Age of drifting Macrocystis pyrifera (L.) C. Agardh rafts in the Southern California Bight.

Macrocystis pyrifera plants that detach from the substratum float to the surface and, if they do not become entangled or wash immediately to the shore, may drift at the surface for an unknown period of time. These rafts provide habitat for a variety of coastal and pelagic fauna. The distances dispersed and the period available for species to utilize these habitats, however, depend on the longevity of the raft and methods for determining the age of rafts are unknown. A method to age drifting M. pyrifera rafts based on a change in length of blades (BL) following detachment is validated here. This technique determines the period of time since detachment and not the actual age of the plant. In general, average BL decreases from initial attached values of 50-60 to about 0 cm, when rafts sink. The rate of aging, or deterioration of BL, is related to water temperature, and sets the period a raft stays afloat. Maximal estimates of ages of rafts were between 63 and 109 days, depending on the exact method used. Their lifetime will limit the distances dispersed by kelp rafts in Southern California, and this methodology will be useful for determining the temporal patterns of abundance of fauna associated with rafts.