MARINE PLANT SUFFOCATION. “While CO2 can readily diffuse from the air through the stomata to the mesophyll cells in terrestrial plants, aquatic plants often have limited CO2 diffusion rates. Factors contributing to slow CO2 diffusion in aquatic plants (and especially in seagrasses) are thick boundary layers around the leaves that are sometimes amplified by the presence of unicellular or multicellular photosynthetic epiphytes that compete for CO2 and the low rate of CO2 transport in water.”
http://bmcevolbiol.biomedcentral.com/articles/10.1186/1471-2148-11-8