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

Macrcrocystis pyrifera plants that detach from the substratum float to the surface and, if they do not become entangled or wash immediately to the sho re, may drift at the surface for an unknown period of time. These rafts pro vide habitat for a variety of coastal and pelagic fauna. The distances disp ersed and the period available for species to utilize these habitats, howev er, 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. T his technique determines the period of time since detachment and not the ac tual age of the plant. In general, average BL decreases from initial attach ed values of 50-60 to about 0 cm, when rafts sink. The rate of aging, or de terioration of BL, is related to water temperature, and sets the period a r aft stays afloat. Maximal estimates of ages of rafts were between 63 and 10 9 days, depending on the exact method used. Their lifetime will limit the d istances dispersed by kelp rafts in Southern California, and this methodolo gy will be useful for determining the temporal patterns of abundance of fau na associated with rafts. (C) 2000 Elsevier Science B.V. All rights reserve d.