[A-HW22] Material transportation and cycling at the land-sea interface: from headwaters to the ocean

[AHW22-P01] Spatial-temporal Behavior and Distribution of Fulvic acid iron in Takahashi River, Okayama Prefecture

*Rohdof Lactem Yengeh¹, Hiroaki Somura¹, Toshitsugu Moroizumi¹, Yasushi Mori¹, Morihiro Maeda¹ (1.Graduate School of Environmental and Life Science, Okayama University, Japan.)

Keywords:Fulvic acid iron, dissolved iron, Fulvic acid, humic substances, spatial-temporal behavior and distribution, Takahashi River

Iron is a crucial trace element that is necessary for the growth of all organisms, including phytoplankton in the coastal and marine environments. It plays a vital role in metabolic processes such as biosynthetic reactions, electron transfer, and nitrate reduction. However, low availability and solubility hinder the full utilization of iron by phytoplankton due to its susceptibility to changes in pH and other environmental conditions. Riverine iron input into the ocean primarily occurs through complexation with low-molecular-weight humic substances, such as fulvic acid iron (FAFe), derived from terrestrial precursor materials like plant litter and decayed animals. However, there is limited understanding of the spatial-temporal behavior and distribution of FAFe in the Takahashi River draining from Mt. Hanami and flowing through the Tamashima plains before entering the Seto Inland Sea. This study aimed to evaluate the spatial-temporal behavior and distribution of FAFe concentrations from the headstream to the downstream of the Takahashi River watershed in the western part of Okayama Prefecture. Regular monthly sampling involved measuring physicochemical parameters, such as surface water temperature, pH, electrical conductivity, oxidation-reduction potential, turbidity, dissolved oxygen, total dissolved solids, and salinity, using the Horiba–U53 series (Japan). Water samples were collected for laboratory analysis and later fractionated through the 1x8 200-400 mesh strong basic anion exchange resin type I (FUJIFILM, Wako pure chemical corporation, Japan) to measure the dissolved mineral components. An ICP emission spectrometer (Shimadzu ICPE-9820, Japan) was used to determine the concentrations of FAFe, dissolved iron (DFe) and other elements. Statistical analyses were performed using the R software version 4.3.2. The mean concentrations of both DFe and FAFe were higher with greater variation in the tributaries compared to the mainstream. Across the four temperate seasons, winter had the lowest concentration, whereas summer had the highest. Multiple linear regression analysis conducted at the headstream, midstream, and downstream indicated that the variation in the concentration of FAFe at the midstream was influenced by many factors.