It is a discrete aquatic ephemeral ecosystem, containing food and what eats the food. Occasionally birds bring things into the pool. These pools form because something below the pool keeps the water in them, e.g., clay pan, bedrock or a volcanic layer. This solid keeps water from percolating through the ground and only evaporation gets rid of the water. What changes in a pool over the year is the volume of water, the temperature of the water, the turbidity of the water, the oxygen concentration, the food availability, crowding and the periods of inundation.
Vernal pools are hotbeds of evolution.
There are lots of things that can't live here. Different species can tolerate different periods of inundation. Plants and algae have to get to the pool first. Then seed shrimp eat the algae. The hatching time of the seed and clam shrimp is important so that they have food to eat when they hatch.