Thermal tolerance of native vs. invasive marine species at the Northern coast of Portugal

Abstract: Two prominent factors posing a major threat to biodiversity in marine ecosystems are climate change and the establishment of invasive species, and the interaction between these two. Because of the upwelling of cold, nutrient-rich water, the coast of Portugal is a thermal refuge and distribution edge for cold-adapted species, making it a particularly important study area from a biodiversity point of view. In this study I investigated the physiological tolerance to heat stress of six different native and co-occurring non-native marine species, red algae; C. crispus and G. turuturu, brown algae; S.polyschides  and U. pinnatifida,  and mussels ; X.securis and M.galloprovincialis,  in the intertidal  rocky coast of Portugal. Dynamic ramping assays with four different durations and intensity of thermal stress was conducted for each specie to determine a so-called thermal death time (TDT) curve and the thermal tolerance parameters CTmax (the temperature that will cause death within one min) and z (the decrease in thermal decay over time) for each specie. To compare the thermal tolerance between the native vs non-native species a linear mixed effects model was applied. This study found a significantly higher thermal tolerance to extreme temperatures expressed as the upper critical temperature CTmax in the invasive red algae G. turuturu and mussel X.securis compared to its native counterparts C.crispus and M.galloprovincialis. The study found no significant difference in the thermal tolerance expressed for the kelp species S. polyschides and U.pinnatifida which had a similar response to the heat challenge. The implication from these the findings for the coast of Portugal, could be that the invasive algae and mussel might better survive an extreme event such as a heatwave then the co-occurring native, while the invasive kelp species would respond in a similar way to the heat challenge. Hence the invasive capacity might depend more on the invasion window after a stressful event and on how well the native species recover from a heat stress. The findings from this study call for further conservation, restoration and monitoring efforts in this area given the biodiversity supported by the important intertidal marine native species. Furthermore, management to avoid spread of the invasive mussel into the Portuguese. coast and limit further increase in abundance of the invasive algae species. These results are highly relevant for projections on how further climate changes might affect the invasive and native species at the coast of Portugal.