K . Hibino, R. van Woesik J. Exp. Mar. Biol. Ecol. 252 2000 1 –14
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2. Materials and methods
2.1. Study location The Ryukyu Islands are an island chain that extend from 308N–1308E to 248N–1238E
in southern Japan Fig. 1. The warm Kuroshio Current largely influences sea surface temperatures. The average sea surface temperature in Okinawa latitude 268N in
summer is 30 and 198C in winter data from the Tropical Biosphere Research Center, Sesoko Island, University of the Ryukyus, allowing the proliferation of reef corals
Nishihira and Veron, 1995 and coral reefs Kan et al., 1995. Although reefs in the northern Ryukyu Islands are typical high latitude reefs, with little carbonate develop-
ment, reef development in Okinawa is extensive.
2.2. Experimental design Three locations on four islands were examined for differences in net carbonate
change: 1 Ikei and Miyagi Islands, located on Okinawa’s east coast; 2 Sesoko Island, on Okinawa’s west coast; and 3 Aka Island, 35 km off Okinawa’s south-west coast.
There were two study sites at each location, one windward and the other leeward Fig. 1. At each site two stations were selected approximately 20–30 m apart. Each station
included two depths 1 and 5 m below low water datum.
2.3. Laboratory and field techniques Carbonate tiles were cut from recently dead massive Porites spp. colonies collected
from the east coast of Ikei and Miyagi Islands. Samples were cut into hexahedral cubes, 4–6 cm wide and 1 cm thick, using a diamond cutter. Each corner on the upper surface
of each tile was filed to 458, taking 3 mm off all sides, in order to avoid any unnaturally acute angles. Any prepared tiles with signs of bore holes or cracks were discarded. The
direction of the coral polyps in the skeleton was arbitrary. After cutting the tiles, sequential numbers were scribed into their bases with an electric drill. Tiles were soaked
in distilled water and cleaned with an ultrasonic cleaner for 30 min. Tiles were then dried for 24 h at 508C in an oven and subsequently weighed to the nearest milligram.
The exposed surface area excluding the base was measured on each tile to the nearest millimeter. To prevent bioerosion from underneath, the base of each tile was covered
with silicon before field placement.
At each depth, 10 carbonate tiles were attached to the reef substrate using under water cement brand name Ronji-patte. Field attachment involved: 1 selecting a flat exposed
carbonate substrate, away from Stegastes spp. damselfish territories, at a gradient of less than 308 to the horizontal; 2 cleaning the substrate with a wire brush to remove
sediment and algae; 3 applying under water cement to the tile base; and 4 pressing the base firmly against the cleaned substrate. This process was repeated for each tile.
Tiles were set at , 1 m apart.
After 90 days 63 days of exposure, tiles were collected. Upon complete removal of
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. Hibino, R. van Woesik J. Exp. Mar. Biol. Ecol. 252 2000 1 –14
Fig. 1. Study locations in the Ryukyu Islands, Japan. TBRC, Tropical Biosphere Research Center; AMSL, Akajima Marine Science Laboratory.
K . Hibino, R. van Woesik J. Exp. Mar. Biol. Ecol. 252 2000 1 –14
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cement and silicon, tiles were bleached with household bleach, and remnant algal filaments were removed with a soft nylon brush to prevent the dislodgment of
encrusting organisms. The tiles were then soaked, dried and weighed as above. Weight change was standardised and presented as milligrams per square centimeter of original
22
surface area, per 3 months of exposure mg cm 3 months note, we extrapolated and
pooled the data over the seasons only when we compared our data with the published
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literature, expressed as kg m year
in Table 3. This process was undertaken once in summer 1996 July–October and once in winter 1996 97 October–January. In total
480 Porites tiles were placed in the field, of which 64 were recovered. Losses were mostly due to a typhoon overpass in summer.
To assess the effect of substrate type, recently dead Acropora hyacinthus plates were collected from approximately a 1-m depth along the west coast of Okinawa. Tiles were
cut from the center of the plate where branch density was highest. In addition, Pleistocene tiles were prepared from a cutting taken from a quarry at Minatogama,
Gushikami village, Okinawa. X-ray diffraction tests of the tiles from the quarry showed high calcite and almost no sign of aragonite, whereas, as expected, the recently dead
corals were all aragonite. Prepared tiles were placed in the lee of Ikei Island over the 1996 97-winter season, following the protocol above.
The mean density of each tile-type was determined by cutting off a cube of
3
approximately 1 cm from each tile. Ten cubes were randomly selected from each tile type. All sides were measured to the nearest 0.1 mm, and their dry weight measured as
above. Density was calculated using the formulae D 5 M V, where D is the density, M is the mass or dry weight and V is the volume. The density of massive Porites spp. was
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1.3560.03 g cm , A
. hyacinthus 1.9760.04 g cm and Pleistocene substrate
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2.4260.11 g cm .
The percentage cover of encrusting calcareous organisms was measured on each tile
2
using the point intercept method. A transparent plastic sheet, etched with 5-mm grids, was placed over the top and sides of each tile. Each organism was categorized as either
coralline algae or ‘others’; the latter included mainly bryozoans and serpulid poly- chaetes.
2.4. Urchin densities The density of echinoids was determined at each site during the 1996 summer season.
The abundance of E . mathaei de Blainville type A was distinguished from E. mathaei
types B, C and D Uehara and Shingaki, 1985; Arakaki et al., 1998 because the former was most likely to graze on the tiles since they are most mobile, and E
. mathaei types B, C and D are quite sedentary Nishihira et al., 1991. Echinoid density was estimated
2
using 10 randomly placed 1-m quadrats at each depth; observations were conducted during daylight hours.
2.5. Data analysis Prior to statistical analyses the original data were tested for normality and for
homogeneity of variances using the normal probability plot procedure and the Levene’s
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. Hibino, R. van Woesik J. Exp. Mar. Biol. Ecol. 252 2000 1 –14
test, respectively. These tests were undertaken on four primary variables: 1 net weight change of carbonate tiles; 2 E
. mathaei densities; 3 calcareous encrustation cover; and 4 substrate type. With the exception of net weight change, the results of the a
priori tests revealed that transformations were necessary. A one-way analysis of variance ANOVA was undertaken on net weight change first testing for significant seasonal
differences note that a four-way ANOVA was not possible, testing season, location, windward leeward and depth, because the summer typhoon dislodged numerous tiles.
There was a significant seasonal difference. Therefore, to avoid inappropriate pooling each successive factor was analysed for each season separately and each variable
required a Ln transformation. Both E
. mathaei densities and substrate types failed a priori tests and subsequent transformation attempts, therefore the non-parametric
Kruskal–Wallis one-way ANOVA was used to test the hypotheses that there were no significant differences in 1 E
. mathaei densities among locations, between windward and leeward reefs and between depths, and 2 percentage net weight change according
to substrate type. Least-squares regression analyses were undertaken to examine whether there was a functional relationship between net weight carbonate change in the summer
season and the density of E . mathaei type A. An arcsinsqrtx transformation was
applied to the percentage cover of calcareous organisms on the tiles and each factor was subsequently tested via one-way ANOVA separately and pooled only when significant
differences were not apparent.
3. Results
