THE INFLUENCE OF HERBIVORES ON POSIDONIA-OCEANICA EPIPHYTES

The biomass seasonality of the seagrass Posidonia oceanica (L.) Delile epiphytes was examined at five sites in NE Spain to assess the relati ve importance of large-scale versus local factors in controlling the s easonal pattern observed. Large-scale seasonal forcing, resulting from changes in light and temperature associated with the solar cycle, was assessed from the coherence of seasonal epiphytic biomass changes amo ng meadows and accounted for 38% of total variance. Most of this varia bility was explained by seasonal variability in Posidonia oceanica sho ot size, as shown by the reduction in the variance explained by season ality when shoot biomass of the host was used as a covariate. Differen ces among sites were also highly significant, both considered as diffe rences in yearly biomass values (accounting for 30% of total variance) and as the differences in the annual time course of epiphyte biomass among meadows, which accounted for 26% of the variability. Nutrient ef fect seemed to be mostly seasonal, thus contributing to the large-scal e variability. In contrast, consumption by herbivores seemed to play a major role in inducing among-site variability of epiphyte biomass, bu t no similar role in influencing Posidonia oceanica shoot size. This i s due to the within-shoot epiphyte distribution, such that the removal of relatively small amounts of leaf standing biomass (e.g. 10-20%) re sults in the removal of a large amount of epiphyte biomass (e.g. 40-50 %). These findings support the hypothesis that epiphyte biomass is pri marily controlled by seasonal changes in seagrass shoot size and hence ultimately by the solar cycle and secondarily by local environmental changes, among which herbivory seems the most important.