Berman Report

### Acoustic measurements of small scale plankton distributions

    INVESTIGATORS:

    J. Green
    NOAA/NMFS
    28 Tarzwell Dr.
    Narragansett, RI 02882
    (401)782-3240
    jgreen@fish1.gso.uri.edu

    Mark Berman
    NOAA/NMFS
    28 Tarzwell Dr.
    Narragansett, RI 02882
    mberman@fish1.gso.uri.edu

#### STATEMENT OF OBJECTIVES

Characterization of the vertical and horizontal distribution of planktonic organisms associated with water column structure using multiple frequency acoustics.

This project employs the TRACOR Acoustic Profiling System (TAPS) to sample the distribution of plankton in the stratified and unstratified waters of Georges Bank. TAPS is a self contained, internally recording acoustic device designed to measure abundance and biomass of copepod-sized particles. TAPS uses concentrically focused multiple transducers to ensonify a 0.1m3 volume centered approximately 1.5 m from the transducer surface. In 1993, '94 and '95, a four frequency instrument (TAPS4) operating at 265, 420, 1100 and 3000 KHz, was deployed on process cruises in late May to early June. In 1995 a more recently developed six frequency system (TAPS6) with two additional transducers operating at 700 and 1850 KHz was also used to provide increased resolution of plankton sizes and compare instrument responses. We anticipate that in the future the 6 frequency instrument will be used routinely.

#### STATEMENT OF WORK

Three different types of deployment have been made, which will yield information on three aspects of plankton distribution. On CTD transects TAPS was deployed with every cast. These data are intended to show how the vertical distribution of the zooplankton changes from the shoal water on the top of the bank to the deeper water on the southern flank. They also provide a time-series that will show how fine scale plankton distributions change diurnally. The second and third series of deployments have been carried out at stations occupied for an extended time period, with the ship either anchored or following a drifter. In these deployments the instrument was repeatedly lowered and raised ('towyoed') through the top 50 m of the water column at 10 m/minute, for a period of 1 hour. Towyos were carried out two to four times per day. When tidal current and turbulence carrying plankton past the instruments are taken into account, this type of deployment will yield data on the horizontal patchiness of the plankton.

Each TAPS observation consists of time, depth, temperature, and volume back-scattering strength from each transducer. TAPS4 recorded one observation every 5 seconds, TAPS6, one observation every 6 seconds. Each instrument was set to average the return of 24 pings into each observations. The differences between the echo strengths at each frequency are used to calculate the number and sizes of the particles in the ensonified area.. This 'inverse calculation' is based on the assumption the plankton reflects sound as fluid filled spheres (this assumption appears to be correct for copepods, but insufficient for larger, rarer plankton groups, e.g. euphausiids). Results of the inverse calculation of the data collected in May of 1993 show that most of the planktonic biomass consisted of particles in a 0.2 mm ESR (equivalent spherical radius) this is equivalent to a copepod with a body length of about 0.8 mm. Pump samples taken after the TAPS casts indicated that these particles were Pseudocalanus minutus, stages C4 through adult. In the stratified area, the P. minutus were found mostly in dense patches associated with the top of the thermocline. Patches ranged in size from about 20 to about 150 m in length, and 3 to 5 m in depth. Biomass concentration within a patch ranged up to 500 mm3/m3. Plankton biomass outside of one of the patches was very low, typically less than 100 mm3/m3. Biomass distribution in the well mixed area was very different. The type of patches observed in the stratified area were largely absent. Biomass was more uniformly distributed, especially away from the surface, but the levels were much higher. This was probably due to acoustic interference caused by large amounts of sediments resuspended by the tidal currents. We are now working to include a second acoustic scattering model (solid elastic spherical scattering) in our analysis software. This will allow us to separate the biomass of the plankton from the apparent biomass due to sediments.

#### SUMMARY OF KEY FINDINGS

Data from our 1993 cruise have been analyzed, and a manuscript describing the results is currently in review. We carried out extensive sampling with both TAPS4 and TAPS6 on two separate cruises in May and June of 1995. Currently we are modifying our analysis software based on the results of our preliminary 1993 work, in preparation for the analysis of the new data.