Effects of reduced salinity on reproduction and germling development in Sargassum muticum (Phaeophyceae, Fucales)
Sargassum muticum has expanded rapidly along southern Scandinavian coasts since its introduction in the mid-1980s. Expansion into brackish areas has been limited, and the establishment of the species in such waters will largely depend on its ability to tolerate hyposaline conditions. Culture and field experiments have been used to examine the effects of reduced salinity on various life-history stages of S. muticum. In culture, fertilization was prevented at salinities of 15‰ and below, but not at 20 - 30‰. In a crossed two-factor culture experiment, germlings of various ages (1, 8, 15 days old) were cultured at various salinities (0, 5, 10, 15, 20, 25, 30‰) for one week and re-exposed to 30‰ to test for ability to recover. Germling survival was significantly reduced after exposure to 5‰ during the first, but not the second or third, weeks after fertilization. One week's exposure to 0‰ was lethal to germlings. Growth of germlings was reduced by exposure to salinities below 25‰, and growth at 5‰ was less than 10% of that at 25 - 30‰. Growth recovery of germlings after re-exposure to 30‰ was positively related to their age during exposure to sub-optimal salinities. To further explore the effects of salinity on growth and reproduction in S. muticum, plants were transplanted to three sites along a salinity gradient in a Norwegian fjord. At the low salinity site (9.5 - 17.4‰) growth and reproduction were slower than at the other sites where salinities were 14.7 - 27.1‰. These results demonstrate that salinity tolerance in S. muticum is lowest during fertilization, and increases with germling age. The salinity requirements of the least tolerant initial life stages will probably constitute a physiological barrier to the expansion of this species into brackish waters.
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