Development of mapping techniques for small seagrass meadows: a case study of Zostera marina and Halodule pinifolia

The present research seeks an appropriate method of monitoring both submerged and exposed seagrasses. A seagrass mapping technique was developed using combination data sets from satellite images, digital maps, SONAR imagery and handheld-GPS data collected from seagrass meadows (Zostera marina) at Shinkawa–Kasugagawa estuary, the Seto Inland Sea, Japan, in September 2011. The entire coverage area of 123,000 m2 was calculated from 638 captured SONAR images. Zostera meadows were observed at water depths of 1.6–5.0 m, over sandy sediments. Zostera was not found at water depths greater than 5.0 m, where the sediments are muddy. The canopy height of Z. marina was estimated to vary between 14 and 87 cm. Additional mapping of intertidal seagrass meadows of Halodule pinifolia was also carried out in Rayong Province, Thailand, by GPS tracking during April 2009 and December 2010. Coverage areas varied between 33,498 and 76,207 m2, and the highest coverage area was found in April 2009. The present study demonstrates that the entire area of both submerged and exposed seagrass meadows can be calculated by simple methods in a short time with acceptable accuracy.