Date of Award

Spring 2024

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Ocean & Earth Sciences

Program/Concentration

Oceanography

Committee Director

Richard C. Zimmerman

Committee Member

Victoria J. Hill

Committee Member

David J. Burdige

Committee Member

Gary Schafran

Abstract

Seagrasses face vulnerability to both global stressors like Ocean Acidification (OA) and climate warming compounded by local stressors such as eutrophication that reduces light availability, leading to a complex dynamic of positive and negative effect on their growth and survival. Increased dissolved aqueous CO2 (CO2(aq)) benefits seagrasses by enhancing photosynthetic and growth rates, but it may increase nutrient demand, potentially depleting nutrient supply, especially in oligotrophic environments.

In this study, the long-term impact of CO2 on Zostera marina L. (eelgrass) were investigated across a gradient of CO2(aq) concentrations (55 – 2200 µM CO2(aq)) and leaf area indices (LAI). The focus was on quantify changes in carbon (C) and nitrogen (N) content, composition, and metabolism in response to CO2-stimulated photosynthesis and growth. Absolute growth rates, shoot sizes, leaf density, sucrose concentrations, and carbon and nitrogen growth demands increased with increasing CO2(aq) availability. However, there was no notable decline in biomass-specific N content of leaf at higher CO2(aq) concentrations primarily due to dilution effects caused by carbon accumulation in thicker leaves, rather than N-limitation. Rather than increasing plant survival in the context of CO2(aq) enrichment, this study found that nutrient enrichment of the sediment reduced plant survival as a result of NH4 + toxicity. In contrast, the high H2S concentrations reaching millimolar levels in sediment, which was stimulated by organic carbon addition, was not particularly toxic to eelgrass.

CO2 effects on N uptake were complicated by changes in canopy architecture due to increasing leaf area index (LAI), affecting N uptake patterns of leaves and roots. Combining a nutrient uptake model with the radiative transfer GrassLight (v2.14) model, this study explored how CO2-driven photosynthesis affected N uptake patterns and requirements for growth. Model predictions across varying LAIs and CO2(aq) concentrations indicated low N demand for eelgrass under all CO2(aq) conditions, with roots playing a key role in N acquisition as CO2(aq) concentrations increased. Overall, my results highlight the importance of photosynthesis in regulating N metabolism and acquisition between the above- and belowground components, and suggest that most eelgrass meadows are unlikely to experience N limitation, even in high CO2(aq) environments.

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DOI

10.25777/e9hb-x931

ISBN

9798382772417

ORCID

0009-0007-4659-0922

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