The upside-down river: Reservoirs, algal blooms, and tributaries affect temporal and spatial patterns in nitrogen and phosphorus in the Klamath River, USA

The Klamath River, located in Oregon/California of the Northwestern U.S., is highly impounded and also experiences large seasonal algal blooms and impaired water quality. We investigated nitrogen (N) and phosphorus (P) constituents for one year (2010-2011) across 193. km of the Klamath River at sites above and below reservoirs and major tributaries to determine the influence of these features on longitudinal and temporal trends in concentrations, loads, and N:P ratios. In general, the headwater lake (Upper Klamath Lake) and reservoirs appeared to be the dominant influence on water quality and nutrient dynamics in the upper river, whereas tributaries appeared to exert stronger influence in the lower river. Overall, high nutrients and poor water quality at upstream sites were ameliorated downstream, however the downstream reductions in N were much greater relative to P. Seasonality appeared to play a major role in the overall appearance and magnitude of longitudinal trends. The greatest upstream-downstream differences occurred during periods of time following large algal blooms in the upper portion of the river. Overall, the amount and composition of N appeared to be strongly driven by algal blooms and biogeochemical conditions such as low oxygen, high pH and warm temperatures in the upper portion of the river, whereas P was more strongly driven by seasonal hydrology. The spatiotemporal influence of reservoirs and tributaries on nutrient flux and nutrient ratios may have significant implications for aquatic communities and ecosystem health. Nutrient objectives should be considered when designing restoration, management, and monitoring objectives for projects involving habitat suitability for anadromous fish and potential dam removal