1. Longitudinal and Seasonal Changes of Organic Matter Sources Through a Semi‐Arid River‐Reservoir System.
- Author
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Yoder, A. M., Baldwin, A., Marvin‐DiPasquale, M., Poulin, B. A., Naymik, J., and Krabbenhoft, D. P.
- Subjects
DISSOLVED organic matter ,WATER quality management ,COLLOIDAL carbon ,WATERSHED management ,OXYGEN in water ,MACROPHYTES - Abstract
The quality and quantity of organic matter (OM) in a river system directly affects ecosystem health; thus, managers benefit from an in‐depth understanding of the drivers and sources of OM. In the Snake River, a highly altered river‐reservoir system in the semi‐arid western United States, OM production and loading are key drivers of reservoir anoxia, which leads to several deleterious processes such as mercury methylation. However, sources and quantities of OM to the Snake River, and the effects of impoundment on OM moving through the river‐reservoir system, are not well understood. Particulate organic carbon (POC), dissolved organic carbon (DOC), particulate nitrogen (PN), chlorophyll a (chl‐a), and δ15N–PN and δ13C–POC isotopic ratios were measured bi‐weekly for over 2 years at four locations through the Snake River Hells Canyon Reservoir Complex to determine spatial and temporal patterns of OM quantities and sources. POC concentrations increased through the riverine zone upstream of the reservoirs, likely due to in situ primary production and/or inputs from tributaries and agricultural drains; then decreased through the most upstream reservoir likely due to particle settling. Isotopic ratios and other OM source indicators (δ15N–PN, δ13C–POC, POC:PN, chl‐a:POC) show that the dominant source of particulate OM was phytoplankton with seasonal terrestrial/macrophytic inputs. Results highlight the effects of major tributary and agricultural drain inputs, primary production, and impoundment on OM composition and concentration through a large river‐reservoir system and may inform water quality management efforts in this and similar systems. Plain Language Summary: The character and quantity of organic matter impact the biogeochemistry, plant growth, and food webs of aquatic systems. Common sources of organic matter in aquatic systems include phytoplankton, macrophytes, benthic algae, and terrestrial soil and leaves. In reservoirs, particulate organic matter may settle out and decompose as flow velocities decrease, releasing nutrients, fueling primary production, and depleting oxygen in the water column. Understanding organic matter sources, forms, and seasonality can therefore inform watershed management efforts to improve water quality. In this study, we analyzed organic matter and its source signatures in the Snake River through the Hells Canyon Reservoir Complex on the Idaho‐Oregon border (USA) over a 2‐year period. We highlight the effects of impoundment and seasonal runoff on dissolved and particulate organic carbon and show that particulate organic carbon sources were primarily associated with phytoplankton with seasonal contributions from macrophytes and (or) terrestrial soil and leaves. Key Points: Riverine zone tributary influx and primary production increased organic carbon and chlorophyll a concentrationsSettling associated with the transition from the lotic to lentic decreased particulate organic carbon and chlorophyll a concentrationsParticulate organic matter was primarily phytoplankton sourced, with seasonal terrestrial and/or macrophytic contributions [ABSTRACT FROM AUTHOR]
- Published
- 2024
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