“Plankton” is a category including a lot of things, most of which are more important in their current role than they would be viable as a protein source for humans. A lot of plankton is microorganisms made entirely out of silica or calcium carbonate (probably not nutritious for humans). A lot more is the eggs and larvae of fish and crustaceans. If humans started eating this, it would cause ecological disaster even faster. I think at some point we’re going to have to normalize eating bugs, at least those can be farmed easily…