Seasonal response of Posidonia oceanica epiphyte assemblages to nutrient increase

The effect of nutrient availability on epiphyte assemblages on the leaves of the seagrass Posidonia oceanica was investigated in an oligotrophic meadow, paying attention to the species composition. The experiment consisted of a year-long monthly column water nutrient addition experiment. Nutrient additions resulted in modest effects on epiphytic biomass (increases only detected in August and September), but in large modifications in the structure of epiphytic assemblages from June to September, with an initial decrease in bryozoan cover (Electra posidoniae) and a stimulation of phaeophytes (Sphacelaria cirrosa, Dictyota spp.) and some Ceramiaceae in summer months. Other variables, such as epiphyte cover, epiphyte nitrogen content, and the algal epiphyte to zooepiphyte ratio also responded to nutrient enrichment during summer, but showed different responses depending on epiphyte composition. In contrast, winter assemblages, largely represented by zooepiphytes and encrusting algae, did not respond in any way to the nutrient treatment. Results support the notion that in oligotrophic temperate systems epiphyte community composition is controlled by nutrient availability from late spring to the end of summer, not only in quantitative, but also in qualitative terms.