Spatial and seasonal variability of the macro-invertebrate community of a rocky coast in Mallorca (Balearic Islands): implications for bioerosion

The number of the main macro-invertebrate species grazing on the microalgal film on rocky coasts was evaluated at six closely located stations on Mallorca (Balearic Islands). The main bioerosive species at the sites studied were Melaraphe neritoides (L., 1758), Patella rustica (L., 1758), Monodonta turbinata (Born, 1780) and Lepidochitona corrugata (Reeve, 1848). The transects considered shared the same general environmental conditions and species pool. The aim of this study was to compare the effects exerted by the vertical gradient with those exerted by other sets of environmental variables. These sets were, namely, (1) inter-seasonal differences, (2) inter-transect differences (comprising degree of wave-exposure and lithological differences) and (3) effects of the micro-morphology (i.e., crevices, basin pools and other small-scale structures). The main null hypothesis verified was that species composition remains constant after assessing the effect of the vertical gradient. This hypothesis is largely rejected here. The main factor modelling the species composition was the vertical gradient (accounting for 31.6% of variability) whereas the percentages of variability yielded by the other sets individually were smaller but significant (P<0.001). Inter-transect differences (including degree of wave exposure and lithology) accounted for 24.9% of the variability. Seasonal differences accounted for 6.3%, small-scale morphology for 4.5%, and wave-height for 1.6%. The main conclusions obtained from these results are that the most basal strip of the coastline undergoes the largest bioerosive rates. Similarly, the sites exposed to wave action will undergo a larger bioerosive impact than the sheltered sites (possibly because they are more damp). These biotic and abiotic effects taken together would result in a maximisation of erosion rates in areas that are porous, exposed and located near the sea.