Environmental control of seasonal and inter-annual variations of periphytic biomass in a North Iberian stream

Department of Plant Biology and Ecology. Faculty of Science and Technology. University of the Basque Country, P.O. Box 644, 48080 Bilbao, Spain

Corresponding author: gvbizigo@lg.ehu.es

Abstract

During 2001 - 2002, we studied periphytic biomass at 5 sites along the Agüera stream (Northern Iberian Peninsula), and compared the results to those from two previous researches (1990 - 1991 and 1992 - 1993). Sampling sites showed large differences in cover, conductivity and nutrient contents, as well as some inter-annual differences, resulting from changes in discharge and in human activities. Most environmental variables showed low seasonality, main variations being produced by floods. Solar radiation changed little during 2001 - 2002, because of the coincidence of a sunny winter followed by a cloudy summer, and seasonal differences in riparian cover resulted in more sunlight reaching the streambed of closed sites during winter, and small differences at the open ones. The growth and abscission of riparian leaves results in a strong light regime at the closed sites, with more sunlight during winter, but this pattern changes from year to year, depending on the weather. Spatial differences and highest values of periphytic biomass were much lower than those measured in 1990 - 91, because of contrasting weather patterns. Variables controlling periphytic biomass in the Agüera change from reach to reach. At open sites flow is the main temporal controller, whereas at closed sites the effects of light availability prevail, thus giving more similar seasonal patterns from year to year. Nutrients seem to play a minor role in temporal variations.