High number of freshwater species at low salinity, and a corresponding high number of marine species at high salinity, enveloping a conspicuous richness minimum at intermediate salinities, has shaped our basic understanding of biodiversity along a coastal salinity gradient for almost 80 years. Visualized as the ‘Remane curve’, this iconic concept was originally based on sedentary macroinvertebrates in the Baltic Sea. To what extent the concept can be generalized, particularly to free-drifting organisms, is currently debated. Here we use ca 16 000 phytoplankton samples from 2 large coastal ecosystems, the Baltic Sea and the Chesapeake Bay, to analyze the relationship between salinity and phytoplankton species richness. Alpha diversity showed a consistent variation along the salinity gradient, with a minimum at mesohaline salinities at around 7 – 9. Rarefied species pools at narrow salinity intervals also showed reduced diversity at intermediate salinities, surrounded by high richness towards both ends of the gradient. The cumulative likelihood of species presence validated the minimum at intermediate salinities. Community composition changed abruptly at the α diversity minimum in the Baltic Sea, while it changed gradually along the salinity gradient in the Chesapeake Bay. We conclude that the Remane concept is in every respect valid for phytoplankton.