Influence of animals on turbulence in the sea

Abstract

Analysis of data on the hydrodynamics of swimming by 100 species, ranging in body mass (M) from bacteria to blue whales, leads to a model of animal-induced turbulence in the ocean. Swimming speeds and Reynolds number (Re) are strongly correlated with body mass, both at typical cruising speeds and at escape speeds associated with predator-prey interactions. We find that animals operating at Re > 1000 typically form schools that are concentrated by many orders of magnitude above their average abundance. We calculate the rate of kinetic energy production by 11 representative species of schooling animals ranging in size from euphausiids to whales, and find it to be of the order of 10 -5 W kg -1 , regardless of animal size. Animal-induced turbulence is comparable in magnitude to rates of turbulent energy dissipation (e) that result from major storms. The horizontal length scale (10 to 1000 m) of energy production rate by animal schools is comparable to the observed fine-scale variability in e. We present detailed case studies of 4 species — Atlantic bluefin tuna, Norwegian herring, northern anchovy and Antarctic krill — all of which have schooling behavior that places them within the zone of maximum seasonal stratification where their energy production rate would be 3 to 4 orders of magnitude greater than the background average rate of turbulent energy dissipation. We conclude that schooling animals are an important source of fine-scale turbulent mixing in the ocean, especially in coastal regions during summer.