Underwater Image Analysis of the High Latitude Scleractinian Coral Alveopora japonica (Eguchi 1968) Occurring in Shallow Subtidal Hard Bottom in Jeju Island: Spatial Distribution Pattern and Association with Macrobenthic Algae

Abstract

High latitude coral Alveopora japonica extends its geographic range from the warm southern area to the northern shallow subtidal in Jeju Island, possibly due to the surface seawater temperature increase and decline in macroalgae. This study investigated the abundance of the coral occurring on the shallow subtidal in the northern Jeju (Shinheung, SH, and Bukchon, BC, Seongsan, SS) and their association with other benthic organisms using underwater photography and subsequent image analysis. Images of macro-benthic organisms appearing on a 1 × 20 m line transect installed at depths of 5, 10, and 15 m were recorded using an underwater digital camera. Species identification and abundance as a percentage area in the transect were estimated using the image analyzing software photoQuad®. The underwater images revealed that of the three sites investigated in this study, A. japonica colonies were most abundant at BC, where the coral accounted for 45.9 % (10 m) and 72.8 % (15 m) of the total transect area. At SS, A. japonica occupied 15.3 % of the total area at 15 m, whereas A. japonica covered less than 1 % of the transect at depths of 5 and 10 m. At SH, A. japonica accounted for 10 % of the total area, whereas the percent cover area was limited to less than 1 % at depths of 10 and 15 m. Dead and bleached colonies of A. japonica could be identified from the images, which accounted for 5.6-11.5 % and 1.8-5.7 % of the coral populations at SH and BC. At SS, the underwater canopy-forming brown algae Ecklonia cava and Sargassum spp. accounted for 20.2 and 24.3 % of the total transect area, respectively at 5 m depth. In contrast, the percent cover of E. cava and Sargassum spp. at SH and BC ranged from 0.1 to 1.8 %, respectively. The underwater images also revealed that at SH, non-geniculate coralline algae dominated the subtidal substrate, ranging from 60.2 (15 m) to 69 % (10 m). Comparatively low percent cover of A. japonica at a depth of 5 m at SS coincided with a high percent cover of underwater canopy-forming brown algae. Such canopy-forming kelps were rare at all depths at SH and BC, where calcium-carbonate secreting coralline algae and the scleractinian coral dominated the bottom. Underwater photography and image analysis used in this study were non-destructive methods that provided qualitative and quantitative information on the surface-dwelling benthic organisms and were considered to be a method of choice in hard-bottom ecosystem analysis.