17:15 〜 19:15
[AHW27-P18] Characteristics of seasonal variation of seagrass-seaweed beds on tidal flats in a coastal island of Seto Inland Sea
キーワード：藻場、干潟、海底湧水、河川流出
Tidal flats and the seaweed beds that inhabit them are one of the most important coastal ecosystems, and at the same time, they are attracting attention as an effective CO2 sink (blue carbon) in the ocean. However, the actual status of carbon sequestration by seagrasses and seaweeds is still largely unknown. In addition, the effects of climate change on the coastal environment have become more pronounced in recent years, and it is important to understand the seasonal changes in seagrasses-seaweeds beds (SSBs) in order to understand the changes in blue carbon associated with climate change.
Therefore, the purpose of this study was to clarify the seasonal variation of SSBs on tidal flats and its factors. The target areas were the northern tidal flat (Hayashi) and the southern tidal flat (Gohonmatsu) of Ikuchijima Island, Onomichi City, Hiroshima Prefecture, where existing studies have revealed the distribution of SSBs during the peak season (summer). We confirmed the seasonal variation of seagrass species (large eelgrass and small eelgrass) and seaweed species (blue-green algae etc.), and estimated biomass and carbon sequestration based on these results. In addition, in order to clarify the factors that contribute to the seasonal variation of SSBs, we discussed the variation in the sedimentation and erosion of the tidal flats and sea water temperature at the site where the quadrats were installed, and their correspondence with changes in meteorological conditions such as precipitation . As a result, the following were found to be the main findings.
1. On the northern tidal flat where the river discharges, the peak season for large eelgrass was around July. On the other hand, on the landward side of the tidal flat, heavy rainfall in spring and summer increases river discharge, resulting in the deposition of coarse-grained sand and gravel onto the tidal flat, which tends to bury or wash away the growing eelgrass and other plants, resulting in a decrease in the amount of cover. However, it was also suggested that buried SSBs may contribute to carbon sequestration on tidal flats.
2. On the southern tidal flat, where there is almost no river runoff, the peak season of small seagrass was observed in late June. The biomass of the seaweed species Ulva was found to be higher from March to October than that of the northern tidal flat, suggesting that the supply of nutrients and stabilization of seawater temperature by submarine groundwater discharge (SGD) prolongs the period of thriving.
3. Comparison of the results between the northern and southern tidal flats showed that the southern tidal flat, which is strongly influenced by SGD, tended to have higher diversity of SSBs and higher carbon storage than the northern tidal flat. These results indicate that increased river runoff may cause disturbance and burial of SSBs, while SGD may create a suitable environment for the survival of SSBs.
Acknowledgement: This research was supported by JSPS Fostering Joint International Research (A) (No. 20KK0262, PI: Mitsuyo Saito) and JSPS Grant-in-Aid for Challenging Research (Exploratory) (No. 22K19869, PI: Mitsuyo Saito).
Therefore, the purpose of this study was to clarify the seasonal variation of SSBs on tidal flats and its factors. The target areas were the northern tidal flat (Hayashi) and the southern tidal flat (Gohonmatsu) of Ikuchijima Island, Onomichi City, Hiroshima Prefecture, where existing studies have revealed the distribution of SSBs during the peak season (summer). We confirmed the seasonal variation of seagrass species (large eelgrass and small eelgrass) and seaweed species (blue-green algae etc.), and estimated biomass and carbon sequestration based on these results. In addition, in order to clarify the factors that contribute to the seasonal variation of SSBs, we discussed the variation in the sedimentation and erosion of the tidal flats and sea water temperature at the site where the quadrats were installed, and their correspondence with changes in meteorological conditions such as precipitation . As a result, the following were found to be the main findings.
1. On the northern tidal flat where the river discharges, the peak season for large eelgrass was around July. On the other hand, on the landward side of the tidal flat, heavy rainfall in spring and summer increases river discharge, resulting in the deposition of coarse-grained sand and gravel onto the tidal flat, which tends to bury or wash away the growing eelgrass and other plants, resulting in a decrease in the amount of cover. However, it was also suggested that buried SSBs may contribute to carbon sequestration on tidal flats.
2. On the southern tidal flat, where there is almost no river runoff, the peak season of small seagrass was observed in late June. The biomass of the seaweed species Ulva was found to be higher from March to October than that of the northern tidal flat, suggesting that the supply of nutrients and stabilization of seawater temperature by submarine groundwater discharge (SGD) prolongs the period of thriving.
3. Comparison of the results between the northern and southern tidal flats showed that the southern tidal flat, which is strongly influenced by SGD, tended to have higher diversity of SSBs and higher carbon storage than the northern tidal flat. These results indicate that increased river runoff may cause disturbance and burial of SSBs, while SGD may create a suitable environment for the survival of SSBs.
Acknowledgement: This research was supported by JSPS Fostering Joint International Research (A) (No. 20KK0262, PI: Mitsuyo Saito) and JSPS Grant-in-Aid for Challenging Research (Exploratory) (No. 22K19869, PI: Mitsuyo Saito).