Observations of 5-MHz acoustic backscattering from Cochlodinium polykrikoides blooms in coastal waters

Abstract

Measurements of 5-MHz volume-backscattering strength of harmful algal blooms were made in summer 2008 at sites off the coast of Geumo island in southern Korea. The most abundant phytoplankton in the blooms was the chain-forming, toxic dinoflagellate Cochlodinium polykrikoides, which accounted for about 70 % of total abundance. The measured backscattering strengths are compared with the abundance of phytoplankton estimated using an optical microscopy from water samples collected simultaneously with the acoustic measurements. The results show that the scattering strengths are highly correlated with dinoflagellate concentrations, suggesting that the acoustic method may be a useful tool for detecting the harmful algal blooms in their initial stage. Finally, the measurements are compared with the scattering strengths predicted by a fluid-sphere scattering model using a single-cell size of C. polykrikoides and the equivalent cell size of a sphere the same size as the chain-forming cells. The results imply that the chain-forming cells should be considered as single body with an equivalent cell size. [Supported by the Korea Ocean Research and Development Institute]