Fig 1.
Location of the study site and 3D model of the artificial boulder reef.
(a) Location of the study site and (b) the man-made boulders reef in Himeshima island. These maps were created by editing public domain, open access base data from the Geospatial Information Authority of Japan (GSI). (c) Orthomosaic image and (d) depth map of the total man-made boulders reef in the study site. The upper side of the 3D model in Figs 1c, d is the shore side, and the lower side is the offshore side. The left-edge areas in the 3D models failed to create 3D constructions because of a lack of photographs. In Fig 1d, solid lines indicate the natural cobble/boulder ridges, and dotted lines indicate the valleys surrounded by the natural cobble/boulder reef.
Fig 2.
Schematic diagram of field survey and underwater photogrammetry.
(a) Multi-camera system (MURAKUMO HANDY) used in photographing for underwater photogrammetry. One of four cameras is a spare one. (b) Camera trails estimated in photogrammetry processing in Metashape. Each pink, yellow, and cyan plot indicate the trail of each camera. Two orange circles show start and end points of photographing. Three orange squares indicate reference points for depth. (c) Flow diagram illustrating the creation of the 3D model through underwater photogrammetry.
Fig 3.
Bottom type of the artificial boulders.
Filled yellow boulders indicate the artificial boulder on the sandy bottom and filled blue boulders indicate the natural cobble/boulder bottom. A total of 550 artificial boulders were placed on the sandy bottoms, and 561 on the natural cobble/boulder bottom. The lower part of the image represents the offshore side.
Table 1.
A list of medium to large sized seaweeds hand-collected on May 26, 2023. Fourteen species of seaweed were collected on May 26, 2023. Colpomenia sinuosa was observed on March 7, but not on May 26 survey.
Table 2.
Vegetation categorization criteria and scoresheet of the macroalgal vegetation. Seafloor vegetation types and vegetation scores were categorized based on the presence of each seaweed species as follows: Sargassum and U. pinnatifida community (score 6), Sargassum community (score 5), S. horneri community (score 4), U. pinnatifida community (score 3), coralline red algae community (score 2), C. sinuosa community (score 1), and bare rock (score 0). In the table, “+” indicates the species were present, “−” indicates absent, and “+/−” indicates that the species wase either present or absent.
Table 3.
Table of the number of artificial boulders with each seaweed species presence, vegetation type, and bottom type (on the sandy bottom and on the natural cobble/boulder bottom). In the table, “+” indicates the species were present, blank indicates absent, respectively.
Fig 4.
Distribution of each seaweed species on the man-made boulders reef.
(a) U. pinnatifida, (b) Sargassum horneri, (c) perennial Sargassum spp., (d) C. sinuosa, and (e) geniculate coralline red algae. The lower sides in the maps represent the offshore side. Filled yellow boulders indicate the artificial boulders on the sandy bottoms and filled blue boulders indicate on the natural cobble/boulder bottom. C. sinuosa was present throughout the man-made boulders reef, but other species tended to be present on sandy bottom in the offshoreside.
Fig 5.
Macroalgal vegetation map and histograms of vegetation scores on the sandy bottom and on the natural cobble/boulder bottom.
The vegetation scores tended to be higher on sandy bottom, and lower on the natural cobble/boulder bottom. The lower sides of the vegetation maps represent the offshore sides.
Fig 6.
Vegetation scores for bottom type and geomorphic indicators.
Plots of (a) vegetation scores and relative height, (b) vegetation scores and surface complexity on sandy bottom and natural cobble/boulder bottom. In these plots, the vegetation scores were shown by adding +0.1 on sandy bottom and −0.1 on natural cobble/boulder bottom for visualization. (c) Boxplots of geomorphic indicators with bottom type and vegetation richness. The box plots display the median (central line), the interquartile range (box), and the whiskers extending to the most extreme data points not considered outliers. Outliers are shown as individual dots. Asterisks denote statistically significant differences (p < 0.01, t-test). Plots of relative height and surface complexity (d) on sandy bottom, and (e) on natural cobble/boulder bottom.
Fig 7.
Macroalgal vegetation of the boulders on the sandy bottoms and high surface complexity (> 0.55).
Dotted lines indicate the valley surrounded by the natural cobble/boulder reef (as in Fig 1d), and solid lines indicate the natural cobble/boulder ridges. Low vegetation scores C. sinuosa community and bare rocks were observed on the artificial boulders on the sandy bottoms in the valley surrounded by cobble reefs (across the dotted lines). Artificial boulders were piled in a white square. Most of the artificial boulders piled on the sandy bottoms exhibited high vegetation scores, though some had low score vegetation C. sinuosa community. The right side of the image represents the offshore side.
Fig 8.
Summary of the macroalgal distribution on the man-made boulders reef.
U. pinnatifida and perennial Sargassum spp. were present on the boulders on the sandy bottom and on the shore side. On the boulders on the natural cobble/boulder bottom, C. sinuosa communities and bare rock dominated, and a few perennial Sargassum spp. and U. pinnatifida present on the high relative height or offshore side near the sandy bottoms. In areas where the boulders on the sandy bottoms were surrounded by natural cobble/boulder ridges, the C. sinuosa community was dominant. On the boulders on the sandy bottom and offshore sides, a rich macroalgal community composed of various seaweed species was observed. S. horneri and geniculate coralline red algae were present only on the offshore side. On some piled boulders, poor vegetation was observed.