The relative growth rates of juvenile Macrocystis pyrifera in southern California kelp forests were substantially reduced during the E1 Nifio of 1982E1 Nifio of -1984. The lower growth rates were correlated with increased temperature and decreased nitrogen availability. Fertilization of juvenile plants with slow-release nitrogen-phosphorus fertilizer increased their growth rates to levels previously observed when temperatures were low and nutrient levels were high. The limitation in growth of M. pyrifera by levels of available nutrients during E1 Nifio was in contrast to the usual limitation of growth by irradiance during non-El Nifio years. Thus, there was a shift in the relative importance of factors controlling growth of juvenile M. pyrifera during E1 Nifio.
Inlroducflon
Growth of juvenile Macrocystis pyrifera in coastal kelp forests of southern California is usually limited by irradiance (Dean and Jacobsen, 1984). Irradiance levels in the San Onofre kelp forest from 1979 to 1982 were generally below the saturation level for juvenile kelp growth, and occasionally below the compensation point (Dean and Jacobsen, 1984). Temperature, nitrogen, or correlated factors limited growth only occasionally. However, oceanographic conditions in southern California changed greatly be~nning in Fall 1982. An "El Nifio" event (i.e., a diminishing of prevailing easterly trade winds in the equatorial Pacific: Cane, 1983) caused a shift in current regimes and an intrusion of offshore surface waters into the coastal southern California region (Fiedler, 1984;