Data from: Phytoplankton species richness along coastal and estuarine salinity continua

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.