PARTICULATE ORGANIC-MATTER AND ECOSYSTEM METABOLISM DYNAMICS IN A TEMPORARY MEDITERRANEAN STREAM

Particulate organic matter (benthic and sestonic) and stream metabolis m were determined in four occasions between January and June 1992 in a second order Mediterranean stream in SW Spain (Arroyo de la Montesina ). We aimed to describe contrasting hydrological periods (presence and absence of surface flow) in terms of particulate organic matter resou rces and dynamics. The relationship between benthic (BOM) and sestonic (SOM) organic matter was examined using the BOM:SOM ratio (= BOM . SO M(-1). d(-1), where BOM is in g . m(-2), SOM is in g . m(-3), and d is the mean depth in metres). The Montesina stream appears to have two d istinct phases related to the hydrological period: 1) Surface water fl ow, when the Montesina appears as a stream and is heterotrophic, with low productivity, low SOM, and high BOM:SOM ratio; and 2) No flowing w ater, when the Montesina appears as a series of superficially-isolated pools and is autotrophic, with higher productivity, higher SOM, and a low BOM:SOM ratio. Thus, the hydrological regime is important in cont rolling particulate organic matter dynamics in the Montesina stream, a nd probably in other temporary Mediterranean streams as well.