Vol. 16: 143-151, 1998 AQUATIC MICROBIAL ECOLOGY Aquat Microb Ecol Published November 27 Heterotrophic metabolism in a forest stream sediment: surface versus subsurface zones Anna M. Romani, Andrea Butturini, Francesc Sabater, Sergi Sabater* Departament d'Ecologia. Facultat de Biologia, Universitat de Barcelona, Avgda Diagonal 645. E-08028 Barcelona. Spain ABSTRACT: The heterotrophic metabolism of a forest stream sediment was studied during leaf fall, immediately after a flood that completely scoured and homogenized the first 10 to 12 cm of sediment. After this event, differences were found between the surface sediment (0 to 3 cm depth) and the sub- surface sediment (7 to 10 cm depth). Higher extracellular enzymatic and respiratory activities were measured in the surface than in the subsurface sediment. The higher heterotrophic activity was related to the higher quantity and quality of the organic matter, which accumulated in the surface sediment (higher chlorophyll and bacterial densities).The heterotrophic metabolism in the surface sediment fol- lowed a marked seasonal pattern that correlated with the variations in the environmental parameters (discharge, nutrients). However, no temporal pattern was observed in the subsurface sediment, less affected by the physical changes (e.g, scouring), where no significant change in any activity was recorded throughout the study period. There was a significant decrease in heterotrophic metabolism in the subsurface sediments. This difference can be attributed to the lower availability of algae-derived matenal in the subsurface sedirnents. KEY WORDS: Biofilm . Metabolism . Sediments . Riparian . Stream . Hyporheos . Ectoenzymatic activities INTRODUCTION Stream biofilms are microbial populations of bacte- ria, algae, fungi, protozoans and micrometazoans embedded in a polysaccharide matrix inhabiting rocks, gravels, wood and sediments (Lock 1993). Microbial biofilms living in the sediments convert allochthonous or autochthonous dissolved organic substances (Nai- man et al. 1987) into particulate form (Kaplan & Bott 1983, Bott et al. 1984).Sediment biofilms develop a key role in the maintenance of the carbon balance in streams (Marxsen 1988, Meyer 1988).In the hyporheic zone, defined as the sediments hydrologically linked to the open stream channel (Findlay 1995) but above groundwater (White 1993), biofilm metabolism and organic carbon cycling are as tightly coupled as in sur- face sediments (Barlocher & Murdoch 1989, Hedin 1990). The metabolic activity measured in the hypo- rheic zones is high (Grimm & Fisher 1984, Hendricks 'Addressee for correspondence. E-mail: ssabater@porthos.bio.ub.es 1993, Pusch & Schwoerbel 1994). The organic matter accumulated in stream sediments is decomposed by bacteria, fungi and occasionally algae, which convert high-molecular-weight molecules to low-molecular- weight ones by extracellular enzymatic hydrolysis (Chrost 1990, Marxsen & Witzel 1991). Metabolism of biofilm sediments is affected by phys- ical factors such as hydrology and permeability (Hak- enkamp et al. 1993, Findlay 1995), leading to hetero- geneity in biological and physicochemical patterns in the hyporheic zone (Palmer 1993). Streambed perme- ability, mainly controlled by grain size (Findlay 1995), favours the water exchange between the surface and the hyporheic sediments (Grimm & Fisher 1984). In this study we focused on heterotrophic metabo- lism in the sediment of a Mediterranean forest stream (Riera Major). Organic matter accumulates in sandy stretches, where the hyporheic zone extends up to 50 cm in depth. Sand sediments are therefore an important place of carbon storage and cycling. Our study started immediately after a severe flood oc- curred in Riera Major in October 1994 that completely O Inter-Research 1998