When is the pH of a pond typically at its lowest?

The pH of a pond is typically at its lowest during the nighttime hours, specifically around midnight. This phenomenon occurs due to the absence of photosynthesis. During the day, aquatic plants and algae absorb carbon dioxide and release oxygen through photosynthesis, which raises the pH level. However, at night, photosynthesis stops, and respiration continues, which consumes oxygen and produces carbon dioxide, leading to an increase in dissolved carbon dioxide levels in the water.

As carbon dioxide dissolves in water, it forms carbonic acid, which lowers the pH. Therefore, the lowest pH levels are generally recorded during the night when photosynthesis is not taking place, culminating around midnight when the consumption of oxygen and the accumulation of carbon dioxide reaches a peak. This natural diurnal cycle is influenced by the daily light conditions, emphasizing the impact of photosynthetic activity during daylight on the water chemistry.