Summary

Samples were collected in the summer from sampling sites on the Union, Tahuya, and Skokomish Rivers between June and August 2004 and samples were collected in the spring from the Union River, Tahuya River, Mission Creek, and three smaller drainage basins in March 2004. Concentrations of nutrients at all sites were low. Ammonia and orthophosphate concentrations were less than the detection limit for most samples, and nitrate plus nitrite concentrations ranged from less than the detection limit of 0.06 to 0.49 mg/L. Concentrations of nitrate plus nitrite were near the detection limit of 0.06 mg/L in the North Fork, South Fork, and mainstem of the Skokomish River. Concentrations of nitrate plus nitrite in the Tahuya River system upstream of Lake Tahuya was 0.17 mg/L, but decreased to 0.10 mg/L or less downstream of Lake Tahuya. Overall, concentrations of nitrate plus nitrite were highest in the Union River ranging from 0.12 to 0.28 mg/L. δ¹⁵N generally fell within the range that encompasses most sources, providing little information on nitrate sources. The δ¹⁵N values increased slightly downstream on the Union River and decreased on the Tahuya River downstream of Lake Tahuya. Most nitrogen in the Union, Tahuya, and Skokomish Rivers was in the dissolved inorganic form. Dissolved organic carbon concentrations in the Tahuya and Skokomish Rivers averaged 0.2 and 0.5 mg/L, respectively, although the concentrations in the Union River averaged 1.3 mg/L. There was little difference in nutrient concentrations at sites U1 and T1 between samples collected in the spring and during summer base-flow conditions.

Lake Tahuya significantly modified the biogeochemistry of nitrogen in the Tahuya River drainage basin. In Lake Tahuya, inorganic nitrogen was converted into particulate and organic nitrogen. Dissolved organic carbon concentrations of 2.6 to 2.7 mg/L were highest downstream of Lake Tahuya, and then decreased to concentrations less than 1 mg/L similar to concentrations in the Skokomish River. Downstream of Lake Tahuya, concentrations of the redox-sensitive ions manganese and iron were elevated in the Tahuya River at sites T7 and T6, suggesting a biogeochemical coupling with the organic matter produced in Lake Tahuya. Iron and manganese concentrations also were elevated in the Union River at sites U3 and U4, as well as for the two sites (S1 and S2) at the mouth of the Skokomish River.

Total nitrogen concentrations near 0.5 mg/L were measured at only two sites: Unnamed Creek at Purdy-Cutoff Road (site S2b) and downstream of Lake Devereaux (site SP5). Nitrate plus nitrite concentrations were highest at site S2b (0.49 mg/L), and dissolved organic carbon concentrations were highest at site SP5 (3.3 mg/L). However, the overall impact by these sites on the nutrient loading to Hood Canal probably is negligible because of their low-flow rates.

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