MEPS 195:101-116 (2000)  -  doi:10.3354/meps195101

Abundance and dispersal of drifting kelp Macrocystis pyrifera rafts in the Southern California Bight

Alistair J. Hobday*

Scripps Institution of Oceanography, 9500 Gilman Drive, La Jolla, California 92093, USA
*Present address: CSIRO Marine Research, Tropical and Pelagic Ecosystems, GPP Box 1538, Hobart, Tasmania 7000, Australia. E-mail:

ABSTRACT: Drifting rafts of Macrocystis pyrifera are formed when attached plants are separated from the substrate. These kelp rafts have the potential to disperse fauna among isolated kelp forests; however, the success of this dispersal mechanism will be influenced by the abundance and the dispersal paths of the rafts. The abundance of kelp rafts in the Southern California Bight (SCB) was estimated from the loss of attached plants, and by direct observation of rafts at sea. Loss of attached adult plants was highest in winter (23%) and lowest in fall (8%) at 7 sites within 2 coastal kelp beds over the period of 1988 to 1998. Raft density at sea differed among 4 cruises in the period from 1995 to 1997, and ranged from 0.78 to 6.96 km-2 (average 3.02 km-2). In the SCB these densities correspond to an instantaneous abundance of between 39000 and 348000 drifting rafts. The potential raft dispersal paths and the frequency of exchanges among kelp forests were evaluated using satellite-tracked drifters. Southward dispersal away from the coast was observed in all months of the year. Dispersal north, both along the coast and away from the coast, was most likely between July and January (summer-winter), while dispersal south along the coast was likely between January and August (late winter and early spring). A definition of a Œsuccessful connection event¹ was adopted, based on a drifter starting less than 10 km and ending less than 5 km from the coast. Under this definition, 45% of all drifter releases were successful. Analysis of the temporal pattern and drift trajectories of the mainland-mainland successes revealed that Pt. Conception was always a boundary for north to south connections, but was open to south to north connections between fall and winter (October to January). Drifters released and remaining north or south of Pt. Conception were successful throughout the year. Connections were not made more than 300 km from a release region, under any criteria for success. The results of this study indicate that while rafts may drift long distances, successful connection of kelp forests through kelp rafts is limited to shorter distances.

KEY WORDS: Algal transport · Benthic inputs · Biogeographic boundary · Dispersal · Macrocystis rafts · Rafting · Satellite drifters

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