Hydrobiologia 506–509: 327–331, 2003. © 2003 Kluwer Academic Publishers. Printed in the Netherlands.
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The distribution of rotifers (Rotifera) within a single Myriophyllum bed Natalia Kuczy´nska-Kippen Department of Hydrobiology, Adam Mickiewicz University, Marcelinska 4, 60-801 Pozna´n, Poland E-mail:
[email protected] Key words: Myriophyllum, rotifers, shallow lake, spatial distribution
Abstract The research on the spatial distribution of rotifers between the central and border part of the Myriophyllum bed (M. verticillatum) was carried out between 1998 and 1999 in the shallow part (approx. 1 m depth) of Budzy´nskie Lake (Wielkopolski National Park, Poland). The comparison of both species composition and the numbers of individuals between both of the examined zones have not revealed statistically significant differences. However, a higher number of rotifer species and their higher densities, as well as increased participation of littoral species were observed in the middle of the vegetation bed. The structure of the dominating species also differed between both areas. Seven rotifer species were found to have significantly greater numbers in the central part of the Myriophyllum bed, while only one species was significantly correlated with the border part of the macrophyte stand. These differences in the behaviour of particular groups of rotifers may be dependent on the structure of their microhabitat and their position in relation to the open water zone. They may also be related to young fish predation in both habitats and better refuge conditions inside the thick macrophyte stand, as well as typical adaptation to littoral or limnetic life. Introduction Various factors are implicated in controlling the distribution of zooplankton community structure in shallow lakes. The differentiation of natural animal communities within various types of water vegetation, differing spatially and morphologically, has already been proved for some groups of invertebrates and young stages of fish assemblages (Downing & Cyr, 1986; Winfield, 1987; Jeppesen et al., 1997). It seems then probable that differentiation may also concern habitat distinctiveness on a smaller scale e.g. the central and the transient zones of a single macrophyte bed. Habitat segregation, usually understood as occupying different depths or following different patterns of dial migration, is a well-known phenomenon in natural zooplankton communities (Makarewicz & Likens, 1975), although there is still a lack of knowledge considering eventual species segregation within two adjacent habitats. The examination considering two differently situated zones of one single bed of submerged macrophytes will assist in answering the questions whether
rotifera populations occupying such habitats differ and what the possible explanations are to such behaviour? Thus, the principle aim of this study is to evaluate the relationships between rotifers and two zones of one macrophyte stand – the central and outer parts. The planktonic animal’s behaviour is always a compromise between costs and benefits, thus avoidance of predation and optimal consumption. Animals will always be faced with a dilemma, what is more beneficial – to be constantly well-fed or to survive? The littoral zone of lakes is able to secure both requirements. A thick conglomeration of aquatic plants creates favourable conditions for concealment, constituting a refuge (Schriver et al., 1995; Phillips et al., 1996; Mazzeo et al., 2003). However, it can provide animals with a nutritional food base, apart from phytoplankton available in the water filling the interstem spaces, especially due to the abundance of periphytic forms and reasonable amounts of detritus, bacteria and protozoans accompanying these algal communities (Gons, 1979; Moore et al., 1994; Jürgens et al., 1994; Theil-Nielsen & Søndergaard, 1999). The examination of the periphyton in Budzy´nskie lake re-
328 vealed that out of all the macrophyte species present, periphyton from Myriophyllum verticillatum was of the highest biomass. At the same time the content of total P and N in the mill-foil periphyton was also highest (Kuczy´nska-Kippen, 2001). The differences in the rotifera behaviour may also be altered by different habitat conditions present in two lake zones, influencing the suitability for typically littoral and pelagic species (Preissler, 1977; Jose-de-Paggi, 1993).
Study area Lake Budzy´nskie is a shallow lake situated in the Wielkopolski National Park (Poland) with an area of 17.4 ha, maximum depth of 2.7 m and a mean depth of 1.4 m (Ja´nczak et al., 1996). A belt of emergent macrophytes (95% of the shoreline) with Typha angustifolia L. and Phragmites australis (Cav.) Steud. surrounds the whole basin of the lake. Nearly 75% of the basin is covered by submerged macrophytes, predominately Chara tomentosa L. and Myriophyllum verticillatum L. The Myriophyllum beds grow in the deeper central parts of the lake, filling the basin from the bottom to the water surface. Small areas of open water divide the Myriophyllum zone into beds of varying sizes.
Methods The research was carried out between 1998 and 1999, from May to October. The object of this examination was Myriophyllum verticillatum, creating dense submerged beds. Samples were collected from two sites: (a) the central part and (b) the border part of the Myriophyllum bed. Zooplankton was taken using a plexiglass core sampler (∅ 50 mm); subsamples from the surface layer (0–1.5 m) were pooled together and the collected material of the volume of 10 l was concentrated using 45-µm plankton net and was fixed immediately with 4% formalin. The ANOVA test was used in order to evaluate the differences in rotifer densities between both examined stations (N = 9). The posteriori Tukey Test was applied in order to find where the differences in time occurred.
Results The comparison of the number of rotifera species and the densities of rotifera communities between the central and the border part of Myriophyllum bed did not reveal any statistical differences (p > 0.05). However, in most cases the central part of the macrophyte stand was characterised by richer species structure and higher numbers of individuals (Fig. 1). Also the analysis of the participation of the littoral species in the total densities of both zones was not significant, although higher abundance was observed in the central part of the Myriophyllum bed. The structure of dominance of both areas differed. The species: Colurella uncinata (O.F. Müller), Conochilus unicornis (Rousselet), Keratella cochlearis Gosse, Lecane bulla (Gosse), L. luna (O.F. Müller), Polyarthra vulgaris Carlin and Trichocerca similis Wierzejski were common for both zones, while Bdelloidea and Lecane closterocerca (Schmarda) dominated only in the middle and Collotheca edentata (Collins), Gastropus stylifer (Imhof), Keratella quadrata (O.F. Müller) and Lecane quadridentata (Ehrenberg) at the edge of the macrophyte bed. The analysis of the habitat preferences revealed that seven species (Lecane closterocerca, L. bulla, Brachionus quadridentatus Hermann, Colurella uncinata, Bdelloidea, Lepadella patella (O.F. Müller), Trichotria tetractis (Ehrenberg)) had significantly higher numbers in the central part of the Myriophyllum bed, while in the border part only one such species was found (Polyarthra remata [Skorikov]) (Fig. 2). An additional analysis of Rotifera distribution within the Myriophyllum bed was carried out, with emphasis on the body size of particular species. Five body size groups were distinguished: I – 45–80 µm, II – 80–120 µm, III – 120–170 µm, IV – 170–250 µm, V – >250 µm. The density variation of Rotifera numbers of particular body size groups between both examined zones was not significant (p > 0.05). The analysis within the border part of Myriophyllum bed of the Rotifera numbers representing particular size groups was the only one of significant character (F4,40=2.98; p=0.03) – the numbers of the smallest rotifers (group I) differed from the medium size group (group III).
Discussion Analysing both the number of species and the density of rotifer communities within a single bed of Myrio-
329
Figure 1. The number of Rotifera species and individuals within the Myriophyllum bed in Budzy´nskie lake.
Figure 2. Habitat preferences of Rotifera species within the Myriophyllum bed in Budzy´nskie lake.
phyllum verticillatum it was noticed that the central area was characterised by higher values when compared to the border part, although the differences were not statistically significant. The differentiation of the rotifera numbers between both the zones may have been the result of the intensity of young fish predation in these two habitats. Phillips et al. (1996), Stansfield et al. (1997), Jeppesen et al. (1997), Jeppesen et al. (1998b) and Wojtal et al. (2003) showed that young fish, undergoing pressure from predatory adult fish and birds, prefer sparse vegetation (which is in the transitional zone of the macrophyte bed), when looking for refuge. This may have been a decisive factor in finding lower Rotifera numbers in the border part of the Myriophyllum bed. Higher densities of rotifer populations inside the plant stand may sug-
gest more advantageous anti-predator refuge or more profitable food conditions present inside the compact vegetation stand. Folsom & Collins (1984) and Walsh (1995) proved that the denser and more complicated morphologically the macrophyte bed is, the more differentiated zooplankton communities are found. The entanglement of the spatial and morphological build of the central part of Myriophyllum stand is followed by creation of numerous differentiated ecological niches (Gliwicz & Rybak, 1976). Moreover, the Myriophyllum bed possessed a much richer source of periphyton compared to other macrophyte species in Budzy´nskie lake (Kuczy´nska-Kippen, 2001), thereby providing the inhabiting organisms with the best conditions. The higher species diversity in the middle part of the macrophyte stand may be connected with the
330 gathering of littoral organisms, which protect themselves from being washed out into the open water zone. Higher participation of typically littoral species in the middle part of the vegetation stand may confirm this hypothesis. A similar situation was observed by Rybak (1960) who, when examining the distribution of crustaceans in the border zone between planktonic and littoral habitats, recorded that in the transitional zone both the zooplankton densities and the number of species were lower. Moreover, this author noticed that the contribution of species, described as littoral, was lower in the border zone of rushes. Furthermore, Wegle´nska & Rybak (1998) observed that littoral species concentrated inside the macrophyte stand during the daytime. Only during the hours of darkness was there any occurrence of movement into the border zone of the plant bed and towards the open water zone. Rotifer communities of the middle and border parts of the Myriophyllum verticillatum stand also reveal a differentiated character with regard to the structure of dominance. Barely 54% of species constitute the community common to both zones. Bdelloidea and Lecane closterocerca, both described as littoral forms, were distinct in the central part of the plant stand. However, the dominating species preferring the border zone were mostly limnetic species. Such a great variety is probably an effect of the situation of both zones in relation to the open water zone. This may be why in the border area of the Myriophyllum stand, having direct contact with free water, pelagic species dominated. Their presence was probably a result of wave action and thus a result of them being washed in from the open water. Additionally, this zone may be treated as a daytime refuge for pelagic plankton, hiding in the transitional zone from predators active during the hours of daylight (Schriver et al., 1995; Lauridsen & Buenk, 1996; Kairesalo et al., 1998). A group of eight Rotifera species, demonstrating significant habitat preferences, was distinguished. The middle part of the Myriophyllum bed possessed seven such species, while the border part only one. Such a situation confirms the greater stability of the centre and the fact that all those specific species were representatives of littoral zooplankton. The only species characteristic of the peripheral zone, was Polyarthra remata, a limnetic species, apparently availing itself of the transitional zone as a daytime refuge. Particular habitat preferences are determined by a number of factors. Cyr & Downing (1988b) suggested that particular kinds of aquatic plant may have an influence – bigger plants with wider leaves are more suitable
for heavier crawling invertebrates, while denser plants with finer leaves for smaller organisms like copepods, cladocerans and also rotifers. Gliwicz & Rybak (1976) hypothesised that the denser the plant stand is, the more isolated the water within it and the more distinct the zooplankton communities. The specific spatial structure of a particular habitat may also influence the type of periphyton available (Gross et al., 2003; Kiss et al., 2003), which may then be preferred by different freshwater organisms. Several studies have suggested the preference of particular macrophyte types by representatives of water fauna, like rotifers (Pejler, 1995), crustaceans (Dvorak & Best, 1982; Cyr & Downing, 1988a, b) and other groups of aquatic organisms (Pip & Stewart, 1976; Lodge, 1985; Sheldon, 1987). Additionally, the distribution of the rotifer densities representing different body size classes between both of the examined zones revealed no significant changes or else the variability was of small significance. The body size of particular species may result in different food preferences. Cyr & Curtis (1999) proved that the body size of individual zooplankton is directly related to its grazing rate and to the range of particle size it can ingest. The lack of positive correlation in the habitat-related body size of rotifers may have been connected with similar food availability due to the small distance between the two parts of the vegetation stand examined. It is also possible that the range in body size of rotifers is not significant enough to concern the impact of size variety of food particles. Another reason for the absence of any differences may be connected with the lack of selective reduction of rotifers by the planktivorous fish, as some are often too small to be spotted and caught efficiently (Scheffer, 2001). Moreover, according to Raffaelli et al. (2000) the habitat architecture may constrain the size spectrum of organisms, thus limiting any possibility of size-differentiation among the inhabiting animals. These differences in the behaviour of rotifers between the central and border parts of the Myriophyllum verticillatum bed seem to be dependent on the structure of the microhabitat and in particular its position in relation to the open water zone. Young fish predation may also play a role, being related to the better refuge conditions found inside the thick macrophyte stand. However, typical adaptations to littoral or limnetic life seem to have an important impact on the special preferences of particular species.
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