Taxonomic composition of zoopsammon in fresh and brackish waters of Estonia, a Baltic province ecoregion of Europe

INTRODUCTION Zoopsammon is a diverse group of organisms living in the interstitial spaces between sand grains at the shoreline. The term psammon is nowadays mainly used to characterize freshwater sandy beach habitats, although it was originally defined as 'a transitional zone between aquatic and soil habitats' (Schmid-Araya, 1998), and is also applicable to brackish and marine beach habitats, where the intertidal zone can be considered as psammon habitat if the sediment consists of sand and is regularly exposed (e.g. Tzschaschel, 1983; Golemansky, 1998; De Smet and Chernyshev, 2006; Alekperov et al., 2007). However, in the marine literature psammic comunities are more often referred to as meiofauna. Psammon communities have received very little attention compared to lower littoral and sublittoral meiobenthos, and psammon has been rarely treated as a single entity. Despite its unstable and very fluctuating environment, the arenal zone hosts a large variety of species (Pejler, 1995; Golemansky, 1998; Gheskiere et al., 2005). Protists, nematodes, rotifers, small crustaceans, tardigrades, gastrotrichs, turbellarians, oligochaetes, and insect larvae are regularly found from psammon habitats (Thane-Fenchel, 1968; Whitman and Clark, 1984; Schmid-Araya, 1998, Kotwicki et al., 2005a, 2005b). From psammic taxa, rotifers have received relatively much attention in freshwater habitats (e.g. Pejler, 1995; Bielariska-Grajner, 2001; Segers and Chittapun, 2001). In marine beach habitats, nematodes tend to be the most thoroughly researched psammic taxa (e.g. Gheskiere et al., 2004, 2005; Liu et al., 2008; Maria et al., 2012, 2013). From the Baltic Sea area some information is available on psammic ciliates (Czapik and Fyda, 1992), testate amoebae (Golemansky, 1998), rotifers (Thane-Fenchel, 1968; Sorensen, 2001), and nematodes (Gheskiere et al., 2005). In Europe, freshwater zoopsammon has received more attention in Poland (e.g. Bielariska-Grajner, 2001; Ejsmont-Karabin, 2003; Nesteruk, 2007; Kalinowska, 2008, 2013; Bielaiska-Grajner and Poznaiska, 2010). At the Estonian water bodies, the coastal zone was sampled already in the 19th century. In these studies, some beach invertebrates were recorded (Eichwald, 1849, 1852; Levander, 1894). However, it is not known where exactly the samples were taken (water or sediment, at waterline/beach or deeper water). First true reports on the interstitial fauna, i.e. zoopsammon, of sandy beach in Estonia date from the 1980s. In these sampling campaigns, Golemansky (1983) investigated psammic testate amoebae from the Estonian coast of the Gulf of Finland, and Kutikova and Haberman (1986) determined the taxonomic composition of rotifers from the arenal zone of Lake Vortsjarv. Recently, some information has been provided about the taxonomic composition and density of zoopsammon communities, their temporal and spatial distribution, and relation with various environmental variables in some Estonian coastal beaches (Lokko et al., 2014) and in two lakes (Lokko et al., 2013; Lokko and Virro, 2014). The aim of this study is to provide an overview and sum up the current knowledge on the taxonomic structure of zoopsammon in Estonian waters. This summarized information serves as a basis of the current status of psammic communities both in freshwater and marine habitats and potentially allows assessing the current status and conservation value of Estonian interstitial beach habitats. MATERIALS AND METHODS The present study is based on two previous studies (Lokko et al., 2013, 2014) and an additional sampling campaign carried out in 2011-2014. Samples were taken from three sandy beaches along the Estonian coast of the Gulf of Finland, Baltic Sea, and from four lakes across Estonia (Fig. 1). The coastal sampling sites were located at Keibu Bay near Nova village (Nova Beach, the westernmost sampling site), at Pirita Beach within Tallinn City, and at a popular beach of Narva Bay adjacent to a small town of Narva-Joesuu (the easternmost site). …