By John Buster, Aaron Pelly, and Sarah Roley, Washington State University, Richland, WA
Excessive nutrient inputs to waterbodies commonly lead to water quality issues, such as the formation of benthic and planktonic harmful algal blooms (HABs). External nutrient loading often occurs because of agricultural fertilizer runoff, poor wastewater treatment, and atmospheric deposition. Disproportionate concentrations of nutrients are sometimes biochemically converted to other forms, such as the transformation of nitrate (NO3–) to chemically inert dinitrogen gas (N2) via denitrification. Excess nutrients can also be assimilated by organisms, which results in the incorporation of nutrients into the tissues of algae, microbes, and plants. In most streams, biological assimilation usually dominates nutrient uptake from the water column before streams discharge into lakes and estuaries. However, assimilation is temporary. When an aquatic plant senesces, shoots of the plant break away from roots and become colonized by microbes that begin to break down organic materials. During breakdown, which includes decomposition, nutrients are released back into downstream surface waters. Investigating the intricacies of nutrient transport, particularly regarding aquatic plant breakdown, furthers our understanding of elemental cycling and may aid watershed management practices. We studied the breakdown rates of water stargrass (Heteranthera dubia) (WSG), a submerged aquatic plant found in the lower Yakima River Basin (YRB), to better understand how aquatic plant breakdown might affect downstream waterbodies.
About the river
Figure 1. Yakima River Basin with agricultural areas. Grieger and Harrison 2021.
Located in central Washington State, the Yakima River Basin (YRB) is a large, semi-arid basin that drains 1,582,681 hectares (ha). The Yakima River that defines the basin is primarily snowmelt-driven. Summers have become warmer and drier due to climate change, resulting in less snowpack in the Cascade Mountains which host the headwaters of the Yakima River. Because of declining mountainous snow, the river has experienced decreased annual flows, leading to the YRB being among the most heavily irrigated regions in the United States. Land use in the lower (southeast) region of the YRB is dominated by agriculture, dairies, and general farming activities. Native to the region, WSG was formerly found along the margins of the Yakima River but now dominates the entirety of the wetted channel in the lower 76 km of the river.
The YRB is like some other semi-arid watersheds in that much more nitrogen (N) has been added to the watershed, particularly in recent years, than can be accounted for in river N export. It was estimated that 537 kg N ha-1 have gone “missing” over a 67-year study period. Although the relative magnitude of various N fates has not been thoroughly investigated in the YRB, the fate is important as it controls whether N remains in soils and groundwater, is lost to the atmosphere as N2, or is taken up by aquatic plants, transformed, and ultimately transported downstream through the river network.
Field study
Recognizing that WSG dominance and external nutrient loading via agricultural runoff have both increased in recent years, we sought to understand the possible link between the two factors and how this relationship may influence downstream waterbodies. To investigate the role WSG plays in downstream N transport, we conducted two seasons of litter bag breakdown experiments.
2023 Field Season
In 2023, we collected WSG samples from the lower YRB near Kiona (Fig. 1). We allowed the samples to air dry, then packed mesh bags with ~5g of WSG. We deployed equal replicates in fine (FS) and coarse sediments (CS). Over a period of 30 days, we collected one mesh bag from each replicate, then dried each bag and collected the final mass.
2024 Field Season
We elected to use live WSG for the 2024 field season because, following senescence, there are still viable rhizomes on the plant which allow it to populate downstream sections of the river. We used a robust net to capture large mats of WSG as they drifted downstream, then placed them in buckets of water. The following day, we used a salad spinner to remove excess water from the WSG mats, then packed ~20g of live WSG in the same type of mesh bags used in the 2023 field study. We followed the same procedure that we used in 2023 with one exception; we collected one bag from each replicate biweekly instead of weekly for a total study period of 70 days.
Results
Our 2023 results show that WSG generally broke down at a steady rate (Fig. 2). However, our 2024 results (Fig. 3) show significant variability from one removal to the next, with samples losing, then gaining mass at times. We found no meaningful difference in WSG breakdown rates by sediment type in both study years – T(10) = -1.0766, P = 0.307.
Figure 2. WSG breakdown over a 30-day study period using dried samples – 2023 season
Figure 3. WSG breakdown over a 70-day study period using live samples – 2024 season
Discussion
Based on preliminary data analysis which suggests that WSG breaks down relatively slowly (compared to similar studies with similar plant types), this submerged aquatic plant may indeed serve as a temporary N sink. Our 2023 data shows that dry, dead WSG broke down at a slow, steady rate, which suggests that, following seasonal plant senescence, there may be a slow release of N into the water column as opposed to a large pulse. However, our 2024 study using live WSG suggests that after these plants break away from their root systems, they are still viable and might resume assimilatory uptake of nutrients after drifting downstream and becoming caught on obstacles such as tree roots, other plants, and rocks. Our 2024 data analysis is ongoing and will include conducting ash-free dry-mass to investigate the true biomass of samples, and C:N analyses to determine elemental concentrations of nutrients at various stages of breakdown and regrowth. By fully investigating breakdown and regrowth, watershed managers, as well as downstream waterbody managers, might better understand the role that WSG fills in the nitrogen cycle as it relates to uptake and release into surface waters.
