SPATIAL AND TEMPORAL PATTERNS OF ARTHROPOD ASSEMBLAGES ASSOCIATED WITH MACROPHYTES IN A LOW-ORDER SUBTROPICAL RIVER (CHACO, ARGENTINA)

The aim of this study was to determine the composition and abundance of arthropod assemblages associated with aquatic vegetation in different sections of a plain river and during different hydrologic conditions. Surveys were carried out in five sites located in the longitudinal gradient of the river (27 degrees 06'49 ''-27 degrees 25'47 '' S and 58 degrees 57'45 ''-59 degrees 27'23 '' W) during rainy and drought periods. According to the physical and chemical features, principal component analysis grouped the sites of the middle section and separated the sites of the low section, which had high conductivity and salinity. Mean arthropod density varied between 197 and 42775 individuals per m(2) of vegetation-covered areas and between 79 y 45542 per 100g of dry plant matter. Total abundance differed significantly between sites, dates, and plant species, and stepwise-multiple regression indicated that plant substrates explain 46% of the variation of total density. In the assemblages, Chironomus sp. and Parachironomus sp. were eudominant, while Cytheridella ilosvayi Daday, Hyalella curvispina Shoemaker, Simocephalus serrulatus Koch, Callibaetis sp. and Ablabesmyia sp. were dominant. The non-metric multidimensional scaling ordination analysis indicated that differences in the abundance of the dominant taxa in the arthropod assemblages were more important across sections than across hydrologic conditions.