Microbial metabolism, enzyme activity and production in the hyporheic zone of a floodplain river.

摘要

Microorganisms are key integrators of biogeochemical cycling and energy flow in ecosystems. Although many factors control the rates of microbial processes, available dissolved organic matter (DOM) is often limiting. This limitation is of key interest in the hyporheic zone of rivers, the subsurface area where surface waters and groundwater mix. Most researchers have focused on the role of advecting DOM from surface water to fuel microbial metabolism along hyporheic flowpaths through gravel bars. Hyporheic flowpaths also occur in large floodplain rivers beneath productive riparian terraces where advecting DOM is rapidly utilized leaving a large proportion of the hyporheic microbial community potentially DOM limited. An alternative source of labile DOM however, is the infiltration of DOM from overlying riparian soils.; I investigated how variation in DOM and microbial activity were related to differences among the successional stages of overlying vegetation and positions along flowpaths in the hyporheic zone of a floodplain terrace on the Queets River, WA. The study had three main objectives: determine seasonal subsurface flow structure and quantify changes in dissolved organic carbon (DOC), determine changes in terrace hyporheic DOM bioavailability using exoenzyme activity, and determine rates of hyporheic microbial production.; I found that hyporheic water in the terrace was mainly derived from surface water. Generally there was insufficient DOC in advecting surface water to support hyporheic respiration in the terrace suggesting that riparian soils were a potential DOM source for microbial metabolism. Exoenzyme analysis of the DOM did not elucidate any patterns either along flowpaths or among overlying riparian patches. Similarly, microbial production was not statistically related to the overlying patch structure or position along the flowpath. Rates of microbial production varied inconsistently along flowpaths suggesting that hyporheic microorganisms were metabolizing other sources of DOM than advecting surface water. Laboratory experiments demonstrated that hyporheic microorganisms could metabolize riparian soil leachates with the same efficiencies as surface water.; Dissolved organic matter and microbial production did not decrease along hyporheic flowpaths as predicted from other studies. These results suggest that DOM infiltrating from overlying riparian soils is an important component of the microbial community in floodplain hyporheic zones.