Arnaud Bertrand, Institut de Recherche pour le Développement, France, addressed the SOMEACOUSTICS symposium with his lecture, From scatters to processes.
Living organisms follow non-random yet non-uniform distributions and tend to aggregate in patches. Both physical forcing and organism behaviour are implicit in the initiation and maintenance of this patchiness, with the latter increasing in importance with each step up the trophic chain. Observations on fine scale ocean dynamics (10 m to kms) do exist but are too fragmented to facilitate the development of comprehensive models over a range of scales. For this reason, the incorporation of these processes in models of marine ecosystem dynamics is still in its infancy, particularly at scales below the mesoscale.
Underwater acoustic techniques have an unrealized potential for multi-component observations of abiotic and biotic characteristics that can overcome previous limitations. A growing number of studies are taking advantage of recent improvements (e.g. multifrequency, broad band echosounders) to simultaneously characterize physical structures (e.g. thin layers, internal waves, eddies) and organisms patterns of distribution across scales from meters to thousands of kilometres. For instance, a series of acoustic-based studies addressed the bottom-up structuring theory and showed how (i) mesoscale (20-100 km) eddies creates pelagic oases for a variety of trophic levels; (ii) submesoscale (1-20 km) fronts modify zooplankton habitat and behaviour; or (iii) internal wave activity (100 m - kms) impact organisms distribution and behaviour.
However, understanding the processes in play and fully quantifying the bottom-up structuring require characterizing physical processes along-scales and quantifying their impact on biological components. Acoustics overcome limitations inherent to other sampling method. For instance acoustics allows for a robust estimation of the oxycline depth of the oxycline depth in regions where an oxygen minimum zone occurs. The vertical deformations of the oxycline reveal the upper ocean turbulence. From this information, available each second along the survey tract, it is possible to extract the upper ocean physical structures along scales (100 m to 100 km) over a large oceanic region. Tens of thousands of physical structures including internal waves or submeso- and mesoscale eddies can be extracted and characterised. The impact of these structures on acoustically-detected zooplankton and fish can then be estimated.
Primary ecosystem interactions were believed to occur at meso- or submesoscale but novel acoustic data reveal that the upper ocean dynamics at scales less than 10 km play the foremost role in shaping the seascape from zooplankton to predators. Ocean surface turbulence creates ephemeral oases where the majority of interaction most likely occurs. Interestingly the aggregation power of these structures is stronger for fish than zooplankton suggesting that behaviour can magnify physically induced spatial structuring.
Figure shows comprehensive description of the pelagic ecosystem
off Peru from acoustic data. (a) Upper volume: acoustically estimated lower
oxycline (in m). Intermediate surface: zooplankton biovolume above the lower
oxycline. Lower surface: fish biomass above the lower oxycline. (b) Acoustic
echogram along a given transect (see a) with the lower oxycline (black solid
line).Redrawn from Bertrand et al. (2014).