Even without viruses, plankton still would’t be able to expand beyond the maximum level water resources support. We need to know why the plankton population actually shrank.

If plankton and viruses are competitors, then the plankton population should’ve grown without the viruses. Instead, it shrank, and this doesn't tell us why.

Viruses kill organisms which in turn feed plankton. Without viruses, the plankton population simply had fewer nutrients available. This explains why the plankton population shrank.

The language here is very weak. We don’t know if bacteria in fact flourished without the viruses, or if plankton are one of the organisms those bacteria can damage.

This doesn’t explain why the plankton population shrank without the viruses. Whether or not many plankton were already infected, we need some other reason why the plankton population shrank further once the viruses were gone.