Seasonal and Diurnal Variability of the Mesozooplankton Community at Ocean Station ALOHA

Michael Landry, Karen Selph and Hussain Al-Mutairi

School of Ocean and Earth Science and Technology, University of Hawaii, Honolulu, HI 96822

Abstract

The mesozooplankton community at Ocean Station ALOHA has been routinely sampled at approximately monthly intervals as part of the HOT Program since early 1994. Measured parameters from mid-day and mid-night, oblique (0-150 m) net tows include total and size fractioned abundances, wet and dry weights, carbon and nitrogen biomasses, and gut pigment contents. Cruise means vary by about a factor of three for estimates of community biomass to almost an order of magnitude for gut pigments. Maxima occur during the summer and minima in autumn or winter. Direct grazing of mesozooplankton on phytoplankton (as inferred from gut pigments) accounts for the loss of only a small fraction of phytoplankton standing stock per day, and variability in mesozooplankton biomass and grazing is not reflected in measured particle fluxes from sediment traps. However, diurnal migrants, generally in the larger size- fractions, contribute significantly to export fluxes by feeding in the euphotic zone and metabolizing at a deeper day-time depth. Mean fluxes associated with migrating zooplankton, computed from day-night differences in biomasses and temperature- and size-corrected metabolic rates, are on the order of 0.2 mol C and 0.03 mol N M^-2 y^-1. These rate estimates vary seasonally from about double the annual mean during spring/summer maxima to half the mean during fall/winter minima.