Presentation Information
[AOS26-P08]Dynamics of Dissolved Organic Matter (DOM) in watershed-scale surveys from the Sakawa River to the Sagami Bay
*hikaru shibata1, Yukiko Senga1, kenji tsuchiya2 (1.Toho university, 2.National Instiute for Environmental Studies)
Keywords:
Dissolved Organic Matter(DOM),Fluorescent DOM,Humification,River-Ocean Continuum
Introduction
Dissolved organic matter (DOM) is a major carbon reservoir in aquatic environments and consists of labile compounds (e.g., proteins and carbohydrates) and refractory aquatic humic substances (AHS) mainly derived from terrestrial plants. DOM plays a key role in marine carbon cycling and undergoes physicochemical and biological transformations during transport from rivers to the ocean, including flocculation, photodegradation, and microbial processing. However, DOM alteration in coastal zones and its transport offshore remain incompletely understood. This study aimed to elucidate DOM transport and transformation from riverine to marine environments. Field surveys were conducted in the Sakawa River and Sagami Bay to determine DOM concentration and composition. In addition, mixing experiments using river water and seawater were performed to examine DOM and bacterial dynamics during estuarine mixing.
Materials and Methods
Water samples were collected from the Sakawa River and Sagami Bay (0–700 m) between April 2025 and January 2026. Mixing experiments were conducted by combining filtered river water and seawater (500 mL each) in sealed flasks and storing them at 25 °C in the dark for 100 days. DOC concentration was determined using a TOC analyzer. The absorbance at 254 nm (Abs) and specific UV absorbance (SUVA) were measured to evaluate AHS content. Fluorescent DOM was characterized using EEM–PARAFAC. Suspended solids and bacterial production, abundance, and cell size were also analyzed.
Results and Discussion
DOC, Abs, and SUVA were higher in summer and peaked in the estuary. In Sagami Bay, DOC and Abs decreased offshore. Terrestrial AHS-like and protein-like components were identified in both systems by EEM–PARAFAC. One terrestrial component was absent in Sagami Bay, likely due to flocculation during estuarine mixing. Fluorescence intensity of terrestrial components decreased offshore, whereas protein-like components were elevated in the estuary. The tryptophan-like component was extremely high in October from the Sakawa River to Sagami Bay. In mixing experiments, DOC and optical indices remained stable, but fluorescence intensities increased. Bacterial abundance initially declined and later increased, while cell size decreased, indicating community shifts. Labile DOM was consumed early, whereas refractory DOM was utilized later. Increases in protein-like and humic-like components suggest bacterial production and humification duringthe experiment.
Dissolved organic matter (DOM) is a major carbon reservoir in aquatic environments and consists of labile compounds (e.g., proteins and carbohydrates) and refractory aquatic humic substances (AHS) mainly derived from terrestrial plants. DOM plays a key role in marine carbon cycling and undergoes physicochemical and biological transformations during transport from rivers to the ocean, including flocculation, photodegradation, and microbial processing. However, DOM alteration in coastal zones and its transport offshore remain incompletely understood. This study aimed to elucidate DOM transport and transformation from riverine to marine environments. Field surveys were conducted in the Sakawa River and Sagami Bay to determine DOM concentration and composition. In addition, mixing experiments using river water and seawater were performed to examine DOM and bacterial dynamics during estuarine mixing.
Materials and Methods
Water samples were collected from the Sakawa River and Sagami Bay (0–700 m) between April 2025 and January 2026. Mixing experiments were conducted by combining filtered river water and seawater (500 mL each) in sealed flasks and storing them at 25 °C in the dark for 100 days. DOC concentration was determined using a TOC analyzer. The absorbance at 254 nm (Abs) and specific UV absorbance (SUVA) were measured to evaluate AHS content. Fluorescent DOM was characterized using EEM–PARAFAC. Suspended solids and bacterial production, abundance, and cell size were also analyzed.
Results and Discussion
DOC, Abs, and SUVA were higher in summer and peaked in the estuary. In Sagami Bay, DOC and Abs decreased offshore. Terrestrial AHS-like and protein-like components were identified in both systems by EEM–PARAFAC. One terrestrial component was absent in Sagami Bay, likely due to flocculation during estuarine mixing. Fluorescence intensity of terrestrial components decreased offshore, whereas protein-like components were elevated in the estuary. The tryptophan-like component was extremely high in October from the Sakawa River to Sagami Bay. In mixing experiments, DOC and optical indices remained stable, but fluorescence intensities increased. Bacterial abundance initially declined and later increased, while cell size decreased, indicating community shifts. Labile DOM was consumed early, whereas refractory DOM was utilized later. Increases in protein-like and humic-like components suggest bacterial production and humification duringthe experiment.