Macroalgae response to grazing pressure under future scenarios of rising temperatures: a nonindigenous species versus a native one.
Supervisors: João Canning Clode Susanne Schäfer (MARE, Madeira)
Among the manifold impacts of global change, non-indigenous species (NIS) are considered one of the most important threats to biodiversity. When finding appropriate environmental conditions for growth and reproduction, NIS can settle and become invasive, generating negative alterations for the original biota and the functioning and services of the ecosystem. European waters host several NIS, commonly macroalgae, and have suffered different episodes of marine bioinvasions, including the most recent case by the Asiatic seaweed Rugulopteryx okamurae. After colonizing several kilometers of the Mediterranean and Atlantic coastlines, this brown macroalgae was recently found on the oceanic island of Madeira. Although its presence is currently molecularly confirmed only on the islands’ north coast, the impacts that R. okamurae may generate are concerning in an ecosystem where native macroalgae communities are already under great pressure. Management strategies of prevention or early control need to be ready for a possible bioinvasion, and their effective design lies in the understanding of the physiology and ecology of the non-indigenous macroalgae. This thesis aims to understand the response of R. okamurae to present and future environmental conditions of Madeira Island. The macroalgae’s performance was tested under the effects of predicted sea surface temperatures and the pressure of an important generalist grazer in the community. Under controlled laboratory conditions, temperatures were established to 20, 24, and 28 ºC, in combination with the presence or absence of the sea urchin Paracentrotus lividus. Additionally, the native macroalgae Halopteris scoparia was exposed to the same treatments to assess the possible competitive advantage of R. okamurae.

Results showed that temperatures alone did not have a significant effect on the metabolic rates of any of the macroalgae. However, R. okamurae showed better Net Primary Production (NPP) than the native species. On the other side, when the grazing pressure was introduced, differences between the two species were drastically evident. While P. lividus did not closely interact with R. okamurae, H. scoparia lost high proportions of its biomass. The effect was most prominent at 24 ºC, where temperature and grazing pressure had a synergic effect, leading to the complete disappearance of the macroalgae. These results give therefore, valuable experimental information about R. okamurae’s physiological response in Madeira’s present environmental conditions, but also in foreseen scenarios of global warming. Moreover, the present work continues to stress the role of sea urchins in modifying the macroalgae community of the island and evidences the low interaction of this generalist grazer with the non-indigenous seaweed. Since R. okamurae is still at the first stages of an invasion in Madeira, this experiment shows that this species presents a metabolic advantage over native macroalgae. Additionally, one of the primary generalist grazers of Madeira has no biological control over this particular species, which could lead to an explosive development of R. okamurae on the island.