A reservoir with a low content of organic matter and carbon dioxide, high oxygen saturation and the complete absence of hydrogen sulphide. It is inhabited by oligosaprobic micro-organisms. An oligosaprobic medium is an aqueous medium rich in dissolved oxygen and (relatively) free of decomposed organic matter. Oligosaprobes are organisms that live in clean water or water that is only slightly polluted with organic matter. In such waters, oxidation processes predominate due to an excess of dissolved oxygen. Nitrates are among the nitrogen compounds present; they contain little carbon dioxide and no hydrogen sulphide.