Voorkomen van libellenlarven in de Nederlandse Delta in relatie tot saliniteit

Witte, R.H. & D. Groenendijk, 1999. The occurrence of dragonfly larvae in the Dutch Delta in relation to salinity. Brachytron 3(2): 3-10 In Europe, no characteristic dragonfly species are known to occur in saline or brackish waters. However, some dragonfly species may succesfully breed in brackish waters in the southwestern part of The Netherlands. At about 225 localities in the mostly brackish Dutch Delta area dragonfly larvae were caught (or exuviae were collected) and salinity values were measured simultaneously. This information was emplified by known literature (figure 1). Results showed that only Ischnura elegans was found to be able to breed successfully in polyhaline (table 1) waters. However, numbers decreased strongly when ion concentrations exceeded 4.5‰. Aeshna mixta (larvae) was found regularly in mesohaline waters with an average ion content of 4.3‰, temporarily increasing during summer to a maximum value of 6.8‰. A maximum of only 2‰ was formerly known for this species in The Netherlands. Aeshna cyanea larvae were found in oligohaline waters, in which the salt concentration increased up to 18‰ during a short period in summer. This observation is highly congruent with results from Denmark, where larvae of Orthetrum cancellatum occurred in brackish waters with an ion content of 13‰. It seems that larvae of several dragonfly species can survive during short periods of high salinity. However, it is highly likely that larval development will be inhibited during these temporary situations. Oviposition in mesohaline waters was recorded for Erythromma viridulum and Enallagma cyathigerum. However, no successful! reproduction could be proven. In Canada E. cyathigerum larvae have been found in brackish waters, as well as Sympetrum danae larvae. Several Sympetrum species are known to be tolerant for brackish water conditions and in the Delta area egg-laying behaviour was even recorded in polyhaline waters during some occasions. In addition, exuviae of Lestes sponsa and L. barbarus were found in oligohaline waters. Larvae of several Sympetrum species and E. viridulum were recorded here also. No other dragonfly species in the Delta area could be traced in brackish waters, although some other species were found in brackish waters elsewhere in Europe.