The influence of herbivores on Posidonia oceanica epiphytes

Abstract

The biomass seasonality of the seagrass Posidonia oceanica (L.) Delile epiphytes was examined at five sites in NE Spain to assess the relative importance of large-scale versus local factors in controlling the seasonal 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 among meadows and accounted for 38% of total variance. Most of this variability was explained by seasonal variability in Posidonia oceanica shoot size, as shown by the reduction in the variance explained by seasonality when shoot biomass of the host was used as a covariate. Differences among sites were also highly significant, both considered as differences 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 effect seemed to be mostly seasonal, thus contributing to the large-scale variability. In contrast, consumption by herbivores seemed to play a major role in inducing among-site variability of epiphyte biomass, but no similar role in influencing Posidonia oceanica shoot size. This is due to the within-shoot epiphyte distribution, such that the removal of relatively small amounts of leaf standing biomass (e.g. 10–20%) results in the removal of a large amount of epiphyte biomass (e.g. 40–50%). These findings support the hypothesis that epiphyte biomass is primarily 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.