Use of a vessel-mounted acoustic doppler current profiler to study currents and zooplankton biomass distribution over the Vancouver Island continental margin

We examine currents and relative zooplankton scattering strengths measured with a vessel-mounted RD Instruments 150 kHz acoustic Doppler current profiler (ADCP) over the southwest Vancouver Island continental margin. Raw ADCP signal strengths are converted into relative zooplankton backscatter intensities using a method of residuals computed from average linear fits to depth profiles, which removes most system-dependent effects. An absolute calibration is not feasible without coincident net tows and a record of system characteristics. The problem of side-lobe contamination of near-bottom zooplankton backscatter estimates is given considerable attention and an objective method is developed to eliminate erroneous backscatter values. Ship-referenced water velocities are converted to absolute (earth-referenced) currents using bottom-tracking measurements of ship velocities. Methods of calculating absolute currents using ship velocities estimated from the change in (GPS) ship position are also examined. It is shown that the GPS positional errors of « ± 300 m lead to velocity errors that are comparable to the currents. Only when bottom-tracking is available are absolute currents reliable. The methods developed to process the data are applied to ADCP data collected in June 1993 over the southwest Vancouver Island continental margin. The currents and relative zooplankton scattering strengths are examined over a 20 hour period along a 14 km survey line which spans the shelf break. Zooplankton concentrations occupied the upper 50 m over the shelf and slope both day and night. Aggregations of scatterers were present over the shelf break at depths of 150-200.m during the day, and migrated to the surface waters at sunset. Mean upwards swimming speeds were 1-1.5 cm/s. Offshore flow and vertical shears of the order of 10⁻³ s⁻¹ on the shelf were correlated with horizontal movements of zooplankton, indicating that the distribution of zooplankton over the shelf and slope was influenced significantly by advection. A strong scattering layer with backscatter intensities 100 times higher than normal surface intensities was advected into the survey area over the shelf a few hours before sunrise on June 30th 1993, and underwent downwards migration just prior to sunrise. Mean downwards swimming speeds were 3-4 cm/s. A southwestward flowing shelf-break current was observed in the upper 50 m with speeds of 15-20 cm/s. The contribution of tidal flows to the net current is determined and is found to be consistent with the predictions of Foreman's (1990) finite-element barotropic tidal model. The ADCP data taken in June, 1993 are examined along eight 100 km long survey lines perpendicular to the coast of Vancouver Island. The eight survey lines span the region from the mouth of Juan de Fuca Strait to the mouth of Barkley Sound. Zooplankton concentrations occupied the upper 50 m over the shelf and slope both day and night. Aggregations of scatterers were present over the shelf break at depths of 150-200 m during the day. A southwestward flowing shelf-break current was observed in the upper 50 m with speeds of 15-20 cm/s. An estuarine type flow was observed at the mouth of Juan de Fuca Strait, with strong (>30 cm/s) outflow in the upper 80 m and weaker landward flow below 80 m. These currents are consistent with previous measurements in the region. The ADCP data taken in June, 1993 are spatially averaged in order to characterize the summer mean zooplankton distribution and current pattern. Surface zooplankton concentrations were observed both day and night over the shelf and slope from Estevan Point to Juan de Fuca Canyon. High concentrations were found near the bottom along the shelf break from Estevan Point to Juan de Fuca Canyon. A southwestward flowing shelf break current with speeds of 15-20 cm/s was persistent along the shelf break from Estevan Point to Juan de Fuca Canyon. Surface outflow from Juan de Fuca Strait was observed to persist up to about 50 km seaward over the shelf. A counterclockwise flowing eddy with a radius of about 30 km was observed over Juan de Fuca Canyon. Current speeds associated with the eddy were strongest near the surface and diminished gradually with depth. Based on the limited data analyzed, it would appear that coastal ocean processes are too affected by high-frequency (tidal period) variability for ADCP surveys to provide synoptic views of the circulation and zooplankton biomass. However, this study demonstrates that the vessel-mounted ADCP is useful for observations of current velocity and zooplankton biomass distribution in confined spatial regions on diurnal time scales.Science, Faculty ofPhysics and Astronomy, Department ofGraduat