Effects of sediment on a rocky intertidal habitat-forming species and its harboured invertebrate assemblages

Sedimentation in rocky shores occurs through natural processes, but it is rising globally due to anthropogenic activities. However, few studies have assessed sediment effects on habitat-forming species and its harboured assemblages. To fill this knowledge gap, the mussel Mytilus galloprovincialis was considered as model habitat-forming to evaluate differences associated with varying sediment loads on mussel density and its invertebrate assemblages harboured. Field data were obtained at two non-urban intertidal rocky shores in the Northern Portuguese coast. First, the relationship between sediment load in mussel clumps and the associated invertebrate assemblages was examined in terms of abundance, richness, Pielou and Shannon indexes and multivariate structure. Then, mussel clumps with low and high sediment load were selected at each rocky shore to test differences on density and size of mussels and assemblage descriptors between them. Sediment on mussels was composed by very coarse and coarse sand and showed a patchy distribution, but density and size of mussels were unaffected by sediment. Our results suggest significant effects of sediment on invertebrate assemblage associated with mussels specifically, decreasing evenness and heterogeneity of assemblages by favouring species tolerant to sediment (annelids, copepods and nematodes) and reducing the abundance of grazers and Nucella lapillus. As grazers and N. lapillus are considered keystone species, changes on them may trigger cascading effects on rocky shore ecosystems. Our results highlight that even sedimentation derived from natural processes may impact intertidal biodiversity, which should be considered when managing coastal interventions specially in the current global change framework that probably will exacerbate sedimentation.