11:00 〜 11:15
[AHW24-08] The terrestrial impact evoked by the land-originated phosphate adhering to coastal calcareous sediments on coral reef ecosystem in Sekisei Lagoo
キーワード：石灰質の底質、リン酸態リン、サンゴ礁
Although the negative effects of terrestrial nutrients on coral reef ecosystems have been widely recognized, a clear consensus on the mechanisms involved has not been established satisfactorily. The phosphate (PO4-P) is rarely detected in surface seawater near the coral reefs, and little is known about the phosphorus dynamics in marine environments. We found that terrestrial phosphate flowing through water systems is bound to build up calcareous sediments in coastal seawater areas, whereas the bound phosphate is readily released into oligotrophic seawater (Iijima et al. 2021). Phosphate concentrations in seawater treated with the sample sediments were much higher than those in the originated oligotrophic natural seawater. The highest phosphate concentration was detected in the sediments collected from the coastal region of urban and agricultural areas. In this study, we collected sediment samples from the Sekisei Lagoon, one of the representative coral reef ecosystems in Japan, to evaluate their impact on the ecosystem.
Calcareous sediments and ambient seawaters were collected in September 2021 at a total of 123 sites in the Sekisei lagoon. The sediments were immediately frozen, and the seawater was refrigerated until analysis on laboratory. The sediments were dried in an incubator at 70 °C and sieved. Sediments (6 g) of a particle size of 0.5 to 1 mm were added with 15 mL of seawater in a 50-mL tube and shaken at 30 °C for 72 h to elute phosphorus. After centrifugation, the supernatant was filtered through a 0.2 µm filter, and the concentration of PO4-P was determined using the molybdenum blue method to calculate the amount of released phosphorus (µM) from the calcareous sediment as the value of accumulated phosphate.
The highest phosphorus concentration of 79.3 µM was detected from the sediment near a shrimp farm located in the southern part of Taketomi Island in Sekisei Lagoon. The phosphorus concentration was maximal at 58.8 μM in the coastal seawater of Kuroshima Island where livestock industry is develop, and more than 10 μM at many sites in reef flat. These results suggest that the phosphates originating from agricultural and/or livestock farming water run-off through submarine groundwater discharge are adsorbed on calcareous sediments in the coastal regions of the islands. On the other hand, dissolved phosphates in the ambient seawaters of the Sekisei Lagoon were below the detection limit (< 0.06 µM) at most sites. Accumulated phosphorus concentrations were higher in shallow water within 5 m depth. We found that the juvenile coral density (individuals/m3) was almost zero when accumulated phosphorus concentrations were over 3 μM. Based on these results, we propose calcareous sediments in reef areas play a crucial role in creating the impact of terrestrial runoff on corals by releasing phosphate into seawater. We need more detailed survey about phosphate bound to calcareous sediments near coasts to manage sustainable coral reef ecosystem services.
Calcareous sediments and ambient seawaters were collected in September 2021 at a total of 123 sites in the Sekisei lagoon. The sediments were immediately frozen, and the seawater was refrigerated until analysis on laboratory. The sediments were dried in an incubator at 70 °C and sieved. Sediments (6 g) of a particle size of 0.5 to 1 mm were added with 15 mL of seawater in a 50-mL tube and shaken at 30 °C for 72 h to elute phosphorus. After centrifugation, the supernatant was filtered through a 0.2 µm filter, and the concentration of PO4-P was determined using the molybdenum blue method to calculate the amount of released phosphorus (µM) from the calcareous sediment as the value of accumulated phosphate.
The highest phosphorus concentration of 79.3 µM was detected from the sediment near a shrimp farm located in the southern part of Taketomi Island in Sekisei Lagoon. The phosphorus concentration was maximal at 58.8 μM in the coastal seawater of Kuroshima Island where livestock industry is develop, and more than 10 μM at many sites in reef flat. These results suggest that the phosphates originating from agricultural and/or livestock farming water run-off through submarine groundwater discharge are adsorbed on calcareous sediments in the coastal regions of the islands. On the other hand, dissolved phosphates in the ambient seawaters of the Sekisei Lagoon were below the detection limit (< 0.06 µM) at most sites. Accumulated phosphorus concentrations were higher in shallow water within 5 m depth. We found that the juvenile coral density (individuals/m3) was almost zero when accumulated phosphorus concentrations were over 3 μM. Based on these results, we propose calcareous sediments in reef areas play a crucial role in creating the impact of terrestrial runoff on corals by releasing phosphate into seawater. We need more detailed survey about phosphate bound to calcareous sediments near coasts to manage sustainable coral reef ecosystem services.