Date of Award


Degree Type


Degree Name

Master of Science in Biological and Environmental Sciences (MSBES)


Cell and Molecular Biology


Cell & Molecular Biology

First Advisor

Bethany D. Jenkins


Global climate and environmental changes have led to declines in oxygen content in both marine and fresh water systems globally. Hypoxia (dissolved oxygen (≤2 mg/L) is enhanced at the sediment water interface, where organic matter and the microbial communities it supports accumulate and boost respiration rates. When dissolved O2 concentrations fall below the threshold for hypoxia, bacterial communities shift to reflect species better suited to low oxygen conditions. Many of these microbes participate in metabolisms that affect water column nutrient cycles by regulating exchange across the sediment-water interface. Understanding community shifts that may impact nutrient cycling and how quickly they occur will inform how feedbacks associated with reduced O2 availability might further contribute to or alleviate hypoxic conditions. This information will help complete the picture for resource managers determining how to mitigate the system-wide impacts of hypoxia. This study addresses 1) how hypoxia impacts benthic microbially-driven nutrient cycling and 2) how hypoxia impacts benthic bacterial communities overall, along with communities of nitrogen-fixing bacteria. The estimated flux of nitrate + nitrite from the sediment into the water column were low to negligible during hypoxia (below detection) when compared to normoxia (0.334 mmol/m2*day), likely due to upregulation of nitrification. However, ammonium flux from sediments was lower during hypoxia (concentrations in the overlying water were below detection limit) than normoxia (0.432 mmol/m2*day), which runs counter to previous work both within Narragansett Bay and in other estuaries. 16S and nifH communities from the same station collected during and after hypoxic episodes in Greenwich Bay, RI, USA, were statistically different (p = 0.046; p = 0.043), with a correlation of 0.652 to measured environmental variables. The conditions associated with hypoxia changed the sedimentary bacterial communities and their functioning, which may have altered the exchange of nutrients with the water column. The opposite response of ammonium to previous observations requires further work to determine additional controls on nutrient fluxes under changing oxygen conditions.

Available for download on Friday, January 12, 2024