Exoenzyme activities as indicators of dissolved organic matter composition in the hyporheic zone of a floodplain river

Abstract

We measured the hyporheic microbial exoenzyme activities in a floodplain river to determine whether dissolved organic matter (DOM) bioavailability varied with overlying riparian vegetation patch structure or position along flowpaths. Particulate organic matter (POM), dissolved organic carbon (DOC), dissolved oxygen (DO), electrical conductivity and temperature were sampled from wells in a riparian terrace on the Queets River, Washington, U.S.A. on 25 March, 15 May, 20 July and 09 October 1999. Dissolved nitrate, ammonium and soluble reactive phosphorus were also collected on 20 July and 09 October 1999. Wells were characterized by their associated overlying vegetation: bare cobble/young alder, mid-aged alder (8-20 years) and old alder/old-growth conifer (25 to >100 years). POM was analyzed for the ash-free dry mass and the activities of eight exoenzymes using fluorogenic substrates. Exoenzyme activities in the Queets River hyporheic zone indicated the presence of an active microbial community metabolizing a diverse array of organic molecules. Exoenzyme activities were weakly correlated with DO, DOC and inorganic nutrient concentrations. Principal components analysis indicated that there were patch effects and that these effects were strongest in the summer. DOM degradation patterns did not change systematically along hyporheic flowpaths but varied with overlying forest patch type in the Queets River hyporheic zone, suggesting that additional carbon inputs exist. We hypothesize that the most likely input is the downward movement of DOM from overlying riparian soils. Understanding this movement of DOM from soils to subsurface water is essential for understanding both the hyporheic metabolism and the carbon budget of streams and rivers.

Citation

Clinton, Sandra M.; Edwards, Rick T.; Findlay, Stuart E.G. 2010. Exoenzyme activities as indicators of dissolved organic matter composition in the hyporheic zone of a floodplain river. Freshwater Biology. 55: 1603-1615.