Carbon cycling across coastal soft sediments: the contribution of macrofaunal communities to seafloor respiration

Coastal ecosystems are biodiversity and carbon-cycling hotspots. However, links between biodiversity and carbon cycling are poorly understood and the role of macrobenthic fauna is often overlooked, despite its contribution to carbon storage and processing and key role in ecosystem functioning. Data that simultaneously quantify macrofauna and link it to seafloor carbon fluxes across coastal soft-sediment habitats are scarce which impedes our ability to assess the contribution of macrofauna to carbon cycling. In this meta-data analysis, we combined data from studies quantifying seafloor respiration over environmental gradients in the Baltic Sea (Finnish Archipelago), with faunal abundance and biomass data, to examine the faunal contribution to seafloor respiration as a measure of carbon cycling across habitat types and seasons. The proportional contribution of macrofauna to seafloor respiration was greater in sandy habitats, whereas overall seafloor respiration rates were highest in muddy habitats with elevated organic matter content. Seasonal patterns in seafloor respiration varied with water depth, while the seasonal variability of faunal respiration decreased with increasing water depth. The relative contribution of fauna was highest in winter, when seafloor respiration was low, and in late summer, when seafloor respiration declined but faunal respiration remained high. Bivalves (e.g. Macoma balthica) and polychaetes (e.g. Marenzelleria spp.) accounted for the largest share of faunal respiration, highlighting their central roles within benthic ecosystems. Macrofauna contribution to total respiration decreased with increasing water depth, organic matter content, and temperatures, indicating that variables enhancing carbon turnover depend on habitat type, biodiversity, and season. The observed patterns emphasize the importance of including biodiversity elements, such as relative species abundance, biomass and functional traits of key taxa in carbon cycling assessments.