Effects of in situ CO2 enrichment on the structural and chemical characteristics of the seagrass Thalassia testudinum

Seagrasses commonly display carbon-limited photosynthetic rates. Thus, increases in atmospheric p CO 2 , and consequentially oceanic CO 2(aq) concentrations, may prove beneficial. While addressed in mesocosms, these hypotheses have not been tested in the field with manipulative experimentation. This study examines the effects of in situ CO 2(aq) enrichment on the structural and chemical characteristics of the tropical seagrass, Thalassia testudinum . CO 2(aq) availability was manipulated for 6 months in clear, open-top chambers within a shallow seagrass meadow in the Florida Keys (USA), reproducing forecasts for the year 2100. Structural characteristics (leaf area, leaf growth, shoot mass, and shoot density) were unresponsive to CO 2(aq) enrichment. However, leaf nitrogen and phosphorus content declined on average by 11 and 21 %, respectively. Belowground, non-structural carbohydrates increased by 29 %. These results indicate that increased CO 2(aq) availability may primarily alter the chemical composition of seagrasses, influencing both the nutrient status and resilience of these systems.