The aquatic environment is the greatest habitat to be continuously exploited by organisms. Liquid water presently covers about 71% of planet Earth, the sea alone occupying 361.3 x 106 km2 of it. The estimated volume of the sea (~1 350 000 000 km3) accounts for 97.4% of all the water on the planet. Taking off the volume stored in the polar ice caps (27.8 x 106 km3) and the amount stored in the ground (~8 x 106 km3), the balance, shared between lakes, rivers and the atmosphere, is less than 0.02%. However, even this fraction, totalling c. 225 000 km3, is overwhelmingly dominated by the volume of standing inland waters: the 13 largest lakes in the world (by volume) alone hold 160 000 km3 (Herdendorf, 1990). At any moment of time, most of this volume is actually so inhospitable to primary producers that it is not conducive to phytoplankton development but, because it is fluid and in persistent motion, all the volume is potentially available, sooner or later. The global rate of the hydrological renewal, in the cycle of precipitation, flow and evaporation, results in an estimated annual loss from the ocean of 353 000 km3, made good by direct precipitation (roughly, 324 000 km3) and net river run-off from the land masses (~29 000 km3). The theoretical replacement time for the ocean is thus around 3800 years.
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