question on building a ecosystem in a jar with water algae & small aquatic animals

if a 1st grade class were to build a simple ecosystem in a jar, and add water, algae, and small aquatic animals that eat algae- then they weigh their algae and the animals at the end of the year. If they were to compare the total weight of all the algae and the combined weight of all the animals, which will weigh more?? EXPLAIN this using terms of energy and nutrients.what observations and inferences would be used to answer this?