Oxygen depleted zones at varying depths in marine waters between 100-1000m are called

photic zone The photic zone or daylight (Sunlit) zone is the part of the water in a lake or sea that is presented to such power of daylight which assigns remuneration point, i.e. the force of light at which the rate of carbon dioxide take-up, or identically, the rate of oxygen generation, is equivalent to the rate of carbon dioxide creation, comparably to the rate of oxygen utilisation, diminishing in this way the net carbon dioxide digestion to zero. Since the photic zone is the place the greater part of the photosynthesis happens, the profundity of the photic zone is for the most part relative to the level of essential generation that happens here of the sea. Around 90% of all marine life lives in the photic zone. A little measure of essential creation is produced somewhere down in the deep zone around the aqueous vents which exist along some mid-maritime edges.

When partially digested carbs reach the colon, bacteria convert part of that food into hydrogen and carbon dioxide gasses

No its just makes the straight guy curious