El Nino-Southern Oscillation 1982-1983: nearshore population, community, and ecosystem responses

Glynn, Peter

El Nino-southern oscillation (ENSO) refers to the interacting global atmospheric and oceanographic conditions that sporadically alter large scale atmospheric pressure systems, wind systems, rainfall, ocean currents and sea level inter alia, largely in the tropical and subtropical latitudes of the Pacific Ocean (18, 191 27, 28, 188, 198, 209, 285). The El Nino event involves a massive advection of warm, low salinity, nutrient ~or surface water to the south, mainly along the coasts of Ecuador and Peru (64, 127, 128, 207). The southerly intrusion of tropical waters during El Nino likely is enhanced by several factors: (a) easterly and poleward propagation of equatorially and coastally guided Kelvin waves; (b) thermocline depression or reduced upwelling, (c) increase in transport of the North Equatorial Counter Current, and (d) raduced or reversed flow of the normally vigorous westerly flowing Pera Current. The widespread extent of El Nino sea warming is indicated in Figure 1, which compares SST conditions at the height of El Nino 1982-83 (June 1983) and in the following year (June/July 1984). During El Nino years, the biological productivity of the euphotic zone declines suddenly and markedly with consequent negative effects on the survival and reproduction of populations at higher trophic levels (13, 14). The distributions of tropical and subtropical organisms also are shifted to higher latitudes at such times. The 1982-83 ENSO was a remarkably strong event (27, 66, 88, 111, 115, 144, 150, 208, 211, 249), probably the strongest of this century. It led to unprecedented disturbances to marine populations, some of which may have long-term (10s to l00s of years) effects. The purpose of this paper is to review the effects of the 1982-83 El Nino on nearshore benthic marine populations and communities, primarily at low latitudes. Many of the species considered here are not commercially important, however, several reviews and research papers on these species are available (e.g., 6, 7, 14, 229, 250, 284). Emphasis will be given to tropical and subtropical biotas because these generally experienced the greatest impacts and also often extended their ranges into higher latitudes. Natural disturbances are potential forces affecting species populations, community dynamics, and community and ecosystem structure (e.g., 35, 186, 192, 194, 200, 247). Equatorial eastern Pacific biotas are within the El Nino disturbance regime, therefore, it is necessary to address the responses observed in 1982-83 in the context of disturbance ecology. Moreover, to develop a deeper understanding of ecologic changes; special consideration is given to the specific causal bases of responses wherever possible. Even though five years have elapsed since the 1983 disturbance, changes and adjustments are still evident in certain populations and communities. Therefore, some of the results reviewed here must be regarded as tentative with additional changes likely.

El Nino-Southern Oscillation 1982-1983: nearshore population, community, and ecosystem responses

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