Major contribution from littoral crustaceans to zooplankton species richness in lakes

A study of pelagic and littoral species richness among microcrustaceans in 2,466 Norwegian lakes recorded 120 crustacean species: 77 cladocerans, 31 cyclopoids, and 12 calanoids, respectively. Very few species were strictly pelagic, and the pelagic crustacean zooplankton species were by far outnumbered by their littoral counterparts. More than two-thirds of the total crustacean species numbers in lakes were accounted for by species with a littoral preference. A considerable number of species occurred with low frequency, and the median total number of crustacean species in the lakes was only 14. A majority of littoral species also occurred commonly in pelagic samples, and vice versa. Some species are truly both littoral and pelagic, and nearly all species occurring in pelagic samples were also common in littoral samples. A high proportion of the common littoral species was only recorded occasionally in pelagic samples, and should thus be considered strictly littoral. There was no significant correlation between lake area and species richness for pelagic or littoral species. Our findings demonstrate the importance of including littoral species when assessing microcrustacean diversity in lakes, and we question the practice of considering species as either pelagic or littoral, because a majority of the recorded species was common in both habitats. It calls for a further discussion of the term ‘‘planktonic’’, since most of the species are at least partly ‘‘semiplanktonic.’’ 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 (Flössner 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, Acknowledgments This paper is based both on a number of published reports and unpublished data and we thank all those who have contributed with data to this study. We also thank our colleagues Bror Jonsson and Odd Terje Sandlund for their most helpful comments to the manuscript. The study has received financial support from CAS (Centre for Advanced Study) and NINA (Norwegian Institute for Nature Research). Limnol. Oceanogr., 51(6), 2006, 2600–2606 E 2006, by the American Society of Limnology and Oceanography, Inc.