Impact of climate change on carbon and nitrogen balance in Zostera Marina L. (Eelgrass)

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.

Jinuntuya M., 2024. Impact of climate change on carbon and nitrogen balance in Zostera Marina L. (Eelgrass). PhD Thesis, Old Dominion University. 200 p. Thesis.

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