Temperature Ecotypes in Seaweeds:Adaptive Significance and Biogeographic Implications

We examined the existence of temperature ecotypes in seaweed species from the North Atlantic Ocean belonging to different biogeographic groups. Temperature ranges for survival, growth and reproduction were determined in culture for a total of 68 isolates belonging to 18 species. From the data, the reproductive seasonality and potential monthly growth yields in situ were estimated, and the adaptive significance of local ecotypes was assessed. Published palaeo-climatic reconstructions of glacial seawater temperatures were used to determine the importance of climatic change for the development of thermal ecotypes.In all cases highest annual growth yields were obtained by the southernmost populations. Although northernmost isolates had improved their growth capacities at low temperatures, local populations are still exposed to severely suboptimal conditions even in midsummer. There was no apparent relationship between the degree of latitudinal displacement during Pleistocene glacial/interglacial cycles and the development of ecotypes.On the other hand, in disjunct parts of the distribution, where populations have been exposed to different types of temperature stress through glacial/interglacial cycles, thermal ecotypes have evolved. In several cold-water species, western Atlantic populations have higher upper temperature limits for reproduction that those from the eastern Atlantic, which enhanced glacial persistence on the American coast. During the glaciation, eastern Mediterranean and Atlantic populations of tropical to warm temperate species were isolated from one another because of the existence of a cold water barrier near Gibraltar. The resultant lack of gene flow, combined with different types of temperature stress on the disjunct refuge populations, has been of major importance for the development of thermal ecotypes in the North Atlantic Ocean.