Encounter Rates in Zooplankton
L. Dzierzbicka-Glowacka
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Institute of Oceanology, Polish Academy of Sciences,
Powstańców Warszawy 55, PL-81-712 Sopot, Poland
Pol. J. Environ. Stud. 2006;15(2):243-257
The influence of turbulence (turbulent kinetic energy dissipation) on predator-prey interactions in zooplankton is discussed with respect to the combined effect of the choice of the turbulent length scale, and size and velocity of predator and prey concentration on the encounter rate. The significance of correct scaling to the turbulent encounter velocity is demonstrated, with three different definitions being considered: the average prey separation, the Kolmogorov scale, and the predator's reactive distance. Numerical investigations using these different definitions of scale were carried out to find the convergence conditions and the behaviour of the scale values for 5-10 mm fish larvae which feed off copepod nauplii in the 104-105 m-3 concentration range. The choice of the turbulent length scale is not important for small predator body sizes <5mm and high prey concentrations in the 107-108 m-3 range, which are reasonable prey densities for a 1-3 mm copepod (i.e. algae and protozoans). Also in the quasi-laminar regime of water flow (l=2πη) and high prey concentrations, the choice of correct scaling is not important. Predators of any body size will forage in such a regime immovably (swimming velocity v ≈0). However, for large larval lengths >10 mm and prey concentrations <106 m-3, the scale can be defined as the average prey separation or as the predator's reactive distance. The effect of turbulence on the encounter rate decreases with the increasing size and velocity of the predator and with prey concentration. A simple one-dimensional prey-predator ecosystem model in the upper mixed layer is presented, which examines the relative importance of turbulence to growth in planktonic consumers. This effect is less for low prey densities <104 m-3, when the initial predator biomass and constant growth rate term have a decisive influence. However, the effect of turbulence on the characteristics investigated increases with rising prey density, in which case the controlling factor is encounter rate and in the case of prey concentration, diurnal migration.
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