Résumé
Coastal environments of the world have been exposed to eutrophication for several decades. Recently, the quality of coastal waters has been gradually and successfully improved; however, this improvement has caused another issue in coastal ecosystem services: oligotrophication. While oligotrophication, with higher water transparency, has recovered benthic macrophyte vegetation, which have been depressed by phytoplankton derived from eutrophication, local stakeholders have suggested that oligotrophication reduces pelagic productivity and, therefore, fishery production in coastal ecosystems. In contrast, oligotrophication with high transparency has recovered benthic primary productivity, including seagrass vegetation. Seagrasses are quite important for climate change mitigation and adaptation, such as through carbon storage, acidification mitigation, and protection from sea-level rise and storm surges, affects which have been welcomed by other stakeholders. Therefore, harmonizing coastal fishery with environmental conservation goals is now essential for the sustainable use of ecosystem services. Here, we present the scope of our study based on an interdisciplinary approach, including ecological actions, socio-economical actions and phycological actions. We chose to focus on the interaction between oyster aquaculture and seagrass vegetation as a typical ecological action. Coastal organisms have adapted their traits to the environment over a long period of time, so restoration of mixed coastal habitats represents reconstruction of the original process of coastal production. Subtidal seagrass vegetation with intertidal oyster reefs is the original mixed habitats in Japan, which would be expected to enhance coastal production by improving the production efficiency without adding nutrients. A simple field experiment with carbon and nitrogen contents and stable isotope analyses revealed that oyster spat cultivated on a tidal flat adjacent to seagrass beds had higher nitrogen contents and higher δ13C ratios than spats cultivated in an offshore area using only pelagic production. This result suggests that utilization of the traditional mixed habitats, which enables oysters to use both pelagic and various benthic production, has potential to sustain food provisioning services for human even in an oligotrophic environment.