Nutrient dynamics in a highly managed reservoir system: considering anadromous sockeye salmon (Oncorhynchus nerka) and nutrient restoration

Abstract

ABSTRACTScott DC, Harris SL, Hebert AS, van Poorten BT. 2017. Nutrient dynamics in a highly managed reservoir system: considering anadromous sockeye salmon (Oncorhynchus nerka) and nutrient restoration. Lake Reserve Manage. 33:14–22.Modeling nutrient flux in Alouette Reservoir, British Columbia, allowed us to (1) determine the relative importance of key factors that influence its nutrient dynamics and (2) evaluate the potential value of returning anadromous salmon to this highly altered reservoir. Annual surface water releases over the dam spillway have facilitated out-migration of juvenile sockeye salmon (Oncorhynchus nerka) produced by resident kokanee parents since 2005, and these juveniles have returned as adult sockeye annually since 2007, thus restoring an anadromous run lost for nearly 90 years. This re-anadromization program may have an effect on the nutrient restoration program, which includes annual additions of nitrogen and phosphorus designed to enhance the recreational kokanee fishery within the reservoir. High flow release to facilitate juvenile out-migration resulted in a net export of nutrients because few adults returned, and a portion of nutrients added to the reservoir were flushed downstream. If only anadromous sockeye are considered, however, nutrient flux was positive in 5 of 6 years for nitrogen (average 2.30 kg; range −1.06 to 5.97), and in 4 of 6 years for phosphorus (average 0.24 kg; range −0.15 to 0.62). Overall, our results show the export of nutrients by the anadromy restoration project represents an average of just 1.54 and 0.60%, respectively, of the seasonal nitrogen and phosphorus budget for the system. Therefore, although the anadromy restoration results in a net loss of nutrients from the system, its effect on the nutrient restoration project is minimal.