Resolving small scale turbulence with acoustic Doppler and acoustic travel time difference current meters from an underwater tower

Abstract

Turbulence is an important factor for planktonic predator-prey interrelationships, as it increases the contact rate between the relatively immobile planktonic organisms. Actual turbulent scales in planktonic predator-prey relationship are those less than the typical planktonic separation distance. For fish larvae feeding on copepod nauplii of concentrations 1-50 nauplii/l the effective turbulence scale is smaller than 10 cm. In regions where the wind-induced turbulence dominates, empirical relationships between wind mixing and wind speed may be applied to calculate rough estimates of the turbulent energy dissipation rate /spl epsi/. In regions where other sources of turbulent energy (tidal mixing, convection, swell in shallow regions and internal breaking waves) are important, the available empirical data are poor, and direct measurements of the turbulence and the derived turbulent frequency spectra are needed. During spring 1995 and 1996 direct turbulence measurements were carried out in Lofoten, Northern Norway, above a shallow plateau of 20-35 m depth where turbulent mixing is mainly generated by tidally and swell-induced mixing. Lofoten is an important feeding area for cod larvae. The turbulence measurements were conducted from a fixed point at the top of 65 m high submarine tower sitting on the sea bed. Three types of ultrasonic current meters, one Doppler current meter and two travel time current meters with different sampling volume were used in the experiment. The results indicate that both instrument types can be used to resolve small scale turbulence. However, Doppler noise limits the performance of the Doppler current meter, while the acoustic pathlength limits the performance of travel time current meters.