The role of micro-organisms in the ecological connectivity of running waters

1. Riparian zones hold a central place in the hydrological cycle, owing to the prevalence of surface and groundwater interactions. Ln riparian transit ion zones, the quality of exfiltrating water is heavily influenced by micro bial activities within the bed sediments. This paper reviews the role of mi cro-organisms in biogeochemical cycling in the riparian-hyporheic ecotone. 2. The production of organic substances, such as cellulose and lignin, by r iparian vegetation is an important factor influencing the pathways of micro bial processing in the riparian zone. For example, anaerobic sediment patch es, created by entrainment of allochthonous organic matter, are focal sites of microbial denitrification. 3. The biophysical structure of the riparian zone largely influences in-str eam microbial transformations through the retention of organic matter. Part iculate and dissolved organic matter (POM and DOM) is retained effectively in the hyporheic zone, which drives biofilm development and associated micr obial activity. 4. The structure of the riparian zone, the mechanisms of POM retention, the hydrological linkages to the stream and the intensity of key biogeochemica l processes vary greatly along the river continuum and in relation to the g eomorphic setting. However, the present state of knowledge of organic matte r metabolism in the hyporheic zone suggests that lateral ecological connect ivity is a basic attribute of lotic ecosystems. 5. Due to their efficiency in transforming POM into heterotrophic microbial biomass, attached biofilms form an abundant food resource for an array of predators and grazers in the interstitial environments of rivers and stream s. The interstitial microbial loop, and the intensity of microbial producti on within the bed sediments, may be a primary driver of the celebrated high productivity and biodiversity of the riparian zone. 6. New molecular methods based on the analysis of the low molecular weight RNA (LMW RNA) allow unprecedented insights into the community structure of natural bacterial assemblages and also allow identification and study of sp ecific strains hitherto largely unknown. 7. Research is needed on the development and evaluation of sampling methods for interstitial micro-organisms, on the characterization of biofilm struc ture, on the analysis of the biodegradable matter in the riparian-hyporheic ecotone, on the regulation mechanisms exerted on microbiota by interstitia l predators and grazers, and on measures of microbial respiration and other key activities that influence biogeochemical cycles in running waters. 8. Past experiences from large-scale alterations of riparian zones by human s, such as the River Rhine in central Europe, undeniably demonstrate the de trimental consequences of disconnecting rivers from their riparian zones. A river management approach that uses the natural services of micro-organism s within intact riparian zones could substantially reduce the costs of clea n, sustainable water supplies for humans.