Hello, I'd like to know what is meant by viral turnover in the following context:

Rough estimates suggest that the lysis of cells by marine viruses removes 20 to 40 per cent of all of the bacteria every day.
This astonishing viral turnover of planktonic microbes fertilizes the water below the sunlit surface of the sea.

turnover here is referring to changing one thing to another. It’s caused by the viruses, so it’s viral turnover. Whatever the products of the lysis are is the fertilization.