Seasonal movement of marine-derived nutrients in riverine surface bed sediment and the floodplain hyporheic zone

In the fall of 2014, 223,425 sockeye salmon returned to the Horsefly River in the Central Interior of BC. During and following the active spawn the river received a pulse of marine derived nutrients (MDN). The value of these nutrients to the functioning of the riverine ecosystem depends on if, where, and for what time period MDN is retained. Re-suspended fine bed sediment and floodplain hyporheic water were collected to determine the potential for MDN to be retained overwinter. During the spawn the proportional contribution of salmon to the fine bed sediment and associated biofilm was 25-50%. Following the winter, isotopic signatures had returned to pre-spawn levels. MDN was also transported into the floodplain hyporheic zone. Hyporheic flow pathways and nutrient concentrations were found to be spatially and temporally variable. More research is needed to determine the proportional contribution of MDN to the hyporheic nutrient pool and the retention time.