Graduate Thesis Or Dissertation
 

Juvenile Pacific Oyster Growth and Food Resources in Eelgrass Habitats of Variable Shoot Density in Tillamook Bay, OR: Ecosystem Interactions for Management Consideration

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https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/mc87pz69b

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  • As the global demand for seafood increases each year, there is a growing need to expand sustainable aquaculture in the interests of food security and resilience of coastal communities. Pacific oysters, Crassostrea gigas, are grown in West coast estuaries - where cultures often overlap with the federally protected native eelgrass, Zostera marina. Understanding how these two ecosystem engineers interact can inform potential tradeoffs among aquaculture and protected species management. This study explores multiple factors that may influence juvenile Pacific oyster growth, including eelgrass density, water quality, and availability of food resources. During the summer of 2021, juvenile oysters were out planted in intertidal habitats ranging from unvegetated mudflat to dense eelgrass vegetation in Tillamook Bay, Oregon, and shell growth rate over the season was calculated. Short-term high frequency time series of water quality metrics (pH, temperature, salinity, turbidity, chlorophyll-a) in dense eelgrass and unvegetated habitat were integrated and compared to evaluate eelgrass effects on water conditions. Bi-monthly discrete water samples were collected in areas of unvegetated, sparse, and dense eelgrass and suspended particulate matter (SPM), particulate organic matter (POM), and chlorophyll-a concentrations were used to characterize food quality and quantity within habitats. Stable isotope values (δ15N and δ13C) measured on oyster tissue were compared to determine if food oyster diet and food sources differed between habitats. Eelgrass shoot density was a significant predictor of oyster shell growth rate, accounting for 7% of the variation, while site was responsible for 55% of the variation. No interaction was present between site and eelgrass density, indicating that both variables influenced oyster growth independently and the effect of eelgrass density was homogenous across the estuary. No evidence of sustained pH buffering was found in eelgrass habitat relative to unvegetated habitat during 4-6 day deployments, but eelgrass consistently influenced turbidity and chlorophyll-a, with site and tidal variation observed in time series. Food concentrations (POM and chlorophyll-a) and seston (SPM) increased with eelgrass density and were highest in sites with intermediate shoot densities ranging from 40-78 shoots/m2. Oyster tissue enrichment in δ15N and δ13C increased with eelgrass density, indicating that oysters in eelgrass consumed different food sources than oysters in unvegetated habitat. Differences in temperature, turbidity, barnacle recruitment, food resources, and water movement likely also influenced differences in oyster growth in addition to eelgrass presence and shoot density. Nonetheless, eelgrass clearly plays an additive role in altering habitat conditions suitable for co-located oysters. Results suggest oysters can benefit from conditions mediated by eelgrass, but there is a density-dependent and context-specific constraint on positive interactions to consider in aquaculture management that requires further research.
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  • Pending Publication
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  • 2023-01-08 to 2024-02-08

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