The distribution and speciation of copper (Cu) in Hood Canal, a fjord in western Puget Sound, Washington, were studied over a 2-year period. Cu is required as a cofactor for many enzymatic pathways, including ammonia oxidation. In Hood Canal, ammonia oxidation is largely dominated by ammonia-oxidizing archaea (AOA), which have high Cu requirements for other processes as well. Dissolved Cu was slightly depleted in the upper water column, and concentrations were almost unchanged from measurements made in the late 1970s, ranging from 4.08 to 6.12 nM. Although this implies that the biological demand is small relative to the large and relatively constant inventory of dissolved Cu, and that Cu limitation is therefore unlikely to influence rates of biological processes, speciation measurements indicated that dissolved Cu is strongly complexed by organic ligands. As a result, bioavailable Cu²⁺ concentrations were considerably lower, varying from 6.14 × 10^-15 M to 1.36 × 10^-12 M. This is a range that encompasses the threshold (<~ 2 × 10^-13 M) for Cu limitation of ammonia oxidation by Nitrosopumilus maritimus SCM1, a representative AOA, in culture (Amin et al., 2013). Furthermore, Cu²⁺ displayed a clear trend over most sampling periods, with Cu²⁺ concentrations one to two orders of magnitude higher below 20m in the deeper, saline waters, and exhibiting minima in the upper 15m. The major freshwater input to Hood Canal is not an important source of ligands, which suggests that the ligands are likely produced biologically in the water column and have slow turnover times. In general, ammonia oxidation rates varied considerably but were lowest in the upper water column where Cu²⁺ concentrations were also lowest. Thus, these findings will facilitate further work to ascertain the relative importance of Cu bioavailability in limiting ammonia oxidation rates versus light inhibition, which has frequently been invoked to account for low rates of nitrification in the upper water column.