American Society of Naturalists

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“Phytoplankton species richness along coastal and estuarine salinity continua”

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Kalle Olli, Robert Ptacnik, Riina Klais, and Timo Tamminen (Aug 2019)

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Phytoplankton diversity-salinity relation follows Remane curve: High richness at fresh and saline ends, low at brackish

Phytoplankton species richness: a minimum at brackish salinity

<i>Monoraphidium</i>, a planktonic green algal genus of freshwater origin, which thrives aslo in the brackish waters of the Baltic Sea. Scale bar = 50 µm.<br/><br/><br />(Credit: Kalle Olli)
Monoraphidium, a planktonic green algal genus of freshwater origin, which thrives aslo in the brackish waters of the Baltic Sea. Scale bar = 50 µm.
(Credit: Kalle Olli)

Throughout the evolutionary time scales, the boundary between freshwater and ocean salinity has been a tough one for aquatic organisms to cross. Compared to freshwater and the ocean, a lower number of species thrive at brackish salinity. This textbook knowledge stems from the classical work by Adolf Remane with benthic macro-invertebrate diversity in the Baltic Sea, and has shaped our understanding of species richness along the salinity gradient for 85 years.

When it comes to phytoplankton, the base of the aquatic food chain, it is surprising how little scrutiny the matter has received. This is partly due to the rareness of high-quality phytoplankton data sets covering the full salinity range from fresh water to the ocean. Recently some researchers have proposed a reversal of Remane’s concept for protists and phytoplankton, with a richness maximum at brackish salinities.

Analysis of sizeable phytoplankton data sets from two large coastal ecosystems, the Chesapeake Bay and the Baltic Sea, unambiguously corroborated the validity of the Remane concept in terms of both alpha and gamma species richness. These analyses, based on >15,000 phytoplankton samples, revealed in both data sets minima in species richness at salinities around 7-9, and increasing diversity towards both limbs of the salinity gradient.

Why does this matter? Biodiversity is the basis of ecosystem functioning and services; hence, understanding the drivers of biodiversity become mandatory for human wellbeing. Our results support the idea that ecological scaling rules apply to microbial diversity in ways similar to what is known from the macrobial realm. The results enable us to predict changes in diversity, and the associated ecosystem functions, in the era of global change, where coastal and estuarine salinity gradients are shifting due to changes in precipitation and hydrology.


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.

Fütoplaktoni liigirikkus rannikumere ja estuaari soolsusgradientidel

Ligi 80 aastat on meie arusaama vee organismide liigirikkusest soolsusgradiendil kujundanud paradigmaks muutunud Remane kõver. Selle järgi on magevee liigirikkus suur, suur on liigirikkus ka ookeanis, kuid vahepealses riimveelises osas on tuntav liigirikkuse madalseis. Adolf Remane sedastas oma seaduspära uurides põhjaeluliste suurselgrootute liigirikkust Läänemeres. Kas ja millisel määral on seaduspära kehtiv teiste organismirühmade puhul, eriti vabalt hõljuvate plankterite puhul, ei ole teada. Me analüüsisime ligi 16,000 fütoplanktoni proovi liigirikkust kahe suure ökosüsteemi, Läänemere ja Chesapeake Lahe soolsusgradientidel. Proovide liigirikkus oli madalaim gradiendi mesohaliinses piirkonnas, soolsusel 7 – 9. Ka harvenduskõverad kitsastes soolsusvahemikes näitasid madalat liigirikkust mesohaliinses piirkonnas ja fütoplanktoni liigirikkus suurenemist nii magevee, kui ookeani soolsuse suunal. Liikide kumulatiivne esinemise tõenäosus soolsuse gradiendil näitas samuti liigirikkuse miinimumi riimveelises osas. Chesapeake Lahes muutub fütoplanktoni kooslus mageveelisest mereliseks sujuvalt kogu soolsusgradiendil. Kontrastina, Läänemere eripäraks on fütoplanktoni koosluse järsk muutus liigirikkuse miinimumi piirkonnas. Analüüsi tulemused näitavad, et Remane printsiip on täiel määral kehtiv ka fütoplanktoni puhul.