Combined Impacts of Climate Warming and Ocean Carbonation on Eelgrass (Zostera marina L.)

The past few decades have accumulated mounting evidence of profound anthropogenic effects on fundamental biogeochemical processes across the planet, especially in coastal environments that support a diverse array of highly productive ecosystems including coral reefs, seagrass meadows, and estuaries.  The ecological significance of seagrasses is largely due to the remarkable degree of adaptation they exhibit to a submerged aquatic existence.  Despite numerous successful adaptations, however, seagrasses have high light requirements that make them vulnerable to anthropogenic disturbances.  The paradoxical vulnerability results largely from their high reliance on dissolved aqueous CO₂ for photosynthesis.  The potential for rising atmospheric CO₂ concentrations to have significant warming impacts on the global climate has long been recognized, but the potential impacts of the “other CO₂ problem”, also known as ocean acidification, have only recently begun to be appreciated.  As with other impacts of climate change, the increased concentrations of dissolved aqueous CO₂ [CO_2(aq)] in the oceans of the world will elicit both negative and positive responses among organisms, ultimately potentiating ecological losers and winners.  This work explores the response of eelgrass to increased CO_2(aq) within the context of a warming coastal ocean using a combination of manipulative experiments, physiological/biochemical investigations and mathematical modeling.  Results indicate that rising CO_2(aq) will increase the high temperature tolerance of plants by improving the Q₁₀ response of photosynthesis relative to respiration, thereby leading to higher growth rates, improved survival of vegetative shoots at high temperature, and even flowering output and seed production.  By focusing on key relationships between environmental parameters that have both negative (ocean warming) and positive (ocean carbonation) impacts on the light requirements and dynamics of carbon balance in these critically important marine angiosperms, we gain predictive insight into how climate change may alter the geographic distribution of this critically important species in a variety of coastal environments that may be subjected to different levels temperature stress combined with ocean carbonation.