Bottoms Beneath Troubled Waters: Benthic Impacts of the 1982-1984 El Niño in the Temperate Zone

Abstract

The massive ENSO that profoundly affected the pelagic community of the northeast Pacific from 1982-84 offered an opportunity to evaluate and scale mechanisms coupling benthic communities to the water column. The California Current system off Central and Southern California had massive changes associated with a deepened thermocline and wanned mixed layer; nutrients, chlorophyll a, and macrozooplankton biomass were extremely low, and transport patterns were altered. These responses were felt throughout the pelagic food chain from the water column to seabirds and pinnipeds. The most pronounced benthic responses were observed in kelp forests. Disturbances of these productive communities were the largest ever recorded and appeared to result from both ENSO-associated storms and low nutrients. In most cases, the main impact was on adults of the giant kelp Macrocystis pyrifera that were either torn out by the storms or starved by the low nutrient conditions. However, at one site the storms killed sea urchins and released the kelps from intense cropping by these grazers. Following disturbance, most kelp forests quickly recovered because of unusually good light penetration in the clear water apparently associated with reduced nutrients. Exceptions were the Macrocystis angustifolia beds that lost their biotic substrata in the storms. Several kelp associated animals were affected, usually via recruitment anomalies, but they also generally recovered quickly. The recruitment of some motile epibenthic species such as Crustacea and some fishes apparently was affected by the ENSO and this may result in modest secondary effects in the future. Few if any ENSO effects were observed in the intertidal zone or among infaunal species. The benthic system did not respond as strongly as the pelagic system with the exception of Macrocystis. Macrocystis grows through the water column and was thus exposed to the full brunt of the storms and low nutrient conditions; it represents a special case for benthic species. Most of the benthic responses that were observed resulted from recruitment anomalies associated with water column processes such as advection and possibly larval starvation. The fundamental differences between benthic and pelagic systems such as relative mobility, longevity, and population turnover times probably contribute to the contrasting results of the ENSO event.