Rhodolith density influences sedimentary organic matter quantity and biochemical composition, and nematode diversity

Rhodolith beds increase the seabed complexity and are hotspots of biodiversity. Despite the crucial ecosystem services provided by rhodoliths, they are threatened by global change and local anthropogenic impacts. In this study, conducted on one of the largest beds of calcareous algae in the world located on the continental shelf of eastern Brazil, we tested whether the higher complexity of the seabed within rhodolith beds could explain the spatial biodiversity patterns of free-living nematodes. Our results show that beds with the highest densities of rhodoliths are associated with higher sedimentary organic matter (OM) contents and by a different biochemical composition. The higher OM nutritional quantity and nutritional quality, as shown by higher biopolymeric C contents and higher values of the protein to carbohydrate ratio, respectively, were associated with higher abundance, biomass, and diversity of nematode genera, thus supporting our hypothesis. Though based on a correlative approach, the results of this study suggest that a decrease in density of rhodoliths caused by human impacts may affect benthic biodiversity and, consequently, the range of ecosystem services they provide.