Seasonal and Diel Changes in Phosphorus Transport and Retention in an Intermittent Agricultural Stream

Phosphorus (P) is an essential nutrient for an aquatic ecosystem, however, excess contributions of P from anthropogenic sources, mainly from agricultural land use, can lead to poor water quality, impair aquatic beneficial uses and, in extreme cases, can lead to the development of toxic algal blooms. P is a commonly studied element in agriculture streams, nevertheless, there is still a lack of understanding about the geochemical, and biological processes driving the retention, release, and transport of P. Geochemical pathways consist of metal-P complexes, and biological pathways consist of direct uptake of soluble reactive phosphorus (SRP). Diel SRP patterns have been observed at Missouri Flat Creek (MFC), a stream flowing through the Cook Agronomy Farm in Pullman, Washington. This suggests potential dynamic P retention and release processes at MFC. The purpose of this research is to assess which of the two pathways drive SRP diel cycling at MFC. Understanding stream P release and retention in stream ecosystems can provide insight into the impacts of management and climate on P export, and lead to realistic restoration goals of these agroecosystems. The objectives of this study were to: (1) Monitor seasonal and diel changes in stream temperature, DO, pH, Soluble Reactive Phosphorus (SRP) concentration and dominant forms of extractable P bound to metals in bed sediments six times throughout the water year (Fall, Spring, and Summer), (2) Quantify seasonal variability in the proportion of soluble and particulate P loading transported in the stream six times throughout the water year (Fall, Spring, and Summer), (3) Quantify the extent to which diurnal and seasonal variation in pH, temperature, DO, flow, stream SRP, and dissolved metal concentration in streams, drive SRP adsorption pathways for each of the 6 sampling days. The results of this study indicate that during the cooler months, Ca/Mg-SRP is the dominant metal-P complex found in sediments, with peak concentration occurring at midday. Ca/Mg-SRP complexes gradually decreases in the warmer month as Fe/Mn-SRP and assimilation becomes more dominant, likely due to increased DO concentration and temperature. In June, low DO concentrations are observed concurrently with higher SRP concentrations and lower sediment Fe/Mn-SRP complexes. Diel cycling of SRP, pH, DO, and temperature was also observed in the spring and summer, making biological assimilation of P a plausible retention pathway. A second, interdisciplinary goal of the project, was to increase the knowledge and understanding of the role stream ecosystems play in watershed management of high school students. This goal was addressed by working with local high school teachers on developing hands-on, stream chemistry and ecology curriculum that can be implemented in the classroom. This outreach effort increased student knowledge and interest in stream chemistry.masters, M.S., Water Resources -- University of Idaho - College of Graduate Studies, 2021-1