Temporal (and Spatial) Scales and Sampling Requirements in Environmental Flows (with Emphasis on Inland and Coastal Waters)

The basic time (and length) scales governing the physical transport and mixing processes in aquatic environments are briefly reviewed in an ecohydrodynamic perspective. Such time scales are: the molecular diffusion time, T_d, the falling particle time, T_f, the mixing time, T_m, the advection time, T_a, and the Kolmogorov (or viscous) time, T_k. For large water bodies, two more time scales can be formulated based on the Coriolis frequency, f_c, and the Kibel frequency, f_k. These time scales form several spectral windows, which correspond to the scales of external forcing or of intrinsic mechanisms, determine the hydrodynamic processes that may significantly interact with the various populations of the aquatic communities, and govern the dynamics of the aquatic system. Motions at the time scales of the weather of the aquatic environment are resonant with the ecosystem dynamics and impose to the ecosystem certain length scales through the process of ecohydrodynamicadjustment. Knowledge of such characteristic time scales facilitates the selection of appropriate strategies for sampling environmental quantities and satisfying the frequency sampling requirements.