In total, 120 crustacean species were identified (77 cladocerans, 31 cyclopoids, and 12 calanoids) from the 2,466 localities included in this study. The highest number of species recorded within any given locality was 47 (1 yr, one to two visits), and the median number of species (pelagic plus littoral) was 14. On the basis of accumulated data, maximum number of species recorded within one lake was 69. A considerable number of species may be classified as rare, in the sense that they occurred in few localities. Cladocerans and copepods were, on average, constituting 70% and 30% of the species respectively (Fig. 1). Species numbers were far higher in littoral than pelagic samples (Fig. 2). Median number of species occurring in pelagic samples (575 lakes) was 6 (maximum 17), whereas median number in littoral samples (728 lakes) was 11 (maximum 41). Corresponding numbers for cladocerans were 4 (maximum 11) for pelagic samples and 8 (maximum 30) for littoral samples. Copepods followed the same pattern with a median number of 2 (maximum 8) in pelagic versus 3 (maximum 11) in littoral samples (Fig. 2). The majority of copepods were cyclopoids. Calanoids were relatively rare, with a maximum species number of four per lake for both pelagic and littoral samples. When assigning species to either the pelagic or the littoral group, both series of samples included a major fraction of rare species (Fig. 3). Among the species recorded in littoral samples, there was a wide variation in their relative frequency of occurrence. Mean frequency of occurrence was 11% only, and only five species occurred in more than 50% of the lakes. This was even more extreme for the pelagic samples, in spite of comparatively lower species richness. On the average, species recorded in the pelagic samples were present in 9% of the lakes, and only three species were found in more than 50% of the lakes. For the pelagic samples, many species were recorded infrequently. When scoring each species for their relative occurrence in the pelagic versus the littoral samples, 23 species occurred with highest frequency in pelagic samples, whereas 86 species were most abundant in the littoral habitat. A large number of species that were common in littoral samples, and then often in high numbers, were more incidentally recorded in pelagic samples. For example, species like Chydorus sphaericus occurred in 67% of the littoral samples, while in no more than 3% of the pelagic samples. Also a very common littoral species like Polyphemus pediculus (present in 54% of the littoral samples) could be considered rare in relative terms in the pelagic samples (11% occurrence). Hence it is hard to arrive at any strict definitions on concepts such as ‘‘rare’’ since this also depends on their relative frequency of occurrence within lakes. However, species such as C. sphaericus and others that are very abundant in littoral samples and very rare in pelagic samples should be regarded as ‘‘pelagic visitors’’. Our data show that only two rare species could be considered strictly pelagic; the cyclopoid Cyclops lacustris and the large calanoid Limnocalanus macrurus. On the other hand, 59 species in total (41 cladocerans, 17 cyclopoids, and 1 calanoid) were assigned strictly littoral, whereas 48 species (30 cladocerans, 10 cyclopoids, and 8 calanoids) occurred regularly in both littoral and pelagic samples. This underlines the problems associated with estimating pelagic or planktonic species numbers.
Except for the two species mentioned above, the species occurring in pelagic samples were also recorded in littoral samples. When plotting the frequency of occurrence for all species that were recorded in both pelagic and littoral samples (note that this will not comprise the full list of species, since many littoral species were not present in the pelagic samples), we observed a distinct branching pattern. One group of species was correlated, i.e., high frequency of occurrence in littoral habitats also means high occurrence in pelagic samples and thus no strong habitat preference, whereas the other group was always uncommon in the pelagic even while being widespread in the littoral samples (Fig. 4). The most frequently recorded species here was Bosmina longispina, which was also the most common species in the littoral samples. This species occurred in 89% of the pelagic samples. However, the majority of species was absent from most localities, in spite of having a wide geographic distribution. This is evident from the fact that in spite of a total of 120 species recorded, median richness was only 14. More than 50% of the species occurred in less than 5% of the samples, even though most of these species would not be considered rare on a national basis since they still often had a widespread spatial distribution.
Average size for the littoral lakes was somewhat less than that of the pelagic lakes. This could represent a bias if, as commonly reported, species number depends on lake area. However, our study does not reveal any correlation between lake area and species richness (p . 0.5, minimum least-square regression), neither for pelagic nor littoral samples (Fig. 5).
DCA ordination of relative abundance data from an 80- lakes subset confirmed that few species were assigned to the positive end of the first axis, representing the pelagic samples, while the great majority of species were found in the opposite (littoral) end (Fig. 6). As B. longispina was found in 78 of 80 lakes and was equally dominant in both habitats, we can assume that species having a higher score along axis 1 are more pelagic, whereas a more negative score indicates preferences for the littoral habitat. DCA
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