High Supply, High Demand: A Fertilizer Waste Release Impacts Nitrate Uptake and Metabolism in a Large River. Issue 12 (25th November 2021)

• Record Type: Journal Article
• Authors: Kelly, Michelle C.; Zeglin, Lydia H.; Husic, Admin; Burgin, Amy J.
• Is Part Of: Journal of geophysical research. Volume 126:Issue 12(2021)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2021-11-25
• Subjects: Nitrogen cycling -- stream metabolism -- diel nitrate -- high‐frequency sensors -- large river; Geobiology -- Periodicals; Biogeochemistry -- Periodicals; Biotic communities -- Periodicals; Geophysics -- Periodicals; 577.14
• DOI: 10.1029/2021JG006469
• Languages: English
• ISSNs: 2169-8953

Abstract

Abstract: Current understanding of the relationship between nitrate (NO3 − ) uptake and energy cycling in lotic environments comes from studies conducted in low‐nutrient (NO3 − < 1 mg‐N L −1 ), small (discharge <1 m 3 s −1 ) systems. Recent advances in sensor technology have allowed for continuous estimates of whole‐river NO3 − uptake, allowing us to address how the relationship between nutrient uptake and metabolism changes over time and space in larger rivers. We used a six‐month, controlled nitrogen (N) waste release into the eighth order Kansas River (USA) as an ecosystem level nutrient addition experiment. We deployed four NO3 − and dissolved oxygen sensors along a 33 km study reach, from February to May 2018, to assess the spatiotemporal relationship between nutrient uptake and stream metabolism during the waste addition. Contrary to our prediction, we did not find evidence of uptake saturation despite an extreme increase in nutrient supply during winter, a period of generally lower biological activity. Although high uptake rates were observed across the study reach, they were uncorrelated to gross primary production. Overall, despite winter temperatures, NO3 − uptake rates were high compared to small streams and rivers. We provide evidence that large rivers can be effective ecosystems for retaining and transforming nutrients, while showing that the fine‐scale mechanisms that regulate nutrient retention in large rivers are still largely unknown. Plain Language Summary: Understanding the link between nutrient and energy cycling in streams and rivers is crucial for water quality management. Despite calls to action over the last decade, our understanding of these processes in large rivers has lagged behind our understanding in small streams. To discover how a large river may respond to increased nutrient loading, we deployed water quality sensors to study a sustained (∼6 months) release of highly nitrogen‐concentrated wastewater from a fertilizer manufacturing plant into the Kansas River. We found that organisms nearest the release site took up nitrogen (as nitrate) at the fastest rates, despite potentially saturating levels of nitrogen supply. We found that photosynthesizing organisms (i.e., primary producers, modeled as gross primary production) were likely responsible for most nitrate uptake, but were limited by wintertime cold water temperatures. After the release ended, and water temperatures increased, there was a more positive relationship between primary producer activity and nitrate uptake at the two sites nearest the release point. Nitrate uptake rates were high in comparison to other smaller streams and rivers, despite cold water temperatures which can slow organism growth. Our work shows that large rivers have the ability to retain and remove nutrients at rates similar to or larger than smaller rivers and streams, but that we don't yet understand the fine‐scale factors that control nutrient retention in large rivers. Key Points: The release of waste from a decommissioned fertilizer plant significantly increased nitrate uptake in the Kansas River Despite high N loading and low water temperatures, the Kansas River removed nitrate at rates equal to or greater than small streams NO 3 ‐ uptake was high when metabolic activity was low, suggesting NO 3 ‐ removal was not due to assimilatory activity alone