Publication Date

2018

Document Type

Thesis

Committee Members

Silvia Newell (Advisor), Mark McCarthy (Committee Member), Chad Hammerschmidt (Committee Member), Justin Chaffin (Committee Member)

Degree Name

Master of Science (MS)

Abstract

Lake Erie experiences annual cyanobacterial harmful algal blooms (HABs), comprised mostly of non-nitrogen-fixing Microcystis, due to excess nitrogen (N) and phosphorus (P) inputs (eutrophication). Lake Erie's watershed is mostly agricultural, and fertilizers, manure, and drainage practices contribute to high nutrient loads. This study aimed to clarify the role of Lake Erie sediments in either exacerbating or mitigating conditions that fuel HABs via recycling and/or removal, respectively, of excess N and P. Sediment-water interface N dynamics were evaluated in low HAB (2016, dry) and high HAB (2017, wet spring and early summer) years. Intact sediment cores and overlying water were collected in the western basin of Lake Erie during the ice-free seasons in 2016 and 2017. Cores were incubated in a continuous-flow system with either no isotope addition (unamended control), a 15NO3- tracer, or a 15NH4+ tracer to help distinguish between N sinks (denitrification and anammox), N links (e.g., DNRA), and N sources (N2 fixation). Sediments were a net source of NH4+ (29.4 ± 7.41 μmol N m-2 hr-1) and ortho-phosphate (2.19 ± 0.52 μmol P m-2 hr-1) to the water column across all sampling sites and times. Net N2 fluxes reflected net N2 fixation (i.e., N2 influx) in spring 2016 and 2017, then switched to net denitrification (i.e., N2 efflux), the primary N removal mechanism, later in the season. On average, western basin sediments were a net N sink (-77.6 μmol N m-2 hr-1), suggesting that the sediments perform a valuable ecosystem service. Extrapolated to the entire western basin surface area, and considering Maumee and Detroit River discharges, western basin sediments can remove approximately 28.9 percent of the average annual total N load. However, denitrification rates were lower during the more severe bloom year (2017) than in 2016, suggesting that large blooms, similarly to high N loading, can inhibit the capacity of sediments to perform that ecosystem service. To mitigate HABs in eutrophic systems, such as Lake Erie, management efforts emphasizing reduction of non-point, agricultural sources of both N and P are necessary because high nutrient loads support larger, more toxic HABs, which limit the capacity of sediments to remove N via denitrification

Page Count

109

Department or Program

Department of Earth and Environmental Sciences

Year Degree Awarded

2018

ORCID ID

0000-0003-1667-3990

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