original scientific paper UD C 593.1(285,2 Blejsko jezero) VERTICAL DISTRIBUTION OF DAPHNIA HYAUNA LEYDIG IN LAKE BLED WITHIN THE FIRST YEAR AFTER THE OCCURRENC E OF DAPHNIA GALEATA SARS Anton BRANCH), Milijan ŠIŠKO & Nataša CORjANC National Institute of Biology, Sl-1000 Ljubljana, Večna pot 111 ABSTRACT Before 1994, only Daphnia hyalina was present in Lake Bled. Intensive study of D. hyalina distribution in the water column of Lake Bled was carried out in 1988. In April 1995, D. galeata was recorded in the lake for the first time. In August, D. galeata built up approximately 25 % of the Daphnia community. From June to November of 1995, we studied vertical distribution of both Daphnia species in the water column of the lake to test two hypotheses: a) are there differences in vertical distribution of D. hyalina between 1988 and 1995 during the 24-hour period; b) are there differences in the vertical distribution of D. hyalina and D. galeata during the day. For 1995 both hypothesis were rejected. Key words: Daphnia hyalina, Daphnia galeata, vertical distribution, eutrophic lake, competition INTRODUCTION Lake Bled is an eutrophic, subalpine lake (470 m a.s.S.; area: 1.47 kmJ, max. depth: 30 m) in the north­western part of Slovenia (Brancelj, 1991; Brancelj & Blejec, 1994). From 1988 onward, zooplankton samples have been taken once a month to check the structure and abundance of the plankton community, with special attention to the Cladocera. Until the end of 1994, only one Daphnia species was found (Brancelj et ai., 1996). According to Negrea (1983), Margaritora (1985) and Glagolev (1986), it was recognised as D. hyalina (Leydig, 1860) (fig. 1). In April 1995, a new form of Daphnia occurred in the lake, eventually recognised as D. galeata Sars, 1864 (Fig. 1). The newcomer was becaming more and more abundant. In August/September it represented approxi­mately 25% of the Daphnia community. The hybrids of the two main forms were also being observed (i.e. Daphnia hyalina x galeata) confirmed also by electro­phoresis (Spaak, pers. com.). The question arises, how these two species will be­have in Lake Bled: will they be subjected to a long-term coexistence or will a competitive exclusion occur? In deep Lake Constance both species show fundamental ecological differences: D. galeata lives epilimnetically and is absent from the lake during the winter, while D. hyalina is present throughout the year and shows pro­nounced diurnal vertical migrations (DVM) (Geller, 1985; Berberovic, 1990). D. hyalina x galeata hybrids also undergo large migrations, whereas D. galeata re­mains in the warm epilimnion (Stich & Lampert, 1981). Spaak (1994) found that the spatial distribution of hybrid and parental taxa in the shallow Lake Tjeukemeer do not indicate the existence of a hybrid zone. In the deep Lake Geneva the different species of the Daphnia com­plex (D. hyalina x galeata), and also different classes, have a distinct DVM which leads to a temporal and spatial segregation (Angeli ef a/., 1995). The difference in vertical distribution of the two co­existing species, D. hyalina and D. galeata, may indi­cate that the food spectra of these two species are differ­ent and that they may differ on the threshold of food concentrations for the maintenance of a viable popula­tion (Geller, 1985). The ability to gather food particles is determined by the filter mesh-size and filtration area (Korinek & Machacek, 1980). In coexisting species this means a reduced competition (8randelberger, 1985; Brandelberger & Geller, 1985). •',:•. Within a particular species, intra-specific competi­ Anion BRANCEt l « .1.: VERTICA L DISTRIBUTIO N O F DAFHNIA HYALINA LEYDI G I N LAK E SIE D .... 93-100 Fig. 1a-f: Differences in body shape in D. hyalina (left) and D. galeata (right) from Lake Bled (Slovenia), a and d: whole specimen (lateral view); b and e: head (frontal view); c and f: detail of head (lateral view). SI. 1a-f: Razlike v obliki telesa med vrstama D. byalina (levo) in D. galeata (desno) v Blejskem jezeru (Slovenija). a in d: celo telo (lateralno); b in e: glava (frontalno); c in f: detajl glave (lateralno). tion can potentially be reduced by a segregation of specimens of different size or status (i.e. juveniles, ovigerous females and adult females without eggs) in the water column (Brancelj & Blejec, 1994). This is usually connected with DVM, which is often induced by the presence of predators (Stich & Lampert, 1981; Dodson, 1990; Ringelberg & Flik, 1995). In this work we tested the hypothesis that spatial seg­regation between D. hyalina and D. galeata appeared after the invasion of D. galeata in Lake Bled in 1995, through a reduction of the vertical habitat range occu­pied by D. hyalina. For this purpose we compared the differences in vertical distribution of D. hyalina before and after the invasion of D. galeata. MATERIAL AN D METHODS Sampling Zooplankton samples were taken at approximately monthly intervals between June and November 1995 in Lake Bled (Slovenia). We sampled a vertical profile at the deepest point of the lake, at 2.5 m intervals from the surface to a depth of 15 rn. Below that depth hypoxia and anoxia regularly appeared during the summer/ autumn. A Friedinger's bottle with a volume of 6.15 I was used. Samples were taken in triplicates at noon and midnight. Samples were preserved in 4% formaldehyde. Laboratory work In the laboratory, specimens of Daphnia from each sample were separated according to the morphological differences of the species (Fig. 1). At each date, the number of specimens of each taxon from each sample was counted and individuals were separated into juve­niles, adult females without eggs and ovigerous females. In both species the body length of all ovigerous females was measured to the nearest 0.01 mm from one sample of each date, depth and time of the day to test for the differences between the species. In addition, the number of eggs per clutch in each specimen was counted. Statistics To represent the differences in daytime distribution of D. hyalina over the samples taken in 1988 and 1995, the median depth of the population and the 1 s i and 3 r " quartil were plotted. The same method was used to compare the differences in day- and night-time vertical distribution of D. hyalina and D. galeata population over time in 1995. The effect of depth in water column and species on the percentage of animals caught was evaluated by two-way ANOVA . As the variables were expressed as a pro­portion in relation to the whole population, the data were arcsinus transformed. Differences between both species in body length and number of eggs per clutch were tested by one-way ANOVA. RESULTS Differences in vertical distribution of D. hyalina be­tween 1988 and 1995 The day-time vertical distribution pattern of the D. hyalina population inhabiting Lake Bled slightly changed between 1988 and 1995 (Fig. 2). In the fake the peripd of the summer thermal stratification lasts, on average, from the middle of May till the end of October. During the thermal stratification in 1988, the median day-depth of the population of D. hyalina in May was only 5 m below the surface, whilst it was to 10-11 m from June to September. The median depth rose again to 6-7 m below the surface in October and November (Fig. 2d). !n 1995 the median day-depth of the population Anton BRANCElj el a!.: VERTICAL DISTRIBUTION Of DAPHNtA HYALINA LEYDIG IN LAKE 8LED ..., 93-500 female s wit h egg s female s withou t egg s 0 -t J i J -t A 1 S i O N i i J0 +• i— t J A h ­ S I O N 'i' ­ 1 2 ­ 4 -­ 6 t 10 t 8 10 -­ 12 -• 12 14 ­ 14 16 16 J juveniles whole population J•••1 J i A S h— H O N 1 1 J 1 J 1 A ( S j O N 2 ­• 4 -• 6 •8 t -a, 10