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

Abstract

During 2001 - 2002, we studied periphytic biomass at 5 sites along the Agüera stream (Northern Iberian Peninsula), and comparedthe results to those from two previous researches (1990 - 1991 and 1992 - 1993). Sampling sites showed large differencesin cover, conductivity and nutrient contents, as well as some inter-annual differences, resulting from changes in discharge andin human activities. Most environmental variables showed low seasonality, main variations being produced by floods. Solarradiation changed little during 2001 - 2002, because of the coincidence of a sunny winter followed by a cloudy summer, andseasonal differences in riparian cover resulted in more sunlight reaching the streambed of closed sites during winter, and smalldifferences at the open ones. The growth and abscission of riparian leaves results in a strong light regime at the closed sites, withmore sunlight during winter, but this pattern changes from year to year, depending on the weather. Spatial differences and highestvalues 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 temporalcontroller, whereas at closed sites the effects of light availability prevail, thus giving more similar seasonal patterns from yearto year. Nutrients seem to play a minor role in temporal variations.