Nutrient attenuation in rivers and streams, Puget Sound Basin, Washington

Abstract

Nutrients such as nitrogen and phosphorus are important for aquatic ecosystem health. Excessive amounts of nutrients, however, can make aquatic ecosystems harmful for biota because enhanced growth and decay cycles of aquatic algae can reduce dissolved oxygen in the water. In Puget Sound marine waters, low dissolved oxygen concentrations are observed in a number of marine nearshore areas, and nutrients have been identified as a major stressor to the local ecosystem. Delivery of nutrients from major rivers in the Puget Sound Basin to the marine environment can be large. Therefore, it is important to identify factors related to how nutrients are retained (attenuated) within streams and rivers in the Puget Sound Basin. Physical, chemical, and biological factors related to nutrient attenuation were identified through a review of related scientific literature. Numerous empirical modeling approaches for estimating nutrient attenuation in streams and rivers also were compiled, and a subset of these models was applied to the Puget Sound Basin. In particular, models based on the physical characteristics of a river reach (RivR-N model) and on the physical and biological features of a river reach (v f model) were used and compared for the 17 major rivers draining to the Puget Sound. Data on the relative amount of instream attenuation (the fraction of nutrient input removed per kilometer of stream reach) showed some common and general themes. Firstly, headwater reaches throughout the Puget Sound Basin tend to be better than the main stems of the major rivers at attenuating nitrate and orthophosphorus (ortho-P). Secondly, rivers are more efficient at attenuating nitrate than ortho-P, probably because of the close relation between phosphorus and suspended sediment, which was not captured fully in the models. Thirdly, when comparing the RivR-N and v f models for nitrate, physical characteristics of the channel may be more effective predictors of relative nitrate attenuation for main stem reaches, whereas biological factors may be more effective predictors in headwater reaches. These results are explained in terms of four primary factors of attenuation: sinuosity, channel slope, specific discharge, and uptake velocity (v f ) of the reach. A simple scoring procedure based on these four factors showed that reaches where attenuation scores were high had higher relative attenuation of nutrients from the RivR-N and v f models. This attenuation "scorecard" can be used to quickly assess the potential for a given reach to attenuate nutrients. Seasonal relative attenuation at three case studies was greater in summer months (July through September) and much lower and almost constant from January through June. An analysis of relative attenuation across a range of nutrient concentrations showed that, at some point, relative instream attenuation is minimized. For nitrate, relative attenuation reached a minimum value greater than 3 milligrams of nitrogen per liter (mg N/L) during low flow and 1 mg N/L during high flow. For orthophosphate, minimum relative attenuation was observed at about 0.1 milligram of phosphorus per liter (mg P/L) for both low- and high-flow conditions. Generally, the temporal dynamics of nutrient attenuation are dependent on the travel time through a given reach, the proportion of flow in contact with the sediment, and the amount of biological activity. Improved understanding of nutrient attenuation in Puget Sound Basin will benefit from the compilation of more detailed data for specific discharge, channel slope, and channel sinuosity in Puget Sound streams and rivers. Additionally, field studies examining upstream-downstream changes in nutrient load and field-based measurements of v f are needed. From a management perspective, preservation and improvement of instream nutrient attenuation should focus on increasing the travel time through a reach and contact time of water sediment (reactive) surfaces and lowering nutrient concentrations (and loads) to avoid saturation of instream attenuation and increase attenuation efficiency. These goals can be reached by maintaining and restoring channel-flood plain connectivity, maintaining and restoring healthy riparian zones along streams, managing point and nonpoint nutrient loads to streams and rivers, and restoring channel features that promote attenuation such as the addition of woody debris and maintaining pool-riffle morphologies. Many of these management approaches are already being undertaken during projects aimed to restore quality salmon habitat. Therefore, there is a dual benefit to these projects that also may lead to enhanced potential for nitrogen and phosphorus attenuation.