Phytoplankton Species Richness along Coastal and Estuarine Salinity Continua

• Published in: The American Naturalist
• Main Authors: Olli, Kalle, Ptacnik, Robert, Klais, Riina, Tamminen, Timo
• Other Authors: Centre for Limnology. Institute of Agricultural and Environmental Sciences
• Format: Article in Journal/Newspaper
• Language: unknown
• Published: 2019
• Subjects: biodiversity; biogeography; phytoplankton; Remane curve; salinity gradient; articles; National Environmental Research Institute
• Online Access: http://hdl.handle.net/10492/7906; https://doi.org/10.1086/703657

The high number of freshwater species at low salinity and the correspondingly 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 Re- mane 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 approximately 16,000 phytoplankton samples from two large coastal ecosystems—the Baltic Sea and Chesapeake Bay—to analyze the relationship between salinity and phytoplank- ton species richness. Alpha diversity showed a consistent variation along the salinity gradient, with a minimum at mesohaline salinities of around 7–9. Rarefied species pools at narrow salinity intervals also showed reduced diversity at intermediate salinities, surrounded by high richness toward both ends of the gradient. The cumulative like- lihood of species presence validated the minimum at intermediate salinities. Community composition changed abruptly at the a diversity minimum in the Baltic Sea, while it changed gradually along the salinity gradient in Chesapeake Bay. We conclude that the Remane concept is in every respect valid for phytoplankton. This study was supported by the Estonian Research Council (grant PUT1574 to K.O.) and Academy of Finland (grants 128987 and 268953 to T.T.). Kevin G. Sellner (Chesapeake Research Consortium) provided data for Chesapeake Bay. Baltic Sea data were provided by the Finnish Environment Institute, Finnish Institute of Marine Research, City of Hel- sinki Environmental Centre (Finland), Institute of Aquatic Sciences (Latvia), Stockholm University (Sweden), Institut für Ostseeforschung Warnemünde (Germany), National Environmental Research Institute (Denmark), and Estonian Marine Institute. Constructive comments from two anonymous reviewers improved the manuscript. ...