Answer:
Step-by-step explanation:
In a lake, oxygenic phototrophs produce new organic material as well as O₂. If primary production rates are very high, the resultant excessive organic matter production can lead to bottom-water O₂ depletion from respiration and the development of anoxic conditions. This in turn stimulates anaerobic metabolisms, including anaerobic respirations and fermentations.
Organic matter that is not consumed in surface layers sinks to the depths and is decomposed by anaerobes.