The hyporheic zone of a spring-fed river and its role in n cycling

Florida is renowned for its abundance of artesian springs.  Despite their enormous ecological and socio-economic value, our understanding of the processes controlling the dynamics of nutrient cycling in Florida’s spring-fed rivers is critically incomplete.  No studies to date document the magnitude of transient storage in these rivers.  This knowledge gap is relevant to our understanding of nitrogen dynamics; nitrate is the primary nutrient of concern due, in large part, to order of magnitude increases in concentration over the past half century.  However, sharp longitudinal declines in nitrate concentration in the Ichetucknee River, underscore a remarkable capacity to remove nitrate. Observed declines are larger than can be explained by assimilation alone, leaving dissimilation, presumably within the hyporheic zone, as the most likely sink.  Preliminary measurements suggest that hyporheic sediments occupy a tremendous volume, much larger in cross-section than the advective channel, and likely play a significant role in N dynamics.  Hyporheic porewater and sediment samples confirm low dissolved oxygen and abundant organic matter necessary for denitrification.  Our objective is to quantify the rates of hydraulic exchange between the channel and the transient storage zone using a conservative tracer.  Preliminary results suggest large transient storage volumes with significant variability within river geomorphic zones consistent with observed longitudinal declines in nitrate.