DIFFERENTIAL REPRODUCTIVE RESPONSES TO FLUCTUATING RESOURCES IN 2 SEAWEEDS WITH DIFFERENT REPRODUCTIVE STRATEGIES

Reproduction is closely tied to environmental conditions and the avail ability of resources, and thus typically varies with season. Consequen tly, perennial organisms that reproduce continuously are generally res tricted to tropical regions with relatively aseasonal climates. The te mperate marine alga Macrocystis pyrifera is a rare exception in this r egard, as most individuals reproduce throughout the year in a seasonal ly variable habitat. Here we measure reproductive responses of the gia nt kelp Mncrocystis during a period in which resources and environment al conditions fluctuated greatly and contrast these responses with tho se of the palm kelp, Pterygophora californica, a sympatric species tha t exhibits strictly seasonal reproduction. The quantity and quality of spore production tracked resource availability within and among years for Macrocystis, but not for Pterygophora. Reproductive allocation an d spore standing stock in Macrocystis were negatively correlated with seawater temperature and positively correlated with the nitrogen conte nt of adult plants. Macrocystis generally displayed two seasonally dis tinct peaks in spore production per year (winter and spring). The only disruption of this pattern coincided with a warmwater El Nino event. Although seawater temperature and the nitrogen content of adults were inversely related in Pterygophora, neither variable was significantly correlated with the quantity or quality of spore production in this sp ecies. Unlike Mncrocystis, Pterygophora exhibited a well-defined repro ductive season in which plants displayed a single broad peak in spore production that varied little in timing and magnitude among years, eve n during El Nino conditions. Spore C/N ratios remained relatively cons tant over time in both species, despite large seasonal fluctuations in C/N ratios of vegetative tissue of adults plants. Nonetheless, spore C/N ratios were positively correlated with seawater temperature in Mac rocystis but not in Pterygophora. Spore viability (swimming and germin ation) varied considerably, and often unpredictably, over time for bot h species. Our results support the general idea that environmental con ditions and resources exert a much greater influence on the quantity a nd quality of reproduction in species that reproduce continuously than on the majority of species that are strictly seasonal in onset of rep roduction The differential responses of Macrocystis and Pterygophora m ay reflect their different morphologies and life-spans. Macrocystis is relatively short lived and may ''hedge its bets'' by reproducing cont inuously rather than risk delaying reproduction. Conversely, since Pte rygophora lives much longer, plants can afford to release spores only during times when the chances for reproductive success are predictably greatest because these plants are likely to reproduce again in subseq uent years.