Sessile macrobenthos (Ochrophyta) drives seasonal change of meiofaunal community structure on temperate rocky reefs

Unlike the soft bottom meiofauna, meiofauna associated to hard substrata is poorly studied, despite its ecological relevance. Since communities of hard substrata are usually characterized by species with different life cycles and strategies from those of soft bottom assemblages, information on hard substrata meiofauna is still needed. In this study, sessile macrobenthos and the associated meiofaunal assemblages of two sites of Portofino (NW Mediterranean) were investigated in two seasons at three different depths on both sub-vertical and inclined reefs. The study aimed to assess the abundance, diversity and composition of the meiofauna and the factors structuring its assemblages. Moreover, as meiofauna is known to be dependent upon the substrate characteristics, the study investigated whether the meiofaunal patterns could be related to the sessile macrobenthos structure and composition, and to which extent. Macroalgae dominated the sessile macrobenthic assemblages, while Nematoda and Copepoda were the main meiofaunal groups. Meiofaunal higher-taxa richness and diversity resulted very high, due to the large number of different microhabitats offered by macroalgae. Macrobenthic assemblages were dominated by Rodophyta and Ochrophyta in summer, the latter dramatically collapsing in winter. The meiofaunal abundance and composition changed significantly with the season, consistently with the sessile macrobenthic assemblages, and resulted strongly correlated with Ochrophyta. Shaping the meiofaunal assemblages, macroalgae appeared to act as ecosystem engineer for the meiofauna.