The dissolved oxygen is a sensitive parameter for the environment quality. It is subjected to important variations in macrotidal estuaries with significant results on the fauna. It's why we can notice the important mullet death note in the Estuary of the Loire during sommer time. The supply of biodegradable organic substances in the maximum turbidity zone led to an important dissolved oxygen demand. It's the example of the Estuary of the Loire where we can watch this main fonction. A detailed study has begun on the dissolved oxygen cycle to compare the biochemical particulate and dissolved oxygen demand. This study collects in situ parameters, either of the environment evolution or of the dissolved oxygen cycle with specifie researches on the rate of degradation of the particulate and dissolved organic substances.