Ken-ichi Hayashizaki1, Hajime Ookawa1,2
1School of Marine BioSciences, Kitasato University, Sagamihara, Kanagawa, 252-0373, Japan
2Hydro Systems Development, Inc., Edogawa, Tokyo, 132-0025, Japan
Recent studies emphasize seaweed ecosystems contribute carbon sequestration as blue carbon. Biomass estimation is important task for blue carbon study. Nowadays sophisticated equipment is available for remote sensing to detect and monitor underwater vegetation with acoustic and optic methods.
Acoustic method includes different type of sonars. Single beam sonar is often used to measure canopy heights. Side scan sonar has wide swath, can cover wider seafloor even in shallow waters. High performance industrial sonars tend to be large and very expensive. Consumers grade sonars are now popular and less expensive, and easy to deploy in the field with compact transducers. In this study we used compact multifunctional, single beam, side scan and down scan sonar with three-in-one transducer.
For optic methods, drones with RGB or multispectral camera are often used. We introduced underwater camera and uplift camera on boat where drone flights are restricted.
We monitored Sargassum meadows at small beaches in Sanriku Ria Coast, northern part of Japan. With consumer grade sonar down scan and sides scan sonar images were precise enough to identify underwater objects, but swath widths were narrow due to low emission power. A multispectral sensor drone was easily deployed in the field and got informative image of bottom in shallow water when the drone could fly. Underwater camera and uplift camera were limited in coverage, but always promising monitoring method.
As a conclusion combination of inexpensive optic and acoustic equipment according to the environmental condition enabled us limited but cost-effective and precise monitoring method on benthic vegetations for blue carbon study.