Multiple environmental stressors and their effects on kelp species distribution

Kelps support productive habitats across temperate and polar latitudes. Despite their wide-ranging distributions, kelps are declining globally as a result of human activities in coastal areas. My thesis takes a multi-scale approach to understanding how kelps respond to multiple drivers of change at various life stages, in an effort to predict the fate of these species-rich habitats in the Anthropocene. I first examine the global state of knowledge about how kelps respond to single and/or multiple stressors using a systematic review. I compiled 'dose-response' curves for kelps from experimental studies. This allowed me to (1) combine this information to create a global, average, stressor-specific, dose-response surface for a widespread kelp species, Saccharina latissima, (2) identify stressors combination values that represent critical threshold and optimal conditions for kelp growth, and (3) identify gaps in our knowledge of the effect of specific stressors on kelp performance. I then shift to the local scale to test the effects of local stressors on kelp distribution. Working in the Bay of Sept-Iles (Québec), I observed a striking contrast in the distribution of two common kelp species in two adjacent habitats; Saccharina latissima occupied both habitats while Alaria esculenta was restricted to a single habitat. An exhaustive comparison of stressor conditions between habitats indicated that light, sedimentation, and/or turbidity might likely drive this pattern. Surprisingly, the habitat-limited species survived when transplanted to the habitat from which it is absent, suggesting that the constraint acts at earlier life stages. This finding led me to test in the laboratory the effects of sediment on spore performance of the same two kelp species. My results show that sediments had a negative impact on spore development but not settlement, with interspecific differences that are not consistent with the observed distribution patterns. This study illustrates how local-scale studies, which match the scale of management, can refine the global picture and are essential for conserving these rich ecosystems.