Importance of dense aquatic vegetation in seasonal phosphate and particle transport in an agricultural headwater stream

Authors: FIELD, HANNAH - The Ohio State University; SAWYER, AUDREY - The Ohio State University; WELCH, SUSAN - The Ohio State University; BENEFIEL, RYAN - The Ohio State University; MATHIE, DAVAN - The Ohio State University; HOOD, JAMES - The Ohio State University; PAWLOWSKI, ETHAN - University Of Minnesota; KARWAN, DIANA - University Of Minnesota; KREILING, REBECCA - Us Geological Survey; JOHNSON, ZACKARY - Duke University; Hanrahan, Brittany; King, Kevin

Submitted to: Water Resources Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/28/2023
Publication Date: 9/5/2023
Citation: Field, H.R., Sawyer, A.H., Welch, S.A., Benefiel, R.K., Mathie, D.M., Hood, J.M., Pawlowski, E.D., Karwan, D.L., Kreiling, R.M., Johnson, Z.I., Hanrahan, B.R., King, K.W. 2023. Importance of dense aquatic vegetation in seasonal phosphate and particle transport in an agricultural headwater stream. Water Resources Research. 59(9). Article e2022WR033782. https://doi.org/10.1029/2022WR033782.
DOI: https://doi.org/10.1029/2022WR033782

Interpretive Summary: In the Midwestern United States, surface ditches or streams adjacent to agricultural lands often have dense stands of aquatic vegetation (e.g., macrophytes) that grow and decay over seasons. These stands of aquatic vegetation also influence physical and biological controls on the transport of nutrients from croplands to larger rivers. This study measured changes in the transport of phosphorus (P) in an agricultural drainage ditch in the Maumee River Basin (Ohio, USA) during the spring, summer, and fall using standard experimental techniques such as constant rate injections of a conservative tracer (i.e., salt, which is not used by biological communities in the stream), dissolved P (which can be taken up or assimilated by biological communities in the stream), and a fluorescent fine particle. We determined how both solutes and particles moved in the stream using a traditional transient storage modeling framework. Briefly, transient storage is a measure of temporary retention of water and dissolved solutes or particles that influences interaction time and can inform understanding of processing in stream ecosystems. Transient storage was greatest during the spring, when thicker vegetation stands caused more pooling and flow stagnation, and decreased through fall, as vegetation stands thinned and shrank. Soluble P uptake lengths (i.e., the distance P traveled before being taken up in the stream) were 8.7 times longer in fall than spring, likely due to declines in biological uptake rates with colder temperatures and immobile zone storage with thinning vegetation. Particle capture lengths also decreased by a factor of 4.3 from fall to spring. With the increasing eutrophication of major waterbodies like Lake Erie and the Gulf of Mexico, it is crucial to understand nutrient transport in small agricultural streams. This study highlights the physical and biological roles of aquatic vegetation in creating seasonally variable immobile zones that slow the flow of nutrients, provide surface area for biofilms, and capture particles that bind nutrients.

Technical Abstract: Agricultural headwater streams and ditches commonly host dense stands of aquatic vegetation that grow and decay over seasons and exert physical and biological controls on the transport of nutrients from cropland to larger rivers. This study examined 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. We quantified transport behavior for both solutes and particles using a traditional transient storage modeling framework consisting of mobile and immobile storage zones connected by a first-order exchange rate constant. Transient storage was greatest during the spring, when thicker vegetation stands caused more pooling and flow stagnation, and decreased through fall, as vegetation stands thinned and shrank. Soluble P uptake lengths were 8.7 times longer in fall than spring, likely due to declines in biological uptake rates with colder temperatures and immobile zone storage with thinning vegetation. Particle capture lengths also decreased by a factor of 4.3 from fall to spring. With the increasing eutrophication of major waterbodies like Lake Erie and the Gulf of Mexico, it is crucial to understand nutrient transport in small agricultural streams. This study highlights the physical and biological roles of aquatic vegetation in creating seasonally variable immobile zones that slow the flow of nutrients, provide surface area for biofilms, and capture particles that bind nutrients.