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Keywords:

  • Okinawa;
  • Ryukyu Islands;
  • coral;
  • coral reef;
  • global warming;
  • typhoon

Abstract

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Regional Setting and Method
  5. 3. Results
  6. 4. Discussion
  7. 5. Conclusion
  8. Appendix A
  9. Acknowledgments
  10. References
  11. Supporting Information

[1] Typhoon-generated storm waves generally cause mechanical damage to coral communities on present-day reefs, and the magnitude and extent of damage is predicted to increase in the near future as a result of global warming. Therefore, a comprehensive understanding of potential future scenarios of reef ecosystems is of prime interest. This study assesses the current status of coral communities on Ibaruma reef, Ryukyu Islands, on the basis of field observations, engineering and fluid dynamic models, and calculations of wave motion, and predicts the potential effects of a super-extreme typhoon (incident wave height,H = 20 m; wave period, T = 20 s) on the reef. On the present-day reef, massive corals occur in shallow lagoons and tabular corals occur from the reef crest to the reef slope. The observed distribution of corals, which is frequently attacked by moderate (H = 10 m, T = 10 s) and extreme (H = 10 m, T = 15 s) typhoons, is consistent with the predictions of engineering models. Moreover, this study indicates that if a super-extreme typhoon attacks the reef in the near future, massive corals will survive in the shallow lagoons but tabular corals on the reef crest and reef slope will be severely impacted. The findings imply that super-extreme typhoons will cause a loss of species diversity, as the tabular corals are important reef builders and are critical to the maintenance of reef ecosystems. Consequently, reef restoration is a key approach to maintaining reef ecosystems in the wake of super-extreme typhoons.

1. Introduction

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Regional Setting and Method
  5. 3. Results
  6. 4. Discussion
  7. 5. Conclusion
  8. Appendix A
  9. Acknowledgments
  10. References
  11. Supporting Information

[2] Tropical typhoons (in the northwest Pacific Ocean), hurricanes (in the northeast Pacific and Atlantic oceans), and cyclones (in the Indian and South Pacific oceans) are the most common and frequent large-scale natural disturbances affecting coral reef ecosystems [e.g.,Woodley et al., 1981; Fabricius et al., 2008; Wilkinson and Souter, 2008; Lugo-Fernández and Gravois, 2010]. Changes in water velocity, sedimentation, turbidity, salinity, and sea-surface temperature are recognized as major factors in the disturbance of coral reefs [Harmelin-Vivien, 1994]. In particular, extreme water velocities and wave heights cause direct mechanical perturbations, such as overturning of massive corals, dislodgement of tabular corals, and breakage of branching corals [e.g., Harmelin-Vivien and Laboute, 1986; Bythell et al., 1993; Kan, 1995; Bries et al., 2004; Gardner et al., 2005; Fabricius et al., 2008].

[3] Numerical models of the effects of global warming can predict changes in the frequency, magnitude, and distribution of tropical typhoons; in particular, these models indicate that the mean intensity of global typhoons will increase significantly in the near future [Oouchi et al., 2006; Meehl et al., 2007]. The results of numerical simulations indicate that increased magnitudes of hydrodynamic disturbance will increase the risk of massive destruction of coral communities; however, the threshold values of water motion at which corals are overturned or dislodged under global warming remain poorly known [Massel and Done, 1993; Madin et al., 2008]. Therefore, a study of the response of coral communities to hydrodynamic perturbations caused by typhoons is important for predicting the future status of coral reefs.

[4] The Ryukyu Islands in the western Pacific are in the middle of a belt of typhoon tracks, and, on average, 7.4 typhoons pass through this area every year [Oouchi et al., 2006] (see also Okinawa Meteorological Observatory, http://www.jma-net.go.jp/okinawa/). Ishigaki Island in the southern Ryukyu Islands is especially prone to typhoons (Okinawa Meteorological Observatory, http://www.jma-net.go.jp/okinawa/), and it also is characterized by one of the highest diversities of coral species in the world [Veron, 1992]. Therefore, this island is ideally suited to studies investigating the effects of typhoons on coral reefs.

[5] The purpose of this study was to predict the response of reef ecosystems on Ishigaki Island to the super-extreme typhoon events that are expected to occur as a result of global warming, in terms of the potential for destruction by the overturning and dislodgment of corals. On the basis of these modeling results, approaches to reef restoration are suggested. Our engineering and fluid dynamic models take into account the shapes and sizes of corals, using data obtained from field measurements and observations. Furthermore, for calculations of wave action, we adapted the CADMAS-SURF (Super Roller Flume for Computer Aided Design of Maritime Structure) [Coastal Development Institute of Technology, 2001] simulation model.

2. Regional Setting and Method

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Regional Setting and Method
  5. 3. Results
  6. 4. Discussion
  7. 5. Conclusion
  8. Appendix A
  9. Acknowledgments
  10. References
  11. Supporting Information

2.1. Ishigaki Island

[6] Ishigaki Island (24°25′N, 124°10′E; Figure 1a) is located in the Ryukyu Islands, approximately 430 km southwest of Okinawa Island. The monthly average sea-surface temperature (SST) ranges from 23.5 in winter to 29.0°C in summer (Japan Meteorological Agency,http://www.data.kishou.go.jp/kaiyou/db/kaikyo/dbindex.html). The tide is semidiurnal, ranging from 2.1 m at spring tide to 1.1 m below mean sea level (MSL) at the mean low-water spring tide at Ishigaki port, Ishigaki Island (Japan Meteorological Agency,http://www.data.kishou.go.jp/kaiyou/db/tide/suisan/). The species diversity of corals on Ishigaki Island (363 species) is high [Veron, 1992].

image

Figure 1. Locations of (top left) the Ryukyu Islands and (top right) Ishigaki Island. (a) Ibaruma reef, located on the northern coast of Ishigaki Island. The solid line shows the location of the survey transect. (b) Distributions of massive corals (in the shallow lagoon) and tabular corals (on the reef crest and upper reef slope) shown on a topographic profile of Ibaruma reef.

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[7] Ibaruma reef (Figure 1b) is located along the eastern edge of Ishigaki Island. The reef is 1600 m wide and contains an enclosed shallow lagoon, reef crest, and reef slope. The lagoon is up to 1000 m wide and has an average depth of 2.6 m at high tide. Spur and groove systems extend seaward from the reef edge. A detailed description and field survey of the reef topography are provided by Hongo and Kayanne [2009, 2010].

[8] Typhoons frequently strike Ishigaki Island during the summer. For example, Typhoon Shanshan (0613) struck Ishigaki Island in September 2006. The maximum instantaneous wind speed was 69.9 m/s and the minimum pressure was 925 hPa (Japan Meteorological Agency, http://www.jma.go.jp/jma/kishou/books/index.html). The significant wave height in the reef region (measured on western Ishigaki Island) was greater than 5 m and the significant wave period was greater than 10 s [Shimizu et al., 2007]; the observation point for these data (24°21′55″N, 124°06′10″E; water depth, 34.8 m) faces the open ocean (East China Sea). Severe wave heights and periods of greater than 10 m and 10 s, respectively, have been occasionally recorded at tide gauges located in the open ocean on the Pacific Ocean side of the Ryukyu Islands during typhoons [e.g., Nagai and Satomi, 2006; Goto et al., 2009, 2010, 2011].

2.2. Sizes and Shapes of Corals Based on Field-Observation Data

[9] Measurements of coral communities on Ibaruma reef were conducted in July 2010, using the line transect method [English et al., 1997]. We measured the sizes and shapes of corals along a single 1600-m-wide transect. Two dominant growth forms, massive and tabular, were selected for study. For the implementation of engineering models, we measured three parameters of massive corals: (1)Lm, length of the major axis; (2) Wm, length of the minor axis; and (3) Hm, height; and we measured four parameters of tabular corals: (1) Dt, diameter of the table; (2) Ht, height; (3) Tt, thickness of the table; and (4) St, diameter of the stem. Figure 2 provides a schematic representation of each parameter. In some cases, the parameters were measured from photographs, obtained using Pentax Optio W90 and an Olympus μ-Tough 8000 underwater cameras. The parameters were measured using the computer program “lenaraf220b.xls” (Microsoft Excel), by tracing the outline of each coral with a mouse. The height, major axis, and minor axis of massive coral, and the height and diameter of tabular coral, were measured by underwater ruler, and the thickness and stem diameter of tabular coral were measured from photographs. The maximum value of each parameter was used for the engineering model.

image

Figure 2. Schematic representation of the measurement parameters of massive (m) and tabular (t) corals. Hm,t, height; Lm, length of major axis; Wm, length of minor axis; Tt, thickness of table; Dt, diameter of table, and St, diameter of stem. The coordinate system is shown in the figure and z axis indicates the direction of flow.

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2.3. Engineering Model of Coral Destruction

[10] We assumed that massive corals on Ibaruma reef were of the unattached type and had a random distribution of lengths of major and minor axes, based on field observations (see Section 3.1). Three main forces act on a coral as it is overturned by a horizontal water flow acting on the center of gravity (Ox, Oy, Oz) projected onto a plane normal to the direction of flow; these are (1) a hydraulic force, (2) a gravitational force, and (3) buoyancy. A massive coral will overturn if the moment of the hydraulic force (Fd) exceeds the vertical force (Fv); the vertical force is a sum of the gravitational force and buoyancy acting on the coral, according to

  • equation image

Fd was estimated using Morison's equation:

  • equation image

where ρW is the density of water, CD is the drag coefficient, u is the water velocity, Am is the projected area of the coral body in a plane perpendicular to the flow, CM is the inertia coefficient, du/dtis the wave-induced acceleration, andVm is the body's volume. Fv is represented by

  • equation image

where ρm is the density of coral and g is the acceleration of gravity. If x is zero, Oy is represented as (see Appendix A):

  • equation image

where Hm is the height of a massive coral. If y is zero, Oz is as follows:

  • equation image

where Wm is the length of the minor axis of a massive coral, because we assume the most severe condition. Massive corals will overturn if the projected area of the major axis about the body in a plane oriented perpendicular to the flow. Therefore, the model of overturning of a massive coral is represented as

  • equation image

[11] We assumed that tabular corals are attached to the substratum on this reef, on the basis of field observations (see Section 3.1). The following model also calculates the potential destruction of corals. A tabular coral will be dislodged when the wave force exceeds the bending stress on the attachment substratum, because the reef substratum is weaker than the coral skeleton [Chamberlain, 1978; Vosburgh, 1982; Madin and Connolly, 2006]. The bending stress (σ) is

  • equation image

where M is the bending moment, IXX is the moment of inertia of the area, and y is the distance from the center of the base to the edge of the stem. The moment M is represented as (see Appendix A):

  • equation image

The moment of inertia IXX is represented as

  • equation image

where d|| is the width of base in the direction parallel to the water flow, and d is the width of the base in the direction perpendicular to the water flow [Madin and Connolly, 2006]. The parameter y is defined as

  • equation image

Combining equations (7)(10), we derived the following dislodgment model for tabular corals:

  • equation image

2.4. Simulation of Wave Velocity and Wave-Induced Acceleration by CADMAS-SURF

[12] To estimate wave forces at the study site (along a single transect), we calculated the wave velocity (u) and the wave-induced acceleration (du/dt) using the CADMAS-SURF model. CADMAS-SURF is a specialized numerical wave-tank model that was released for open use in 2001 [Coastal Development Institute of Technology, 2001]. The model is useful in assessing the threshold of destruction for structures (e.g., sea walls) and contributes to coastal management because it calculates wave velocity, wave acceleration, and wave form (enabling the user to determine the wave height, reflected waves, and wave overtopping). The governing equation in the model is based on the extended Navier–Stokes equations for a two-dimensional wavefield in porous media. The water free surface is evaluated by the volume of fluid (VOF) method. The high-Reynolds k –ε2 equation is used for the turbulence model. A staggered grid system is used for the discretization of governing equations using the finite difference technique. The simplified marker and cell (SMAC) method is used to solve the continuity and momentum equations. The time resolution for the present analysis is set to the optional value (0.001 s). Some structures (e.g., sea walls and patch reefs) are also set in the model. Moreover, the software package that houses the model is easily installed on a personal computer. Consequently, the model has been successfully applied to complicated phenomena such as wave breaking, and the results have been reported by scientists and engineers [e.g., Karim and Tingsanchali, 2006; Kawasaki et al., 2007; Miyata et al., 2009; Fujii et al., 2010].

[13] The model calculates wave velocity and acceleration, based on three input parameters: (1) incident wave height, (2) incident wave period, and (3) topographic inputs, such as water depth. Wave velocity and acceleration were sampled at intervals of 0.001 s. The maximum recorded values were for engineering model of coral destruction.

[14] Model outputs are given as arbitrary grid data. The accuracy of grid measurements of wave velocity and acceleration is dependent on the depth profile. Water depth was measured at 5-m intervals, with a precision of 0.01 m, using topographic data obtained byHongo and Kayanne [2009, 2010]; thus, for the modeling we used a horizontal grid interval of <5 m and a vertical grid interval of 0.5 m. To estimate wave forces on corals, we used a seafloor grid, as corals are benthic organisms.

[15] We assumed a wave height (H) range of 10–20 m and a wave period (T) range of 10–20 s. Severe typhoons, with significant wave heights greater than 10 m and wave periods greater than 10 s, frequently strike the Ryukyu Islands [Shimizu et al., 2007], and wave heights of 20 m and periods of 20 s have been assumed previously as possible maxima [Yamashita et al., 2008]. Therefore, we assume that three types of typhoons are possible: (1) present-day moderate typhoons (H = 10 m, T = 10 s), (2) present-day extreme typhoons (H = 10 m, T = 15 s), and (3) occurrences in the near future of super-extreme typhoons under conditions of global warming (H = 20 m, T = 20 s).

3. Results

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Regional Setting and Method
  5. 3. Results
  6. 4. Discussion
  7. 5. Conclusion
  8. Appendix A
  9. Acknowledgments
  10. References
  11. Supporting Information

3.1. Distribution and Shapes of Corals on Ibaruma Reef

[16] We measured 50 massive corals (e.g., Porites spp., and Cyphastrea spp.), which are dominant in the shallow lagoon on Ibaruma reef at water depths of less than 4 m (Figure 1b). The massive corals are generally not attached to the seafloor. There is no relationship between coral size and water depth or distance from the reef crest; consequently, the following ratios are taken from all the data. The ratio of Lm to Wm is approximately 0.9, while the ratio of Lm to Hm is approximately 0.4 (Figure 3a); thus, massive corals are characteristically hemispheroidal. Relationships between Lm and Wm and between Lm and Hm are approximately linear, and are represented by

  • equation image
  • equation image
image

Figure 3. Relationships among the skeletal characteristics of corals on Ibaruma reef. (a) Relationship between major axis length and minor axis length (solid circles) and height (solid triangles) in massive corals. (b) Relationship between diameter of table and height (solid circles) in tabular corals.

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[17] The projected area of coral body in the direction of flow (Am) and the body's volume (Vm) are given as, respectively:

  • equation image
  • equation image

[18] We measured 71 tabular corals (e.g., Acropora digitifera and Acropora gemmifera); measurements on 12 of the specimens were taken from photographs. Tabular corals are dominant on the reef crest and the upper reef slope, at water depths of less than 10 m (Figure 1). There is no relationship between coral size and water depth or distance from the reef crest; consequently, the following ratios are taken from all the data. An increase in Dt corresponds to an increase in Ht (Figure 3b). The ratio of Dt to Ht is approximately 0.2 (Figure 3b), and is given by

  • equation image

[19] Table thickness (Tt) ranges from 0.03 to 0.06 m (0.04 m ± 0.01 m; mean ± SD; n = 12) and stem diameter (Dt) ranges from 0.05 to 0.14 m (0.1 m ± 0.03 m; mean ± SD; n = 10).

3.2. Wave Propagation Over the Reef From CADMAS-SURF

[20] Wave motion on Ibaruma reef during typhoons, as deduced using CADMAS-SURF, is as follows. As a wave breaks on the reef, its height decreases between the upper reef slope and the reef crest (Figure 4a). The rapid decrease in wave velocity is commensurate with the energy loss associated with wave breaking (Figures 4b and 4c). As a wave propagates into the shallow lagoon, wave velocity shows a decrease to near zero (Figures 4b and 4c). However, the behavior of water flows during super-extreme typhoons is more complex than that during typical typhoon events.Figures 4b and 4c show that the influence of wave properties (acceleration and velocity) is greatest on the reef crest and reef slope.

image

Figure 4. Calculated predictions of acceleration and velocity of waves along the transect across Ibaruma reef during typhoons. (a) Change in wave height on the reef. Wave-breaking occurs at the reef edge, whereas the shallow lagoon is characterized by calm conditions. (b) Maximum acceleration and (c) maximum wave velocity along the reef transect. Black circles, moderate typhoons (incident wave height,H = 10 m; incident wave period, T = 10 s); black triangles, extreme typhoons (H = 10 m and T = 15 s); black squares, super-extreme typhoons (H = 20 m and T = 20 s).

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3.3. Engineering Model Using Observed Data From Corals

3.3.1. Massive Corals

[21] The prediction model for the overturning of a massive coral of a given size (Lm), derived by combining equations (6) and (12)(15), is given by

  • equation image

where CD is the coefficient of drag (approximately 1.0 [Madin and Connolly, 2006]), CM is the inertia coefficient (approximately 2.0), g is the gravitational acceleration (9.8 m/s2), ρm is the density of coral (approximately 1400 kg/m3), and ρw is the density of seawater (1025 kg/m3). The water velocity and the acceleration along the Ibaruma reef transect were given by CADMAS-SURF. The predicted thresholds for the overturning of massive corals during moderate, extreme, and super-extreme typhoons are shown inTable 1 and Figure 5.

Table 1. Summary of Water Velocity, Acceleration, and the Overturning Threshold With Respect to the Shapes of Massive Corals on Ibaruma Reef, Ishigaki Islanda
  Category
  Moderate Typhoon: H = 10 m, T = 10 sExtreme Typhoon: H = 10 m, T = 15 sSuper-extreme Typhoon:H = 20 m, T = 20 s
Length of Coastline (m)Depth (m)Wave Velocity u (m/s)Acceleration du/dt (m/s2)Length of Major Axis (m)Height (m)Wave Velocity u (m/s)Acceleration du/dt (m/s2)Length of Major Axis (m)Height (m)Wave Velocity u (m/s)Acceleration du/dt (m/s2)Length of Major Axis (m)Height (m)
  • a

    Here “0.2” indicates that corals for which the height or the length of the major axis is less than 0.2 m will be overturned. “Stable” indicates that the corals will continue to grow if other environmental conditions are favorable for growth. “All-overturned” indicates that all the corals will be overturned.

00.00.00.0stablestable0.00.0stablestable0.00.0stablestable
100−1.40.00.0stablestable0.00.0stablestable0.00.0stablestable
200−2.00.00.0stablestable0.00.0stablestable0.00.0stablestable
300−2.20.00.0stablestable0.00.0stablestable0.00.0stablestable
400−2.50.00.0stablestable0.00.0stablestable0.00.0stablestable
500−2.50.00.0stablestable0.00.0stablestable0.00.0stablestable
600−2.90.00.0stablestable0.00.0stablestable0.00.0stablestable
700−2.80.00.0stablestable0.00.0stablestable1.40.60.40.2
800−2.30.00.0stablestable0.00.0stablestable2.30.80.80.4
900−3.20.00.0stablestable0.00.0stablestable1.80.90.60.3
1000−1.20.00.0stablestable0.00.0stablestable1.40.90.40.2
1100−1.00.00.0stablestable0.00.0stablestable3.11.51.50.7
1200−1.00.00.0stablestable2.71.91.40.74.63.810.24.4
1300−0.73.32.62.51.13.43.44.41.98.66.5All-overturnedAll-overturned
1400−7.21.51.30.40.21.71.70.70.43.42.83.01.3
1500−13.53.12.01.80.83.42.52.61.25.33.17.73.3
image

Figure 5. Calculated predictions of the overturning of massive corals on Ibaruma reef during conditions of moderate and extreme present-day typhoons and super-extreme typhoons expected in the near future under global warming conditions. “Stable” indicates that the massive corals are not overturned by the effects of the typhoon. “All” indicates that all corals are overturned by the effects of the typhoon. Values indicate the threshold heights at which massive corals will be overturned. In the shallow lagoon, the model predicts that corals will be stable during moderate and extreme typhoons, whereas small corals on the reef slope will be overturned. Extreme and super-extreme typhoons will overturn corals on the reef crest, and super-extreme typhoons will overturn corals in areas of the lagoon proximate to the reef crest. “Observed distribution” shows that massive corals are generally distributed in the shallow lagoon at the study site. “Calculated distribution” shows the data based on calculations.

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[22] In the shallow lagoon, most massive corals are stable during moderate and extreme typhoon events (Figure 5). During super-extreme typhoons, however, corals on the seaward side of the shallow lagoon less than 0.4 m in height will be overturned. An increase in the intensity of a typhoon causes an increase in the risk of overturning. For corals distributed on the reef crest and the upper reef slope, all categories of typhoons are predicted to overturn corals (Figure 5).

3.3.2. Tabular Corals

[23] The prediction model for the dislodgement of a tabular coral with given dimensions is derived by combining equations (11) and (16), yielding

  • equation image

where CD = 1.0, CM = 2.0, ρw = 1025 kg/m3, St = 0.1 m, Tt = 0.04 m, and d|| = d = St (i.e., the shape is circular).

[24] We calculated the value of σ for corals with Dt values of between 0.1 and 0.3 m (Table 2), to estimate the relationship between an increase in the size of corals and their risk of dislodgement. Although the strength of the coral skeleton is greater than a dozen MPa [Chamberlain, 1978; Tunnicliffe, 1982; Vosburgh, 1982; Madin, 2005], corals can be dislodged when σ exceeds 0.2 MPa [Madin, 2005; Madin and Connolly, 2006]. This implies that the strength of the attachment plane between a coral and the substratum is weaker than the skeletal strength of coral [Madin, 2005; Madin and Connolly, 2006]. Moreover, the species composition of tabular corals and their habitat at Lizard Island on the Great Barrier Reef [Madin, 2005; Madin and Connolly, 2006] are similar to those of the present study site. Therefore, we assumed that tabular corals are dislodged when σ exceeds 0.2 MPa.

Table 2. Summary of Water Velocity, Acceleration, and Dislodgement Threshold With Respect to Tabular Corals on Ibaruma Reef, Ishigaki Island
  Category
  Moderate Typhoon: H = 10 m, T = 10 sExtreme Typhoon: H = 10 m, T = 15 sSuper-extreme Typhoon:H = 20 m, T = 20 s
Length of Coastline (m)Depth (m)Wave Velocity u (m/s)Acceleration du/dt (m/s2)σ (MPa), Diameter of Table = 0.1 mσ (MPa), Diameter of Table = 0.3 mThreshold of Diameter of Table (m), σ = 0.2 MPaaWave Velocity u (m/s)Acceleration du/dt (m/s2)σ (MPa), Diameter of Table = 0.1 mσ (MPa), Diameter of Table = 0.3 mThreshold of Diameter of Table (m), σ = 0.2 MPaaWave Velocity u (m/s)Acceleration du/dt (m/s2)σ (MPa), Diameter of Table = 0.1 mσ (MPa), Diameter of Table = 0.3 mThreshold of Diameter of Table (m), σ = 0.2 MPaa
  • a

    Here “0.1” indicates that corals in which the diameter of table is greater than 0.1 m will be dislodged. “Stable” indicates that corals will continue to grow provided that other environmental conditions are favorable for growth.

00.00.00.00.00.0stable0.00.00.00.0stable0.00.00.00.0stable
100−1.40.00.00.00.0stable0.00.00.00.0stable0.00.00.00.0stable
200−2.00.00.00.00.0stable0.00.00.00.0stable0.00.00.00.0stable
300−2.20.00.00.00.0stable0.00.00.00.0stable0.00.00.00.0stable
400−2.50.00.00.00.0stable0.00.00.00.0stable0.00.00.00.0stable
500−2.50.00.00.00.0stable0.00.00.00.0stable0.00.00.00.0stable
600−2.90.00.00.00.0stable0.00.00.00.0stable0.00.00.00.0stable
700−2.80.00.00.00.0stable0.00.00.00.0stable1.40.6<0.1<0.11.0
800−2.30.00.00.00.0stable0.00.00.00.0stable2.30.8<0.1<0.10.7
900−3.20.00.00.00.0stable0.00.00.00.0stable1.80.9<0.1<0.10.7
1000−1.20.00.00.00.0stable0.00.00.00.0stable1.40.9<0.1<0.10.9
1100−1.00.00.00.00.0stable0.00.00.00.0stable3.11.5<0.10.10.5
1200−1.00.00.00.00.0stable2.71.9<0.10.10.54.63.8<0.10.10.3
1300−0.73.32.6<0.10.10.43.43.4<0.10.10.48.66.50.10.50.1
1400−7.21.51.3<0.1<0.10.81.71.7<0.1<0.10.73.42.8<0.10.10.4
1500−13.53.12.0<0.10.10.53.42.5<0.10.10.45.33.1<0.10.20.3

[25] An increase in the size of tabular corals causes an increase in the magnitudes of σ (Table 2). On the reef crest and the upper reef slope, corals with a diameter of table less than 0.4 m are stable during moderate and extreme typhoons (Figure 6). During super-extreme typhoons, however, an increase in the wave action in this zone causes dislodgment of tabular corals with a diameter of table greater than 0.1–0.4 m, depending on the site. Consequently, most tabular corals on the reef crest will be dislodged by the effects of a super-extreme typhoon; only corals with a diameter of table of less than approximately 0.1 m will survive such an event. All tabular corals located in the shallow lagoon will likely withstand the effects of all categories of typhoons (Figure 6).

image

Figure 6. Calculated predictions of the overturning of tabular corals on Ibaruma reef. See the legend in Figure 5 for an explanation of symbols. Values indicate the threshold lengths of the diameter of table at which tabular corals will be dislodged from the substrate. On the reef crest and upper reef slope, corals with a diameter of table of less than 0.4 m are stable during moderate and extreme typhoons, for a threshold (σ) of 0.2 MPa. During super-extreme typhoons, an increase in wave action on the reef crest will cause the dislodgment of corals with a diameter of table greater than 0.1 m.

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4. Discussion

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Regional Setting and Method
  5. 3. Results
  6. 4. Discussion
  7. 5. Conclusion
  8. Appendix A
  9. Acknowledgments
  10. References
  11. Supporting Information

4.1. Correspondence Between Calculated and Observed Distributions of Corals in Response to Moderate and Extreme Typhoons

[26] The distributions of corals on Ibaruma reef predicted by our engineering and fluid dynamic models are consistent with field observations. Given that moderate (H = 10 s, T = 10 s) and extreme (H = 10 m, T = 15 s) typhoons frequently strike the reef, our findings imply that the distributions of corals are generally constrained by the effects of wave action. Our calculations show that whole massive corals are stable in the shallow lagoon on the reef during typhoon events (Figure 5); this is consistent with the observation that various sizes of massive corals, including those greater than 2 m in height, are found in the lagoon (Figure 3a). According to the calculations, most whole massive corals on the reef crest and reef slope should be overturned and removed by moderate and extreme typhoons (Figure 5). This result is also consistent with field observations, showing that large unattached massive corals are absent from the reef crest and slope.

[27] The calculated distribution of tabular corals is also consistent with observed distributions on the present-day reef. Our calculations indicate that tabular corals with a diameter of table greater than 0.4 m will be dislodged on the reef crest and reef slope by moderate and extreme typhoons (Figure 6); this is consistent with our field observations indicating that the diameter of table of present-day corals in this zone is less than 0.4 m (Figure 3).

[28] There is a discrepancy between model predictions and observations of the distribution of tabular corals in the shallow lagoon. Our calculations show that tabular corals can grow in the shallow lagoon, but that they are absent. In addition to wave action, a variety of factors may explain this discrepancy. In Indo-Pacific regions, tabular corals are generally distributed on the reef crest and reef slope [Montaggioni and Braithwaite, 2009]. Distribution patterns are influenced by various controlling factors, such as salinity, temperature, light intensity, nutrients, sediment runoff, seawater chemistry, species interactions, and wave action [Sheppard et al., 2009]. These factors may explain the distribution of tabular corals on Ibaruma reef.

4.2. Future Scenarios: Effects of Super-Extreme Typhoons on Coral Communities

[29] The landward side of shallow lagoons will become a refuge for massive corals when super-extreme typhoons, a consequence of global warming, strike the Ibaruma reef in the near future (Figure 7). Even if catastrophic typhoons strike the reef, the dissipation of wave energy in the shallow lagoon is such that massive corals on the landward side of the lagoon will not be overturned. Small massive corals will, however, be overturned on the seaward side of the lagoon. Thus, overall, massive corals (e.g., Porites) will likely thrive in the shallow lagoon under the conditions of global warming, although their size will be limited by water depth.

image

Figure 7. Present-day and future scenarios for the Ibaruma reef after exposure to typhoon events. (a) The present-day reef is frequently struck by moderate and extreme typhoons. Massive corals occur throughout the shallow lagoon and tabular corals occur on the reef crest and upper reef slope. (b) Future scenario of the reef affected by super-extreme typhoons under conditions of global warming. Massive corals will survive in the landward region of the shallow lagoon, but small massive corals will disappear in the seaward region of the shallow lagoon. Most large tabular corals will disappear from the study site. The hatched area indicates the demise of large tabular corals from the reef crest and upper reef slope, and the demise of massive corals from the seaward side of the shallow lagoon. (c) Restoration plan for a reef affected by super-extreme typhoons under conditions of global warming. If large massive corals are transplanted to the seaward side of the shallow lagoon, and robust tabular corals are transplanted to the reef crest and upper reef slope, the corals will survive the effects of super-extreme typhoons under conditions of global warming.

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[30] In contrast to the situation for massive corals, super-extreme typhoons under conditions of global warming are likely to destroy and remove most tabular corals from Ibaruma reef (Figure 7), although very small tabular corals (diameter of table less than approximately 0.1 m) would likely survive. The reef crest and upper reef slope are generally characterized by high wave-energy conditions, and the corals are directly affected by the water motion. If the habitat of tabular corals extends to the shallow lagoon, then the corals will survive; however, conditions are assumed to be inadequate for the survival of tabular corals in lagoon-type environments.

[31] The distribution patterns of massive and tabular corals are similar to those in other Indo-Pacific regions [Montaggioni and Braithwaite, 2009; Nakamura and Nakamori, 2009; Pichon, 2011]. This implies that the scenario we have developed for Ibaruma reef is likely to be representative of the region as a whole, although the intensities of typhoons may vary in different areas.

[32] The reef crest and upper reef slope are generally characterized by high species diversities of corals [e.g., Sheppard, 1980; Fujioka, 2002; Hongo and Kayanne, 2010]. The structural corals, which include the tabular corals, contribute to the variability in physical conditions, resources, and habitats required by many other organisms [Huston, 1994]. Moreover, the corals are one of principal reef builders in this region [Camoin et al., 2004; Hongo and Kayanne, 2011; Hongo, 2012]. Consequently, if tabular corals suffer a dramatic decline throughout the Indo-Pacific region in the near future, many marine organisms will also disappear from the reefs, causing a reduction in the species diversity of these reef ecosystems.

4.3. Implications for Restoration on Coral Reefs

[33] Present-day coral reefs are faced with potential losses worldwide, influenced by typhoons, climate change, and anthropogenic impacts [Gardner et al., 2003; Wilkinson and Souter, 2008]. Therefore, various approaches have been employed to study the restoration of coral reefs, mostly involving direct transplantations of juvenile corals [Yap, 2004; Rinkevich, 2005; Omori, 2011]. However, corals established by transplantation are frequently destroyed by typhoons as, for example, in the detachment of corals during Typhoon Caloy (May 2006) in the Philippines [Shaish et al., 2010].

[34] Some studies show that the transplantation of large coral fragments increases the probability of their survival [e.g., Soong and Chen, 2003]. Our study indicates that large massive corals will survive in the shallow lagoon; consequently, this environment is a potential habitat for transplantation attempts. However, larger tabular corals are likely to be dislodged during a typhoon (Table 2). Therefore, our findings imply that the selection of appropriate coral size and sites is important for successful reef restoration efforts (Figure 7).

[35] A variety of materials (e.g., wire, adhesives) have been used in reef restoration efforts to attach coral transplants to substrates [e.g., Clark and Edwards, 1995; Omori, 2011]. Our study indicates that an increase in the size of tabular corals is related to a decrease in the strength of the attachment force (σ); therefore, if large tabular corals are transplanted, an increase in the fixation strength of the coral to the substrate is important for successful establishment of the colony. For example, if σexceeds 0.8 MPa using artificial techniques, robust corals on the reef crest will survive super-extreme typhoons under the conditions of global warming (Figure 7). Consequently, the corals will contribute to maintaining the coral-reef ecosystem.

4.4. Limitations of the Model and Proposed Improvements

[36] Our practical model would be applicable to other reefs in the Indo-Pacific and Caribbean regions, and would be useful in predicting the effect of storm waves on corals in the present climate and in a future climate under global warming. However, the model has some limitations and requires the following improvements.

[37] 1. The proposed model is applicable for examining the destruction of corals on reefs worldwide, although further validation and improvement of the model is required through water tank experiments to investigate the response of corals to wave action.

[38] 2. The model can contribute to our understanding of the relationship between wave action and coral survival in two-dimensional analyses; however, corals are often influenced by lateral flows such as diffracted waves due to topographical effects. In order to understand the complex behavior of waves, it is necessary to measure the three-dimensional (3D) topography as well as the distribution of corals in the area of interest. In addition, 3D-wave analysis (e.g., 3D numerical simulations and 3D water tank experiments) would be required to investigate the effect of the complex behavior of waves on corals.

[39] 3. The model assumed wave heights of 20 m and periods of 20 s generated by super-extreme typhoons under conditions of global warming at the present study site; however, wave height and period are influenced by oceanic conditions in the Indo-Pacific and Caribbean regions. Furthermore, the track locations and frequencies of typhoons are expected to be affected by global warming. Consequently, it is necessary to set wave heights and periods for various oceanic conditions using the general wave model CADMAS-SURF.

[40] 4. The dislodged model of tabular corals assumed threshold values of 0.2 MPa. However, the threshold probably varies among species and with the local setting because the strength of coral is generally greater than 10 MPa [Chamberlain, 1978; Tunnicliffe, 1982; Vosburgh, 1982; Madin, 2005]. Moreover, ocean acidification under global warming would weaken coral skeletons. Consequently, to precisely evaluate the impact of waves on tabular corals, a range of thresholds should be considered for various reefs in the Indo-Pacific and Caribbean regions before applying the model.

5. Conclusion

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Regional Setting and Method
  5. 3. Results
  6. 4. Discussion
  7. 5. Conclusion
  8. Appendix A
  9. Acknowledgments
  10. References
  11. Supporting Information

[41] This study predicted mechanical damages to massive and tabular corals subject to the effects super-extreme typhoons under conditions of global warming that are likely to occur in the near future. Our results, based on engineering and fluid dynamic models, and wave action effects modeled by CADMAS-SURF, are consistent with the observed distributions of corals on Ibaruma reef on Ishigaki Island, which is prone to frequent present-day strikes by moderate and extreme typhoons. Massive corals are found in the shallow lagoon on the reef, while tabular corals are distributed on the reef crest and upper reef slope. Our results indicate that if a super-extreme typhoon (incident wave height, 20 m; wave period, 20 s) strikes the reef in the near future, massive corals will survive on the landward side of the shallow lagoon, but most tabular corals will be severely destroyed and removed from the reef. Moreover, these findings imply that a super-extreme typhoon will cause a reduction in the species diversity of reef ecosystems, as the habitat of tabular corals sustains large numbers and diversities of marine organisms.

[42] An understanding of the behavior and the hydrodynamic effects of typhoons is important for reef restoration efforts. Tabular corals in particular are at risk of devastation by extremely large tropical cyclones, such as typhoons and hurricanes, that strike reefs worldwide. Our results indicate that an increase in the size of tabular corals increases the risk of dislodgement. Consequently, this study underscores the importance of choosing appropriate coral size, sites, and stabilization techniques to ensure the successful direct transplantation of corals and to maintain coral-reef ecosystems.

Appendix A

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Regional Setting and Method
  5. 3. Results
  6. 4. Discussion
  7. 5. Conclusion
  8. Appendix A
  9. Acknowledgments
  10. References
  11. Supporting Information

[43] The center of gravity O(x, y, z) of a massive coral, at the time of overturning, is given by

  • equation image

The wave force on a microdomain of a tabular coral is given by

  • equation image

where A(y) is the projected area about the body in a plane oriented perpendicular to the flow.

[44] The bending moment of a tabular coral is estimated by integrating the wave force on the microdomain throughout the projected area and volume; the equation is represented as

  • equation image

where H is the table thickness plus the thickness of the stem. Integration of the area and volume domains is represented by, respectively:

  • equation image

and

  • equation image

Acknowledgments

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Regional Setting and Method
  5. 3. Results
  6. 4. Discussion
  7. 5. Conclusion
  8. Appendix A
  9. Acknowledgments
  10. References
  11. Supporting Information

[45] Financial support for this research was awarded to C. Hongo by the Japan Science Society (Sasakawa Scientific Research Grant 22-716), the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan, (Grant-in-Aid for Scientific Research on Innovative Areas: “Coral Reef Science for Symbiosis and Coexistence of Humans and Ecosystems under Combined Stresses”; 20121006), and the Ministry of the Environment, Japan (Environmental Research and Technology Development Fund; S9), and to K. Goto by the Japan Society for the Promotion of Science (22240084). We thank Hideaki Yanagisawa, Dennis Baldocchi (Editor, JGR-Biogeosciences), an anonymous Associate Editor, and two anonymous reviewers for important comments that significantly improved the clarity of the results.

References

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Regional Setting and Method
  5. 3. Results
  6. 4. Discussion
  7. 5. Conclusion
  8. Appendix A
  9. Acknowledgments
  10. References
  11. Supporting Information

Supporting Information

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Regional Setting and Method
  5. 3. Results
  6. 4. Discussion
  7. 5. Conclusion
  8. Appendix A
  9. Acknowledgments
  10. References
  11. Supporting Information
FilenameFormatSizeDescription
jgrg922-sup-0001-t01.txtplain text document2KTab-delimited Table 1.
jgrg922-sup-0002-t02.txtplain text document2KTab-delimited Table 2.

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