Zooplankton species richness in lakes is, as for functional groups in other ecosystems, a key measure not only of the biological status, but also a link to ecosystem functioning, food web complexity, and ecosystem stability. Most studies on zooplankton diversity in lakes include species sampled by net-hauls in open waters, i.e., species assumed to be truly planktonic. Rather few studies include detailed studies of the littoral species, and comparative studies of pelagic and littoral species diversity are almost nonexistent. This is partly due to the higher complexity and frequently more demanding taxonomy of the littoral taxa, but also because there are no clear-cut boundaries separating the pelagic and littoral habitats.
The terms ‘‘crustacean zooplankton’’ as opposed to ‘‘littoral crustaceans’’ are commonly used by limnologists to describe species that are strictly living in open waters, or species associated with the surface of macrophytes/sediments, respectively. There is, however, no sharp boundary between these two groups. According to Pennak (1966), true zooplankters are found not only in the pelagic but also among macrophytes. He emphasizes that ‘‘true’’ here refers to zooplankters that are swimming about in the water and are not associated with a plant substrate at the time of capture. In the macrophyte zone, however, there is undoubtedly a much larger number of niches available because of the spatial, nutritional, and food-web roles of the vegetation (Pennak 1957). The absence of strict categorical boundaries obviously has implications for diversity or species richness estimates of zooplankton in lakes, since these to a varying degree will include both pelagic and semilittoral species, depending on lake size, morphometry, and sampling method.
Some crustacean families like Chydoridae and Macrotricidae tend to be primarily littoral (Flo¨ ssner 2000). On the other hand, vertical movements of epiphytic and benthic microcrustacea have been observed in several studies (e.g., Whiteside 1974, Meyers 1984). Meyers (1984) suggested that many chydorid Cladocera are facultative planktivores that move upward at night to utilize phytoplankton. Low concentrations of edible phytoplankton may preclude vertical migration. Many epiphytic and benthic microcrustaceans also exhibit strong vertical movements with declining oxygen concentrations (Meyers 1980; Tinson and Laybourn-Parry 1985). In a study of horizontal migration, Lauridsen et al. (2001) found that Bosmina spp. and Holopedium gibberum were evenly distributed between the littoral and the pelagical in deep and shallow fishless lakes, but that their near-shore densities were lowest in the presence of fish. Macrophyte-related and benthic cladocerans concentrated either in the littoral or were evenly distributed between the littoral and the pelagic, irrespective of depth and the presence of fish. The boundary between littoral macrophyte beds and open waters may also be an important daytime refuge for potentially migrating pelagic cladocerans (Lauridsen and Buenk 1996). Such vertical and horizontal migrations render strict division between true zooplankton and littoral/benthic species difficult. In this study we analyzed species number of microcrustaceans in a large data set covering a wide range of localities that have been sampled both for pelagic and littoral crustaceans or both habitats pooled, to test the importance of including littoral microcrustacean species when assessing diversity. On the basis of this we also discuss the validity of the terms ‘‘planktonic’’ and ‘‘littoral’’ crustaceans.