Marked seasonal reproduction is a widespread phenomenon in seaweeds in temperate waters strongly influenced by seasonal variations. In the Baltic Sea, however, the perennial brown alga Fucus vesiculosus exhibits two periods of reproduction: early summer (May-June) or late autumn (September-November). In this study, we compared the geographical distribution, the reproductive biology and the recruitment success for F. vesiculosus that reproduce during either of these two periods on the east coast of Sweden. Both monocultures and mixed populations of summer- and autumn-reproducing F. vesiculosus occurred along the mainland coast from the southern parts of the Stockholm archipelago to the southernmost part of Sweden. F. vesiculosus reproduced only during summer around the coasts of the large islands of bland and Gotland. After reproduction, in July and January respectively, plants that reproduced in summer abscised their receptacles and part of their branches, while plants reproducing in autumn abscised only their receptacles. In laboratory experiments, short-day treatments (light: dark 8: 16 h) were shown to initiate receptacles in summer-reproducing plants, whereas plants reproducing in autumn initiated receptacles independently of short- or long-day (light: dark 16: 8 h) treatment. Plants reproducing during summer produced more eggs (21.0 x 10(4) eggs/g(-1) frond mass) of smaller size (0.067 mm) than plants reproducing during autumn (8.9 x 10(4) eggs/g(-1) frond mass, eg size 0.070 mm). However, recruitment success in the field was similar irrespective of the period of reproduction. The differences in the photoperiodic response, the differences in reproductive output and egg size between summer and autumn plants, and the observation that a single individual kept the same pattern of reproduction over a period of at least three reproductive seasons, may indicate that the two strategies of reproduction in F. vesiculosus are expressions of two distinct genotypes.
