Perspectives and predictions on the microbial ecology of the hyporheic zone

1. Studies of hyporheic microbial ecology have suggested an important role for hyporheic microbial processes in stream ecosystem functioning. Using ev idence from microbial communities in other aquatic habitats, some predictio ns are made concerning the diversity of microbial types and microbial proce sses likely to occur in the hyporheic zone, and the relative importance of these various types to the hyporheic ecosystem. 2. It is predicted that the biofilm growth form of interstitial micro-organ isms will create a variety of microniches, allowing coexistence of a great diversity of microbial types, and promoting the activity of some otherwise poor competitors. It is further predicted that the confluence of reduced gr oundwaters and aerobic surface waters will favour chemolithotrophic process es in the hyporheic zone, but that these will contribute significantly to h yporheic production only if surface water is very low in dissolved organic carbon, or the groundwater is extremely reduced, such as by the influence o f riparian wetlands. A variety of anaerobic respiratory pathways, such as n itrate, ferric ion, sulphate and even methanogenic respiration will be empl oyed in the hyporheic zone, with biofilm dynamics permitting these to occur even in aerobic sediments. Anaerobic pathways may account for a significan t proportion of total hyporheic organic matter mineralization. 3. The role of fungi in hyporheic dynamics is, as yet, almost completely un studied. However, it is expected that they will be important in breaking do wn buried particulate organic matter (POM), which may account for a large p roportion of total stream POM. 4. Physicochemical conditions in hyporheic sediments appear to be highly he terogeneous, and this heterogeneity may be very important in the cycling of certain nutrients, especially nitrogen, which involves a series of steps r equiring different conditions. 5. Various new techniques are now available by which biofilm dynamics and i n situ microbial processes may be measured. Studies are recommended of inta ct microbial communities both at the microscale of the biofilm and at the s cale of the heterogeneities occurring in hyporheic sediments. Studies are n eeded that measure actual rates of microbial processes under in situ condit ions.