The eggs and larvae of marine animals tend to require water with more dissolved oxygen in it than the adults do. if water flows from a freshwater source into a salt water bay, what effect would this have on the population of marine animals?

The population will drop.

Explanation:

This is because the fresh water of high water potential will lower the solute potential of the salt water when they mixed up; to create a HYPOTONIC ENVIRONMENT for the larvae and the eggs.

Therefore, the higher water potential drives in water into the larva and egg membrane, down the water gradient by OSMOSIS. This made the cells and its organells  to swell up until the elastic limit of the cell membrane is exceeded.

The products busted (eggs and larvae) once the elastic limit / stretching limit is exceeded   leading to death  of larvae and destrction of the egg, with eventual drop in population  of the  marine animals; since  reproduction has been truncated( old animals are not replaced with young ones from the eggs and larvae.)