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Marine Ecology Progress Series

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MEPS 148:269-279 (1997)  -  doi:10.3354/meps148269

Population dynamics of giant kelp Macrocystis pyrifera along a wave exposure gradient

Graham MH, Harrold C, Lisin S, Light K, Watanabe JM, Foster MS

Sporophyte recruitment, holdfast growth, and mortality of giant kelp Macrocystis pyrifera were measured seasonally on permanent transects at 3 sites (protected, intermediate, and exposed) along a wave exposure gradient on the Monterey Peninsula, central California (USA) between 1988 and 1991. The constant presence of cold, nutrient-rich water and the relative absence of other kelps and large grazers allowed the dynamics of M. pyrifera populations to be examined under conditions in which wave exposure was highly variable and influences of other abiotic and biotic factors were minimized. Recovery of M. pyrifera populations from decreased adult density (presumably due to storm-induced mortality; adult density was negatively correlated with storm activity) was a 2-stage process requiring the establishment of juvenile populations and conditions suitable for juvenile growth to adult size. Sporophyte recruitment was negatively correlated with M. pyrifera canopy cover, and thus appeared to be related to irradiance. Recruitment was low and continuous under a temporally stable M. pyrifera canopy at the protected site. At the intermediate and exposed sites, canopy cover was more variable, canopy loss was greater, and durations of low canopy cover were longer than at the protected site, resulting in episodic sporophyte recruitment. These distinct patterns in sporophyte recruitment resulted in continuous juvenile populations at the protected site and intermittent juvenile populations at the intermediate and exposed sites. Growth of juveniles to adult size required additional irradiance probably due to greater light requirements for juvenile growth than for sporophyte recruitment. We observed that juveniles grew to adult size when canopy cover was low and adults were below a threshold density estimated at ~10 plants 100 m-2, but juveniles also occasionally grew to adult size following periods of low canopy cover only. Episodic sporophyte recruitment at the intermediate and exposed sites resulted in decreased coincidence of the 2 recovery stages; adult densities were often decreased below threshold prior to the establishment of juvenile populations. Recovery time, that required by populations to return to densities above threshold, was equal to the time lag between occurrence of the 2 recovery stages and was therefore greatest at the more exposed sites. Comparisons between central and southern California M. pyrifera populations suggest that by altering recovery time, variable frequency and magnitude of storm disturbance may result in different periodicities of adult population cycles.

Giant kelp · Macrocystis

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