| Popis: |
Agricultural headwater ditches and streams are frequently maintained by removing woody riparian vegetation, leading to seasonal growth of aquatic vegetation that influences the transport of water and nutrients from cropland to larger rivers. This study examined seasonal changes in the transport of phosphorus (P) in an agricultural drainage ditch in the Maumee River Basin (Ohio, USA) by conducting constant rate injections of a novel tracer mixture [conservative salt (Cl as NaCl), dissolved P (KH2PO4), and a fluorescent fine particle (Dayglo AX-11-5 Aurora Pink®)] in spring, summer, and fall as aquatic vegetation grew and decayed. I modeled retention and transport behavior for solutes and particles using a traditional transient storage approach consisting of mobile and immobile storage zones, connected by a first-order exchange rate constant. Transient storage of solutes and particles was greatest during the spring, when thicker vegetation stands caused more pooling and flow stagnation, while transient storage decreased through fall as reed grasses decayed and vegetation stands became thinner and smaller. Nutrient spiraling lengths were 8.7 times longer in fall than spring, likely due to declines in both biological uptake rates with fall senescence and transient storage in shrinking vegetation stands. With the increasing eutrophication of major waterbodies like Lake Erie and the Gulf of Mexico, it is crucial to better understand how nutrients move through agricultural headwater systems. This study highlights the physical and biological roles of aquatic vegetation in creating immobile zones that slow the downstream movement of nutrients, increasing the assimilation of dissolved nutrients, and filtering particle bound nutrients. Because these processes are seasonal, the relationships between travel times of soluble and particle-bound nutrients are also strongly seasonal, with the greatest disparity in travel times occurring in the spring, when nutrient export is typically greatest. |
