Spatio-temporal variation in macrofauna community structure in Mediterranean seagrass wrack

Many Mediterranean beaches accumulate wrack of Posidonia oceanica L. leaves and rhizomes, creating an ecotone between marine and terrestrial systems that provides essential ecosystem services (e.g. shoreline protection, nutrient supply to adjacent dune systems). However, little is still known about the diversity of macrofauna that lives in this wrack and how its community structure varies in space and time. In this study, we assessed the variation in macrofauna composition on three Mallorcan beaches (Balearic Islands, Western Mediterranean Sea) characterized by different geomorphological features. For an entire year, we seasonally sampled P. oceanica wrack on each beach and identified the animal species living in them. We found up to 20 different species belonging to four functional groups: (A) detritivores/fungivores/scavengers, (B) phytophagous, (C) secondary consumers, and (D) tertiary consumers. Species composition and diversity indices showed strong differences among beaches associated with wrack dynamism. Both variables revealed also a significant temporal variation, with highest individual abundance in summer and lowest in winter. Detritivores were the most abundant group, and their contribution to community structure represented over 75% through the study period. Springtails, in particular, contributed the most (45%) to spatial and temporal changes in community structure, followed by the oligochaete Enchytraeus sp. (12.6%) and the sand hopper Talitrus saltator (4.6%). Our study builds towards a more robust understanding of these marine-terrestrial ecotones, demonstrating the existence of complex and dynamic macrofauna communities which can provide more ecological information about the food webs of these ecosystems and the services they provide.