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Loyola University Chicago

Department of Biology

Michael Hassett Thesis Defense: Project Description

Title: The influence of eastern oyster (Crassostrea virginica) reef restoration on nitrogen cycling in a eutrophic estuary.


Oysters (Crassostrea virginica) are functionally extinct in the urbanized Hudson-Raritan estuary (HRE) in New York City, however, oyster restoration is promoted to mitigate nitrogen (N) pollution via oysters’ filtration and excretion. Seasonally, I took 12 sediment cores (45 cm2) adjacent to and 10 m away from a recently constructed reef in the HRE.  Cores were incubated in flow-through chambers with site water containing (1) no amendments, (2) 15N-ammonium, or (3) 15 N-nitrate, from which I calculated coupled nitrification-denitrification and direct denitrification as isotope-enriched N2. Coupled denitrification was minimal at all sites while direct denitrification was elevated near the reef, suggesting organic matter in oyster waste stimulated direct denitrification of water column nitrate, however, high variability in field samples precluded statistical significance. To further investigate these effects, I designed a laboratory study to test how oysters influence direct and coupled denitrification in oligotrophic and eutrophic sediments, creating a 2x2 aquaria testing matrix with oyster presence and trophic state as factors while repeating the isotope treatments and sediment cores methods used in the field study. Overall, oyster presence increased direct denitrification, but caused only small changes in coupled denitrification, suggesting that oyster reef restoration may be useful for removing nitrate from coastal ecosystems.



This research was funded by National Science Foundation DEB 0918152 to Timothy Hoellein and Chester Zarnoch, and by the Hudson River Foundation to Timothy Hoellein, Chester Zarnoch, Denise Bruesewitz, Brett Branco, and Wayne Gardner. Thank you to my thesis adviser Timothy Hoellein and committee members Martin Berg, John Kelly, and Christopher Peterson for their comments on this thesis. Thanks also to Steven Polaskey, Matthew Girard, the Hoellein Lab at Loyola University Chicago, and Christopher Calderaro for their assistance in the laboratory. Thank you to Joseph Schluep, Allison Mass Fitzgerald, Doris Law, and Samantha Lindemann for their assistance in this research.  Finally, thank you to my family and friends for encouraging me throughout the thesis writing process. 



Michael Hassett was born and raised in southeast Michigan.  He attended Michigan State University in East Lansing, MI and earned a Bachelor of Science degree in Fisheries and Wildlife in 2009 with classwork focusing on fisheries and aquatic ecology. While an undergraduate, he was selected for a study abroad program in Antarctica and a NSF-REU summer fellowship at Fordham University in Armonk, NY. After graduating, he worked as an intern at the Arizona Game and Fish Department in Pinetop, AZ and at the U.S. Fish and Wildlife Service's Fish Technology Center in Warm Springs, GA. At Loyola, Michael taught introductory biology laboratory classes in the Department of Biology.


Committee Members 

Dr. Timothy Hoellein 

Dr. Martin Berg

Dr. John Kelly

Dr. Christopher Peterson





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