitrifyers will be characterized in terms of growth under different conditions typically encounterd across the Great Lakes. The project will yield valuable information and insight into the operation of the nitrogen cycle under conditions that promote stoichiometric imbalances. Broader Impacts: The Laurentian Great Lakes are a valuable regional resource and an immense reservoir of planetary fresh water. Lake Superior is often considered to be relatively pristine but the ultimate source of the N converted to nitrate in the lake is as yet unknown and may involve past changes to the watershed or other anthropogenic factors. This project will support the training of a Postdoctoral researcher and graduate students both in Minnesota and in Ohio. The student line allocated to BGSU will be reserved for a graduate of the BGSU-Lorain County Community College (LCCC) University Partnership B.S. in Biology program, an innovative program created to provide the residents of Lorain and surrounding counties, which form part of Ohio's economically-depressed "rust belt", a chance to enroll in bachelors programs while attending LCCC. Students enrolled in the University Partnership participate in NSF-funded undergraduate research and this project will afford an opportunity for a LCCC student, many of whom are considered "non-traditional", to work towards earning a graduate degree.