Date of Award
Master of Science in Oceanography
Nutrient loading into coastal waterbodies causes excessive phytoplankton growth that increases organic matter supply to the benthos and decreases availability of dissolved oxygen. Areas of hypoxia (<2mg/l dissolved oxygen) form on seasonal timescales in coastal zones and alter microbial metabolisms in the benthic region. Nutrient efflux from sediment can serve as a major nutrient source for pelagic primary production in shallow coastal areas, creating positive feedbacks in eutrophication. Here, we examined benthic nutrient fluxes under hypoxic and oxic conditions in Greenwich Bay, RI, the Northern Gulf of Mexico, and through mesocosm manipulations using in situ benthic chambers. We also used sediment cores to further elucidate sediment nitrogen and phosphorus cycling processes. Phosphate efflux was elevated at some sites under hypoxia but not others, neither supporting nor refuting previous research that suggests phosphate movement in sediment is controlled primarily by redox conditions. In the mesocosm experiment, the sediment effluxed ammonium and consumed NOx at elevated rates under hypoxia, implying that hypoxia promoted organic matter remineralization, denitrification, and/or dissimilatory nitrate reduction to ammonium (DNRA). The effect of hypoxia on nitrogen flux in the two natural study sites was inconclusive: in August 2021, ammonium was released from sediments while in July of 2020, ammonium was consumed by sediment. Both were intermittently hypoxic periods in Greenwich Bay. Ammonium was also released during the oxic sampling period, October of 2020. In the Northern Gulf, the sediment released both nitrate and ammonium non-linearly with nearly no concentration change during the first few hours of all benthic chamber deployments but evidence of release into the chamber later in the deployments. Environmental variables, including the presence of microbial mats and macrofauna, likely created conditions for non-diffusive sediment efflux in the Gulf. In addition, the supply of labile organic matter and the strength and duration of hypoxia may have played a role in regulating sediment flux in all study sites.
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This work is licensed under a Creative Commons Attribution 4.0 License.
Mucci, Nicole, "Benthic nutrient fluxes response to seasonal hypoxia in coastal zones" (2023). Open Access Master's Theses. Paper 2431.
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