Seasonal variations in nutrient concentrations and speciation in the Chena River, Alaska

Abstract

To better understand the seasonal controls on nutrient abundances, speciation, and fluxes in a watershed underlain by discontinuous permafrost, we collected water samples biweekly from the Chena River during 2005–2006 to measure inorganic and organic N, P, and Si in dissolved and particulate phases. Nitrate concentrations were low (8–14 μM) during the winter and summer dry seasons but were elevated during the spring freshet (15–24 μM). Ammonium varied from 8 to 13 μM during the winter but dropped dramatically during the ice‐open season to 0.1–3 μM. Phosphate was very low throughout the year (ranging from 0.03 to 0.3 μM), reflecting the pristine condition of the watershed. Dissolved silica was high in the winter and reached its minimum during the spring freshet. DIN was the dominant species in the total N pool (60%), followed by DON (30%) and PN (10%). Most of the phosphorous was present in the particulate phase (74%), with phosphate and DOP only comprising 19% and 7%, respectively. Seasonal variations in nutrient concentrations and speciation were mostly controlled by the hydrological flow regime and biological activity in the river. Annual nutrient export fluxes from the Chena River during 2005–2006 were 51.1 × 106 mole‐N, 1.4 × 106 mole‐P, and 197 × 106 mole‐Si, corresponding to an annual yield of 9.8 × 103 mol‐N km−2, 0.28 × 103 mol‐P km−2, and 37.9 × 103 mol‐Si km−2, respectively. Within the annual export fluxes, the spring freshet contributed about 18% of TN, 27% of TP, and 10% of Si, while the winter season contributed 11% of TN, 12% of TP, and 20% of Si. Continued climatic warming in northern watersheds will likely increase the export of nutrient species from watersheds.