Water Resources Research Act Program

Details for Project ID 2018OH564B

Maumee River sediments as a nitrogen source or sink to Lake Erie: the competing roles of ammonium recycling and denitrification

Institute: Ohio
Year Established: 2018 Start Date: 2018-03-01 End Date: 2019-02-28
Total Federal Funds: $39,998 Total Non-Federal Funds: $53,784

Principal Investigators: Mark McCarthy

Project Summary: Since the mid-1990s, Lake Erie has experienced toxic cyanobacterial blooms due to high nutrient (nitrogen and phosphorus) inputs from its tributaries. These blooms negatively affect aquatic life, pets and livestock, and humans, as evidenced by the shutdown of the City of Toledo’s water treatment plant in August 2014. These blooms differ from those observed in the 1960s and 1970s, prior to phosphorus (P) loading reductions, in that the organism most commonly responsible for algal blooms in the western basin of Lake Erie (Microcystis) cannot fix atmospheric nitrogen (N). The Maumee River has the largest drainage area into the western basin of Lake Erie and is most responsible for high N and P loads to Maumee Bay and the western basin. In summer 2017, as in some previous years, a significant Microcystis bloom was observed within the river prior to discharge into Lake Erie (Toledo Blade; Sept 23, 2017). The Maumee River watershed is highly agricultural, and fertilizers, manure, and drainage practices contribute to high nutrient loads. Studies evaluating the amount and form of P and phytoplankton in the Maumee River and Lake Erie have been conducted extensively in recent years (e.g., Michalak et al. 2013), but N studies are comparatively rare. Ammonium (NH4+) may be the key N form for harmful cyanobacterial blooms (e.g., Monchamp et al. 2014), but it is challenging to measure accurately because of insufficient water sampling and handling procedures, which allow rapid microbial cycling processes to continue occurring in water samples between sample collection and filtration or analysis (McCarthy et al. 2013). We will quantify removal and recycling pathways of both nitrate (NO3-) and NH4+ in Maumee River sediments to determine whether the sediments are a N source or sink to the water column. We will determine net sediment NH4+ fluxes (possible source or sink), denitrification and anammox (sinks), dissimilatory NO3- reduction to NH4+ (DNRA; possible recycling source), heterotrophic N fixation (source), and net phosphate (PO43-) fluxes at the sediment-water interface. Sediment cores will be collected at four sites in the Maumee River between Antwerp and Maumee in May (or earlier if feasible), June, July, August, and September 2018 to span runoff and discharge variability over the course of the season. Core incubations from each site will be conducted in a continuous-flow system with duplicate cores of either no isotope addition (unamended control), a 15NO3- stable isotope tracer, or a 15NH4+ tracer. From the 15NO3- addition, denitrification and heterotrophic N fixation rates will be calculated from 15N-N2 production patterns, and potential DNRA will be determined as 15NH4+ production. Using the 15NH4+ tracer, anammox and NH4+ uptake and regeneration rates will be determined. Net oxygen, inorganic (NO3-, NO3-, NH4+) and organic (urea) N, and PO43- fluxes will be measured in all cores. From these measurements, we will evaluate the natural capacity of Maumee River sediments to remove excess N, possibly helping mitigate eutrophication, versus N being recycled within the system, possibly exacerbating eutrophication. These results are important for Lake Erie and Maumee River watershed managers and will help determine the role of riverine N cycling in promoting, or possibly mitigating, harmful algal blooms in Lake Erie.