TEMPERATURE ECOTYPES IN SEAWEEDS - ADAPTIVE SIGNIFICANCE AND BIOGEOGRAPHIC IMPLICATIONS

We examined the existence of temperature ecotypes in seaweed species f rom the North Atlantic Ocean belonging to different biogeographic grou ps. Temperature ranges for survival, growth and reproduction were dete rmined in culture for a total of 68 isolates belonging to 18 species. From the data, the reproductive seasonality and potential monthly grow th yields in situ were estimated, and the adaptive significance of loc al ecotypes was assessed. Published palaeo-climatic reconstructions of glacial seawater temperatures were used to determine the importance o f climatic change for the development of thermal ecotypes. In all case s highest annual growth yields were obtained by the southernmost popul ations. Although northernmost isolates had improved their growth capac ities at low temperatures, local populations are still exposed to seve rely suboptimal conditions even in midsummer. There was no apparent re lationship between the degree of latitudinal displacement during Pleis tocene glacial/interglacial cycles and the development of ecotypes. On the other hand, in disjunct parts of the distribution, where populati ons have been exposed to different types of temperature stress through glacial/interglacial cycles, thermal ecotypes have evolved. In severa l cold-water species, western Atlantic populations have higher upper t emperature limits for reproduction that those from the eastern Atlanti c, which enhanced glacial persistence on the American coast. During th e glaciation, eastern Mediterranean and Atlantic populations of tropic al to warm temperate species were isolated from one another because of the existence of a cold water barrier near Gibraltar. The resultant l ack of gene flow, combined with different types of temperature stress on the disjunct refuge populations, has been of major importance for t he development of thermal ecotypes in the North Atlantic Ocean.