Some viruses found on the oceans surface can infect and kill plankton. How would this affect the fish population and human health?

Fish eat plankton, so if they are gone then the oceanic food chain would corrupt. ultimately the fish population would go down and there would be less fish in the ocean for humans to eat. also plankton clean the waters and get rid of any bacteria. so getting rid of plankton would mean unsanitary waters where more fish would die.