5:15 PM - 6:30 PM
[ACG41-P06] Stratigraphy of seagrass bed at Bise in Okinawa Island.
Keywords:Seagrass beds, Coral reefs, Foraminifera, Intertidal flats, Ryukyus
Coral reef seagrass bed is a community of seagrass formed in the lagoon off the beach, where fish and benthic organisms live and spawn, and play a role in the carbon cycle in shallow coastal area (Nellemann et al., 2009; Unsworth et al., 2018).
The surface substrate of the seagrass beds in Bise, Okinawa Island, was muddy sand sediments and a living branch coral (Montipora digitata). It has been proposed that seagrass and coral are in a symbiotic relationship by the rhizome of Thalasia hemprichii is entwined with the underground part of Montipora digitata in this area (Ninomiya et al., 2016). In addition, past aerial photographs show that the almost seagrass beds in Bise was disappeared in the 1960s, but recovered rapidly after 1980s.
It is necessary to clarify how the seagrass beds, which is an ecologically highly unique place and is thought to be sensitive to environmental changes in the coral reef lagoon, has accumulated during the process of coral reef development in the Holocene. It is very important to conserve the seagrass beds and consider the factors that form the zonal trend of coral reefs including the seagrass beds.
In this study, we used the five sediment cores (maximum penetration depth 253 cm) collected from the seagrass beds in Bise by hand boring. We focused on coral fossils and large benthic foraminifera contained in the core sediments to clarify the sedimentary structure of the seagrass beds.
The top part of the core sediment (0-40 cm; 157 cm below MSL) was muddy sand sediments indicating seagrass bed. The bottom part below 86 cm was a coarse-graine sediment including many fossil corals (Acropora sp.), and 40-86 cm was a transition facies in which muddy sand including fossil Acropora fragments.
As a result, it was clarified that the habitat of branch coral (Acropora sp.) was buried by muddy sand sediments and gradually formed a modern seagrass beds.
The surface substrate of the seagrass beds in Bise, Okinawa Island, was muddy sand sediments and a living branch coral (Montipora digitata). It has been proposed that seagrass and coral are in a symbiotic relationship by the rhizome of Thalasia hemprichii is entwined with the underground part of Montipora digitata in this area (Ninomiya et al., 2016). In addition, past aerial photographs show that the almost seagrass beds in Bise was disappeared in the 1960s, but recovered rapidly after 1980s.
It is necessary to clarify how the seagrass beds, which is an ecologically highly unique place and is thought to be sensitive to environmental changes in the coral reef lagoon, has accumulated during the process of coral reef development in the Holocene. It is very important to conserve the seagrass beds and consider the factors that form the zonal trend of coral reefs including the seagrass beds.
In this study, we used the five sediment cores (maximum penetration depth 253 cm) collected from the seagrass beds in Bise by hand boring. We focused on coral fossils and large benthic foraminifera contained in the core sediments to clarify the sedimentary structure of the seagrass beds.
The top part of the core sediment (0-40 cm; 157 cm below MSL) was muddy sand sediments indicating seagrass bed. The bottom part below 86 cm was a coarse-graine sediment including many fossil corals (Acropora sp.), and 40-86 cm was a transition facies in which muddy sand including fossil Acropora fragments.
As a result, it was clarified that the habitat of branch coral (Acropora sp.) was buried by muddy sand sediments and gradually formed a modern seagrass beds.