Comment by HillRat

The problem is that the ocean is not amenable to random sampling in the same way that, say, Twitter accounts are; phytoplankton are going to be differentially-distributed depending on where in the euphotic zone you're sampling, which latitudes, distance to land, time of year, and so on, which is why longitudinal biodiversity studies are carried out in specific areas. It's vanishingly unlikely that 500 water samples of unknown provenance (and I'm curious why they weren't sampling using the standard method of towed plankton nets) can demonstrate a global extinction event of all plankton, though it could indicate a loss of biodiversity in certain areas. God knows we're working hard to empty out our oceans, but I'm not sure that this paper demonstrates what they claim.