Short time dissolved oxygen dynamics in shallow water ecosystems

Dissolved oxygen evolution over time holds information about the recent and past history of the physics, the chemistry and the biology of aquatic systems. On the daily time scale various phenomena seem to repeat with a sort of regularity, leaving a trace in the further trend of oxygen dynamics. These are the wind calm of nocturnal and early morning hours, the advent of the afternoon breezes, the micro- and macro-algae diurnal production, the chemical and biochemical processes. On the larger seasonal scale, dissolved oxygen shows an almost periodical trend. In this case, a greater dependency on the climatic events leads to the colder and windy season, while the warm and hot period is closely related to the vegetable production, which frees remarkable amounts of oxygen in the water through photosynthesis. Result of experimental measurements of dissolved oxygen, acquired with an automatic equipement on a very short time scale (30 s), are shown and discussed. The experiments were run to detect oscillating phenomena characterized by a period which is shorter than the daily occurrence, as well as stochastic phenomena, led by almost periodical drifts, which contribute substantially to the oxygen dynamics on the longer time scale. To determine these pseudo-periodicities, or time variation for the oscillation period, the analysis of the temporal distances between local maxima of oxygen concentrations has been performed. This kind of analysis is based on the hypothesis that we are dealing with a dissipative dynamical system whose evolution is similar to the behavior of some non linear oscillating systems with a control, where the control can be represented by a simple function of the temporal interval between two consecutive peaks. The result of the analysis suggests the existence of dynamics on a 6–8 h time scale.