Interacting effects of local and global stressors on mussel beds and ecosystem functioning

Biogenic habitats, such as mussel beds, provide various functions in their associated ecosystems. However, these habitat-forming species are exposed to cumulative impacts as the number and diversity of anthropogenic stressors increase, particularly in estuarine ecosystems. Experiments designed to test the effect of single and multiple interacting stressors on mussel beds and associated biotic components are rare (i.e. in situ experiments are uncommon, as they usually occur in laboratory settings). We conducted a field experiment in the St. Lawrence estuary (Québec, Canada) to address this gap. We transplanted blue mussels (Mytilus spp.) to mimic mussel beds and exposed them to increased nutrient concentrations and thermal stress at three intervals (6.5, 10.5 and 15 weeks) during May through September. For each transplant, we evaluated epizoic microalgal biomass (as pigment biomass), microbial activity and oxygen uptake, and mortality levels and energy content in the tissues of Mytilus spp. following three exposure times. No effects were found for chlorophyll a biomass, microbial activity and oxygen uptake, and mortality in mussels. In contrast, we found thermal stress and nutrient input interacted to create antagonistic and synergistic effects on energy content in Mytilus spp. at different exposure times and exerted additive effects over time on phaeopigments and the ratio of chlorophyll a/phaeopigments. Our work highlights the importance of combining multiple biological components (i.e. multiple biological responses measured at different scales of biological complexity) and different experimental approaches to capture the complexity behind stressor interactions.