Epiphyte response to in situ manipulation of nutrient availability and fish presence in a Posidonia oceanica (L.) Delile meadow

Abstract

Epiphytes are important components of the biomass and productivity of Posidonia oceanica meadows. Bottom-up mechanisms may promote epiphyte biomass through a bottom-up mechanism. At the same time, epiphytes represent an essential resource for higher trophic levels of seagrass food webs. Posidonia oceanica meadows host a diverse assemblage of fish that feed directly on the leaves, on the epiphytes, and on the mesograzers inhabiting the meadows. In this study, we experimentally evaluate the overall effect of fish community and increased water column nutrient availability on seagrass and the associated epiphytes. Our results show a large increase of epiphyte biomass in P. oceanica shoots after 2 months of nutrient addition. Increased nutrient concentrations also resulted in a reduction of shoot size. Fish exclusion did not affect epiphyte biomass under ambient or increased nutrient availability. Although herbivorous fish bites were present in 6% of the shoots, the percentage of shoots with fish bites did not respond to the higher leaf nutrient content found in the increased nutrient treatments. Consumption marks of gastropod herbivores in the leaves were present in 78% of the shoots; however, grazer activity did not modify the response of epiphytic biomass to nutrient addition. These results highlight the importance of nutrient impact in oligotrophic meadows, where bottom-up processes seem to be more relevant in the control of epiphyte and seagrass growth than top-down processes.