Spatial-temporal changes in the physico-chemical characteristics of the Agura river (Basque Country and Cantabrian region, Spain) were studied over six months, a period that included a long period of unusual drought and the return to more normal hydrological conditions. The chemical composition of the water was affected by water flow. It wasinfluenced either directly through the drainage characteristics at the level of different parts of the surrounding basin,or indirectly through the action of biotic communities on their environment. From these results, some spatial and/ortemporal models have been developed for the dynamics of the different variables in this study.

In the Lea River basin, 47 taxa of caddisflies have been found belonging to 32 genera and 15 families. Rhyacophilidae (5 species), Hydropsychidae (6 species), Limnophilidae (4 species) and Polycentropodidae (4 species) were the dominant families ; Hydropsyche pellucidula being the most abundant species. Using all these taxa as entities, an ordination of the River Lea Basin has been made by means of Correspondance Analysis. The ordinations with higher ecological meaning were obtained on the plane of the second and third axes, which explained about 21 % of the total variance. According to this ordination the following zones characterized by different communities of caddisflies could be identified in this basin : the headwater of the main river, the small tributaries and a downstream sequence from site 1 to site 5 of the main river as well as a downstream sequence from site 1 to site 2 of its principal tributary. Along the second axi,s a seasonal succession from spring to summer and autumn could be observed in the main river.