Non-linear effects of macrofauna functional trait interactions on biogeochemical fluxes in marine sediments change with environmental stress

Biogeochemical fluxes in marine sediments are profoundly influenced by species that bioturbate and bioirrigate the sediments. However, functional traits associated with these activities encompass a wide range of behaviours that have different consequences for the movement of particles and solutes. Interactions between infaunal species of different functional groups and benthic biogeochemical fluxes may be context-specific, requiring multiple studies; yet, to date these experiments are rare. In a laboratory experiment, we incubated specimens of Macomona liliana, a facultative deposit-feeding bivalve, and Macroclymenella stewartensis, a head-down conveyor- belt feeding polychaete, both separately and together, and measured fluxes of nutrients and oxygen. Both species are common in New Zealand estuaries and often coexist. The addition of thin surface layers of mud generated 3 treatment levels (0, 3 and 6 mm thickness). The presence of M. liliana and M. stewartensis enhanced benthic fluxes compared to control treatments. Oxygen uptake and nitrogen cycling stimulation due to their interaction were modelled, based on the results of single-species treatments, and then compared to results of multiple-species treatments with no animals. The effect of the interaction of the 2 organisms proved to be stronger than the additive effect of each species. This study demonstrated the central role of functional trait inter - actions for ecosystem functioning and its non-linear nature, highlighting the importance of testing actual effects against prediction based on trait analysis and the incorporation of these community effects in future research and models of ecosystem function and service delivery across marine habitats.