Influence of coastal upwelling-downwelling variability on tropical euphausiid abundance and community structure in the inshore Mexican central Pacific

Abstract

The effect of wind-induced coastal upwellings on tropical euphausiid abundance and community structure was investigated in the Mexican central Pacific (19° N, 105° W) during a monthly time series (1996−1998). Eight species were identified, of which Euphausia distinguenda contributed between 88 and 90% of the total euphausiid abundance, and E. lamelligera contributed ~7%. The hydrographic structure (<200 m depth) and euphausiid species composition had strong seasonality patterns associated with the upwelling (February to May) and downwelling (July to November) periods. Redundancy analysis of euphausiid abundance and community structure as a function of the environmental variables revealed that coastal upwelling index, salinity at 10 m depth, and temperature explained most of the euphausiid abundance variability. Stations sampled during intense upwelling periods had the highest abundance of E. distinguenda and E. lamelligera juveniles and adults. Their abundance was strongly and positively correlated with salinity and abundance of nano- and microphytoplankton, but was negatively correlated with surface temperature. Larvae of E. distinguenda and the oceanic species Nematoscelis gracilis (downwelling ensemble) were strongly associated with warm waters of low phytoplankton abundance. The hepato-somatic index (ratio of hepatopancreas length to carapace length) of E. distinguenda and E. lamelligera adults was significantly larger during mixed and semi-mixed than during stratified periods, providing a useful proxy for euphausiid health and trophic condition. Wind-induced upwelling−downwelling are significant coastal processes that influenced seasonal euphausiid abundance and species composition in this tropical ecosystem, while the strong and brief El Niño event of 1997−98 had only a relatively moderate effect in comparison with that observed on euphausiids from transitional (northwest of Mexico) and temperate (Pacific USA) ecosystems.