Seagrass-bivalve facilitative interactions: Trait-mediated effects along an environmental gradient.

Facilitative interactions are important forces in shaping community structure and function, and understanding how they respond to environmental changes has become an increasing concern in ecology. Lucinid bivalves play a significant role in seagrass meadows, through a mutualism in which the seagrass provides habitat and oxygen via the roots, while the bivalves and their associated bacteria eliminate sulfides from pore water, improving thus plant performance. In this study, we evaluated how this mutualism is modified along a gradient of organic matter content in the sediment, in a coastal bay dominated by Cymodocea nodosa meadows. We used a correlative approach, seeking statistical association between sediment organic matter content, lucinid abundance, and plant traits. Lucinid abundance was higher in vegetated that in bare areas. In vegetated areas, lucinid abundance decreased as organic matter content in the sediment increased, decrease seemingly associated to modification in plant traits, i.e. root abundance and morphology. In organic-rich sediments, roots are less abundant and less branched, reducing the potential habitat for lucinids and suggesting a weakening of the interaction. This finding contributes to our understanding of how facilitative interactions can be modified along human disturbance gradients, and how disturbances can reduce resilience of seagrasses through this modification.