ABSTRACT:

CO₂ is a critical and potentially limiting substrate for photosynthesis of both terrestrial and aquatic ecosystems. In addition to being a climate-warming greenhouse gas, increasing concentrations of CO₂ will dissolve in the oceans, eliciting both negative and positive responses among organisms in a process commonly known as ocean acidification. The dissolution of CO₂ into ocean surface waters, however, also increases its availability for photosynthesis, to which the highly successful, and ecologically important, seagrasses respond positively. Thus, the process might be more accurately characterized as ocean carbonation. This experiment demonstrated that CO₂ stimulation of primary production enhances the summertime survival, growth, and proliferation of perennial eelgrass Zostera marina from the Chesapeake region, which is regularly impacted by summer heat stress. The experiment also quantified the logarithmic response to CO₂ in terms of shoot proliferation, size, growth and sugar accumulation that was fundamentally consistent with model predictions based on metabolic carbon balance derived from short-term laboratory experiments performed with other eelgrass populations from cool ocean climates and other seagrass species from tropical and temperate environments. Rather than acting in a neutral fashion or as an independent stressor, increased CO₂ availability can serve as a quantitative antagonist to counter the negative impact of climate warming on seagrass growth and survival. These results reinforce the emerging paradigm that seagrasses are likely to benefit significantly from a high-CO₂ world.