Graduate Thesis Or Dissertation
 

Ecosystem Functions of Pacific Northwest Estuaries: The Role of Ocean and Watershed Drivers in Eelgrass and Coho Salmon Dynamics

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  • Interest is expanding for the potential role of estuaries, particularly seagrass and salt marsh habitats, to sequester carbon, mitigate ocean acidification, and support abundant fisheries. The important functions of estuaries are part of a broader set of ecosystem services, or benefits to humans, which are regulated by ocean and watershed dynamics. The aim of this dissertation is to investigate ocean and watershed dynamics to better understand the key drivers and consequences of ecosystem change in Pacific Northwest (PNW) estuaries. Chapters 2, 3, and 4 of my dissertation address gaps in our understanding of ecosystem change in PNW estuaries and improve our ability to address estuary management challenges in the face of climate and other human-driven changes. Chapter 5, although distinct from my other chapters, aligns with the theme of my dissertation, which is to understand the drivers and consequences of climate and human-driven changes for important coastal organisms and habitats. In Chapter 2, I investigate the relative role of ocean, estuary, and watershed drivers in determining macrophyte (eelgrass Zostera marina and ulvoid macroalgae) production among four PNW estuaries (Willapa Bay, WA, Netarts Bay, OR, Yaquina Bay, OR, and Coos Bay, OR) between 2006-2019. Significant environmental change has occurred over this time period, including the longest marine heatwave event (anomalously elevated water temperature) on record in the northeast Pacific Ocean. Substantial changes in macrophyte ecosystems were observed, however responses differed among the four estuaries. These findings provide a framework for predicting the response of macrophytes in PNW estuaries to environmental change and indicate that some estuaries may be more resilient to these changes than others. In Chapter 3, I investigate the drivers and consequences of recent declines in eelgrass and ulvoid macroalgae in South Slough, Coos Bay, Oregon via a record of macrophyte communities and environmental variables of ocean, estuary, and watershed factors from 2004-2019. Although it was not possible to reconstruct the exact mechanism for macrophyte loss, drivers included increasing temperature (water and air) and increases in watershed disturbance and turbidity in the ecosystem over the 15-year period. Consequences of the loss of macrophytes included an overall decline in ecosystem metabolism and reduced influence of macrophytes on water quality, including ocean acidification and hypoxia (pH and dissolved oxygen). Even when controlling for differences in macrophyte biomass, ecosystem metabolism and OAH mitigation potential differed between locations within the estuary. Changes in these relationships over time as macrophyte biomass declined shows that daytime amelioration of low pH is most achievable when macrophyte biomass is high. Therefore, the efficacy of macrophyte mitigation of ocean acidification depends on environmental conditions that favor sustained and highly productive eelgrass communities across the estuarine gradient. In Chapter 4, I focus on Oregon coast coho salmon (Oncorhynchus kisutch) as a beneficiary of productive PNW estuaries. Recent research suggests that coho salmon may rely more heavily on estuarine habitats than was previously thought but little is known about the relative role of estuary versus upland habitat for the production of coho salmon. To do this, I used multivariate statistics to explain variation in Oregon coast coho salmon production across 21 populations and 23 years (1994-2016). Of the proportion of variance in coho salmon production that was statistically explained, nearly 48% was associated with estuarine factors, including the area of emergent and tidal swamp wetlands. Comparatively, 51% of the variability in coho salmon production was associated with upland stream habitat metrics. This is the first study to quantify the importance of estuaries, and specific estuarine habitat types, for coho salmon and contributes to a greater understanding of how to prioritize conservation and restoration actions for salmon recovery. The final chapter of my dissertation (Chapter 5) was completed in partial fulfillment of the requirements for Oregon State University’s NSF Research Traineeship (NRT) program and the Risk and Uncertainty Quantification in Earth Systems minor. This study was the result of a collaboration between students from multiple disciplines (biological sciences, social science, and mathematics) aimed at understanding the effect of changing ocean conditions on Dungeness crab catch and the susceptibility of U.S. west coast port communities that participate in this lucrative fishery. We found that future sea surface temperature scenarios for 2080 reduced Dungeness crab catch by 30-100%, depending on the amount of warming, with greater declines at southern ports compared to northern ports. However, ports showed varying levels of risk (a combination of exposure and community susceptibility) that do not necessarily align with latitude or fishery management boundaries. The results of this chapter suggest that successful adaptive fishery management in a time of ocean change should rely on an understanding of projected fishery changes as well as the risk to coastal communities relative to the current and future ranges of the fisheries.
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  • Pending Publication
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  • 2020-09-20 to 2021-10-20

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