Turbulence, Acoustic Backscatter, and Pelagic Nekton in Monterey Bay

Abstract

During August 2006 aggregations of nekton, most likely small fish, intersected microstructure survey lines in Monterey Bay, California, providing an opportunity to examine biologically generated mixing. Some aggregations filled the water column, 80 m deep, and extended 100?200 m along the survey track. Others were half that size, and some were much smaller. Acoustic energy backscattered from the aggregations was measured with a calibrated echosounder and yielded volume backscattering strength Sv values of ?80 to ?60 dB re 1 m?1. Turbulent dissipation rates ? were 10?6?10?5 W kg?1 in the more intense aggregations. Within these, ? was much more uniform than turbulence measured outside the aggregations and varied with S?. Three similar aggregations contributed half of the average ? in 142 profiles taken along a 5-km-long survey line during a 12.5-h tidal cycle. Turbulence within aggregations differed markedly from that outside in the following three ways: (i) Thorpe scales, that is, root-mean-square overturning lengths, were much smaller than Ozmidov scales, LOZ ≡ (?/N?3)1/2, the upper limit for overturns limited by stratification. (ii) Spectra of small-scale shear matched the universal shape only in the viscous high-wavenumber rolloff. At lower wavenumbers, the shear spectra had slopes closer to k1 than to the k1/3 Kolmogorov slope with a corresponding velocity spectra peak near 8 cpm. (iii) Temperature gradient spectra matched portions of the universal Batchelor spectrum that could be resolved, but their magnitudes were smaller than those in turbulence produced by flow instabilities. Average mixing efficiency Γ was 0.0022 within aggregations, compared with 0.23 outside. This 100-fold decrease in efficiency compensates for a 100-fold increase in ? to produce no net change in diapycnal diffusivity K?. Because the composition and behavior of the aggregations were not confirmed?that is, whether the nekton had regular spacing and swimming patterns characterizing schools?it is not possible to know whether one or many turbulent states were sampled within aggregations. If these observations are representative, mixing in aggregations may be more interesting than it is important, but the data are too few for definitive conclusions.