TEN YEARS QUANTITATIVE DATA ON A POPULATION OF RHINOGLENA FERTtiENSIS (BRACHIONIDAE, MONOGONONTA) A. HERZIG Institut fiir Limnologie, asterreichische Akademie der Wissenschaften, A- 1090 Wien, Berggasse I 8/r 9, Austria Keywords: Rhinoglena, winter plankton, shallow lake, abundance, resting eggs, distribution Abstract Some information, based upon a ten years study, is given on the ecology of Rhinoglena fertiiensis, a cold stenothermic rotifer. The largest numbers were always found during winter months under ice. A remarkable increase in population density is re- corded throughout the investigation and is explained by the good quality of the food supply and a parallel increase in its abun- dance. The dynamics of this species appear to be closely con- nected with water temperature gradients. The distribution of Rhinoglena fertiiensis is discussed in connection with water chemistry. Introduction The cold stenothermal genus Rhinoglena inhabits the pelagial of small and/ or shallow lakes (Ruttner-Kolisko, I 974). It appears from September/ October to May and, up to now, only in athalassic, saline waters of low concentra- tion. The first description is found in Varga (I 929) from Neusiedlersee. In Germany, it was first found by Remane between 1934-1936 in Siisser See, Central Germany accord- ing to Althaus (1957) who confirmed this record during her studies on the rotifer fauna of nine athalassic, saline water bodies in the surroundings of Halle/ Saale. Loffler (1959) and Peschek (1961) observed R. fertiiensis in small, shallow and saline lakes east of Neusiedlersee(Seewinke1, Burgenland, Austria) and Zakovsek (1961) noticed it again in Neusiedlersee during her studies from 1950-1952. According to Bartok (1959) it is probable that R. fer- Gensis could be found in water bodies of Southern Slo- vakia. All authors agree that R. fertiiensis can be defined as a cold stenothermal species with a main occurrence in the winter months. In Neusiedlersee it is a typical planktonic animal found in the open lake, rarely occurring near the Phragmites belt and occasionally appearing in small ponds within the reed belt, as long as these are connected by channel to the open lake. A short survey of the species composition and abun- dance of rotifers in Neusiedlersee (1968-1975) already exists (Herzig, I 979). The aim of this paper is to report on the changes in the population dynamics of R. fertBensis over ten years together with possible reasons for these. Material and methods The population studied has formed the major component of the winter plankton of Neusiedlersee since 1973. A detailed description of the limnology of this lake can be found in Loffler (I 979). Some chemical date can be seen in table 2. The 1968/69 winter samples were taken at three differ- ent depths using a Ruttner sampler but in 1970/ 72 a hand pump was used and since 1975 vertical net hauls (30 pm mesh size) through I .55 m maximal depth. The latter had the advantage of an integrated sample without clogging of the net since most of the particles are smaller than 30 pm. The sample interval varied between 2 weeks in autumn and spring and 4 weeks in winter. All samples were counted in petri dishes under a dissecting microscope. More details about sampling, subsampling and counting techniques can be found in Herzig (1974, 1979). In order to obtain a reasonable estimate of the popula- tion in the lake 8-20 separate stations have been sampled 161 Hydrobiologia 73, 161-167 (1980). 0018-8158/80/0732-0161$01.40. o Dr. W. Junk b.v. Publishers, The Hague. Printed in the Netherlands.