Geographical Review of Vol. 71 (Ser. B), No. 1, 72-82, 1998

Sandy Sediment Distribution on Coral Reefs and Beaches at Several Islands of the Ryukyu Island Arc

Hideo YAMANOUCHI Department of Geography, Faculty of Education, Gunma University Maebashi 371-8510, Japan

Abstract: In the Ryukyu Island Arc, bioclastic sands containing foraminifera, e.g. Baculogypsina and Calcarina with spines on their tests, are abundant in sands of beaches and reefs. Attrition of the spines of Baculogypsina and Calcarina clarifies habitat and dispersion of these foraminifera at Yoshihara reef in Ishigakijima, where water flow clearly exists due to a distinct channel in the reef flat. At Sesokojima reef where no distinct channel exists, the method of using the attrition of spines on these bioclastic sands is applicable for evaluating transport courses of the bioclastic sands. This method provides a clue to elucidate gain and loss of beach sands in reefal coasts of the Ryukyu Island Arc.

Key words: coral reef, sediment, bioclastic sand, Baculogypsina, Calcarina, Ryukyu Island Arc

Bradach (1961) showed that large amount of Introduction corals were eaten and exhausted by reef fish. Crozier (1918) reported the amount of bottom Fringing coral reefs develop around almost sediment ingested by holothurians. Concerning all islands of the Ryukyu Island Arc in South deposits around coral reefs, Roberts et al. (1981) western Japan (Environment Agency 1996). referred to the effect of Callianassa. Though Many larger benthic foraminifera live on the such biological effects are known to play a sig reefs and are important components of beach nificant role, the author considers the physical sand deposits of this region (Figure 1). Exis processes of waves and currents as the most tence of Baculogypsina and Calcarina in the effective agency of supply and transport of Pacific region has been reported in many papers sandy sediments. (e.g. Todd 1960; Murray 1973). Baculogypsina In recent years, several investigations of sedi and Calcarina are common at the Ryukyu ments on the lagoon bottoms of New Caledonia Island Arc from north of were conducted (Charily et al. 1988), and pre (Kuwano 1956a), to south of Ishigaki-Iriomote cise distribution maps were made (Chevillon Islands (Hatta and Ujiie 1992). Saito and and Clavier 1988; 1990, Chevillon and Richer Yamanouchi (1972) pointed out the existence of de Forges 1988). Chun et al. (1997) examined Baculogypsina at Tairajima of Tokara which lagoon-shelf sediment exchange by storms has a narrow coral reef less than 100m in using foraminiferal assemblges as tracer. width. Transport of sediments to outer reef slopes Sakai and Nishihira (1981) discussed valuable before and after storms is difficult to survey, biological data on Baculogypsina and estimated but Kan (1994) proved successful in his attempt the annual sediment production. Hohenegger and obtained valuable data. However, there is (1994) reported a detailed distribution of living little data available about sandy sediment on larger foraminifera on and off the coral reef the reefs and beaches. at Sesokojima in the central Ryukyu Island Saito and Yamanouchi (1973) noted the attri Arc, and mentioned that Baculogypsina and tion of tests on Baculogypsina at the reefs of Calcarina are abundant in the reef flat deposits. Tairajima and Kuchinosima. In Japan, only There are many papers referring to the bio one study using Baculogypsina as an indicator logical effects of sediments at coral reef regions. of drifting sediments was carried out at Sediment Distribution on Coral Reefs and Beaches 73

aerial mapping and field surveys. Reef and beach profiles were clarified by measurement of depth and distance from shorelines using staffs and marked nylon strings. Wherever possible sediment samples were collected according to grid points. Particle and composition analyses of sediments were carried out. After being washed and dried, the samples were sieved with the Ro Tap type sieving ap paratus. The samples were weighed, analyzed and gained M4 as a mean diameter and a as a sorting coefficient of samples (Stoddart 1978). Very coarse sands of 1-2mm in diameter were removed from the sieved samples. Approximately 1000 particles of each sample were then classified into coral fragments, shell fragments, lithic fragments, foraminifera: espe cially abundant Baculogypsina and Calcarina, other foraminifera: Amphistegina, Marginopora and others. The number of other types such as echinoide spines, crab shell and miscellaneous was insignificant. The number of each particle was counted and proportions of components were obtained. Among the foraminifera, Baculogypsina and Calcarina which originally have spines on their tests were classified into grade-A character ized by fresh spines, grade-B characterized by partly broken spines and grade-C without Figure 1. Components of beach sand (very spines (Figure 2). Because this method clarifies coarse sand: ƒÓ=1-2mm) at some islands of the Ryukyu Island Arc. differences between these foraminiferal sands 1: Coral fragments 4: Baculogypsina caused by the attrition of sand particle spines, 2: Shell fragments 5: Calcarina the author refers to this as "attrition grade 3: Lithic fragments 6: Others method" hereafter. The components and the attrition grade of each point were determined, Yoronjima by Akiyama (1979). However, quan enabling distributional maps based on the pro titative information concerning the relation portions of all points. Habitat and trans ship between beach and reef deposits is still port course of sands were presumed by these insufficient. maps. To confirm water flow, the author set On the basis of the attrition of the spines sediment traps on sandy bottoms in the moat of on Baculogypsina and Calcarina, the author at the reef. A currentmeter was used to determine tempts to clarify transport courses of bioclastic water movement at some points in the moat. sands on some islands in the Ryukyus with To confirm the habitat of Baculogypsina and special reference to the existence of channels in Calcarina, dying method by the Rose Bengal the reef flats (Yamanouchi 1988). (Kuwano 1956b) and the Sudan Black B (Walker et al. 1974) were used. Method Results and Discussion Topographic divisional maps of several reefs and beaches were made by topographic and Figure 1 shows all mean proportions of sever 74 H . Yamanouchi

Figure 2. Three attrition grades of foraminifera: Baculogypsina and Calcarina. Baculogypsina sphaerulata (Parker & Jones) . 1: Grade-A, 2: Grade-B, 3: Grade-C. Calcarina gaudichaudii (d'Orbigny) 4: Grade-A, 5: Grade-B, 6: Grade-C.

Figure 3. Particle size of sediments on the reef and beach at Okidomari reef in Okinoerabujima . Source: Yamanouchi, 1982. al sand samples which the author collected Baculogypsina is less than 2 mm in diameter . from the beaches at the islands of the Ryukyu Judging from the analyses of collected sand Island Arc. Figure 3 shows the particle size of samples, a high ratio of the components of sediments on a transect at a right angle to a foraminiferal sands in coarse sands is typical shoreline at Okidomari reef in Okinoerabujima for beach and reef in the Ryukyu Island Arc . in the central Ryukyus, indicating a high ratio The reef of is the northern of sands 1-2mm in diameter, as well as that at most coral reef in this region, and this 200m Kasari reef in Amamiohshima shown in Figure wide reef shows that Baculogypsina are domi 4. Figure 4 also indicates that at the Kasari reef nant on the beach and reef as shown in Figure 5 the Calcarina sands 1-2mm in diameter are (Yamanouchi 1980). According to the attrition dominant, and the size of most Calcarina and grade method, the outer reef flat area 75m and Sediment Distribution on Coral Reefs and Beaches 75

Figure 5. Characteristics of sands on the reef and beach at Kuchinoshima. Source: Yamanouchi, 1980.

Figure 4. Particle size and components of sands on the reef and beach at Kasari reef in Amamiohshima. Source: Yamanouchi, 1980.

Figure 6. Topographic divisional map of Yoshihara reef in Ishigakijima. Source: Yamanouchi, 1984. 76 H. Yamanouchi

Figure 7. Distribution of each grade of attrition of Baculogypsina and Calcarina at Yoshihara reef in Ishigakijima. Source: Yamanouchi, 1984. beyond from the shore is supposed to be the reef at Ishigakijima (Yamanouchi 1984; Yama habitat of Baculogypsina because of the high nouchi et al.1989) and eastern reef of Ikeijima ratios of grade-A found among the compo (Yamanouchi and Hasegawa 1988). The explana nents. Otherwise, it is clear that the sands at tion of the Yoshihara reef at Ishigakijima fol near shoreline are affected by turbulence and lows below. attrition by breakers in periods of high tide as Figure 6 shows a very clear channel in the grade-C particles dominate. middle of Yoshihara reef at Ishigakijima. Com Next, the distributional pattern of sands on ponent analysis of the collected samples in the reefs and beaches was examined. As exam this area reveals that the highest ratio of the ples of reefs having a clear channel, the author grade-A of Baculogypsina exists on the reef surveyed Okidomari and Ohtsukan reefs at crests on both sides of the channel (over 60%), Okinoerabujima (Yamanouchi 1982), Yoshihara whereas in the channel head the lowest ratio Sediment Distribution on Coral Reefs and Beaches 77 of the grade-A (10%) and the highest ratio of the grade-C (20%) exist as shown in Figure 7. In the case of Calcarina, a similar pattern of the attrition grades is recognizable in Figure 7. These features indicate that living Baculogypsina and Calcarina on the reef crests move into the moats and offshore, and in the channel head spine attrition may be attributed to the existence of a strong current. Though not as clear as in Yoshihara at Ishigakijima, a similar distribution pattern of each grade of Baculogypsina and Calcarina in relation to the channel was observed at Okidomari reef at Okinoerabujima, and eastern reef at Ikeijima. On the other hand, the author surveyed the reef and beach of Sesokojima as an example of a reef with no clear channel (Yamanouchi 1993). This is a wide reef 400m maximum in Figure 8. Surveyed reefs and beaches at width at the northwestern coast of the island, Sesokojima. and a narrow reef less than 200m in width at Source: Based on maps of GSI l: 25,000 topographic map, Sesokojima and Nago. the northern coast (Figure 8). The upper left rectangle indicates the Yamazato et al. (1974) and Nakasone (1974) studied area shown in Figure 9, and the present valuable findings in their paper on to upper right one indicates that in Figure 13. pography and benthic animals on this reef. An other paper (Nishihira, 1974) which reporting data about oceanographic conditions offshore of this reef does not include data on conditions within the reef area. It is difficult to find the channel by topography and depth of this reef (Figure 9). On the northwestern reef, Baculogypsina and Calcarina live on the reef crest and some part of the moat, because grade-A of Baculogypsina and Calcarina is concentrated in these areas (Figure 10). Otherwise, grade-C of Baculogypsina and Calcarina is dominant on the beach, partly due to drifting on the reef flat. Judging from these distributional maps (Figure 10), it can be recognized that the sediment drift occurred from the shores to the reef crest across the moat around the transects C, because of Figure 9. Topographic divisional map of the high percentages of grade-C of Baculogypsina northwestern part of Sesokojima on the reef crest. As a whole, these f oram reef. iniferal sands were transported from the reef crests to shores by waves and to the offshore by in the moat in the northwestern coast of tidal current. Sesokojima (Yamanouchi 1993). Each trap is This trend supports the results obtained by made with a crossed screen net of fine mesh sediment traps (Figure 11: Yamanouchi 1993). fixed by 5 pins on the hard rubble square plate Sediment trap is fixed on the flat sandy bottom of 20cm•~20cm. Figure 12 shows current 78 H . Yamanouchi

Figure 10. Distribution of each grade of the attrition of Baculogypsina and Calcarina on the northwestern reef of Sesokojima.

directions to north, south, east, and west at each method. The author believes this disparity is spot as deduced from trapped sediment weight. due to the short interval of one day and one In the results, the directions of vector night survey by traps. Figure 10 shows that analyses show northwestward to north by another longshore current also exists in the northwestward at the point on transect A and moat from transect X to transect Z. B, west by southwestward to south by south On the northern reef (Figure 13), presence westward at the point on transects C and D, of grade-A suggests that the habitat of respectively. It is recognized that strong Baculogypsina and Calcarina are in the moat 50 offshoreward flow exists between B and C m or more off shore (Figure 14). This fact is transacts. This result does not completely con supported by the method of coloring with the form with the result obtained by attrition grade Rose Bengal (Figure 15) (Kuwano 1956b) . In Sediment Distribution on Coral Reefs and Beaches 79

Figure 11. Sediment trap.

utilizing this method, living foraminifera are tainted red and therefore it can be distin

guished from the non-living one. The method Figure 12. Survey results by using the sedi of coloring by the Sudan Black B (Walker et al. ment traps on the moat bottom at 1974) was also tried, but the author could not the northwestern reef of Sesokojima. distinguish easily between colored and non (June 25-26, 1991) colored particles. Solid arrows show drift directions sug In spite of the small relief on the reef flat, it gested by trapped sediment weight. Plain arrows show the direction of vector was ascertained that drifting current occurred of each location. at the middle point of the transect of this north ern reef by distributional maps produced by attrition grade method (Figure 14). And it appears that the location of transect T is the center of dispersion for these foraminiferal sands. The result based on the sediment traps for this area was also supported by understand ing of sediment movement (Figure 16). Results obtained by vector analyses reveal that the direction of vector at the point on transect T is northward, while it is north by northeastward at transect U, and that at transect S is south by southwestward. From these results, it is possi ble to find the outline of dispersion course of sediments. The author believes that distinct Figure 13. Topographic divisional map of the offshore flow exsits at transect T, and water northern reef of Sesokojima. flow converges from both sides of transects U 80 H. Yamanouchi

Figure 14. Distribution of each grade of the attrition of Baculogypsina and Calcarina on the northern reef of Sesokojima.

and S.

Conclusion

The author surveyed the distribution of sandy sediment on the coral reefs and beaches paying special attention to the attrition grade changes of foraminif era Baculogypsina and Calcarina which originally have spines. And it was recognized that the distributional trend could help explain that the origin and their dispersion are related to the topography of the channel. Moreover, even in the case of the reef with no clear channel, this attrition grade method is valuable for understanding sediment transport course. This study is still in the preliminary stage. Further collection of quantitative data is neces sary to establish more precise sedimentological Figure 15. Map showing the probability of trends of these sands. In the future, it will be living Baculogypsina and Calcarina important to make accurate measurements of inferred from the Rose Bengal the volume of deposits on the beach, particular staining method. ly before and after storm events. Sediment Distribution on Coral Reefs and Beaches 81

Figure 16. Survey results by using the sediment traps on the moat bottom at the northern reef of Sesokojima. (July 22-23, 1997) The same legend as that in Figure 12.

Coral Reef Symposium, Townsville. 3: 425-430. Acknowledgements Chevillon, C., and Richer de Forges, B. 1988. Sedi The author would like to express his sincere thanks ments and Bionomic mapping on soft bottoms to Dr. Kiyoshi Yamazato, Meio University, former in the south-western lagoon of New Caledonia. Director of Sesoko Marine Science Center of the Proceedings of the Sixth Coral Reef Symposium, Ryukyu University for support of this study. Thanks Townsville 2: 589-594. are due to Prof. Johann Hohenegger of Wien Univer Chevillon, C., and Clavier, J. 1990. Preliminary sity and Kazuhiko Sakai of Ryukyu University, for sedimentological results on Chesterfield lagoon many valuable comments, Shigekazu Mezaki, Akira (New Caledonia). Proceedings of Eighth Interna Maekado, Hitoshi Hasegawa, Tatsuo Nakai, Toshio tional Society for Reef Studies Congress, Noumea.: Kawana, Tatsuo Takahashi, Hironobu Kan, Kiyoshi 173-178. Ichikawa for assistances during field surveys and Chun, L., Brian J., and Blanchon, P. 1997. Lagoon discussions, Prof. Kazuo Yoshikawa for photography shelf sediment exchange by storm-evidence from of foraminifera, Prof. Leonid G. Yoffe of Gunma foraminiferal assemblages, east coast of Grand University for revising this manuscript. The author Cayman, British West Indies, Journal of Sedimen wishes to express his gratitude to unknown referee tary Research, Section A 67: 17-25. for many kind comments. Crozier, W. J. 1918. The amount of bottom material (Received Jan. 22, 1998) ingested by holothurians. Journal of Experimental (Accepted June 4, 1998) Zoology 26: 379-389. Environmental Agency. 1996. Distributional Map of References Coral Reefs Nos. l-4. (JE) Hatta, A., and Ujiie, H. 1992. Benthic foraminifera Akiyama, Y. 1979. Destruction process of star sand as from coral seas between Ishigaki and Iriomote an indicator for sand drifting-An example on islands, Southern Ryukyu Island Arc, North coral reef at northeastern part of Yoronjima. western Pacific. Bulletin of the College of Science, Chin Kagaku (Geographical Sciences) 31: 33-40. University of the Ryukyus 54: 163-287. (JE) Hohenegger, J. 1994. Distribution of living fo Bradach, J. E. 1961. Transport of calcareous frag raminifera NW of Sesoko-jima Okinawa Japan. ments by reef fishes. Science 133: 98-99. Marine Ecology 15: 291-334. Chardy, P., Chevillon. C., and Clavier, J. 1988. Major Kan, H. 1994. Typhoon effects on sediment move benthic communities of the south-west lagoon of ment on reef edges and reef slopes. In Recent New Caledonia. Coral Reefs 7: 69-75. Advances in Marine Science and Technology, ed. Chevillon, C. and Clavier, J. 1988. Sedimentological Bellwood, O., Choat, H., and Saxena, N. 191-201. structure of the northern lagoon of New Cal Townsville: 94 Pacon International and James edonia. Proceedings of the Sixth International Cook University. 82 H. Yamanouchi

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