Final answer:
Cellular respiration by fish would lead to a lower pH in an aquatic ecosystem as carbon dioxide produced during respiration forms carbonic acid when dissolved in water.
Step-by-step explanation:
The question is aimed at understanding which processes in an aquatic ecosystem would not lead to a decrease in pH levels. Cellular respiration by fish is an activity that actually contributes to lowering the pH in water. When fish undergo cellular respiration, they produce carbon dioxide (CO2) as a byproduct. This CO2 can dissolve in water to form carbonic acid (H2CO3), which dissociates into bicarbonate (HCO3-) ions and hydrogen (H+) ions, the latter increasing the acidity of the water and thus lowering the pH.
So, cellular respiration by fish would not be the correct answer if one is looking for an activity that does not lower pH. Instead, other factors such as the presence of photosynthetic organisms would increase pH as they consume CO2 during photosynthesis.