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The aim of this study was to examine relations between the density and species richness of rotifers and environmental factors in 55 small water bodies in the Wielkopolska region. Canonical correspondence analysis (CCA) revealed that typically pelagic rotifers (species of the genera Brachionus, Keratella, or Polyarthra) occurred in larger and deeper reservoirs in conjunction with open water and helophytes. Their distribution was conditioned by the presence of fish, by the lack of overshading, and high concentrations of phosphorus. Littoral rotifers (Cephalodella, Lecane, or Lepadella) were typical of small surface and shallow ponds and of areas with a high degree of spatial complexity – elodeids. They preferred fishless water bodies with strong overshading and high transparency of water. The distribution of pelagic species was dependant on high concentrations of chlorophyll a, while littoral species depended on high concentrations of dissolved organic matter. Variance partitioning extracted the type of habitat and the associated degree of habitat heterogeny as very strong predictors of rotifer distribution in mid-field reservoirs.
The composition and dynamics of zooplankton (Rotifera, Crustacea) communities were studied in a dystrophic lake (Drawieński National Park, northern Poland). The investigated lake was a typical mid-forest lake of a small area (ca. 0.65 ha) but relatively deep (Zmax = 6.8m) and covered with a peat (Sphagnum sp.) mat. The study was made in the shallow part of the lake (Z = 0.5 m). Zooplankton was collected twice in August 2004, in triplicate subsamples, taken from three stations (1. under the peat mat, 2. the transitional zone between the peat mat and open water area and 3. open water zone) from two different sites within the same lake. The distance between sampling stations within a transect was ca. 1.5 m. The whole area under study was not greater than 10 m2. Therefore the results concern the very small-scale distribution of zooplankton. The aim of the study was to find out whether spatial segregation of the zooplankton community and the dominating species between the Sphagnum mat and open water zone as well as in the transitional zone between both zones takes place in a dystrophic lake and whether the moss mat can be considered as an anti-predator refuge. Both the species number and zooplankton densities differed between the stations along a transect, being the highest (40 zooplankton species and mean 150 ind l–1 for the whole zooplankton community) in the peat mat and lowest (12 species and 72 ind l–1) in the open water zone. Humic-water species constituted 24% of the species composition of rotifer and 14% of the crustacean community. Cladocerans prevailed numerically over rotifers. Dominating species – Bdelloidae, Keratella cochlearis Gosse, Polyarthra vulgaris (Carlin), Synchaeta pectinata Ehrenberg, Trichocerca insignis Carlin, Alonella excisa (Fischer), Ceriodaphnia quadrangula (O.F. Muller) – revealed a differentiated pattern of spatial distribution. The mean Shannon-Weaver biodiversity index of zooplankton was not notably high and amounted to 1.45. The highest values were found in the peat mat (mean – 1.76 for rotifers and 0.67 for crustaceans), while the lowest values were found in the open water (0.99 and 0.36 respectively). These results suggest that in the site connected with Sphagnum moss in a humic lake more diverse and abundant zooplankton occurs in relation to other habitats. The differences in zooplankton distribution between the peat mat and the open water zone of the dystrophic lake seems to be affected by biological interactions which relate to predator presence, both vertebrate and invertebrate, and competition between large cladocerans and smaller rotifers. Due to the dominance of larger forms of zooplankton it may be supposed that invertebrate predators may have a more pronounced effect. The habitat within the Sphagnum moss can be considered as a predictable refugium.
The research on the distribution of species of the Lecane genus among different types of macrophytes (including rushes, nymphaeids and two zones of submerged macrophytes) in comparison with open water was carried out for three years in a shallow lake (Lake Budzyńskie, western Poland; an area – 17.4 ha, maximum depth – 2.7 m and a mean depth – 1.4 m) in order to determine the possibility of their competition and of co-existence. The distinct species of submerged macrophytes create separate vegetation beds and patches in the lake. The size of a particular macrophyte bed did not exceed the area of 5 m. Zooplankton samples were collected between 1997 and 1999 (from April to October, at about 2-week intervals) in the shallow part (approx. 1m deep) of a lake. Nymphaeids were only sampled during the 1998 and 1999. Samples were taken at each site using a plexiglass core sampler (∅ 50- mm). Subsamples of a volume of about 1.5 l from the surface layer (0–1.5 m) were sampled from randomly chosen places within each macrophyte patch. Six Lecane species were analysed (Lecane bulla (Gosse), L. closterocerca (Schmarda), L. flexilis (Gosse), L. furcata (Murray), L. luna (Müller) and L. lunaris (Ehrenberg)). L. bulla dominated at most of the examined stations each year. Detailed seasonal analysis of the abundance of particular species of the Lecane genus in most cases revealed the replacement character of their occurrence. The sudden increase in the numbers of one species caused a simultaneous decrease of another within the same macrophyte stand. A distinct replacement pattern was observed for L. bulla, which was replaced by L. closterocerca or L. luna and for another two pairs of species (L. closterocerca with L. furcata and L. flexilis with L. luna). At the same time, pairs of species such as L. closterocerca–L. lunaris (statistically positive correlation was found in the case of Chara bed – rs = 0.70; P = 0.007), L. flexilis–L. furcata (within Typha – rs = 0.58; P = 0.048) and also L. luna–L. furcata (in the Myriophyllum bed – rs = 0.80; P = 0.001) exhibited a similar pattern of seasonal changes without, however, revealing the exchange occurrence between each other. The pattern of species replacement within a genus is an example of the competitive exclusion of closely related species. The nature of the seasonal distribution of species of the Lecane genus, replacing each other over a period of time, may be connected with the niche overlap of particular species, which results in time segregation. Exploitative competition cannot be excluded when describing such behaviour.
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