Chironomid (Diptera: Chironomidae) assemblages in three Alpine glacial systems

During the melting season (June-September) from 1996 to 1998, three glacial streams (Conca, Niscli and Cornisello) and one small stream (the main tributary to the Conca stream) fed by snowmelt, groundwater and rain were studied. The survey area is located in the central-eastern Italian Alps. In all 18 stations were sampled and all, except one, were located above the tree line (about 2000 m a.s.l.) within areas stretching from the source to about 2.5 Km downstream. At each station, geomorphological, physical, chemical and biological data were collected. 110 taxa were identified, belonging to Chironomidae (55), other Diptera (17), Ephemeroptera (7), Plecoptera (15), Trichoptera (8), Crustacea (4), Oligochaeta, Nematoda, Tricladida and Hydracarina. Chironomidae were the dominant group as number of taxa and relative abundance in the four investigated streams. Within the Chironomidae, 1 Tanypodinae, 18 Diamesinae, 34 Orthocladiinae and 2 Chironominae were identified. Faunal data were classified using Two-Way Indicator Species Analysis while the relationships between the environmental factors (33 in all) and the faunal components were explored through a canonical correspondence analysis (CCA) or redundancy analysis (RDA), depending on the gradient length. The three glacial streams, though sharing a similar geological and climatic conditions, appeared rather different in the water temperature pattern, discharge, current velocity, suspended sediments and substrate composition. This heterogeneity allowed to highlight difficulties in the selection of the most important factors affecting faunal succession. One factor (i.e. channel stability in the Conca and Cornisello streams), in fact, may play different roles in structuring the faunal composition downstream of the snout in different glacial streams or even in the same stream in different years. 19 environmental variables were selected as significant explanatory factors for the invertebrate assemblages in the four investigated streams: altitude, glacial influence, distance from the source, slope, discharge, current velocity, Pfankuch index (= channel stability), substrate composition, water depth, mean water temperature, temperature range, conductivity, pH, silica, sulphate, nitrate, BPOM, presence of the chrysophyte Hydrurus foetidus, presence of mosses. All these variables changed along an upstream-downstream gradient, generally associated with the first ordination axis in the CCA/RDA diagrams. High sulphate concentration and presence of mosses were the two variables more associated with the non-glacial condition. The CANOCO model explained less than 50% of the total faunistic variance. Samples were generally separated more per site than per date. TWINSPAN faunistic patterns appeared partly depending on these environmental factors, fitting with the CCA/RDA gradients. A strict relationship between Diamesa species and high altitude, short distance from the glacial snout, high slope, high current velocity, low mean water temperature and abundance of Hydrurus foetidus was common to the three glacial streams. The two variables indicated by the model by MILNER & PETTS (1994) as the most important for the species succession, channel stability and maximum water temperature, were selected among the most significant factors affecting the faunal pattern using different data bases, but in all cases they explained a relatively low % of the total faunistic variance (< 9%). The longitudinal pattern of taxa richness and succession, with some exceptions, was in concordance with the predictions by the model. This research emphasised: - the complexity of the interactions among environmental factors and between environmental factors and fauna living in high mountain streams (these interactions may be different in different streams and/or periods); - a wider ecological tolerance of some species; Diamesa spp., for example, known as cold stenothermal, are able to maintain consistent populations also above their upper temperature limit. Other species, as Orthocladius (E.) rivicola gr. or Micropsectra atrofasciata, not commonly found in the kryal biotopes, were found numerous in some sites and dates in glacial sites; - a wider geographical distribution of some species as Diamesa wuelkeri Serra-Tosio and D. geminata cf. Kieffer; - difficulties in the identification to species level of Diamesa larvae, pupae and adults also due to the high morphological variability within the same species (ecotypes); - the complexity of the life cycle of Diamesa species, characterised by opportunism (r-strategy) and, at least some species, by multivultinism.