Impact of a macrofaunal ecosystem engineer on its assemblage and its habitat in mixed sediments as assessed through manipulative experiments

Abstract

Ecosystem engineers can modify the environment, affecting the associated community. Studies of mussels as ecosystem engineers have been conducted mostly in the rocky intertidal zone, while those in sedimentary intertidals have been relatively scarce. The present study assessed the effect of varying densities of Brachidontes rodriguezii on local environmental variables (substrate weight, water content, organic matter, weight of sediments adhering to mussel byssus) and ecological attributes of the associated community (richness, abundance, diversity, evenness, and assemblage structure) at different intertidal levels, sites, and seasons, in a mixed-sediment intertidal region in San Antonio Bay (Patagonia, Argentina). We performed two independent manipulative field experiments at low and mid intertidal levels at two sites -Punta Verde (PV), with relatively higher current velocities and coarser grain sizes than Punta Delgado (PD)- in both winter and summer. Manipulative experiments consisted in reducing mussel density to 50% in the low intertidal and increasing that density to up to 100% in the mid zone. We hypothesized that local environmental variables and ecological attributes associated with B. rodriguezii depended on mussel density. The reduction in B. rodriguezii density increased the evenness and reduced the abundance at PD plus reduced the richness at PV. Also, when reducing mussel density, the assemblage structure differed in water content and organic matter at both sites between treatments and seasons. The sediment weight adhering to mussel byssus in PD and substrate weight in PV were the best descriptors of the assemblages. The increase in mussel density, however, produced a weak effect on the ecological attributes of the community, and the assemblage structure did not correlate with the environmental variables considered. Our results demonstrated a stronger engineering effect of B. rodriguezii at reduced, but not increased, mussel density. This study revealed that the ecological attributes associated with B. rodriguezii both were density-dependent and varied with sites along the year in mixed-sediment intertidal zones.