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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