Seaweed responses to climate change: predictions, observations, and knowledge gaps

Kelps and other macroalgae in the Salish Sea are sentinel species for detecting the effects of climate change on marine ecosystems. However, the connections between changes in climate and changes in the distribution and abundance of benthic algae are complex. Warming, particularly in the intertidal where high temperatures can be stressful, is generally expected to result in seaweed mortality and contraction of species’ upper limits. Detailed time-series data support this contention; the upper limit of an intertidal turf contracts downshore during rare heat waves. Historical data from the Strait of Juan de Fuca suggest that this is true for intertidal kelps as well, as the upper limits of several kelp species have shifted downshore since the 1950s. However, surveys of other seaweeds at the same sites did not reveal coherent patterns in non-kelp species over the same time period. Into the future, patterns of seaweed distribution and abundance will be driven by multiple stressors including temperature and ocean acidification, and the novel combinations of these stressors may produce unexpected outcomes. Furthermore, important climate-driven changes in herbivore populations will have far-reaching effects on seaweeds. At this stage, we have accumulated a wealth of (usually) single-factor / single-species laboratory studies that identify some potential leverage points for change, and some historical data confirming ongoing changes in the field. Future work must focus on comprehensive experiments that can mimic realistic combinations of climate change stressors in communities that contain important producers and consumers, and provide better links between what is observed in the laboratory and in the field