The relative importance of hydrological and biological factors in driving the diel distribution of the copepod Cyclops abyssorum from small to large scale is evaluated here in a lake where a complex hydrodynamics is at work and a clear environmental gradient is present. A set of statistical tools (which include the proposed "partial variance") are used for characterizing the scale patterns of distribution and their daily trend. A day -night aggregation/dispersion process is observed at any of

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... e spatial scales and directions considered. The value of the fractal dimension of the copepod distribution (D-horizontal = 1.89) suggests that short-range effects prevail over long-range ones in affecting the overall complexity of distribution. The geostatistical analysis shows that individuals form isotropic swarm-like assemblages of a few metre diameter during daytime which relax during night. The cohesion among population members of different age and sex also shows a daily fluctuation, with separation distance increasing vertically during daytime and horizontally during night. The present study shows how visual predation affects the whole structure of patchiness and explains the diel spacing among population fractions, whereas food availability prevails over water transport in driving the copepod distribution at a large scale.