Macroinvertebrate communities along a third-order Iberian stream

Many studies have tried to elucidate the main factors influencing macroinvertebrate community composition and abundance in streams (e.g. Hart 1992, Dudgeon 1994, Giller & Twomey 1993, Maridet et al. 1998, Tierney et al. 1998, Pardo 2000, Doisy & Rabeni 2001). Considerable attention has been paid to food resources (e.g. Hawkins & Sedell 1981, Cowan & Oswood 1984, Boulton & Lake 1992, Friberg 1997, Wallace et al. 1997) because invertebrate community structure is thought to be tightly linked to the nature and availability of these resources (Vannote et al. 1980). Additionally, environmental disturbances may exert a great influence on the macroinvertebrate communities (e.g. Resh et al. 1988, Townsend 1989, Townsend & Hildrew 1994). Variations in stream flow are seen as a major disturbance in these systems (e.g. Poff & Ward 1989, Yount & Niemi 1990, Robinson & Minshall 1998). The influence of floods on biotic communities seems to be related with their intensity, frequency and predictability (e.g. Minshall 1988, Resh et al. 1988). In 1990, Basaguren et al. (1996) examined the macroinvertebrate assemblages of a heterotrophic and an autotrophic reach of the Agüera stream (northern Spain). The differences in fauna between both sites were explained as a result of inter-site differences in food and flow refugia availability. Furthermore, the results of that previous work suggested that the temporal patterns of macroinvertebrate communities were determined by both food and flow regime. In this paper we extend our analysis of macroinvertebrate communities to 3 reaches of the Agüera catchment: 2 shaded (firstorder, and third-order) reaches, and an open third-order reach. Our main hypothesis was that inter-site differences in community features such as species composition and functional organization would reflect spatial differences in food availability. Moreover, we expected that floods would influence the temporal patterns of macroinvertebrate communities by decreasing total numbers of invertebrates, while increasing density of some opportunistic taxa.