Modern assemblage changes of benthic algae as a result of hypersalinization of the Aral Sea

Abstract

The benthic algal coenosis, consisting of microphytobenthos, macrophytes, and microepiphytes, exhibited changes related to gradually increasing salinity in the Large Aral Sea from 2002 to 2005. Since 2000, the diversity has decreased from 159 species [Mirabdullayev, E.M., 2004. Succession of the ecosystems of the Aral Sea during its transition from oligohaline to polyhaline water body. The Dying Aral Sea, Selected Papers from the 35th International Liege Colloquium on Ocean Dynamics. J. Mar. Syst. 47 (1–4), 101–107.] to 38 species of microepiphytes, including mobile forms. We investigated species composition, assemblage structure, and spatial distribution. The salinity of the Large Aral Sea, which recently separated into two parts (a shallow eastern and a deep western basin connected by a channel to the north), ranges from 95–130‰, with the highest values in the eastern basin and in the channel. The algal flora of the Large Aral Sea currently consists primarily of diatoms. Diatom species previously restricted to the eastern basin ( Amphora coffeaformis, A. acutiuscula, and Synedra tabulata var . parva) spread into the western basin and species adapted to salinity < 90‰ ( Actinocyclus ehrenbergii) are gradually disappearing from the assemblages. However, the majority of the remaining algal flora species have adapted to the steadily increasing salinity. Some of the euryhaline species observed in the western basin exhibit a salinity tolerance > 90‰, which has not been described before. As a result of increasing salinity (91 to 97‰) in the western basin surface water layer, a replacement of “western” by “eastern” epiphytic type assemblages started in 2004–2005.