Algae are the bottom of the ocean food chain. Everything interacts with it. But algae's happy to grow in a bowl of water left in the sun.

Lots of things eat krill and small fish. They're near the bottom of the foodchain too. In addition to algae, krill are opportunistic omnivores who often consume detritus. But their primary diet is algae. Small fish tend to be pretty similar.

It's not that other things don't interact with algae or krill or small fish, it's that those groups are the foundation bedrock of the ocean ecology. And single celled organisms like algae are tough as nails in aggregate. Couldn't kill them all if we tried. Pool owners will be familiar with the struggle.