The Role of Reproductive Synchrony in the Colonization Potential of Kelp

Abstract

Dispersal is a key element in the recovery of populations that have been locally destroyed by disturbance. Surprisingly, many sessile species that seemingly have limited potential to disperse often rapidly colonize areas that have been recently disturbed. The synchronous release of propagules during periods that promote advection may extend the dispersal of such species and promote their rapid colonization. Alternatively, rapid recovery may result from colonization by dormant stages that survive disturbance. Here we test for reproductive synchrony as a way of extending the dispersal potential of two common seaweeds, the kelps Macrocystis pyrifera and Pterygophora californica. Synchrony in spore release in their case is likely to be particularly useful in extending colonization distance because fertilization in these species occurs after spore dispersal; synchrony increases the chance of fertilization by increasing the concentration of spores. We also evaluate experimentally the relative importance of dormant stages vs. recently settled spores in accounting for the rapid recovery of local kelp populations following severe disturbances. Reproductive synchrony was evaluated by following weekly changes in the reproductive condition of adult kelp. The degree of reproductive synchrony in both Macrocystis and Pterygophora was significantly greater than that expected under conditions of asynchronous reproduction. In Macrocystis, periods of synchronous spore production and release occurred sporadically over a 2-yr period. At least 75% of the sampled population exhibited the same directional change in reproductive condition in 38 of 82 sample periods. Episodes of significant spore release varied in duration from as little as 1 wk to as much as 2 mo. The sharpest decline in reproductive condition occurred during a severe storm. In contrast to Macrocystis, relatively well defined cycles of spore production, maturation, and release were observed in Pterygophora. More than one cycle was observed within a single reproductive season, and each cycle lasted ∼3–4 wk. The different patterns and degrees of synchrony observed between the two species likely reflect the degree to which their production of spores is influenced by environmental conditions; spore production in Macrocystis is greatly influenced by fluctuations in seawater temperature and nutrients while spore production in Pterygophora is not. Results from field experiments comparing the recruitment of small plants among rocks placed in the kelp bed for varying lengths of time indicated that microscopic life stages of Macrocystis and Pterygophora have little capacity for dormancy, and that the vast majority of recruitment resulted from recently settled spores. These results contrasted with those observed for the annual brown alga Desmarestia ligulata which showed a dormancy period of several months. Our findings suggest that mechanisms such 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 time due to disturbance.