Mid-level tide pools are more stable when it comes to temperature and salinity, since the tide replenishes the water during every tidal cycle. However, changes in pH can also be stressful. Mid-level pools generally have more algae growing in them, then the upper tide pools. At night when photosynthesis stops, carbon dioxide can build up in the pool creating an acidic (lower pH) environment. When photosynthesis resumes in the daylight the algae consume the carbon dioxide and the tide pool becomes more alkaline (higher pH).