Hydrobiologia 506–509: 209–212, 2003. © 2003 Kluwer Academic Publishers. Printed in the Netherlands. 209 Deep chlorophyll maximum by Ceratium hirundinella (O. F. Müller) Bergh in a shallow oxbow in Hungary I. Grigorszky 1 , J. Padis´ ak 2 , G. Borics 3 , C. Schitchen 1 & G. Borb´ ely 1 1 Botanical Department, University of Debrecen, H-4010 Debrecen, P. O. Box 14, Hungary E-mail:gege@tigris.klte.hu 2 Department of Limnology, Veszpr´ em University, H-8200, Veszpr´ em, P. O. Box. 158, Hungary 3 Environmental Protection Inspectorate for Trans-Tiszanian Region, H-4025 Debrecen, Piac utca 9/b, Hungary Key words: Ceratium hirundinella (O. F. Müller) Bergh, mesotrophic oxbow, stratification, DCM Abstract The stability of the water column permitted stable stratification of the shallow (maximum depth: 3.75 m) oxbow, Kecskészugi-Holt Körös in summer 2000. During the stratified period a deep chlorophyll maximum (DCM) was found at depths getting 60–180 μmol m −2 s −1 photosynthetically active radiation (PAR). The phytoplankton was dominated by Ceratium hirundinella and it is concluded that the development of the DCM largely resulted from the behavioural aggregation of this motile flagellate. Introduction Vertical aggregation of different algal taxa in cer- tain layers of stratified water columns have been described from many freshwater and marine envir- onments and it is frequently referred to as a deep chlorophyll maximum (DCM). Various processes may result in a DCM: different sinking mechanisms of phytoplankton (Veinrick et al., 1973; Padisák et al., in press a), selective zooplankton grazing (Longhurst, 1976), physiological adaptation in the ratio of cel- lular carbon to chlorophyll (Kiefer et al., 1976), in situ growth (Veinrick et al., 1973; Padisák et al., in press a) and ‘behavioural’ aggregation of flagellates (Cullen, 1982). The studies on the vertical distribution of phytoplankton or chlorophyll demonstrated that the mechanisms and persistance of DCM may exhibit dif- ferences depending on geographical location (Cullen & Eppley, 1981). This paper presents the vertical distribution of phytoplankton and chlorophyll a where a DCM was found despite the moderate depth of the mesotrophic oxbow. Materials and methods The investigations were carried out in the Kecskészugi- Holt Körös, Körös area, Békés county, SE Hungary (46 ◦ 57 ′ 13.96 ′′ N and 20 ◦ 49 ′ 28.73 ′′ E), a small (sur- face area: 12 ha, maximum depth: 3.5 m, mean depth: 2.2 m) oxbow (Pálfy, 1995). Samples were collected with a hard polivinyl plastic tube monthly between January and December 2000. During the stratified period from the beginning of June to the end of July samples were obtained biweekly. Samples were taken in 25 cm increments down to the bottom between 13.00 and 14.00 hours. Euphotic depth correspond- ing to 1% penetration of subsurface light were derived from multiplying Secchi-disc readings by 2.5. Relative water column stability (RWCS) was cal- culated by comparing the density gradient (based on surface and bottom temperatures) of the whole wa- ter column to the density difference between 4 ◦ C and 5 ◦ C pure water (density data from Perry, 1950) using the following formula: RWCS = D b − D s D 4 − D 5 where D b is the density of bottom waters, D s is the density of surface water, and D 4 and D 5 are the dens- ities of water at 4 ◦ C and 5 ◦ C, respectively (Padisák et al., in press, a). The biovolume of the species was calculated geo- metrically using the mean dimensions of 10 specimens for the abundant species and published data (Taylor & Wetzel, 1988) were applied for rare species. The ni- trate and ortho-phosphate concentrations were meas-