Contrasting benthic ecological functions in deep-sea canyon and non-canyon habitats: Macrofaunal diversity and nutrient cycling

Understanding ecosystem functioning and stability of continental slope environments requires assessing the effects of habitat heterogeneity (canyon and non-canyon habitats) on benthic macrofaunal communities and ecosystem functioning. We evaluated the importance of habitat heterogeneity and phytodetrital enrichment in canyon, channel, and inter-canyon habitats of the Gulf of Maine, Northwest Atlantic. Using an ROV, we collected sediment cores at five seafloor sites in the Gulf of Maine (663–969 m depth). Over ∼24 h, we incubated ∼6 cores per site, three of which were enriched with the diatom Chaetoceros muelleri. For all core treatments, we measured macrofaunal composition, functional diversity indices (functional richness, evenness, and divergence), and benthic nutrient fluxes (ammonium, nitrate, phosphate, and silicate), as well as oxygen consumption. We also dedicated two non-incubated cores to quantifying sedimentary and chemical-physical characteristics. We found that benthic communities differed among the three habitats, but these differences and our phytodetrital additions did not translate to significant differences in rates of benthic fluxes among habitats., expect for nitrate and phosphate fluxes. Total organic carbon and chlorophyll-a explained most of the variation in oxygen consumption, which was similar in enriched and control cores. We conclude that large-scale habitat heterogeneity contributes to variation in benthic community composition, but with no consistent effect on nutrient fluxes. These results also suggest that phytodetrital deposition influences oxygen consumption but plays a lesser role in short-term nutrient cycling in deep-sea heterogeneous habitats, perhaps masked by other environmental variables. Moreover, sediments with high functional richness and relatively high functional divergence may sometimes lack the specific trait types that rapidly enhance nutrient regeneration and thus contribute to ecosystem functioning of deep-sea sediments.