Chironomus species (Diptera: Chironomidae) in the profundal benthos of Spanish reservoirs and lakes: factors affecting distribution patterns

SUMMARY 1 The taxonomy, distribution and habitat preferences of Chironomus larvae from profundal benthos of reservoirs and natural lakes in Spain are presented. The 114 water bodies studied were medium to large reservoirs and lakes throughout Spain, and were visited on at least two occasions (summer and winter). Out of the natural lakes investigated, Chironomus larvae were found only in one karstic lake (Banyoles), and to date, these have not been found in the high mountain lakes of the Spanish Pyrenees or Sierra Nevada, or in Spain's largest oligotrophic lake (Sanabria). 2 Multiple regression with environmental data from reservoirs showed that the density of Chironomus larvae was inversely correlated with water depth, temperature and sulphide content of the bottom water, but positively correlated with alkalinity and particulate organic nitrogen (PON) content of bottom water. Thus, the density of Chironomus larvae was higher in shallow reservoirs from the eastern part of Spain (calcareous water and higher PON), but markedly lower in southern reservoirs (higher temperature and higher sulphide). 3 According to morphological and cytological characteristics, three species of this genus have been identified in the samples examined: C. bernensis, C. plumosus and C. nuditarsis. Reasons for the absence of other widespread European species are discussed. 4 Using canonical correspondence analysis to relate species and environmental variables, C. bernensis showed a significant preference for reservoirs with higher oxygen concentrations in bottom water and lower total reactive phosphorus (TRP) content at the surface than those reservoirs where C. plumosus-type were found. This suggests that the smaller C. bernensis prefers mesotrophic reservoirs and the larger C. plumosus-type (C. plumosus + C. nuditarsis) prefers eutrophic reservoirs. Taking into account the TRP/depth index for those samples collected at depths below the thermocline, and thus, where oxygen can be a limiting factor, C. bernensis and C. nuditarsis were present in reservoirs in which the index was low ( 1:800 for C. nuditarsis), while C. plumosus thrived in reservoirs with higher index values (<1:20). These differences may reflect the preference of C. bernensis and C. nuditarsis for detrital material, while C. plumosus prefers phytoplankton. These observations are in agreement with the depth distribution of each species: C. plumosus was restricted to reservoirs less than 46 m deep, whereas the other two species (i.e. C. bernensis and C. nuditarsis) were found up to depths of 65 and 80 m, respectively. 5 Data from a transect (5–20 m deep) in Lake Banyoles are consistent with the general pattern described in reservoirs. In this lake, a high density of C. bernensis was recorded from littoral and sublittoral areas with abundant oxygen (above or close to the thermocline), whereas C. plumosus was scarce in several parts of the lake, especially in oxygen-deficient areas during the summer. This karstic, oligotrophic lake has a low TRP/depth index, and therefore, we expected C. bernensis to be dominant in the littoral, as is the case. Nevertheless, the severe depletion of oxygen caused by meromixis, and the high hydrogen sulphide concentration benefit C. plumosus in deeper zones. Chironomus nuditarsis was scarce in Lake Banyoles and only found at considerable depths.