Multiscale variability of tropical ocean zooplankton biomass

The variability of zooplankton biomass on a scale of thousands, hundreds, and tens of kilometres was estimated from the data bank of over 40 expeditions to the Atlantic and Indian Oceans and seas of the Mediterranean basin. Thirty oceanographic grid surveys were used to estimate variability on a scale of hundreds of kilometres. Continuous records within these grids were used to estimate zooplankton variability on the scale of tens of kilometres, and high-resolution sampling was used to investigate variability of kilometre scale. In the multiscale variability of zooplankton biomass, the maximum variability was observed on the thousands of kilometres scale, with the quantitative values of biomass represented in a form of normalized variance. The local peak of variability of zooplankton biomass in the range of hundreds of kilometres, in the tropical ocean, was probably due to the enhanced water dynamics with the same scale range. This activity is linked with open-ocean mesoscale eddy fields in the tropical zones of both oceans. The other typical feature of the structure of the zooplanktonic fields is its spatial anisotropy, which indicates different properties of spatial variation of parameters along directions through space. The anisotropy was evaluated by means of two-dimensional spatial autocorrelation functions. Two-dimensional correlation ellipses of the zooplankton biomass fields were orientated by their main axes in accordance with the direction of transport of the main water mass, the direction of motion of the eddies, and the orientation of divergence or convergence zones. The spatial heterogeneity of zooplankton biomass distribution in the horizontal plane can be characterized by the frequency of occurrence of patches. On a log scale the frequency of occurrence of different size patches diminishes proportionally with their linear size. Zooplankton biomass is distributed more heterogeneously than that of phytoplankton (chlorophyll a). Synchronous measurements of the three-level system "phytoplankton-mesozooplankton-flying fish" (where each component acts as a prey item for the next one) exhibit the same trend of spatial autocorrelation functions, diminishing over the trophic levels on a scale from hundreds to tens of kilometres. This means that relatively heterogeneous fields of predators exist on more uniformly distributed fields of their prey.