10:15 AM - 10:30 AM
[AHW22-06] Spatial distribution of phosphate oxygen isotope in tidal flats using passive sampling for understanding phosphate supply to seaweed beds.
Keywords:Blue carbon, Phosphate oxygen isotope
Introduction
Seagrass and seaweed beds inhabiting coastal areas are attracting attention as "blue carbon" due to their high capacity for carbon dioxide sequestration (Fourqurean et al., 2012). Phosphorus (P) can be a limiting factor for primary production in aquatic ecosystems. Therefore, understanding P cycle in coastal areas is important for managing the seagrass and seaweed beds. The tidal flats located in the intertidal zone create a complex water cycling with high temporal and spatial variations. Consequently, understanding the P supply process is challenging.
Phosphate oxygen isotope (δ18OPO4) could be a useful tool for understanding the P cycling in tidal flat because it can estimate the P sources and the degree of biological processes (Paytan and McLaughlin, 2012). In recent years, passive sampling methods have been developed to collect PO4 in freshwater directly by installing PO4 adsorbent (Ishida et al., 2022a). However, it is not clear whether it can be used in seawater systems and the spatial distribution of time-integrated δ18OPO4 obtained by the passive sampling has information on P supply process to seagrass and seaweed beds.
The purposes of this study were to examine the applicability of the passive sampling method in seawater systems and the utility of the spatial distribution of δ18OPO4 for understanding the P supply process to seagrass and seaweed beds.
Material and method
The investigation was conducted at the tidal flats of Ikuchijima Island in Onomichi City, Hiroshima Prefecture, Japan. The tidal flats, spanning about 300 m, are abundant with seagrass and seaweed. On the western side, there are a spring, while no continuous surface water flows into the tidal flat. Piezometers have been installed on the tidal flat for groundwater monitoring and sampling. There are mountain forests and citrus orchards on the upper part of the tidal flat.
For the passive sampling, zirconium-loaded resin was prepared following Ishida et al. (2022a). The mesh bags filled with 20 ml of the resin were placed on the bottom of the tidal flat (15 sites) and inside the piezometer (2 sites) from August 2 to 30, 2023. The mesh bags were similarly installed in spring of the western side (1 site), the seawater without spring water influence (1 site), and monitoring well for groundwater (1 site).
The δ18OPO4 of PO4 adsorbed on the resin was measured following Ishida et al., (2022a, b) using a TC/EA-IRMS (a Delta V advantage via ConFlo IV, Thermo Fisher Scientific) at the Research Institute for Humanity and Nature. The analytical precision (±SD) was less than 0.4‰.
Results and discussion
The passive sampling successfully recovered PO4 in the range of 4.52 to 75.9 µmol/bag, enabling δ18OPO4 analysis at all sampling sites and indicating the applicability of the passive sampling in the marine systems.
The δ18OPO4 values on the tidal flat bottom ranged from 17.0‰ to 18.8‰ with a spatial variability of 1.8‰, exceeding the analytical precision of the δ18OPO4 analysis. The result suggests that the spatial differences in P sources and biogeochemical processes within the tidal flat is reflected in the δ18OPO4 distribution. The δ18OPO4 values for seawater (17.0‰) and spring (18.5‰), potential P sources to the tidal flats, were close to the minimum and maximum δ18OPO4 values at the tidal flat bottom, respectively. The δ18OPO4 value of groundwater (14.9‰) was lower than that of the spring water in the tidal flat. The simplest explanation for the spatial distribution of δ18OPO4 on the tidal flat bottom is that it is determined by the mixing of seawater and spring water. However, biogeochemical processes involving isotope fractionation should also influence the δ18OPO4 distribution. In the future, further investigation of detailed water cycling, and biological activity will be essential for a comprehensive understanding of P dynamics in the tidal flat environment.
Seagrass and seaweed beds inhabiting coastal areas are attracting attention as "blue carbon" due to their high capacity for carbon dioxide sequestration (Fourqurean et al., 2012). Phosphorus (P) can be a limiting factor for primary production in aquatic ecosystems. Therefore, understanding P cycle in coastal areas is important for managing the seagrass and seaweed beds. The tidal flats located in the intertidal zone create a complex water cycling with high temporal and spatial variations. Consequently, understanding the P supply process is challenging.
Phosphate oxygen isotope (δ18OPO4) could be a useful tool for understanding the P cycling in tidal flat because it can estimate the P sources and the degree of biological processes (Paytan and McLaughlin, 2012). In recent years, passive sampling methods have been developed to collect PO4 in freshwater directly by installing PO4 adsorbent (Ishida et al., 2022a). However, it is not clear whether it can be used in seawater systems and the spatial distribution of time-integrated δ18OPO4 obtained by the passive sampling has information on P supply process to seagrass and seaweed beds.
The purposes of this study were to examine the applicability of the passive sampling method in seawater systems and the utility of the spatial distribution of δ18OPO4 for understanding the P supply process to seagrass and seaweed beds.
Material and method
The investigation was conducted at the tidal flats of Ikuchijima Island in Onomichi City, Hiroshima Prefecture, Japan. The tidal flats, spanning about 300 m, are abundant with seagrass and seaweed. On the western side, there are a spring, while no continuous surface water flows into the tidal flat. Piezometers have been installed on the tidal flat for groundwater monitoring and sampling. There are mountain forests and citrus orchards on the upper part of the tidal flat.
For the passive sampling, zirconium-loaded resin was prepared following Ishida et al. (2022a). The mesh bags filled with 20 ml of the resin were placed on the bottom of the tidal flat (15 sites) and inside the piezometer (2 sites) from August 2 to 30, 2023. The mesh bags were similarly installed in spring of the western side (1 site), the seawater without spring water influence (1 site), and monitoring well for groundwater (1 site).
The δ18OPO4 of PO4 adsorbed on the resin was measured following Ishida et al., (2022a, b) using a TC/EA-IRMS (a Delta V advantage via ConFlo IV, Thermo Fisher Scientific) at the Research Institute for Humanity and Nature. The analytical precision (±SD) was less than 0.4‰.
Results and discussion
The passive sampling successfully recovered PO4 in the range of 4.52 to 75.9 µmol/bag, enabling δ18OPO4 analysis at all sampling sites and indicating the applicability of the passive sampling in the marine systems.
The δ18OPO4 values on the tidal flat bottom ranged from 17.0‰ to 18.8‰ with a spatial variability of 1.8‰, exceeding the analytical precision of the δ18OPO4 analysis. The result suggests that the spatial differences in P sources and biogeochemical processes within the tidal flat is reflected in the δ18OPO4 distribution. The δ18OPO4 values for seawater (17.0‰) and spring (18.5‰), potential P sources to the tidal flats, were close to the minimum and maximum δ18OPO4 values at the tidal flat bottom, respectively. The δ18OPO4 value of groundwater (14.9‰) was lower than that of the spring water in the tidal flat. The simplest explanation for the spatial distribution of δ18OPO4 on the tidal flat bottom is that it is determined by the mixing of seawater and spring water. However, biogeochemical processes involving isotope fractionation should also influence the δ18OPO4 distribution. In the future, further investigation of detailed water cycling, and biological activity will be essential for a comprehensive understanding of P dynamics in the tidal flat environment.