11:00 〜 13:00
[AHW24-P04] Sources and dynamics of phosphorus in the sediments of Lake Biwa using phosphate oxygen isotope analysis
キーワード：リン、安定同位体比、地下水
Introduction
Phosphorus (P) input through groundwater discharge plays a significant role in nutrient condition in the coastal area, causing eutrophication or mitigating oligotrophic conditions. (Slomp and Van Cappellen, 2004). Therefore, its biogeochemical cycling in underground environment is important in proper land management and understanding of natural systems. Groundwater P flows into the coastal bottom environment, reacting in the sea/lake sediment. The P in the sea/lake sediments could be used to evaluate the P supply from groundwater.
Recently, phosphate oxygen isotope ratio (δ18OPO4) has been used as a promising tool to elucidate the P cycling (Paytan and McLaughlin, 2012). Previous studies showed the possibility to evaluate P sources and metabolism by organism in some ecosystems. However, it is not clear whether δ18OPO4 is useful for evaluating the P cycling of in underground environment, because few research has applied the δ18OPO4 analysis for underground P cycling (Neidhardt et al., 2018).
The purposes of this study were to clarify the P concentration and its δ18OPO4 value in lake sediments and coastal groundwater of Lake Biwa and to evaluate the P supply through groundwater.
Material and method
The investigation was conducted on the eastern shore of Lake Biwa. Shallow (1.8-10.0 m) and deep (24.7-27.5 m) groundwater was collected from monitoring wells in the University of Shiga Prefecture (Hikone City, Shiga Prefecture) in May 2019 and October 2020. Lake sediments were collected in October 2020 using a core sampler from 4 site near the University of Shiga Prefecture, from the shore to offshore. The collected sediment cores were cut into 5 cm pieces and freeze-dried for chemical and isotope analyses.
The groundwater samples were immediately filtered through a 0.5 μm cartridge filter for δ18OPO4 analysis. A portion of the samples were filtered through a 0.2 μm membrane filter for chemical analysis and stored in a freezer until analysis. To extract labile P, iron-bound P, authigenic P and detrital P in the sediment samples, the sequential extraction method (SEDEX) was applied (Ruttenberg, 1992). The P concentrations in the extractions and soluble reactive P (SRP) in the water samples were measured using the molybdenum-blue method.
The δ18OPO4 analysis was conducted on the groundwater and SEDEX extractions. The PO4 in the samples were converted to Ag3PO4 according to Tamburini et al., (2010). The δ18OPO4 values were measured using a TC/EA-IRMS at the Research Institute for Humanity and Nature.
Result and discussion
The groundwater, especially in the deep aquifer, was characterized by high SRP concentration (0.74-6.78 mmol L-1) and low ORP value (-80-66 mV). This result suggests that SRP in groundwater is formed by PO4 release from lithological sediment associated with the reductive dissolution of iron. The groundwater would supply large amounts of dissolved iron and PO4 to the lake sediments. The δ18OPO4 values in groundwater ranged from 12.7‰ to 15.1‰ in shallow groundwater and 16.8‰ to 17.4‰ in deep groundwater.
The total inorganic P content (sum of each extraction fraction) in the lake sediments increased from shore to offshore and the main P fraction changed from authigenic P to iron-bound P. Also, the δ18OPO4 values of the iron-bound P increased from shore (13.4‰-13.9‰) to offshore (15.3‰-16.9‰). In the authigenic P (14.8‰-6.4‰) and detrital P (13.9‰-17.4‰) fractions, there was no clear relationship between the δ18OPO4 values and the distance from the shore. The spatial distribution of P accumulation and δ18OPO4 values of iron-bound P fraction in the lake sediment may reflect the supply of iron and P by shallow and deep groundwater: deep groundwater with high concentrations of SRP and dissolved iron and high δ18OPO4 affect the offshore lake sediments, while shallow groundwater with low δ18OPO4 upwells along the coast. These results suggest that the supply of P and iron via groundwater promotes the accumulation of iron-bound P in the offshore lake sediments.
Phosphorus (P) input through groundwater discharge plays a significant role in nutrient condition in the coastal area, causing eutrophication or mitigating oligotrophic conditions. (Slomp and Van Cappellen, 2004). Therefore, its biogeochemical cycling in underground environment is important in proper land management and understanding of natural systems. Groundwater P flows into the coastal bottom environment, reacting in the sea/lake sediment. The P in the sea/lake sediments could be used to evaluate the P supply from groundwater.
Recently, phosphate oxygen isotope ratio (δ18OPO4) has been used as a promising tool to elucidate the P cycling (Paytan and McLaughlin, 2012). Previous studies showed the possibility to evaluate P sources and metabolism by organism in some ecosystems. However, it is not clear whether δ18OPO4 is useful for evaluating the P cycling of in underground environment, because few research has applied the δ18OPO4 analysis for underground P cycling (Neidhardt et al., 2018).
The purposes of this study were to clarify the P concentration and its δ18OPO4 value in lake sediments and coastal groundwater of Lake Biwa and to evaluate the P supply through groundwater.
Material and method
The investigation was conducted on the eastern shore of Lake Biwa. Shallow (1.8-10.0 m) and deep (24.7-27.5 m) groundwater was collected from monitoring wells in the University of Shiga Prefecture (Hikone City, Shiga Prefecture) in May 2019 and October 2020. Lake sediments were collected in October 2020 using a core sampler from 4 site near the University of Shiga Prefecture, from the shore to offshore. The collected sediment cores were cut into 5 cm pieces and freeze-dried for chemical and isotope analyses.
The groundwater samples were immediately filtered through a 0.5 μm cartridge filter for δ18OPO4 analysis. A portion of the samples were filtered through a 0.2 μm membrane filter for chemical analysis and stored in a freezer until analysis. To extract labile P, iron-bound P, authigenic P and detrital P in the sediment samples, the sequential extraction method (SEDEX) was applied (Ruttenberg, 1992). The P concentrations in the extractions and soluble reactive P (SRP) in the water samples were measured using the molybdenum-blue method.
The δ18OPO4 analysis was conducted on the groundwater and SEDEX extractions. The PO4 in the samples were converted to Ag3PO4 according to Tamburini et al., (2010). The δ18OPO4 values were measured using a TC/EA-IRMS at the Research Institute for Humanity and Nature.
Result and discussion
The groundwater, especially in the deep aquifer, was characterized by high SRP concentration (0.74-6.78 mmol L-1) and low ORP value (-80-66 mV). This result suggests that SRP in groundwater is formed by PO4 release from lithological sediment associated with the reductive dissolution of iron. The groundwater would supply large amounts of dissolved iron and PO4 to the lake sediments. The δ18OPO4 values in groundwater ranged from 12.7‰ to 15.1‰ in shallow groundwater and 16.8‰ to 17.4‰ in deep groundwater.
The total inorganic P content (sum of each extraction fraction) in the lake sediments increased from shore to offshore and the main P fraction changed from authigenic P to iron-bound P. Also, the δ18OPO4 values of the iron-bound P increased from shore (13.4‰-13.9‰) to offshore (15.3‰-16.9‰). In the authigenic P (14.8‰-6.4‰) and detrital P (13.9‰-17.4‰) fractions, there was no clear relationship between the δ18OPO4 values and the distance from the shore. The spatial distribution of P accumulation and δ18OPO4 values of iron-bound P fraction in the lake sediment may reflect the supply of iron and P by shallow and deep groundwater: deep groundwater with high concentrations of SRP and dissolved iron and high δ18OPO4 affect the offshore lake sediments, while shallow groundwater with low δ18OPO4 upwells along the coast. These results suggest that the supply of P and iron via groundwater promotes the accumulation of iron-bound P in the offshore lake sediments.