Intense upper ocean mixing due to large aggregations of spawning fish

Abstract

Small-scale turbulent mixing plays a pivotal role in shaping the circulation and a broad range of physical and biogeochemical ocean processes. Despite advances in understanding geophysical processes responsible for this mixing, the nature and importance of biomixing -turbulent mixing caused by marine biota -remains controversial. A major source of uncertainty pertains to the efficiency of biomixing -the fraction of the turbulent energy produced through swimming that is spent in mixing the ocean vertically -, which the few in situ observations available suggest to be much lower than that of geophysical turbulence. Here, we shed light on this problem by analysing 14 days of continuous measurements of centimetre-scale turbulence in a coastal upwelling area. We show that turbulent dissipation is elevated 10 to 100-fold (reaching 10 −6 -10 −5 W kg −1 ) every night during due to the swimming activity of large aggregations of anchovies that gather regularly over the spawning season. Turbulent mixing is invigorated concurrently to dissipation, and occurs with an efficiency comparable

Limnology and Oceanography Bulletin Volume 33 Number 2 May 2024 1‐95

“…However, proof that they actually do make a significant contribution to turbulent diffusivity in the ocean does not yet exist, and that proof will require in situ observations of physical impacts that can be unambiguously attributed to animal swimming. The best efforts to date have relied on fortuitous encounters with aggregations of swimming animals, work that has incidentally been recognized with the 2023 Ig Nobel Prize in Physics (Fernandez Castro et al 2022). However, those studies are difficult to generalize, since in some cases the animals were not migrating vertically and in each case the primary focus has been on the energy dissipation that could be correlated with the presence of animals.…”

Smart toilets, bored students, why crowds gather, and tasty rocks—it’s the 2023 Ig Nobel Awards

“…• Physics: scientists from France, Galicia, Spain, and Switzerland for measuring the extent to which ocean water mixing is affected by the sexual activity of anchovies13…”

Isopycnal eddy stirring dominates thermohaline mixing in the upper subpolar North Atlantic

“…Profiles of potential temperature (θ), practical salinity (S) and surface-referenced potential density (σ θ ) with 1 m vertical resolution were derived by bin-averaging the CTD output. The dissipation rates of turbulent kinetic energy (ε) and thermal variance (χ) were computed from the microstructure shear and temperature measurements, respectively, with a vertical resolution of 1 m from overlapping data segments of 4 m length following Piccolroaz et al (2021);Fernández Castro et al (2022).…”

“…Generally, ε and χ θ were determined by integration of the shear and temperature gradient spectra over the well-resolved wavenumber ranges, and the variance outside those ranges was recovered using empirical spectral forms (Nasmyth and Kraichnan, respectively) (Fernández Castro et al, 2022) (Fig. S1).…”