Fluxes and quality of dissolved organic matter within forested and human-impacted watersheds
Year: 2022
Type: dissertation
Abstract: Aquatic fluxes of carbon and nutrients link terrestrial and aquatic ecosystems within the global carbon cycle. Optical properties of natural waters provide insights into the chemical quality of dissolved organic matter. This dissertation is comprised of three chapters quantifying the amount and quality of aquatic organic carbon in two distinct study areas. The first study site is watershed #9 in the Sleepers River Research Watershed, a forested temperate watershed in Vermont, USA (Chapters 1 and 2). The second study area is a region of industrial salmon aquaculture activity adjacent to rivers and streams in southern Chile (Chapter 3). Chapter one assesses the relationships between rainstorm event size and the relative fluxes of carbon and nitrogen in throughfall, stemflow and streams. Dissolved organic carbon (DOC) and total dissolved nitrogen (TDN) concentrations in each flow path were measured for 23 events in a forested watershed in Vermont, USA. Concentrations of DOC and TDN in throughfall and stemflow decreased exponentially with increasing precipitation, suggesting that precipitation removed a portion of available sources from tree surfaces during the events. Relationships of water yield and solution concentrations with rainstorm event size were used to estimate DOC and TDN fluxes for 76 events across a 2-year period. For most events, throughfall and stemflow fluxes greatly exceeded stream fluxes, but the imbalance narrowed for larger storms (>30 mm). The largest 10 stream events exported 40% of all stream event DOC whereas those same 10 events contributed 14% of all throughfall export. Approximately 2 to 5 times more DOC and TDN was exported from trees during rain events than left the catchment via streams annually. The diverging influence of event size on tree versus stream fluxes has important implications for forested ecosystems as hydrological events increase in intensity and frequency due to climate change. Chapter two assesses the optical quality of tree-derived dissolved organic matter (DOM) in the Sleepers River Research Watershed. To increase understanding of the factors controlling tree-derived DOM quality, DOM optical properties, dissolved organic carbon (DOC) concentrations, and calcium concentrations were measured in throughfall and stemflow for 17 individual rain events during summer and fall seasons. DOC and calcium were enriched in throughfall and stemflow relative to rain. A multiway model developed using absorbance and fluorescence spectroscopy characterized DOM quality in throughfall as containing a higher percentage of ultraviolet fluorescence while stemflow was characterized by increased humic-like fluorescence. DOM quality did not appear to be influenced by storm size, but percent of ultraviolet fluorescence increased in throughfall during autumn senescence. Given the large DOM fluxes from trees to catchment soils, future modeling of DOM transport, soil ecology and soil biogeochemistry should consider the influence of differing DOM quality in throughfall and stemflow across species and seasons. Chapter three quantifies the enrichment of nutrients (nitrogen and phosphorus) and DOC in land-based salmon aquaculture facility effluents compared to adjacent and downstream rivers and lakes. Salmon aquaculture is an important economic activity globally where local freshwater supplies land-based salmon aquaculture facilities cultivating early life stage salmon. However, the effluents of these facilities are often minimally treated and have the potential to contribute to eutrophication of receiving waters. Effluents from 23 facilities were monitored for nutrient concentrations and dissolved organic matter quantity and quality over a 5-year period in southern Chile. Dissolved organic carbon and nutrients were enriched in effluents compared to stream waters. Aquaculture effluents were also enriched in protein-like, ultraviolet DOM fluorophores. Principal component and correlation analysis revealed that tryptophan-like fluorescence was a good predictor of total N and P in effluents, but the strength of significant linear relationships varied among individual facilities. Agreement between laboratory fluorescence and a portable fluorometer indicated the utility of in situ sensors for monitoring of both tryptophan-like fluorescence and covarying nutrients in effluents. Thus, continuous in situ sensors are likely to improve industry management and allow more robust estimates of aquaculture-derived nutrients delivered to receiving waters. The results presented in this dissertation contribute to a growing body of knowledge regarding the sources, transport, and fate of DOM in aquatic environments. As land use and climate change continues to alter hydrologic event characteristics and forest structure, improved mechanistic understanding of event controls on watershed fluxes is critical to understand future ecosystem and carbon cycle function of forests, their soils and the streams that drain them. This work demonstrates how optical properties of DOM can also indicate changing water quality and provide clues to the sources and processes controlling aquatic carbon transport. Given the high variability of DOM across time and space, high resolution studies in all systems are required to better understand, monitor, and manage freshwater resources. Most useful will be studies that characterize chemical quality of DOM allowing the sources and fate of aquatic carbon to be traced. In situ instruments measuring absorbance and fluorescence in natural waters provide a means to do this at high resolution for both DOM and correlated water quality parameters.--Author's abstract
DOI: 10.17760/d20432744
Author: Kevin A. Ryan
Language: English
Subfield: Environmental Chemistry
Field: Environmental Science
Domain: Physical Sciences
Open Access status: gold
Funder: Fulbright Chile
OpenAlex ID: W4292937539