The benthic zone is the region of sea (or ocean) that includes the topmost sediment layer and some water layer over it. This ecological zone is dominated by anaerobic bacteria. This region is deficient in sun light, since the sun's rays are incapable of penetrating the great depths where benthic zone exists. The productivity of this region is pretty low, since photosynthesis is not possible in this area. The organic matter from shallower depths slowly falls down towards this zone. This organic matter consists of dying plants and animals, apart from any organic matter added by other sources (such as rivers merging into ocean). When this falling matter is greater in quantity than what can be utilized by benthic organisms, hydrogen sulfide and methane are generated. This entire process results in consumption of dissolved oxygen and causes prevalence of anaerobic conditions. Under such conditions (anaerobic and low productivity), anaerobic bacteria dominate the microbial population.
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