The sampling for assigning littoral, pelagic, or total species richness was performed in separate localities. Owing to the very large sample size as well as geographical overlap in sampling sites, we believe that our conclusions are robust. The sum species numbers for pelagic and littoral samples correspond well with the total species numbers for the pooled samples. It is likely, however, that a more intensive sampling effort for each locality would have revealed more littoral species relative to pelagic species. This because the littoral is a more heterogeneous habitat than the pelagic and therefore offers a higher number of potential niches but also more sampling constraints relative to the homogeneous pelagic.
One would also expect a higher occurrence of species that are typically considered littoral being represented in the pelagic of small and shallow lakes. While species number usually increases with the area of habitat (Browne 1981; Fryer 1985; Dodson et al. 2000), we found no correlation between lake area and species richness neither for pelagic nor littoral species. This accords with a previous study based on a different sample of Norwegian lakes (Hessen et al. 2006), where the absence of area–richness relation was accredited to the comparatively short time since the last glaciation, as well as a topography with mountain barriers that may constrain species dispersal. Consequently, many localities may be unsaturated with species (i.e., not in ‘‘equilibrium’’). Local lake characteristics may also override any habitat–size effect (cf. Whittaker et al. 2001). The low mean number of species in the Norwegian lakes may also be explained by the high number of ultraoligotrophic localities, many situated in mountain areas.
There is no straightforward way of separating pelagic and littoral microcrustaceans, and the term zooplankton commonly also includes littoral species. Wetzel (2001) stated that ‘‘littoral zooplankton communities are made up of a diverse assemblage of protozoans and other protists, rotifers, and microcrustaceans. Many of these animals are strongly associated with the sediments or macrophytes and are not truly planktonic, or are only intermittently planktonic’’. Littoral microcrustacean communities have been assigned to three general groups in the littoral zone: the strictly plant-associated species, those free-swimming among the large plants, and the sessile species mainly living on or in littoral sediments (e.g., Pennak 1966; Whiteside et al. 1978; Fairchild 1981). Few of these strictly benthic or epiphythic microcrustacea enter the water column, even at night (Paterson 1993). On the other hand, truly planktonic species are common in areas within littoral zones devoid of vegetation but are rare within areas with dense macrophyte vegetation (Wetzel 2001). However, even predominantly pelagic species may utilize macrophyte boundaries as a refuge from fish predation (Lauridsen and Buenk 1996). Fryer (1985) suggested that members of littoral taxa like the chydorids may occur in planktonic samples by acting as opportunistic riders on filaments of blue-green algae (Fryer 1985). Our study confirms that there is no distinct boundary between most pelagic and littoral species. We believe that one reason for the high number of rare visitors in the pelagic samples was the generally high number of littoral species, of which some occasionally would be driven offshore by wind and waves.
Our study demonstrated that the microcrustaceans could be divided into three groups: a large group of strictly littoral/benthic species, another fairly large group that ranged freely among the pelagic and the littoral habitat, and a very few species that were strictly pelagic. The higher richness of littoral species clearly demonstrates that pelagic samples will only represent a modest fraction of total microcrustacean richness in lakes. The common presence of typically littoral species in pelagic samples creates a problem in the assessment of richness, since, if only a small fraction of the littoral species exploits the pelagic habitat, their presence would result in an overestimate of species richness in the pelagic zone. Since most studies on zooplankton richness in lakes are based on pelagic samples only, richness estimates are highly vulnerable to the input from littoral species. Not only will this contribution vary in space and time, but it could also differ with lake-specific properties such as the volume-to-surface ratio, the hypsographic profile, wind and mixing events, lake productivity, etc. This means that zooplankton species richness estimates based on strictly pelagic samples should be judged with some caution.
The dominance of littoral species reflects the higher niche diversity in littoral habitats compared with the homogenous pelagic habitat. Littoral cladocerans show highly diverse habitat preferences, including submerged macrophytes, sediments, sand and rock surfaces, as well as the water column (Pennak 1966; DiFonzo and Campbell 1988; Paterson 1993). Thus, the high richness of littoral species may reflect an adaptive radiation not only with regard to feeding preferences, but also for spatial niches. It may also reflect that the evolutionary origin of the several species-rich microcrustacean taxa like the chydorids is littoral (cf., Fryer 1968). It should be noted that, because of the spatial diversity and potential constraints on adequate and representative sampling procedures in the littoral zone, it is more likely that the true number of species is underestimated in the littoral than in the pelagic habitat. Hence, our reported dominance of littoral species should be seen as minimum estimates.
Though many crustacean species occur in a large fraction of both pelagic and littoral samples, this does not necessary imply that they are also numerically abundant. Species like Holopedium gibberum and Cyclops scutifer occurred in a large fraction of the localities, both in pelagic and littoral samples. Judged from their numerical abundance, however, their pelagic preference became clearer since they had high dominance score in pelagic samples while less so in the littoral samples. In contrast the large calanoid Heterocope saliens, which also occurred in a high fraction of both pelagic and littoral samples, was more typically littoral judged from the dominance score, i.e., when it occurred in littoral samples it often did so in large numbers. Dominance score had less effect in the littoral end of the first axis, since these species were so strongly associated with the littoral zone. In conclusion, our study demonstrates not only that littoral or semilittoral species are major contributors to microcrustacean biodiversity in lakes, it also underlines the problems of separating between ‘‘true’’ pelagic zooplankton species and their littoral counterparts when assessing species richness on the basis of pelagic samples.