In situ CO2 enrichment alters the nutrient and carbohydrate content of seagrasses: Parallels to terrestrial research

Seagrasses commonly display carbon-limited photosynthetic rates. Thus, increases in atmospheric pCO₂, and consequentially oceanic CO₂(aq) concentrations, may prove beneficial. While ex situ mesocosm experimentation has provided some evidence for this, we know relatively little in regards to in situ responses, stressing the importance of field experimentation. This study examines the effects of in situ CO₂(aq) enrichment on the widely distributed tropical marine angiosperm, Thalassia testudinum. We utilized a series of clear, open-top chambers to continuously manipulate seawater carbonate parameters around a shallow seagrass bed, and replicate CO2(aq) forecasts for the year 2100. After 6 months, seagrass growth responses to CO₂(aq) enrichment were relatively weak, while plant nitrogen (N) and phosphorus (P) content strongly declined, increasing leaf C:N and C:P ratios. Elevated CO₂(aq) additionally increased rhizome non-structural carbohydrates (NSC). Our results follow trends documented in terrestrial plants, and suggest that seagrasses stand to undergo a number of physiological and ecological changes under CO₂(aq) enrichment, altering the future functionality of these systems.

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