11:15 AM - 11:30 AM
[AHW24-09] Estimation of the terrestrial phosphate loading by integrated hydrological modelling for evaluation of phosphorus accumulated in calcareous sediments
Keywords:Terrestrial phosphate loading, Accumulated phosphorus in sea sediment, Coral reef ecosystems
Groundwater is one of the most common measures for transporting nutrients linked to human activities in islands consisting of Ryukyu Limestone. Although nutrient enrichment in groundwater and submarine groundwater discharge in subtropical and tropical regions have been reported, little is known about the relationship between nutrient enrichment and coral reef ecosystem degradation. We found that terrestrial phosphate flowing through water systems is bound to calcareous sediments in coastal seawater areas. In this study, we carried out several investigations focusing on the coastal area at southern part of Okinawa Island to test the hypothesis that terrestrial phosphate is transported via groundwater and rivers and accumulated in the calcareous sediment, thus deteriorating the coral reef ecosystem.
We estimated the phosphate load from the groundwater and surface water by using the combination of field observations and numerical simulations. Groundwater level and river discharge were measured for about a year. An integrated three-dimensional hydrological simulation model (GETFLOWS) was calibrated in the region based on observed data. We surveyed the juvenile coral recruit at four sites with different pollution levels in the region. Calcareous sediments and ambient seawaters were collected in March and July 2021 at a total of 78 sites in Southern part of Okinawa Island principally occupied by agricultural fields.
The highest phosphorus accumulation level of 72 µM was detected in the sediment in the proximity of submarine groundwater discharge. Additionally, a high level of accumulated phosphates was detected at the mouth of Shiramizu and Yuhi rivers in Gushichan and Minatogawa coast. Based on the relationship between the number of recruited juvenile corals and accumulated phosphorus concentrations in the sediment, we estimated the threshold of accumulated phosphorus at which corals are able to survive to be approximately 3 µM. In addition, we found a correlation between the amount of terrestrial phosphate loading and phosphorus accumulated in the sediments, based on which the acceptable daily load of phosphate from the terrestrial environment to keep accumulated phosphorus below 3 µM in the sediments in this research area was estimated to be approximately 100 g-P/m3/day. The extent to which the amount of terrestrial loads should be regulated differs depending on the targeted regions. In marine protected areas where coral reef ecosystems need to be protected, it is necessary to estimate acceptable terrestrial phosphorus loads that can keep accumulated phosphorus in sediments below 3µM, and to implement necessary terrestrial load reduction countermeasures.
We estimated the phosphate load from the groundwater and surface water by using the combination of field observations and numerical simulations. Groundwater level and river discharge were measured for about a year. An integrated three-dimensional hydrological simulation model (GETFLOWS) was calibrated in the region based on observed data. We surveyed the juvenile coral recruit at four sites with different pollution levels in the region. Calcareous sediments and ambient seawaters were collected in March and July 2021 at a total of 78 sites in Southern part of Okinawa Island principally occupied by agricultural fields.
The highest phosphorus accumulation level of 72 µM was detected in the sediment in the proximity of submarine groundwater discharge. Additionally, a high level of accumulated phosphates was detected at the mouth of Shiramizu and Yuhi rivers in Gushichan and Minatogawa coast. Based on the relationship between the number of recruited juvenile corals and accumulated phosphorus concentrations in the sediment, we estimated the threshold of accumulated phosphorus at which corals are able to survive to be approximately 3 µM. In addition, we found a correlation between the amount of terrestrial phosphate loading and phosphorus accumulated in the sediments, based on which the acceptable daily load of phosphate from the terrestrial environment to keep accumulated phosphorus below 3 µM in the sediments in this research area was estimated to be approximately 100 g-P/m3/day. The extent to which the amount of terrestrial loads should be regulated differs depending on the targeted regions. In marine protected areas where coral reef ecosystems need to be protected, it is necessary to estimate acceptable terrestrial phosphorus loads that can keep accumulated phosphorus in sediments below 3µM, and to implement necessary terrestrial load reduction countermeasures.