The impact of climate change on the geographical distribution of habitat-forming macroalgae in the Rías Baixas.

In the current scenario of climate change characterized by a generalized warming, many species are facing local extinctions in areas with conditions near their thermal tolerance threshold. At present, the southern limit of the geographical distribution of several habitat-forming algae of cold-temperate affinities is located in the Northwest Iberian Peninsula, and the Rías Baixas may be acting as contemporary refugia at the range edge. Therefore, it is necessary to analyze future changes induced by ocean warming in this area that may induce changes in macroalgae populations. The Delft3D-Flow model forced with climatic data was used to calculate July-August sea surface temperature (SST) for the present (1999-2018) and for the far future (2080-2099). Mean daily SST was used to develop and calibrate a mechanistic geographical distribution model based on the thermal survival threshold of two intertidal habitat-forming macroalgae, namely Himanthalia elongata (L.) S.F.Gray and Bifurcaria bifurcata R. Ross. Results show that H. elongata will become extinct in the Rías Baixas by the end of the century, while B. bifurcata will persist and may occupy potential free space left by the decline in H. elongata.

Invasion of Sargassum muticum in intertidal rockpools: patterns along the Atlantic Iberian Peninsula.

Spatial patterns of non-indigenous species show scale-dependent properties. Sargassum muticum is an invasive macroalga widely distributed along the Atlantic Iberian Peninsula. Despite being quite abundant from Norway to South Portugal, there is little information about its patterns of distribution, particularly at a large spatial scale (i.e. thousands of kilometres). Here, we examined the spatial variation in the invasion success of S. muticum from rockpools at multiple spatial scales using a hierarchical design. In addition, we analysed how the richness of native assemblages was related to its invasion success and how this relationship changed over different scales. Most of the variation in the invasion success was found at the smallest scales of pool and plot. Furthermore, the invasibility of native macroalgal assemblages was related to the native species richness, but causes that determined invasion success could not be separated from the effects provoked by the invader. Results suggest that small-scale (centimetres to metres) processes contribute considerably to the heterogeneity of S. muticum invasion success.

Physical factors driving intertidal macroalgae distribution: physiological stress of a dominant fucoid at its southern limit.

Climate change is driving species range shifts worldwide. However, physiological responses related to distributional changes are not fully understood. Oceanographers have reported an increase in ocean temperature in the northwest Iberian Peninsula that is potentially related to the decline in some cold-temperate intertidal macroalgae in the Cantabrian Sea, namely Fucus serratus. Low tide stress could also play a role in this decline. We performed one mensurative (in situ) and two manipulative (in culture) experiments designed to evaluate the interactive effects of some physical factors. The first experiment analysed field response to low tide stress in marginal (mid-Cantabrian Sea and northern Portugal) versus central (Galicia) populations of F. serratus. Then a second experiment was performed that utilized either harsh or mild summer conditions of atmospheric temperature, irradiance, humidity, and wind velocity to compare the responses of individuals from one marginal and one central population to low tide stress. Finally, the combined effect of sea temperature and the other factors was evaluated to detect interactive effects. Changes in frond growth, maximal photosynthetic quantum yield (F(v)/F(m)), temperature, and desiccation were found. Three additive factors (solar irradiation, ocean and air temperatures) were found to drive F. serratus distribution, except under mildly humid conditions that ameliorated atmospheric thermal stress (two additive factors). Mid-Cantabrian Sea temperatures have recently increased, reaching the inhibitory levels suggested in this study of F. serratus. We also expect an additive secondary contribution of low tide stress to this species decline. On the northern Portugal coast, ocean warming plus low tide stress has not reached this species' inhibition threshold. No significant differential responses attributed to the population of origin were found. Mechanistic approaches that are designed to analyse the interactive effects of physical stressors may improve the levels of confidence in predicted range shifts of species.