Oxidation-reduction potential (ORP) manifests a similar trend to that of dissolved oxygen; when DO is high, ORP tends to be large as well. Generally the upper part of the bottom mud layer of lakes is oxidized, the reduction potential is gradually lowered from the middle layer. It is believed that this is because the respiration of microbes and bottom feeding animals dissolves organic materials to generate inorganic substances that consume DO, whereas the (lower) reduction layer contains microbes which dissolve organic materials through anaerobic respiration and fermentation; consume nitrates through a denitrification process; and generate hydrogen sulfide through sulfuric acid reduction and methane from methane fermentation.