Environmental preferences of Polysiphonia subtilissima (Ceramiales, Rhodophyta) in transitional, oligohaline and fresh waters of the East Adriatic coast

Polysiphonia subtilissima is originally a marine macroalga with a wide tolerance for salinity and therefore present on nearly all the coastlines of the planet. However, the transition of P. subtilissima toward freshwater ecosystems has been noticed worldwide, with numerous authors recording oligohaline and intertidal populations of notable size, although only three localities (Florida, Jamaica and Spain) with truly freshwater populations have been registered to date. This paper reports large populations of P. subtilissima in transitional, oligohaline and freshwater ecosystems of the Adriatic Sea Basin (Croatia, Southeast Europe). The alga was recorded in salinity ranging from 0.42 to 4.59 in the waters of the Neretva River Valley, and at 18.2 in transitional waters in the Zrmanja River. Along with P. subtilissima, the freshwater tropical red alga Compsopogon caeruleus has been recorded in the Neretva River Valley, being the first record of this species for Croatia and SE Europe. This particular combination of species has previously been recorded in freshwater localities only in Florida, Jamaica and Spain. Measured environmental parameters and species composition were analysed using canonical correspondence analysis and Gaussian response model, which showed that both species prefer waters with temperatures above 20 degrees C, high water conductivity, pH values above 7.5, and elevated water turbidity.