Effects of physical structure and processes on thin zooplankton layers in Mamala Bay, Hawaii

Abstract

Vertically thin layers of zooplankton were found to be common and recurring features in Mamala Bay on the south shore of the island of Oahu, Hawaii. The formation, maintenance, and vertical displacement of these thin layers are, in part, a function of regional physical oceanographic processes. The purpose of this study was to quantify general thin zooplankton layer characteristics in Mamala Bay and the underlying physical environment in which they occurred. We utilized a 2 mo time series of acoustic backscatter measurements from a calibrated acoustic Doppler current profiler (ADCP) to identify thin scattering features; took biological samples; and collected vertical profiles of physical, optical, and biological characteristics of the water column. In general, thin zooplankton lay- ers were associated with increased water column stability. Stratification at the study site was low rel- ative to other coastal regions around the continental USA where thin layers have been observed. Instances of significant stratification were short-lived, and possibly as a result, thin layers were shorter in duration. Diurnal surface heating accounted for much of the observed stratification, and the breakdown of stratification during civil twilight corresponded with a decrease in thin zooplankton layer formation. Biological and optical measurements taken during a focused shipboard profiling experiment over the course of the study suggested a mechanism for zooplankton layer formation in which zooplankton converged to graze on a thin phytoplankton layer.