5:15 PM - 6:30 PM
[MIS11-P03] Phosphate release mechanisms in the Obitsu River Estuary: Investigation by field observation and laboratory experiment
Keywords:estuary, phosphate
[Introduction]
Estuary is known to be one of the most productive ecosystems in the world. One of the reasons for active primary production may lie in that phosphate (PO43-) is produced autogenously in estuaries. The main mechanisms of PO43- release in estuaries are supposed to be (1) release of adsorbed PO43- due to reductive reaction of iron oxyhydroxides, (2) ion-exchange reaction between abundant anions in seawater and PO43- in soil particles, and (3) weathering of soil particles under anoxic conditions. However, few studies have studied on how much these mechanisms contribute to PO43- release in estuaries. In this study, we investigated these 3 mechanisms of PO43- release by field observation in the Obitsu River Estuary and laboratory experiment.
[Materials and Methods]
Field observation: The study site is the estuary of the Obitsu River in Chiba Prefecture. Six sites in the estuary and three sites in the creek of the tidal flat were selected, and surface waters and sediments were sampled in July, August, and October 2020. Porous cups were installed in the sediment and pore waters were collected with a syringe. Sampled surface waters and pore waters were analyzed for total phosphorus (TP), dissolved phosphorus (DP), PO43- (only DP and PO43- in pore waters), Fe2+, NH4+, S2-, anions (NO2-, NO3-, Cl-, and SO42-) concentrations, and sediments were measured for water content, ignition loss, TP, and Inorganic phosphorus (IP) concentrations.
Laboratory experiment: 10 g of sediment, collected at two sites in the estuary (R5) and creek (C3) in October, were enclosed in DO bottles with 100 mL of the following 5 solutions: ultrapure water (control), 10 ‰ artificial seawater, ultrapure water with hydroxylammonium chloride (HA) as a reducing agent, 10 ‰ artificial seawater with HA, and ultrapure water with CO2 aeration to accelerate carbonic acid weathering. Three replicates were incubated for 48 or 72 hours to observe changes in dissolved oxygen, PO43-, Fe2+, S2-, and SiO2 concentrations in the bottles over time.
[Results and Discussion]
Field observations in October showed that PO43- concentrations were significantly higher (49 µmol/L) at C3 than at all other sites in the estuary, although elevated Fe2+ concentrations were detected in R5, R6, and C3. The reason for the high concentrations of PO43- only at C3, may be that high concentrations of S2- produced by the sulfate reduction binds to Fe2+ and precipitates as FeS, repressing Fe2+ to retain PO43-. Therefore, the main contribution of PO43- release is likely (1) reductive reaction of iron oxyhydroxydes. In addition, there was no increase in PO43- concentration with increasing salinity as indicated by EC, indicating that the contribution of PO43- release via (2) ion exchange is small.
In the sediment (R5) incubation experiment, the highest increase in PO43- concentrations were observed in bottles added with HA and seawater+HA. PO43- concentrations increased up to 5.8-6.7 µmol/L in 12 hours and then remained constant, suggesting that reductive reaction of iron oxyhydroxides greatly contributed the increase in PO43-, which is consistent with the result of field observation. On the other hand, PO43- concentration remained lower in bottles added seawater and aerated CO2 than even in the bottle added with ultrapure water. Therefore, the possible processes of PO43- release via (2) ion exchange and (3) weathering are most likely to be small.
Conclusively, the most important mechanism of PO43- release in Obitsu River Estuary is (1) reductive reaction of iron oxyhydroxydes. Furthermore, concurrent sulfate reduction may be additionally important to promote PO43- release.
Estuary is known to be one of the most productive ecosystems in the world. One of the reasons for active primary production may lie in that phosphate (PO43-) is produced autogenously in estuaries. The main mechanisms of PO43- release in estuaries are supposed to be (1) release of adsorbed PO43- due to reductive reaction of iron oxyhydroxides, (2) ion-exchange reaction between abundant anions in seawater and PO43- in soil particles, and (3) weathering of soil particles under anoxic conditions. However, few studies have studied on how much these mechanisms contribute to PO43- release in estuaries. In this study, we investigated these 3 mechanisms of PO43- release by field observation in the Obitsu River Estuary and laboratory experiment.
[Materials and Methods]
Field observation: The study site is the estuary of the Obitsu River in Chiba Prefecture. Six sites in the estuary and three sites in the creek of the tidal flat were selected, and surface waters and sediments were sampled in July, August, and October 2020. Porous cups were installed in the sediment and pore waters were collected with a syringe. Sampled surface waters and pore waters were analyzed for total phosphorus (TP), dissolved phosphorus (DP), PO43- (only DP and PO43- in pore waters), Fe2+, NH4+, S2-, anions (NO2-, NO3-, Cl-, and SO42-) concentrations, and sediments were measured for water content, ignition loss, TP, and Inorganic phosphorus (IP) concentrations.
Laboratory experiment: 10 g of sediment, collected at two sites in the estuary (R5) and creek (C3) in October, were enclosed in DO bottles with 100 mL of the following 5 solutions: ultrapure water (control), 10 ‰ artificial seawater, ultrapure water with hydroxylammonium chloride (HA) as a reducing agent, 10 ‰ artificial seawater with HA, and ultrapure water with CO2 aeration to accelerate carbonic acid weathering. Three replicates were incubated for 48 or 72 hours to observe changes in dissolved oxygen, PO43-, Fe2+, S2-, and SiO2 concentrations in the bottles over time.
[Results and Discussion]
Field observations in October showed that PO43- concentrations were significantly higher (49 µmol/L) at C3 than at all other sites in the estuary, although elevated Fe2+ concentrations were detected in R5, R6, and C3. The reason for the high concentrations of PO43- only at C3, may be that high concentrations of S2- produced by the sulfate reduction binds to Fe2+ and precipitates as FeS, repressing Fe2+ to retain PO43-. Therefore, the main contribution of PO43- release is likely (1) reductive reaction of iron oxyhydroxydes. In addition, there was no increase in PO43- concentration with increasing salinity as indicated by EC, indicating that the contribution of PO43- release via (2) ion exchange is small.
In the sediment (R5) incubation experiment, the highest increase in PO43- concentrations were observed in bottles added with HA and seawater+HA. PO43- concentrations increased up to 5.8-6.7 µmol/L in 12 hours and then remained constant, suggesting that reductive reaction of iron oxyhydroxides greatly contributed the increase in PO43-, which is consistent with the result of field observation. On the other hand, PO43- concentration remained lower in bottles added seawater and aerated CO2 than even in the bottle added with ultrapure water. Therefore, the possible processes of PO43- release via (2) ion exchange and (3) weathering are most likely to be small.
Conclusively, the most important mechanism of PO43- release in Obitsu River Estuary is (1) reductive reaction of iron oxyhydroxydes. Furthermore, concurrent sulfate reduction may be additionally important to promote PO43- release.