Harry Smit & Henk van der Hammen


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Summary
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 were hardly been investigated. This resulted in publications on the coastal dunes (Smit & Van der Hammen 1992),  quackfens (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). Moreover, three new species for science could be described (Smit & Van der Hammen 1992b; Smit 1996a).

The database
The database contains some 4.900 locations, with nearly 59.000 records. We used a 5 x 5 km 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 Lake IJssel 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 collected not far from there homes. Moreover, these old locations are difficult to localise. Only for rare species, occurring in less than ten 5x5 km squares, we added old data from the notebooks of Besseling, which are deposited in the Zoological Museum in Amsterdam. Included in the notebooks of Besseling are the data of Romijn. Furthermore, data from C. Davids have been used. 
Our recent data show a bimodal distribution over the year, as we sampled twice a year, in spring and in summer.

The list of Dutch water mites
We give a revised list of the Dutch water mites. 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).
For each species a complete bibliography is given of the Dutch publications. Not included in this bibliography are the so called "grey" literature, e.g. unpublished reports. In table ** the number of species known from The Netherlands can be found, published by Besseling (1964), Davids (1979) and this publication. The Dutch list contains nowadays 51 genera, with 238 species and 2 subspecies. 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.

Description of the species
For each species its occurrence is given, using the following division:

very rare  species occurs in 10 or less 5x5 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 ib more than 500 squares

The seasonal periodicity is given for species which show a preference for either spring or (late) summer/autumn.

Environmental parameters
For species occurring more than five times, a range of the following environmental parameters is given:
 
 

Parameter	Abbreviation	Unity
Acidity	pH
Bicarbonate	HCO3-	mg/l
Chloride	Cl-	mg/l
Sulphate	SO4-	mg/l
Total phosphate	tP	mgP/l
Nitrate	NO3-N	mgN/l
Kjeldahl nitrogen	NH4-N	mg/N/l
Sodium	Na+	mg/l
Magnesium	Mg+	mg/l
Calcium	Ca2+	mg/l
Potassium	Ka+	mg/l
Iron	Fe2+	mg/l
Oxygen	ZVP	%
Width		dm
Depth		dm
Thickness mud layer		cm

(Chloride+sulphate)/(chloride+sulphate+bicarbonaat)  index 1
Chloride/(chloride+sulphate)                                     index 2
Calcium/(Sodium+Potassium+calcium+Magnesium)  index 3

For each parameters is given:

n number of records
Mean mean
Std standaarddeviation
Min minimum value
P10 10-percentile value
P50 50-percentile value (mediaan)
P90 90-percentile value
Max maximum value

All data are from the provinces North- and South-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". Moreover, 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.
 

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