Theses and Dissertations

Date of Award

5-2022

Document Type

Dissertation

Degree Name

Ph.D.

Department

Marine Sciences

Committee Chair

Kelly M. Dorgan, PhD.

Abstract

Shallow coastal sediments are sites of intense respiration and organic matter breakdown. Macroinfauna bioturbate and bioirrigate sediments which supplies microbes with oxygen and newly deposited organic material from surface sediments, facilitating microbial remineralization of organic matter. These processes depend heavily on the concentration of dissolved oxygen in overlying water. Shallow water oxygen patterns often follow a diel cycle as dissolved oxygen decreases at night due to respiration and then increases during the day with photosynthesis, creating recurring suboxic conditions that are potentially stressful to organisms. Sediment oxygen flux is known to depend on ambient dissolved oxygen concentration, but behavioral responses of macrofauna to low oxygen can be complex and diverse, introducing variability into sediment metabolism rates. This dissertation research examined the effects of diel changes in dissolved oxygen on macrofaunal behavior and activities and corresponding changes in sediment metabolism throughout the diel cycle. I constructed a simple laboratory system to manipulate dissolved oxygen concentrations into a diel pattern and exposed sediment infauna to repeated diel oxygen cycles. Sediment mixing in all three of the tested taxa decreased overall throughout the experiment and over two diel cycles, but also varied proportionally with oxygen within each diel exposure. Behaviors did not show significant variation with the diel cycle, though this is likely because behaviors relevant to sediment x mixing activity were not easily detected or quantified with the employed methods. These results indicate that experiments quantifying sediment mixing by macrofauna that occur in fully oxygenated conditions may not be representative of in situ rates, and that it may require more even than a single diel cycle for representative rates to emerge. To better understand how natural macrofaunal assemblages affect sediment metabolism when exposed to diel cycling oxygen, I conducted a field sediment metabolism experiment. Flow-through sediment metabolism chambers were constructed and deployed to measure in situ sediment oxygen consumption. The presence of macrofauna drove overall greater and more variable rates sediment oxygen demand particularly at night, presumably due to fauna responding to low oxygen by increasing their irrigation activity. This research shows that in coastal sediments, variation on small temporal and spatial scales interact to affect sediment metabolism. Sediment metabolism, a key ecosystem function, is controlled by complex networks of interactions and feedbacks between biogeochemical and ecological processes. This research sheds new light on the connection between oxygen concentration and oxygen consumption in these dynamic, productive marine systems and improves our understanding of the role of macrofauna in modulating that relationship.

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