The response of giant kelp (Macrocystis pyrifera) in southern California to low-frequency climate forcing

The nutrient climate on the inner shelf off southern California changed markedly across the 1976–1977 North Pacific climate regime shift. With respect to giant kelp (Macrocystis pyrifera) canopies off southern California, the nitrate climate shifted from relatively replete conditions prior to the regime shift to depleted conditions afterward, and the dynamics of 14 giant kelp forests appeared to change as a result. The response of giant kelp to nutrientreplete years before the regime shift was dampened compared to their response afterward. The sensitivity of these kelp-forest canopies to nutrient limitation appears to have increased since the regime shift. This intensification of physical control after 1977 is evident in the strong correlation of seawater density (st) and M. pyrifera density. The linear fit of the percent of time the 25.1 st isopycnal bathes the inner shelf, accounted for , 71% of the variability in kelp density off Point Loma, and the median depth of this isopycnal has deepened , 5 m since the regime shift. The wave climate also intensified beginning in the early 1970s. The dampened kelp response prior to the regime shift was likely due to greater biological control of kelp canopies via consumer and competitive processes (i.e., biological modulation) or decreased physical control at possibly many trophic levels. Our results