Material and methods
This study is based on samples of microcrustaceans from 2,466 localities, covering the entire mainland of Norway (58u39 to 71u49N) and spanning a wide range in terms of altitude, area, pH, and conductivity: 1.0–1,837 m above sea level (mean 434, median 307 m), 0.001–363 km2 (mean 2.1, median 0.2 km2), 3.8–9.9 in pH (mean 6.2, median 6.5), and a conductivity from 0.41 to 200 mS m21 (mean 4.7, median 2.4 mS m21). Total organic carbon (TOC) and total P was only recorded for a limited subset of localities, but confirmed the dilute and low-productivity nature of these pristine lakes with TOC ranging from 0.2 to 14.8 mg L21 (mean 3.1, median 2.1 mg L21) and total P from 0.13 to 69 mg L21 (mean 17, median 8 mg L21).
The data set includes lakes that are sampled only for pelagic zooplankton (575 lakes), only littoral samples (728 lakes), or both (1,163 lakes). In the last category pelagic and littoral samples were (unfortunately) pooled when they entered the database. There should be no a priori sampling bias with regard to these three categories of samples. The great majority of lakes (.90%) have been sampled only once or twice during summer or early autumn. Some lakes have been more intensively studied over 2 or more years. For these lakes, a single year was randomly picked as representative of that locality to avoid a bias in species richness for these lakes. For lakes sampled over several years, cumulative species number may be almost 50% higher than that of single samples (Arnott et al. 1998). However, many of the species contributing to the cumulative richness in our study were rare and perhaps transient visitors (for example occurring in single samples represented with a single individual over a 10-yr period). For zooplankton sampling, a net haul (27.5–30-cm diameter, 90 mm net) was in most lakes taken from the deepest part from the bottom to the surface. This method ensures a high number of individuals and also an almost complete species list. The littoral species were sampled by a net haul horizontally at low speed (about 25 m min21) both outside and inside vegetation stands whenever possible in each lake. A smaller (10-cm diameter, 90 mm) net was used where vegetation was too dense to allow use of the regular net.
All crustaceans were identified to species; rotifers were not included in this survey. Cladoceran species were identified in accordance with Flo¨ ssner (2000), whereas copepods were identified after Kiefer (1978). The taxonomic affinities remain vague for some of the cladoceran species. This holds especially for the Daphnia group, e.g., Daphnia longispina, where recent screening of genetic affinities by use of allozyme studies and mitochondrial or nuclear markers show that D. rosea and the melanistic alpine D. umbra should be separated from D. longispina s. str. (Schwenk et al. 2004; Hobæk 2005). Some species, notably D. galeata, commonly hybridize with other species (Schwenk et al. 2001; Hobæk et al. 2004), and there is no doubt that further genetic screening will reveal taxonomic revisions both for the daphnids as well as for other groups. However, these somewhat unclear taxonomic affinities for some species would not have major consequence for species richness per se in this large data set.
The pelagic and littoral samples were scored for presence or absence at the species level and each species was assigned to one of three categories: those present in pelagic samples only, those present in littoral samples only, and those present in both habitats. Great care was taken in rinsing the sampling gear between sampling in different localities. In spite of this, samples might become contaminated and 13 species that were abundant in the littoral samples while only recorded in a single pelagic sample (,0.2% of the samples, and then often in a typically dried condition) were omitted from combined pelagic–littoral category. Data on relative abundance for species both in pelagic and littoral samples from the same localities were obtained from a subset of 80 lakes. Relative abundance for each species was assigned to three categories; ,1%, 1–10%, and .10% of total number. The highest recorded abundance observed from a total of four samples (two from early summer and two from late summer) were used as input for each species. Patterns of distribution, relative abundance, and absence/presence were summarized by detrended correspondence analysis (DCA) (Hill 1979) using the program CANOCO (ter Braak and Smilauer 1998) with down-weighing of rare species. By using pelagic and littoral samples as input we expected the first axis to separate between the two main habitats. Since species that are far apart correspond to sites that are dissimilar in species composition, our aim was to visualize which species was associated with the two respective habitats on the basis of dominance score.
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