Effects of environmental variability on trophic interactions and food web structure in a pelagic upwelling ecosystem

The dietary compositions of 18 species of pelagic nekton were examined from purse seine collections made during 4 (1981 to 1984) oceanographically contrasting summers in the coastal upwelling zone off Oregon and Washington, USA. Euphausiids, hyperiid amphipods, decapod larvae. pteropods, and larval and juvenile fishes were the dominant prey categories consumed during all years by this assemblage of nekton, although their relative proportions varied among the years. Considerable differences were observed in food habits, diet overlap, and food web structure depending upon the prevailing oceanographic conditions. The moderate to low upwelling year of 1981 showed a generally high overall trophic diversity and a low level of d ~e t overlap and food web complexity. During the relatively strong upwelling year of 1982, euphausiids were the dominant food consumed, resulting in high d ~e t a r y overlap and low trophic diversity and food web complexity. During the warm and low productivity El Niiio year of 1983, marked changes were observed in the taxonomic composition of the diet of many species. The diets contained many species of southern origin leading to a high diversity of prey and low overall dietary overlap. A late occurrence of strong upwelling in 1984 resulted in a trophic diversity and overlap that were intermediate to the other years Although some species preyed on species of lower trophic level during strong upwelling condit~ons, the overall trophic level was lowest during 1983 and 1984 due to the influx of large numbers of pelagic zooplanktivores.