Spatial patterns feeding temporal predictions: forecasting the impact of climate change in the subtidal seaweed communities of the northern Iberian Peninsula.

Supervisors: Jose M. Gorostiaga, Isabel Diaz (PiE-UPV/EHU).
Seaweeds sustain highly diverse and productive ecosystems in temperate rocky seashores and thus, are one of the most ecologically and socio-economically important habitats in temperate waters. They are habitat formers, supporting many species, and they also provide many ecosystem goods and services. Hence, a change in their abundance or even the disappearance of biogenic species will have a cascade effect throughout the entire ecosystem. These organisms are exposed to environmental conditions that naturally vary through time and space at different scales. However, the planet is currently facing a change in climate induced by anthropogenic activities whose pace of shifting has no precedent. The present study examines present and future spatial distribution patterns based on field data on subtidal macrophyte assemblages (composition and species abundance) from Atlantic and Mediterranean coasts of the Iberian Peninsula and environmental data from the open OCLE database. The first main finding of the present study was that water temperature was the most relevant factor in shaping macroalgal assemblage distribution along the Iberian Peninsula coasts, whilst nutrient availability played a secondary role by modulating the thermal effect in coastal stretches with the coldest and intermediate temperature ranges. For the first time and based on these two explanatory factors, an assemblage-environment-relationship model was built for the Iberian Peninsula in order to project assemblage shifts under future climate change scenarios, RCP 4.5 and RCP 8.5. Under both scenarios, we predict that assemblages occurring at intermediate temperatures and nitrate concentrations (central Cantabrian communities) will become more similar to those occurring in the warmest and most oligotrophic area of the northern Iberian Peninsula, the easternmost part. Moreover, northwest macroalgal assemblages will remain differentiated from the rest of Iberian Peninsula. This differentiation will be greater under the most pessimistic scenario (RCP 8.5), under which the central and eastern communities of the Iberian Peninsula will show higher resemblance to those of the Mediterranean region than those of the northwest. Therefore, the study hypothesis of a potential future mediterraneanization of macroalgae assemblages from northern Iberian rocky shores was partially supported by the results. This mediterraneanization/tropicalization will lead to a flattening of the sea bottom landscape in a shift from three-dimensional biogenic habitats towards a bidimensional space dominated by algal turfs and crusts, with subsequent negative consequences for ecosystem functioning and biodiversity. Northwest coast will remain as a refugee for spatially complex vegetation with colder affinity seaweeds. Nevertheless, future research is needed in order to improve community shift projections, including in the model other crucial factors, particularly dealing with herbivory and predation, data on temporal changes in macroalgal assemblages in response to environmental fluctuations, and higher spatial replication.