Mesozooplankton biomass and composition in the equatorial Pacific along 180°

Abstract

Latitudinal variations in mesozooplankton biomass and composition were investigated along an equatorial transect (8°S–8°N, 180°) in October–November 1996. This study also included intensive sampling (3‐hour intervals for 48 hours) for diel variations in mesozooplankton vertical distributions at 3°S and the equator. Most of the study took place in the high‐nutrient, low‐chlorophyll (HNLC) area that stretched between 7°S and 5°N. Mesozooplankton latitudinal distributions were influenced by the passage of a tropical instability wave during the equatorial time series station, which brought lower mesozooplankton biomass from the northeast, contrasting with the lack of a similar effect on concentrations of phytoplankton and particulates. South of the equator, the distributions of the mesozooplankton showed variable patterns with respect to chlorophyll and surface nitrate concentrations that could be ascribed to different states of the HNLC ecosystem. Very low diel variations of mesozooplankton biomass in the 0–50 and 0–100 m depth strata, a shallow vertical distribution, and the dominance of the larger size fraction (500–2000 μm) appear to be typical of the equatorial Pacific HNLC mesozooplankton and contrast with tropical oligotrophic ecosystems. Effects of such characteristics are a low active carbon export and a continuous predatory pressure.