Acoustical and Optical Methods in Arctic Zooplankton Studies

Abstract

Concurrent acoustical and optical measurements have a great potential to describe zooplankton distribution over large temporal and spatial scales. It is difficult to collect complete information on zooplankton distribution with traditional methods (e.g. nets), that provide discrete and low resolution data on distribution of zooplankton biomass, abundance, as well as community structure of zooplankton. Acoustic sounding makes environmental studies fast, non-intrusive, and relatively cheap with high temporal and spatial resolution. LOPC delivers real-time information on zooplankton abundance and size spectra. In this review we present the results of study on zooplankton distribution in two fjords of Spitsbergen in the summer of 2013. Data for this study was collected during simultaneous profiling with high frequency (420 kHz) echosounder and LOPC along the main fjord axes. Zooplankton size spectra obtained by LOPC were used as input parameters in "high-pass" model of sound scattering on fluid-like particles. Model output values of acoustic backscattering strength were compared with values obtained by echosounding. In most cases there was a good agreement between measured and modeled values, except conditions of very low zooplankton abundance and events of fish presence. Zooplankton size structure is helpful in validating and refinement of "high-pass" acoustic model for specific set of scatterers. This gives a possibility to determine the theoretical backscattering strength of zooplankton. Implementing two complementary methods allows to obtain fast and more complete information on zooplankton distribution.