NITRATE REMOVAL IN THE HYPORHEIC ZONE OF A SALMON RIVER IN ALASKA

Pacific boreal streams and riparian zones are believed to receive significant N loads that are derived from the ocean in the form of decaying sockeye salmon (Oncorhynchus nerka). Using a small stream in south-central Alaska we examined whether the associated riparian forest could take up the pulse of marine-derived nitrogen (MDN) entering the hyporheic zone from spawning and dying sockeye salmon. We evaluate the relative importance of riparian uptake and denitrification in nitrate-N removal in hyporheic sediment. We found that maximum biological removal of nitrate peaked within 1 h of water entering the hyporheic zone, decreasing exponentially with subsurface flow duration. Plant and microbial uptake reached 14 mu g NO3--N L-1 min(-1) and denitrification reached 4 mu g NO3--N L-1 min(-1) during the initial 2 h of transit time. Our results reinforce the hypothesis that MDN from Pacific salmon can be transferred to riparian zone via hyporheic flow. Most nitrate-N removal along hyporheic flow paths is by plant and microbial uptake (the respective contributions could not be determined). Denitrifying bacteria are present and active in the hyporheic zones of this well-oxygenated Alaskan stream but their contribution to the nitrate-N removal is small compared to plant and microbial uptake in such nitrate-N poor environment. Copyright (C) 2008 John Wiley & Sons, Ltd.