Latitudinal distribution of mesozooplankton in the off-equatorial northeastern Pacific before and after the 1998/99 La Nina event

Abstract

Factors affecting mesozooplankton distributions in the northeastern tropical Pacific Ocean were investigated using data obtained along a meridian line (5 degrees-12 degrees N, 131.5 degrees W) in the summers of 1998, 1999, and 2003. The survey periods corresponded to a sharp transition between the 1997-1998 El Nino and 1998-1999 La Nina events, the 1999 La Nina event, and near-normal conditions after the moderate 2002-2003 El Nino in the equatorial Pacific. A strong upwelling in the divergence zone from 10.5 degrees to 11 degrees N caused a shoaling of the thermocline depth (similar to 30 m), resulting in increases in nitrate and phytoplankton chlorophyll a (chl-a) concentrations, and, in turn, mesozooplankton abundance during the La Nino in 1999. In contrast, in 1998, remnants of El Nino characteristics, deeper thermocline depth (60-150 m) and warm surface water (> 28 degrees C), led to low concentrations of nitrate, chl-a and low mesozooplankton abundance, except in the convergence zone around 7N. The thermocline depth and nitrate concentration obtained during the near-normal period in 2003 corresponded to intermediate values as compared to those obtained during El Nino and La Nina conditions. Interannual changes in the position and strength of ecotones, such as divergence and convergence zones, affected mesozooplankton community structure and cyclopoid-to-calanoid ratios along the 131.5 degrees W meridian line. The clustering pattern of the mesozooplankton community was mostly characterized by calanoid (mainly Clausocalanus sp.) and cyclopoid (mainly Oncaea sp.) copepods, accounting for most of the observed differences among groups during the study period. Cyclopoids and calanoids were more abundant in 1999 than in 1998 or 2003, with a sharp increase to the north, while they were less abundant to the north in 1998 and 2003. The cyclopoid-to-calanoid ratio peaked in the convergence zone in 1998 and the divergence zones in 1999 and 2003, apparently due to the strength and location of the ecotones. Principal component analysis (PCA) with environmental factors and dominant mesozooplankton groups showed that dominant groups were affected by nitrate and chl-a concentrations in 1998, by sigma-t (water density), nitrate and chl-a concentrations in 1999, and by sigma-t, salinity and chl-a concentration (except siphonophores) in 2003. Latitudinal distribution of thermocline depth before and after the 1998/99 La Nina event showed a distinct interannual difference. The abundance of mesozooplankton in the divergence zone in 1999 was distinctively higher than abundances found in the convergence and divergence zones in 1998 and 2003, which resulted from the shallow thermocline depth due to an intensified upwelling during the strong 1998-1999 La Nina event. (c) 2007 Elsevier Ltd. All rights reserved.