Question

what most likely causes the trends in oxygen concentration shown in the graph above?
oxygen concentration in the water of a lake
the water becomes colder at night and thus holds more oxygen.
respiration in most organisms increases at night.
more organisms are respiring at night than during the day.
photosynthesis produces more oxygen than is consumed by respiration during the day

Explanation:

During the day, photosynthesis occurs (plants produce oxygen), and at night, only respiration occurs (organisms consume oxygen). The graph shows oxygen concentration decreasing at night (when there's no photosynthesis) as respiration by organisms continues.

Answer:

Photosynthesis produces oxygen during the day (increasing oxygen levels), and at night, only respiration (which consumes oxygen) occurs in organisms, leading to the oxygen - level trends shown in the graph.