U.S. GLOBEC: Turbulent Mixing on Georges Bank
Dave Hebert (GSO/URI) and Neil Oakey (BIO/DFO)

As part of the U.S. GLOBEC GEORGES BANK project two cruises were undertaken in the spring of 1995 to study the effects of stratification and mixing on larval fish recruitment. During each cruise, two anchor stations were occupied: one in a shallow, well-mixed region and another in deeper water near the main stratification mooring where stratification was just starting to develop during the first cruise and was established by the time of the second cruise. The proposed work for the next two years consists of continued analysis of the data collected during our cruises to address three goals: to determine the processes and dynamics responsible for the vertical structure of the mixing rates, to determine the bottom stress at each location and a parameterization to explain the variability of the stress and to determine the relationship between the biological patchiness and turbulent mixing rates.

It is necessary to address the first two goals in order for modellers to properly simulate the circulation and mixing on Georges Bank. It is also necessary to properly simulate the tidal mixing. The observed depth dependence of the mixing rate and its variation as a function of time and location will be examined using data obtained from the free-fall profiler EPSONDE. The dynamics responsible for the vertical structure and phase delay of the mixing rate will be determined. Bottom stress and drag coefficient estimates will be obtained from both ADCP and EPSONDE data. By examining the turbulent kinetic energy dissipation rate, bottom stress and related quantities with the velocity direction and the bottom structure at each site, parameterizations for the drag coefficient may be determined. With results from this analysis, it may be possible to determine the drag coefficient over all of Georges Bank using the ADCP data collected on the broad-scale surveys. Mixing processes must be understood and properly simulated in the numerical models before addressing the more difficult problems of prey-predator interaction under realistic conditions.

One of the basic goals of U.S. GLOBEC is to understand prey-predator interactions and the influence of physical processes on these interactions. Different assumptions on the feeding dynamics, distribution of plankton and turbulence lead to different conclusions on the importance of turbulence on fish larvae mortality (Rothschild and Osborn~1982, Davis et al.~1991, MacKenzie et al.~1994). Unfortunately, the direct interaction of turbulence and species' abundance is not possible with our data. However, the turbulence may have an indirect effect. For example, turbulence levels may affect the patchiness of prey (and predator) concentrations. The relationship between turbulence level and predator/prey distribution will be examined with biological and physical data collected on the cruises.

During June 1995, Mark Berman and Jack Green used the TAPS acoustic system to estimate plankton abundance through out the water column. The hour-long time-series provide data on the horizontal patchiness of the plankton as they are advected past the ship. Additional TAPS samples taken every two hours allow us to examine the fine-scale plankton variability over longer time periods. On both cruises, the CTD package contained a transmissometer and fluorometer which will also allow us to examine the relative distribution of plankton throughout the cruises.

Working with Mark Berman, Jack Green, Lew Incze and others, we will compare the distributions of plankton to the level of turbulence and determine if there are any relationships. If so, we will attempt to determine the basis of any relationships. Also, we will work Incze on theoretical aspects of fish larvae feeding behaviour in turbulent fields, especially concerning the patchiness of prey due to turbulent motions.