Atlas van de Nederlandse watermijten (Acari: Hydrachnidia)

Atlas of the Dutch water mites (Acari: Hydrachnidia) Few biogeographic studies have been published on water mites. Only Lundblad (1962) has published an atlas of the Swedish water mites. So far, there are no complete publications on the distribution of Dutch water mites. Acarologists who worked on Dutch water mites, i.e. A.C. Oudemans (1858-1943), G. Romijn (1868-1930) and A.J. Besseling (1898-1968) have collected in a limited geographical area only. Besseling, therefore, classified most of the water mites as rare or very rare. Davids (1979) adapted this to the knowledge of that time. Since this last publication, many new data have been collected in The Netherlands, including biotopes which had hardly been investigated. This resulted in publications on the coastal dunes (Smit & Van der Hammen 1992), quagfens (Smit & Van der Hammen 1996) and springs and streams (Van der Hammen & Smit 1996). We also tried to collect hyporheic water mites, however, without results so far. Many data have been collected by the regional water authority boards, for which we identified and checked many specimens. The result of our work is a number of new species for the Dutch fauna and some taxonomical changes (Smit & Van der Hammen 1990b, 1992c, Smit & Duursema 1993, Smit et al. 1993a, Van Maanen et al. 1997). Furthermore, three new species for science could be described (Smit & Van der Hammen 1992b, Smit 1996a). The database contains some 4900 localities, with 60.000 records. We used a 5x5km-grid to show the distribution of the species. In The Netherlands 1666 of these so called hour-squares can be found with fresh - brackish water, of which 1120 have one or more records of water mites. We stored our data in the program Ecolims, including data on watertype and co-ordinates of the Dutch Topographical Survey. We have gained an almost complete coverage. Only some brackish areas, the large rivers (Rhine, Meuse) and the lake IJsselmeer have limited or no data. However, most of these areas are poor in water mite species. We did not include many old data. Usually, these data do not contribute much to our knowledge, as the earlier acarologists usually collected close to their homes. Furthermore, these old localities are difficult to locate. Only for rare species, occurring in less than 10 5x5kmsquares, we have added old data from the notebooks of Besseling (deposited in the Zoological Museum in Amsterdam). Included in these notebooks are the data of Romijn. Furthermore, data from C. Davids have been used. Our recent data show a bimodal distribution over the year (figure 3), as we sampled twice a year, in spring and in summer. We give a revised list of the Dutch water mites on p. 6-13. Previous reviews have been published by Besseling (1964), Van der Eijck (1977), Davids (1979, 1980) and Mol (1984). We re-identified a number of species from the collection of Besseling. We followed the classification of Viets (1987), except for Hydrachna and Piona, where no subgeneric classification is given. For the subgeneric classification of Unionicola we followed Vidrine (1986) and Harvey (1998). On the family level one change has been made: the Wettininae have been raised to family level (Harvey 1998). The genus Hydrochoreutes has been placed in its own subfamily, the Hydrochoreutinae (Smith & Cook 1991). The number of species known from The Netherlands in earlier publications was 188 (Besseling 1964) and 213 (Davids 1979), respectively. The Dutch list now contains 51 genera, with 234 species and including six subspeies not belonging to the nominate form. Five species and one subspecies new for the Dutch fauna are given: Piersigia limophila, Torrenticola anomala, Piona disparilis, Piona conglobata coacta, Forelia spatulifera and Arrenurus sculptus. On the other hand, four species have been omitted: Oxus oblongus, Piona annulata, Forelia koenikei and Arrenurus pugionifer. The description of the species includes the following topics. Nomenclature: For each species a complete list of synonyms used in Dutch publications is given, as well as all publications in which records of the species in The Netherlands have been published. Not included in this bibliography is the so-called ‘grey’ literature, e.g. unpublished reports. Occurrence and distribution: For each species its occurrence is given, using the following division: very rare species occurs in 10 or less 5x5km-squares rare species occurs in 11-29 squares rather rare species occurs in 30-100 squares rather common species occurs in 101-250 squares common species occurs in 251-500 squares very common species occurs in more than 500 squares The seasonal periodicity is given for species which show a preference for either spring or (late) summer/ autumn. Only old records of species which occur in less than ten squares or species which have not been found recently, have been included on the maps. To distinguish these records from recent observations a division is made between data before and since 1970. This is a convenient year because the recent extensive research on the distribution of Dutch watermites started in 1978-1979. All data of environmental parameters are from the provinces Noord- and Zuid-Holland only. In these provinces, situated in the Holocene part of the country, fringe zones occur from Pleistocene to Holocene and from coastal dunes to the low lying ‘polders’. Furthermore, very brackish parts can be found, as well as clay and peat soils. Therefore, the data given in this paper are not suitable for species which have their optimum in acid waters or in streams. Summary: Each species account is concluded with a short summary in English. The diversity of the water mite fauna is highest in the seepage region between the Holocene and Pleistocene parts of The Netherlands. The reasons are probably the better water quality, the low chloride content and the relatively high habitat diversity caused by peat digging activities in earlier centuries. In parts of The Netherlands with only lentic waters, chloride and nutrient levels are the main factors that determine the species richness of the water mite fauna. For example, in ditches with a bad water quality the maximum number of species is 10; when the quality is optimal, more than 20 species are possible. Since the beginning of the 20th century the diversity of the water mite fauna in lotic waters has deteriorated strongly. Of the rheophilic species 40% has disappeared and nowadays 80% is rare or very rare. The main reasons are canalization and eutrophication. In The Netherlands, the surface waters are classified in different types, mainly based on morphology and lentic-lotic divisions. Distinguished are: springs, brooks, rivers, fens, marshes, cattle pools, larger pools, ditches, small canals, large canals, lakes, dune waters and city waters. The water mite fauna of each of them has been calculated and compared with each other. Only species present in more than 10% of the number of localities belonging to a special water type is given (table 84).