What are epiphytes and how do they affect seagrasses?

Epiphytes and epibionts are a diverse and dynamic community of organisms that grow on and around seagrass leaves. They primarily consist of micro- and macroalgae, but also include invertebrates such as sponges, crustaceans, barnacles and spirorbid (coiled tube) worms. In healthy seagrass beds unaffected by nutrient pollution, epiphyte loads are low and generally consist of calcareous algae, diatoms, and spirorbid worms. Epiphytic algae require nutrients and photosynthesize just like seagrasses. As nutrient pollution increases, the amount of macroalgal epiphytes present on seagrass leaves increases as well. Older seagrass leaves tend to be more heavily epiphytized since there has been more time for the epiphytes to establish and grow. Heavy epiphyte loading is detrimental to seagrasses by reducing the quantity and quality of light available to photosynthetic tissues in the seagrass leaves, which can limit growth. Grazing by invertebrate herbivores is a primary mechanism at limiting epiphyte growth on seagrasses; a reduction in epiphyte grazers results in heavier epiphyte loads than excess nutrient input would. Algal epiphytes are a main food source for small invertebrates and the base of the food web for this important ecosystem.