Particle size dependence on current velocity

Abstract

The concentration and size of particles in coastal oceans is of great ecological importance, for example for light penetration and thus primary production. A common tool to determine particle sizes and concentrations is the Laser In Situ Scattering and Transmissometry (LISST). Previous studies with LISST instruments have found large variations in particle sizes and a possible influence on the measurement by current shear. To determine the strength of this influence, we conducted a cruise in the German Bight. We determined particle sizes and concentrations using a Sequoia LISST-200X, as well as the encounter velocity and the direction with which the water hits the instrument frame using an Acoustic Doppler Velocimeter. The encounter velocity was modulated by drifting and steering of the ship, leading to minimal velocities of 0.1 m s⁻¹ during drift and maximum velocities of 0.6 m s⁻¹ during steering. We found that the determined particle size is strongly dependent on the encounter velocity and the orientation of the instrument. The determined particle size decreased by 17–56 μm per increase in 0.1 m s⁻¹ encounter velocity. Identifying and exploring two hypotheses, we assume that large particles are destroyed by the current shear at high velocities. We propose that for future LISST measurements, the encounter velocity with which the water hits the instrument be taken into account and reduced as far as possible. In addition, we propose measurements under controlled conditions that can accurately determine the extent of the influence of encounter velocity on particle size determination.