Higher diversity of deposit-feeding macrofauna enhances phytodetritus processing

The link between biodiversity and ecosystem functioning is an important question that remains unresolved, particularly in marine systems, in which cycling of organic matter by benthic organisms is of global significance. Direct observations of specific resource use by each species in single- and multispecies communities, as quantified by stable isotopes, facilitates a mechanistic understanding of the importance of each species for ecosystem functioning. We tested the effects of altered biodiversity (species richness) of deposit-feeding macrofauna on incorporation and burial of phytodetritus in combinations of three species representing natural communities found in the sediments of the species-poor Baltic Sea. The three species, two amphipods and a bivalve, had different rates of incorporation and burial and different needs for carbon (C) and nitrogen (N). The amphipods exhibited clear resource partitioning in sympatry, as a result of vertical separation in the sediment and consequent differential use of food. Communities of several species incorporated more C and N than expected from the respective single-species treatments, due to higher incorporation by surface feeders in multispecies treatments. Community incorporation of N in the most diverse treatment even exceeded N incorporation by a single-species treatment of the best-performing species, showing transgressive over-yielding. This over-yielding was primarily due to positive complementarity in all treatments. Diverse soft bottoms are also likely to be more productive in the long run, as species-specific traits (subsurface feeding) preserve fresh phytodetritus by burying it to depths in the sediment at which the mineralization rate is low. The more diverse sediment communities showed more efficient trophic transfer of phytodetritus, a finding of general significance for understanding biological processes driving the transformation of nutrients and energy in benthic ecosystems.