Journal of Experimental Marine Biology and Ecology 252 2000 1–14
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Spatial differences and seasonal changes of net carbonate accumulation on some coral reefs of the Ryukyu Islands,
Japan
K. Hibino, R. van Woesik
Department of Marine Sciences , University of the Ryukyus, Senbaru 1, Nishihara, Okinawa 903-0213,
Japan Received 9 September 1999; received in revised form 6 April 2000; accepted 20 April 2000
Abstract
This study sought to understand short-term spatial changes in accretion and erosion on experimental carbonate blocks on three coral reefs of the Ryukyu Islands, Japan. The principal
objectives were to differentiate net accretion erosion according to season, location, depth and substrate-type. At all locations the summer season showed more positive net weight changes and
higher coralline algal coverage than the winter season. Windward reefs revealed higher net accretion and higher coralline algal coverage than leeward reefs. Massive Holocene Porites
blocks showed highest net loss, followed by Pleistocene carbonate and Holocene Acropora blocks. High population densities of Echinometra mathaei de Blainville were recorded on reefs
adjacent to large human populations and overall net carbonate loss significantly correlated to densities of E
. mathaei type A.
2000 Elsevier Science B.V. All rights reserved.
Keywords : Coral reefs; Bioaccretion; Bioerosion; Carbonate; Echinometra mathaei; Coralline algae
1. Introduction
Coral reefs are a culmination of contemporary processes that have produced net carbonate accretion on ancient foundations throughout the Holocene Adey, 1978;
Hopley, 1982; Davies, 1983. Net reef growth is a product of accretional, sedi- mentological and erosional processes. Accretion may be biological, through the growth
Corresponding author. Fax: 181-98-895-8552. E-mail address
: b984138sci.u-ryukyu.ac.jp R. van Woesik. 0022-0981 00 – see front matter
2000 Elsevier Science B.V. All rights reserved.
P I I : S 0 0 2 2 - 0 9 8 1 0 0 0 0 2 1 4 - 8
2 K
. Hibino, R. van Woesik J. Exp. Mar. Biol. Ecol. 252 2000 1 –14
of framework building corals and other calcareous organisms, or physical or microbial through mineralisation of existing framework, or geological through sediment accumula-
tion and in-filling Smith and Kinsey, 1976; Glynn, 1997. Reefs may erode because of the activity of urchins Bak, 1990, 1994; Mokady et al., 1996, herbivorous fishes
Bellwood, 1995, endolithic sponges, bivalve molluscs, and polychaetes Davies and Hutchings, 1983; Hutchings, 1986; Scoffin, 1992; Glynn, 1997. Erosion of carbonate
can also occur through physico-chemical processes, either by physical abrasion by waves or suspended sediment Ball et al., 1967, or by geochemical shifts, when the addition of
carbon dioxide causes an acid shift in the water chemistry, enhancing calcium carbonate dissolution Gattuso et al., 1998; Kleypas et al., 1999.
Grigg 1982 and Grigg and Epp 1989 reported a threshold for coral reef growth, which they termed ‘the Darwin Point’. At this threshold net reef production is at
equilibrium with tectonic subsidence, and production below this threshold invokes ‘reef drowning’, where reef growth is unable to keep pace with apparent sea level. Grigg
1982 suggested that this threshold exists at latitude 298N at the northern end of the Hawaiian Archipelago. Along similar lines, Done et al. 1996 argued that human
impacts might reduce net reef production beyond a threshold, where erosion exceeds production. With a caveat that reef growth is dependent on sea surface temperatures,
‘healthy’ reefs may be viewed as those inclined to net accretion, whereas ‘unhealthy’ reefs tend more toward net erosion.
Reef accretion traits may be, on the one hand, reflected in the coral community characteristics Van Woesik and Done, 1997, and or the population structure of the
fishes and sea urchins McClanahan and Muthiga, 1989. Indeed, recent studies have shown that anthropogenic impacts to coral reefs may induce phase shifts in benthic
species composition Done et al., 1996 that could alter the bio-constructional processes and the net function of the reef. A shift from net accretion to net erosion may change the
reef topography that may lead to changes in reef associated organisms McClanahan, 1994; Grigg, 1995; Van Woesik et al., 1999. On the other hand, an assessment of the
state of reef development may be made not through an assessment of live coral dynamics but through an experimental assessment of net carbonate changes. Indeed, in a
global synopsis, coral reefs in 1997 supported on average only 33 live coral cover Hodgson, 1999, the majority of the reefs were carbonate and unconsolidated sediment.
Bioerosion estimates are common for coral reefs Trudgill, 1983; Bak, 1990; Kiene and Hutchings, 1992; Eakin, 1996, although emphasis has recently shifted to spatial
variability within and between locations Glynn, 1988; Kiene and Hutchings, 1994; Peyrot-Clausade et al., 1995. Studies on seasonal variability, however, are few. Our
hypothesis is that there are no significant spatial or temporal relationships between net weight carbonate change and the following variables: location, depth, season and type of
carbonate. Our aims were to: 1 determine the mean net weight change in carbonate over 3 months in different seasons, at different locations, and at two depths; 2 compare
net weight carbonate change using Porites and Acropora substrate and carbonate from two different geological epochs; and 3 assess whether there is a relationship between
the number of sea urchins, Echinometra mathaei de Blainville, a herbivore known for its erosional effects and prolific in the Ryukyu Islands, and net carbonate change.
K . Hibino, R. van Woesik J. Exp. Mar. Biol. Ecol. 252 2000 1 –14
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2. Materials and methods
