Catastrophe, recovery and range limitation in NE Pacific kelp forests: a large-scale perspective

Abstract

The 1997-98 El Nino was one of the strongest on record and resulted in widespread losses of the giant kelp Macrocystis pyrifera (Agardh) along the west coast of North America. Draw- ing on a rich history of studies that have shown abnormally large waves and warm nutrient-poor water associated with El Ninos to negatively impact giant kelp populations at some locations in south- ern and Baja California, we examined (1) how these impacts scale up when considered across the species' geographic range in the NE Pacific Ocean and (2) if these impacts are generalizable over broad spatial scales. Working at 56 sites in 14 study locations over a 3 yr period (1997 to 2000), we examined how giant kelp populations were impacted by and recovered following the 1997-98 El Nino over a ~1500 km span along the west coast of North America. Our results indicate that while nearly all giant kelp disappeared from the southern one-third of the species' range along the coast of Baja California, Mexico, and heavy losses occurred throughout the central one-third of the species' range in southern California, USA, only minor impacts were observed throughout the northern one- third of the species' range in central California. Further, although highly variable among regions, these impacts were similar and generalizable among locations within each region. Our results also suggest that, as has been observed in local-scale studies, this large-scale variability in giant kelp mortality was driven by large-scale patterns in ocean temperature (nutrient concentration) and wave intensity. Recovery following El Nino, in contrast, was variable at multiple spatial scales and although not directly tested here, presumably influenced by numerous factors such as proximity to upwelling areas, competition with other algae, grazing, and propagule availability. Further, variability in the rates of recovery among locations resulted in a generally slow recovery of giant kelp throughout most of Baja California, and residual large-scale impacts of the El Nino were still evident 2 yr after the El Nino ended. As global climate change may lead to increases in the frequency and intensity of El Ninos, our findings have broad implications for the ways in which ecosystems might be expected to respond to them and provide a measure by which their impacts to giant kelp ecosystems may be compared among events.