Int J Earth Sci (Geol Rundsch) (2003) 92:511–519 DOI 10.1007/s00531-003-0317-z ORIGINAL PAPER W. E. Piller · B. Riegl Vertical versus horizontal growth strategies of coral frameworks (Tulamben, Bali, Indonesia) Received: 5 April 2002 / Accepted: 1 December 2002 / Published online: 16 April 2003 Springer-Verlag 2003 Abstract Two different coral framework structures located in a shallow subtidal area on the east coast of Bali are described in this study. One structure is a typical coral carpet with a distinct internal succession of coral taxa and growth forms. It starts with a variety of coral species exhibiting massive, tabular, branching, and platy growth forms settling on volcanic boulders and cobbles. The main body of the coral carpet is composed almost monospecifically of Acropora cf. vaughani, which has filled all accommodating spaces up to the low-water sea level. Mostof this carpet died during the bleaching event of 1998 and the resultant dead Acropora framework is now capped by a platy Montipora assemblage. Some of the Acropora branches within the dead carpet, however, are still alive and display active growth. The Montipora cover protects the dead Acropora framework against mechanical and biological destruction. The few still growing Acropora branches may also contribute to the strength of the framework. The second coral framework is made up almost monogenerically of Montipora. One species of Montipora is of a laminar growth form and produces whorl-like colonies. Within this framework, only part of the Montipora colonies are dead; however, these are intensively fragmented. The fragments have been rapidly settled by a platy Montipora species, which has stabilized the fragments. In this case, the fragment shedding of the Montipora offers the opportunity for progradation of the framework on these fragments. Concerning the Acropora carpet, similar examples from the fossil record of the Miocene era of Spain and Austria have been reported. W. E. Piller ()) Institut fr Geologie und Palontologie, Universitt Graz, Heinrichstrasse 26, 8010 Graz, Austria e-mail: [email protected] B. Riegl National Coral Reef Institute, Oceanographic Center, Nova Southeastern University, 8000 N. Ocean Drive, Dania, FL, 33004, USA Keywords Coral ecology · Coral carpet · Acropora thicket · Montipora framework · Actuopaleontology · Bali Introduction On the east coast of Bali, near Tulamben, no true coral reefs have formed. Since the area is rich in corals and exhibits flourishing coral growth on rocks, one of the reasons for the absence of reefs is likely due to repeated disturbance by lava-flows emanating from Mount Gunung Agung. In protected bays, however, a dense cover of coral frameworks is found on volcanic substrate. Most of these dense coral stands were heavily affected by the bleaching event caused by the 1997–1998 El Nio Southern Oscillation (Wilkinson 2000) and died. This event has offered the opportunity to investigate these dead frameworks by excavating parts of it and studying their internal composition and successions. These studies may provide insight into the growth of modern non-reef coral frame stones, offering the possibility of developing models for fossil counterparts. Study area The study area is located at the E–NE corner of Bali (Indonesia), a few hundred meters east of the village of Tulamben (see Fig. 1). The particular study sites are located immediately in front of a Hindu Temple (see Fig. 1) and are easily accessible from the shore. The coastal area is covered by a lava flow which has been reworked along the beach forming coarse volcanic boulders and cobbles. A few to several tens of meters off the coast, the shallow water area is densely settled by scleractinian corals growing from a depth of 1–3 m up to the surface of the water. The little embayment directly in front of the temple is framed by two old lava flows that emanated from Mount Gunung Agung, reaching the sea in 1963 (local oral tradition and Ruddiman 2001, p. 379). The solidified lava formed steep sea cliffs of 2–10 m in 512 Fig. 1A, B General location of the study area. C Detail of the area around Tulamben. D Location of the two study sites (circles) east of Tulamben with bathymetry. Light-gray shaded areas in C and D represent abundant coral growth (C and D after Pickell and Siagian 2000) height (cf. Pickell and Siagian 2000). The bay is densely covered by a coral thicket predominantly made up of branching Acropora colonies. Adjacent to the eastern section, the neighboring bay is densely settled by foliaceous Montipora colonies. This area is not as densely covered as that of the Acropora thickets. The gullies between the Montipora areas have not been settled by corals, and the Montipora forms clusters of up to several hundreds of square meters in size. Both Acropora and Montipora thickets have attained a thickness of more than 1 m. Materials and methods The area was documented by photographic transects starting at the coast and moving seawards. After an intense survey, two sites within dense Acropora thickets were selected. Here, two trenches were excavated using a pickaxe, hammer and shovel in order to study the growth successions from the substratum up to the surface of the coral thicket. Detailed measuring in the field was followed by both a thin-sectioning of the corals and overgrowing organisms, as well as further measurements taken with a light and electron microscope. 513 Results Acropora thickets The Acropora-dominated thickets exhibit dense growth and cover an area of several thousands of square meters, which is only interrupted in some places. The thickness of approximately 80 cm, is relatively uniform, increasing slightly, however, just offshore from the beach. Generally, the upper surface is relatively flat in correlation to the low-water sea level. Vertical succession in Acropora thickets The excavations of the two trenches within the thicket display a clear succession of corals that are similar in both examples (see Fig. 2). The base The base of the thicket is formed by a substratum of volcanic boulders and pebbles. These represent fragments of the lava rocks which, because the bay is open to the east and thus exposed to winter storms, have become rounded due to frequent mobilization caused by wave action. This mobilization among the larger components has caused sand and gravel of a volcanic material to occur. Initiation to termination sequence The basal sequence in the excavated framework shows a combination of massive, tabular, branching, and encrusting corals in direct contact with the volcanic cobbles and pebbles (see Figs. 2 and 3). The species found to encrust individual rocks were faviids and most frequently, Favites pentagona. Acropora palifera was found to encrust several cobbles with the same colony, thus leading to a binding of these cobbles and a stabilization of the substratum. Acropora cf. hyacinthus was found to first attach to a single cobble and then to overtop several others. While the tabular species did not actively bind several cobbles, as did the Acropora palifera, which has a broader encrusting base, the overshadowing of several cobbles nevertheless sheltered them from wave action and stabilized them. Acropora cf. vaughani, which forms the framework of the thicket (see Fig. 4), started to grow simultaneously with the other basal corals, and, somewhat later, even grew on some of them, particularly on the platy ones. Later on, this species had overgrown and buried all the corals of the initial assemblage. At an early stage, such frameworks are still low and punctuated in many areas by large boulders, which have the potential of becoming mobile in heavy seas, thus having the capacity to destroy the frameworks. As the Acropora thicket grows, these Fig. 2 Schematic drawing of internal succession of the Acroproa carpet boulders become more and more enveloped until they are totally covered and stabilized. This dense thicket forms a more or less continuous coral framework of up to a thickness of 0.8–1 m . Viewed from the outer surface and from the excavated lateral surfaces, this framework is composed of corals that are apparently dead. Approximately 20 cm of the uppermost portion of the Acropora branches is intensively encrusted with coralline algae. In a few places, however, several live branch tips were found inside the dead and encrusted framework of Acropora cf. vaughani (Riegl and Piller 2001). These live tissues mostly occurred approximately 10 cm below the upper surface of the frame and, as ascertained by visual assessment, were of normal color and vitality. The branchlets observed were primarily 514 Fig. 3 Initial association of the Acropora carpet. A General view of the excavated trench with volcanic boulders at the base and small branching Acropra in between, a crustose coral layer on the boulders, and branching Acropora cf. vaughani thicket. B Basal volcanic pebbles encrusted by faviids, which are overgrown by a laminar coral colony and the Acropora cf. vaughani thicket. C Detail of B showing the crustose and massive faviid species. D Small branching Pocillopora verrucosa, Acropora palifera and Favites pentagona form the very base oriented laterally, since upward growth was blocked by competing branches of the same colony (Riegl and Piller 2001). The surface of the dead Acropora framework was covered by laterally continuous colonies of platy Mon- tipora (see Figs. 2, 4). These Montipora colonies grew on the tips of the Acropora cf. vaughani branches, bridging the space between branches and providing rigidity and protection for the dead framework (see Fig. 4). In places, the Montipora did not only cover and bridge the Acropora 515 Fig. 4 A Top part of the Acropora cf. vaughani thicket overgrown by encrusting Montipora with protuberances. B Top view of the Acropora carpet with the cover of encrusting Montipora. C Detail of top of the encrusting Montipora with short protrusions branches, but also the branch tips of the Acropora were encrusted for a few centimeters. Besides these protrusions, resulting from the encrustation of the Acropora branches underneath, the upper surface of the Montipora cover exhibited vertically arranged short branches (sensu Veron 2000) of a few centimeters in length (see Fig. 4C). Individual platy Montipora colonies did not fuse laterally (see Fig. 4B). In depressions and gullies within the thicket and surrounding areas, relatively little Acropora rubble was found. Only the immediate margin of the thickets exhibited minor amounts of shed rubble. Horizontal succession (landward–seaward) The beaches in the Tulamben area are made up of coarse black sand covered by rounded lava pebbles and cobbles (see Fig. 5A, B). On these coarse volcanics between the beach and the Acropora thicket described above, a coral association similar to the initiation assemblage at the base of the thicket was found. Cobbles and boulders were primarily encrusted with faviids that rarely overgrew more than one cobble and thus generally did not lead to binding of the substratum. Frequently, however, acrop- orid, pocilloporid and montiporid colonies settled in the fissures in between adjacent boulders and then encrusted both boulders with their basal plates, further stabilizing the boulders with their upward growing branches wedged into free spaces (see Fig. 5). Later, many colonies grew big enough to envelop the boulders and thus began to stabilize them. Finally, Acropora growth became denser, representing the beginning of the thicket (see Fig. 5C). Montipora frameworks In the slightly more sheltered bay east of the Acropora thicket a framework of densely spaced, laminar Montipora colonies of whorl-like growth were found (see Fig. 6A). The total coverage of this framework was several thousands of square meters, with a thickness of about 1 m in the portions that were better developed. Although no excavations were carried out (due to a generally high concentration of living colonies), in adjacent areas, Montipora was seen encrusting over several cobbles or boulders, forming whorls in the fissures between the cobbles. Although this particular Montipora area had suffered less mortality than the Acropora thicket nearby, and about half of the framework 516 fragments were not transported any further and remained angular and unrounded. The talus and the rubble beds in between the colonies were densely settled by the encrusting Montipora species which overgrew the surfaces of the dead Acropora framework. Thus, the Montipora rubble layers were stabilized by the binding activity of another encrusting Montipora species (see Fig. 6C). In addition to stabilization, the encrustation prevented further destruction via bioerosion and offered the opportunity for the Montipora to prograde over its own fragments (see Fig. 6C). Discussion Acropora thicket Fig. 5 Pioneer coral associations along the shore. A Initial encrustation of volcanic pebbles by numerous faviids and small Acropora colonies between the pebbles. B Small Acropora and faviids between volcanic pebbles. C Initial Acropora thicket already covering most of the volcanic components was still alive, there were enough dead colonies to allow for a significant disintegration of skeletal material to take place. The Montipora colonies tended to break into shingle-like fragments filling the free spaces in between the colonies and, where no free space was available, forming a talus in front of the framework (see Fig. 6B, C). Except for this shedding into the talus, the Montipora The vertical succession within the thicket and the composition of coral assemblages between the beach and the thicket represent a good example of Walther’s law: the basal coral assemblage in the thicket (a mixture of massive, tabular, branching, and platy corals), and the assemblage between the beach and the thicket are the same and represent a pioneer coral association. Some of the corals were able to overgrow adjacent volcanic boulders and pebbles (e. g., Acropora palifera), others grew on a single boulder but overshadowed several adjacent blocks. Both growth strategies offer protection against damage by hydrodynamic energy and also stabilize the substratum. This stabilization makes the upward growth of dense Acropora cf. vaughani possible. Dense upward growth of Acropora cf. vaughani can only be observed in some of the more protected parts of the coastal strip. The reason for this is believed to be wave energy, which is, at least during winter storms, too high to allow for long-lasting colonization. A few tens of meters from the beach, at a depth of only 1–2 m, the energy level seems to already be favorable for permanent and upward coral growth. These corals cover wide areas reaching the neap tide sea level, and therefore are a good example of catch-up coral frameworks (Neumann and Macintyre 1985). Since this dense thicket had caught up to the water level, it was exposed to high temperatures and irradiance levels and, as a consequence, had become almost entirely bleached to death during the positive sea-surface temperature anomaly associated with the unusually strong El Nio Southern Oscillation of 1997–1998(Schaffelner, personal communication). The dead tips of this Acropora were quickly and intensively encrusted with coralline red algae followed by settlement and coverage by platy Montipora. These platy corals served to cap the upper surface of the framework and, by keeping bioeroders away and accreting actively, helped to maintain the integrity of the largely dead Acropora framework. Despite the encrustation and shading by the coverage, several live and flourishing branch tips of Acropora cf. vaughani were still found. This indicates three strategies followed by the coral. While in healthy colonies most live 517 Fig. 6 Montipora carpet. A Detail of the laminar Montipora species forming upright whorls. B Margin of Montipora carpet with talus of Montipora fragments. C Detail of a Montipora carpet margin with the main frame of the laminar Montipora species and the fragment-talus colonized by encrusting Montipora species with protrusions tissue is concentrated in the uppermost layer of the framework closest to light, at least some cryptic tissue remains intact after the surface of the thicket dies off. These cryptic branches may provide a nucleus for the recovery and resettlement of the surface by re-growth from below. Even if these cryptic branches are unable to reach the surface again, their further growth and production of new branchlets will continue to fill free spaces inside the framework. This may help to increase the density, mechanical stability and, consequently, the longevity of the framework. From a more geological perspective, different aspects of this Acropora thickets may be considered to be important: – The distinct internal zonation within the thicket (from more robust at the base to branching growth forms above) might be interpreted as a record of changing energy conditions. However, the robust initiation assemblage stabilizes the volcanic pebbles, which is a prerequisite for the establishment of a branching thicket. – If only seen in outcrop, the succession observed here at the top part of the thicket (platy corals covering a 518 branching thicket) could be interpreted to suggest a deepening of the environment, since platy growth is often interpreted as a typical low -light adaptation (e.g., McCall et al. 1994; Schuster and Wielandt 1999). In reality, the mechanism leading to this succession is nearly the opposite: full utilization of accomodating spaces by Acropora cf. vaughani reflects a catch-up scenario. Subsequently, what happens is the death of the Acropora thicket due to the encrustation of coralline algae and bleaching, followed finally by settlement by—better adapted or opportunistic (?)— platy Montipora. – Due to capping by the Montipora, the branched Acropora-framework is protected against biological and physical destruction; and because of the cryptic, still living Acropora branches, the framework may even gain additional strength due to further internal growth. Both aspects are a clear indication that this framework, although affected by heavy winter storms with waves of several meters height, may remain cohesive even after it has died (Schaffelner, personal communication). This is supported by low amounts of Acropora rubble in and around the thickets, which indicates that individual broken Acropora skeletons do not live long among volcanic cobbles. When these move during storms, they grind the softer carbonate rubble to sand. Terminologically the described structures may be called a coral thicket, coral carpet, or incipient reef. Since we cannot predict future evolution of the structure, the term coral carpet is preferred (Riegl and Piller 2000a). In the geological record, coral carpets are classified with biostromes, particularly as autobiostromes (Kershaw 1994; Sanderstrm and Kershaw 2002). The Acropora structures studied are therefore an excellent example of a modern coral carpet as already recorded in a few other places within different reef provinces (e.g., Red Sea: Piller and Pervesler 1989; Riegl and Piller 1997, 1999, 2000a; Arabian Gulf: Riegl 1999; Caribbean: Geister 1983; Miller et al. 2001). Since coral studies of the Cenozoic usually focus on reefs sensu stricto (bioherms), neither modern nor ancient coral carpets are well documented. The modern example described, however, matches the description of fossil coral carpets from the Miocene very well. Other good examples come from the Tortonian (Upper Miocene) of southeastern (Almanzora corridor: Martin et al. 1989) and southern Spain (Granada Basin: Braga et al. 1990). In these examples (called reefs by Martin et al. 1989 and Braga et al. 1990), several cycles of frameworks have developed in which corals started to grow on coarse clastics (deltaic conglomerates) and exhibit an internal succession (from Porites to Tarbellastrea). These cycles are interpreted as being the result of ecological successions, and colonization by Tarbellastrea was only possible after the stabilization of the substratum by Porites (Martin et al. 1989). From the Middle Miocene Leitha Limestone of Austria (Central Paratethys), Riegl and Piller (2000b) report several types of coral carpets (biostromes). In one of these coral carpets the authors relate the internal succession (corals and encrusting red algae) to ecological disturbances, e.g., mortality events. Also, in this fossil case, no breakdown of the framework occurred due to either encrustation of branching corals by algae and capping of the whole structure by encrusting corals (Riegl and Piller 2000b). Montipora frameworks The growth form of the framework building Montipora is very open and the platy skeleton is thin and fragile. Since part of the framework had died and no distinct encrustation by other organisms was observed, this led to rapid disintegration and fragmentation by wave action and bioerosion. Open areas between Montipora colonies and the surrounding areas were covered by a thick talus of these fragments, which remained angular and unrounded. Therefore stabilization had to occur rapidly after fragment shedding. This means that colonization of the fragments by platy Montipora was fast. The importance of this capping sequence is stabilization and defense against intense bioerosion that could otherwise remove the framework and talus. By forming the dense rubble talus, stabilized by subsequent encrustation, the Montipora framework is an example of progradation by fragmentshedding. This means that fragmentation and shedding of one Montipora species produces substrate for settlement and encrustation by another Montipora species. In doing so, this mechanism favors and speeds up lateral growth of the framework, which may again be classified as a coral carpet. From a geological perspective, contrary to the Acropora carpet, most of this carpet may be preserved as a bed of densely packed Montipora fragments with encrusting Montipora, probably in situ, in between. In terms of Kershaw’s biostrome classification (1994), this type would by classified as autoparabiostrome or even as parabiostrome, although disruption of coral does not necessarily relate to storm activity. Acknowledgements This study was funded by the Austrian Science Foundation via grant P-13165-GEO. 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