A faunistic and ecological characterization of the water mites (Acari: Hydrachnidia) of the Branew River (central-eastern Poland)

A characteristic feature of the Hydrachnidia communities of the Branew River, distinguishing its fauna from that of other Polish rivers, was the very high abundance of species of the genus Lebertia . Species of this genus accounted for as much as 53.1% of the collected material. The most numerous was Lebertia rivulorum , which was caught in numbers not found in other Polish rivers. Lebertia inaequalis and L. oblonga were also abundant. The largest synecological groups were rheophiles and rheobionts (94.9% combined). More individuals (1494) and species (27) were caught in the lentic zone of the river than in the lotic zone (1291 ind., 25 sp.). The species most associated with the lotic zone was L. rivulorum . This species was caught mainly on a substrate of gravel and stones with a small amount of sandy sediments, sparsely covered with Elodea canadensis . The species most associated with the lentic zone was Forelia variegator . The most abundant species in the Branew River, Hygrobates setosus , was caught in much higher numbers in the littoral zone than in the central part of the river. The high similarity of fauna was observed between the lotic and lentic zones, and much lower similarity between sites. The results indicate intensive species migration in the transverse profile of the river and low migration in the longitudinal profile. Higher species diversity was recorded in the lentic zone than in the lotic zone of the river – both in the river as a whole and at the individual sites. In the anthropogenically transformed stretch of the river (straightened riverbed, concrete dam, and concrete bottom), species diversity was significantly lower (H’ = 1.83) than in the natural stretch (H’ = 2.37). The results confirm literature data describing the negative impact of such transformations on Hydrachnidia communities. Despite anthropogenic transformations in parts of the river, the structure of the fauna (a very large proportion of rheobionts and rheophiles), as well as the physicochemical parameters of the water, is indicative of the good ecological condition of the river.