Influence of Salmon Carcass Placement in Red Alder Riparian Areas on Stream Chemistry in Lowland Western Washington

Abstract

Populations of Pacific salmon Oncorhynchus spp., which historically returned large quantities of marine-derived nutrients (MDN) to Pacific Northwest streams and riparian areas, are now much lower. In response, salmon carcasses are now being placed in streams and riparian areas to compensate for the loss of MDN, particularly organic N and P. Our objectives were to determine (1) whether placement of chum salmon O. keta carcasses in riparian areas dominated by red alder Alnus rubra, an N-fixing tree, changed stream chemistry and (2) whether carcass placement affected water quality. Study sites were Brown and Le Bar creeks, two small streams in the Skokomish River valley in western Washington. The amount of N in carcasses placed in riparian areas in Brown and Le Bar creeks in November 1999 was estimated to be 22.4 and 59.8 kg/ha, respectively, while the amount of P was 2.7 and 7.2 kg/ha. Monthly stream grab samples were taken between September 1999 and October 2000 upstream and downstream of salmon carcass placement areas and analyzed for NO₃-N, NH₄-N, PO₄-P, pH, electrical conductivity, and alkalinity. The maximum concentration was low for NO₃-N (0.32 mg/L), NH₄-N (0.10 mg/L), and PO₄-P (0.026 mg/L). Salmon carcass placement did not significantly change stream chemistry. This could have been because of the particular amounts of N and P added or uptake by soil organisms, riparian vegetation, and stream processes. Substantial amounts of nutrients in salmon carcasses can be added to the system with little influence on stream water quality. Red alder in the riparian areas did not strongly influence the stream concentration of NO₃-N.