Interactions among sediments, organic matter, and microbial activity in the hyporheic zone of an intermittent stream

Interactions between surface and subsurface water in intermittent streams are poorly understood. We predicted that surface discharge patterns would influence retention and transport of fine sediments and particulate organic matter in a first-order intermittent stream, which in turn would affect microbial activity at different depths in the sediment. We measured sediments, dissolved and particulate organic carbon, and microbial and bacterial biomass and activity at three depths (surface and 20 and 40 cm) over a period spanning low and high flows at five stations on an intermittent stream. Discharge influenced physicochemical and sediment characteristics at the upstream stations with coarse substratum. In the finer sediments of the lower reaches, an active hyporheic microbial assemblage primarily governed sediment and organic dynamics. With decreasing discharge and increasing retention of fine sediments and particulate organic carbon, greater microbial hydrolytic activity in bed-sediments occurred downstream. Dissolved oxygen, organic carbon, fine sediments, microbial biomass, hydrolytic activity, and bacterial biomass declined with depth, and changed over time, apparently in response to varying discharge. We conclude that discharge and substratum particle size may interact to control organic dynamics and hyporheic microbial activity in a Ist order stream.