THE ROLE OF REPRODUCTIVE SYNCHRONY IN THE COLONIZATION POTENTIAL OF KELP

Dispersal is a key element in the recovery of populations that have be en locally destroyed by disturbance. Surprisingly, many sessile specie s that seemingly have limited potential to disperse often rapidly colo nize areas that have been recently disturbed. The synchronous release of propagules during periods that promote advection may extend the dis persal of such species and promote their rapid colonization. Alternati vely, rapid recovery may result from colonization by dormant stages th at survive disturbance. Here we test for reproductive synchrony as a w ay of extending the dispersal potential of two common seaweeds, the ke lps Macrocystis pyrifera and Pterygophora californica. Synchrony in sp ore release in their case is likely to be particularly useful in exten ding colonization distance because fertilization in these species occu rs after spore dispersal; synchrony increases the chance of fertilizat ion by increasing the concentration of spores. We also evaluate experi mentally the relative importance of dormant stages vs. recently settle d spores in accounting for the rapid recovery of local kelp population s following severe disturbances. Reproductive synchrony was evaluated by following weekly changes in the reproductive condition of adult kel p. The degree of reproductive synchrony in both Macrocystis and Pteryg ophora was significantly greater than that expected under conditions o f asynchronous reproduction. In Macrocystis, periods of synchronous sp ore production and release occurred sporadically over a 2-yr period. A t least 75% of the sampled population exhibited the same directional c hange in reproductive condition in 38 of 82 sample periods. Episodes o f significant spore release varied in duration from as little as 1 wk to as much as 2 mo. The sharpest decline in reproductive condition occ urred during a severe storm. In contrast to Macrocystis, relatively we ll defined cycles of spore production, maturation, and release were ob served in Pterygophora. More than one cycle was observed within a sing le reproductive season, and each cycle lasted similar to 3-4 wk. The d ifferent patterns and degrees of synchrony observed between the two sp ecies likely reflect the degree to which their production of spores is influenced by environmental conditions; spore production in Macrocyst is is greatly influenced by fluctuations in seawater temperature and n utrients while spore production in Pterygophora is not. Results from f ield experiments comparing the recruitment of small plants among rocks placed in the kelp bed for varying lengths of time indicated that mic roscopic life stages of Macrocystis and Pterygophora have little capac ity for dormancy, and that the vast majority of recruitment resulted f rom recently settled spores. These results contrasted with those obser ved for the annual brown alga Desmarestia ligulata which showed a dorm ancy period of several months. Our findings suggest that mechanisms su ch as reproductive synchrony that extend the distances over which kelp spores can effectively colonize are likely to play a critical role in the dynamics of kelp populations, which often fluctuate greatly in ti me due to disturbance.