Complex sentence: Because cold water holds more air than warm water, small bubbles will appear in water with a high dissolved oxygen content that is heated or depressurized, because this reduces how much dissolved gas the water can hold.
Split into simpler sentences: Because cold water holds more air than warm water, small bubbles will appear in water. It has a high dissolved oxygen content that is heated or depressurized, which reduces how much dissolved gas the water can hold.