Contribution to the 32nd/33rd/34th and 35th BUFUS Newsletter;
Authors: Pierre MADL
with contributions from:
Arnfried ANTONIUS: Chapter IV
Karl KLEEMANN: Chapter V
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Introduction:(implemented Feb. 2005) - Main Page

Modern tropical coral reefs flourish along the circum-tropical equatorial region since approximately 50·E6 years.1.1 These reefs are characterized by both the last interglacial episode (some 120·E3 years ago) and the initiation of growth of modern reef structures featuring hermatypic corals (less than 9·E3 years ago).1.2 Tropical coral reefs occupy roughly 0.09% of the overall littoral ocean surface and can rightly be termed as the Jewels of the otherwise oligotrophic (low-nutrient) tropical oceans.1.3 The living tissue of reef-building coral is just a thin veneer, measuring several millimeters in thickness. Yet, this extraordinary biofilm produces everything that is useful about coral reefs. Indeed, it formed the most massive structures in the world made by living creatures (including humans) and created limestone structures up to 1300m strong (remote Pacific Atoll Islands) or in other cases form an almost uninterrupted chain stretching over 2000km in length (such as the Great Barrier Reef).

Coral reefs are among the most biologically diverse and productive ecosystems in the world. They form communities that can hardly be outstripped in species diversity and their interaction by any other ecosystem both above as well as under the water line. Corals are among the few organisms that lend habitat and resources to millions of species of protists, plants, animals, and humans. Coral reefs are found in over 100 countries, mostly in the less economically developed tropical regions with at least 100·E6 people having direct access (while an estimated 500·E6 people live within 100km of a coral reef - fig1.0).1.4a It is estimated that tens of millions of people are directly or indirectly dependant on reefs worldwide.1.5a Whether based on fisheries, aquaculture, or tourism, the people of atolls in particular not only obtain their protein intake from the reef, their livelihood is entirely dependent on it.

 


Fig.1.0: Number of people living within 100km of coral reefs (95kB) 1.4b

It is therefore the aim of this report to focus on the anthropogenic component that interferes with the fragile natural balance in the long-term survival of this particularly vulnerable ecosystem. The following few pages of the first chapter will briefly summarize general aspects of tropical reefs. The second chapter focuses on the coral animal, its phenotypic characteristics, and the physiological importance of coral endosymbionts. Chapter three highlights the dynamic balance of a reef system when stressors of various kinds interact with the benthic coral community. The ongoing shift in community stability and the reduced resilience of coral to bioerosive prokaryotes (i.e. so-called diseases) is illustrated in chapter four, while chapter five provides an insight into the bioerosive properties of higher taxa, of naturally occurring predator-prey relationship among coral species and of inter-specific competition within the animal kingdom. The rapidly growing capitalization of coral-reefs and their associated effects are at the center of attention in chapter six. The final chapter summarizes the previous chapters by emphasizing the importance of sustainability, management, and appeals to a new attitude towards our environment and to coral reefs in particular.

 

Chapter – I:

Geographical distributions: The fundamental factor governing the concentration of tropical reefs along the circum-tropical belt is sunlight. Across the equatorial regions, and throughout the year, it irradiates the surface at a high angle. This intense influx of solar energy heats up equatorial air causing it to expand and rise to higher levels within the troposphere. In this process cooler bottom air from both the northern and southern hemisphere are sucked in. Because of the planets spin (rotating eastward) these bottom winds effectively move westward rather than directly north or south. The frictions of these trade winds literally blow the ocean surface layer (equatorial currents) westward. On their westward path, these equatorial currents are blocked by the continents (in both the Atlantic and the Pacific Ocean) and along with the Coriolis-effect, 1.5b are forced to move clockwise in the Northern hemisphere and counter-clockwise in the Southern hemisphere (fig.1.1a). This effect not only determines the direction of ocean surface currents, it also forces typhoons, tropical cyclones, or hurricanes to generally move from east to west (the planet rotates eastward below the atmosphere in which these spinning storms are suspended).

 

 


Fig.1.1a: GLobal Atmospheric wind patterns (83kB) 1.8a

As a result, the combined effect cause coral reefs to extend latitudes on the western sides of oceans than on the eastern side. On the other hand, impoverished coral-reef communities are more easily replenished with recruits from easterly located reef regions (presuming that they are in healthy conditions), especially at higher latitudes bordering the eastern shores of the continents.1.6 Because of this ocean current pattern, the western shores of the continent are characterized by upwelling, whereas downwelling dominates along eastern shores (fig.1.1b). This distinction between nutrient-rich upwelling and oligotrophic downwelling is also reflected by the trophic relationship of these regions, in that the grazing pressure on benthic communities off the Pacific coast of Panama for example (where upwelling occurs) is 25x greater than on the Caribbean side of Panama.1.7
The physical environment limits the distribution ranges and diversity of coral species, controls most morphological variation in coral species, and is more important than biological mechanisms in driving evolution. Physical-environmental parameters that generate biogeographic patterns, tend to be either latitude-correlated (including temperature, light, reef/non-reef habitats and boundary currents), or non-latitude-correlated (including non-boundary sea surface circulation, substrate availability water quality and nutrients, regional ecology and regional dispersion barriers). Physical-environmental gradients tend to be less complex and more readily answered. Databases containing species-level distribution compilation are available. The primary data, derived from original field-work, are extensive and taxonomically detailed, covering all known coral species throughout the full length of both the east and west coasts of Australia, as well as the east coast of Asia from the Philippines to mainland Japan (see fig.1.1b1). 1.8e



Fig.1.1b: GLobal Ocean Surface Currents, Coral Distribution and Species Richness (190kB) 1.8b

 


Fig.1.1b1: Latitudinal attenuation of coral diversity (95kB) 1.8d

Reef builders & Trophic Relationships: A variety of corals (especially colonial scleractinians, helioporans, and milleporans), along with coralline algae are the major constructors of the reef framework, whereas the crevices within the carbonate framework are filled by minor contributors, which include solitary scleractinians, stylasterine and tubiporan corals, sclerosponges, sessile molluscs (vermitid gastropods, chamid bivalves, oysters, etc.), bryozoans, foraminifera and algae. Due to the three-dimensional complexity, niche diversification is especially pronounced and confirms the principle of “diversity begetting diversity”. This is particularly evident in the abundance and importance of symbiotic interactions; i.e. compared to temperate environments, the seasonally less variable nature of the tropics reflect a high degree of specialization and abundance of often complex and specific symbioses. 1.9

Trophic Complexity: Although reefs are settled in an oligotrophic environment, they are still capable of sustaining a lush environment. Every task that regards production, consumption, and decomposition of organic matter within the reef involves many species doing it. Thus, the abundance, diversity, and degree of specialization of grazers and predators seem to exceed that of detrivores. It is not surprising then that coral reef-food webs are long and intrinsically interrelated. This implies that the total ecological efficiencies of conversion of primary production (photosynthate) into animal flesh are low compared to many other (marine) ecosystems (fig.1.c). Consequently, higher trophic levels control productivity at lower levels; i.e. grazing and predation control plant and prey populations.1.10a



Fig.1.1c: Relative biomass accumulation of the world's major ecosystems (100kB) 1.8c

Thus, the combined areal biomass of all heterotrophs (from bacteria to whale sharks) in reef communities approximates that of autotrophs (pyramids of biomass for reef communities are steep sided), and the body sizes of the organisms comprising most of this biomass are small (fig.1.d). As a result, heterotrophic respiration is the major pathway by which organic material is turned over in coral-reef systems (recycled back to inorganic form). The large photosynthetically active area of reef surfaces, the great abundance of light and inorganic carbon, and the small size of most of the plants (with high specific productivities) makes it so worrisome when supporting populations, i.e. corals die off on a vast scale.1.10b



Fig.1.1d: Ataxonomic relationship (95kB) 1.10c

In comparison, marine systems that have especially high productivities, such as those with substantial upwelling, are characterized by relatively few species and simple, short food webs. Since tropical coral reefs typically establish themselves in oligotrophic waters characterized by downwelling, their extreme endogenous productivity (high biomass and high diversity) can only be derived from efficient nutrient recycling through highly complex food webs.1.11 Thus it comes of no surprise that greater yield at a given trophic level from a system can only be obtained if lower trophic levels are harvested. The Peruvian upwelling for example consists only of two trophic levels (see fig.1.2).1.12
Tropical reefs on the other hand are characterized by high productivity and low yield. Yet, far less than 1% of the gross primary productivity is converted to production that is meaningful for human consumption. Part of this low net system yield in terms of the major bulk of the biomass are rooted in the larger number of trophic levels in coral reefs (see fig.1.2). Compared to the previously mentioned upwelling regions of the world, which have a characteristic 2-level food web, coral reefs with six trophic levels definitely can be considered as one of the most complex marine ecosystems.1.14a

Of the estimated 2-3% of gross primary productivity (GPP) that becomes net primary productivity from a coral-reef community, perhaps ¾ of that constitutes the net system yield, which is unusable by humans, while approx. 15% is accumulated in reef structures. The remaining 10% (of those 2-3% GPP) of a coral reef (that is <1%!) constitutes usable secondary production, of which much is algae and invertebrates.1.14b
The degree of nutrient recycling within coral-reef communities is extreme. Chlorophyll has been found to be present only 2-6% of the food mass, whereas much of the zooplankton is “recycled” rather than drifting through the system. As will be oultined further belowe, faecal material from fishes may be eaten and recycled through five fishes before it reaches the seafloor to be consumed by corals or other invertebrates. This is in stark contrast to pelagic regions where it is exported below the euphotic zone.1.14c
As one moves up the coral-reef food web, much of the assimilated energy is redirected to maintain the sandwiched trophic levels in-between (refer right-hand insert in fig.1.12). One option to increase yield would then suggest targeting lower trophic levels. The impact seems reduced, but more of it has to be harvested in order to obtain the bulk masses for economic or nutritional requirements.1.14d

 


Fig.1.2: Trophic complexity among ecosystems (162kB) 1.13

Microbial Loop:1.16a Nutrient recycling within the coral-reef community begins at the microscopic scale. The term "microbial loop" refers to the bacteria-mediated recovery through uptake and metabolism of dissolved organic matter (DOM). Without the microbial loop, DOM generated by excretion, exudation, and diffusion is "lost" from the trophic system. These microbes at the trophic base are in fact so diverse that their food-webs are virtually confined to a given substratum. Just to name a few: in decomposing organic matter (detritus), within the first few centimetres of the sediment, in the water column over coral reefs, on the muco-poly-saccharide layer (MPSL) of the coral animal, etc. Indeed, the smaller the size of an organisms, the greater its importance in the processes of production and decomposition (compare fig.1.1d).
Via heterotrophic bacteria and microscopic bacteriovores, such as protozoans and meiofauna, the microbial loop efficiently recycles dissolved organic matter (DOM) that leaks from primary producers (such as algae, photosynthetic bacteria, corals, etc.). Heterotrophic bacteria are mainly sinks as they mineralize more than 50% of assimilated DOM). As shown in fig.1.3b, the microbial loop is responsible for the decomposition of almost all of the organic material reaching coral-reef surfaces. Yet still, the microbial loop supports much growth at higher trophic levels, leaving little to be buried as organic sediment1.16b Since microbes are R-strategists, they not only have fast generation times but also have large surface to volume ratios.1.16c This enables them to respond rapidly in numerical and metabolic terms to changes in the supply and quality of organic material. This competitive advantage though, comes with a price - the smaller the organism, the smaller not only the pools of materials, but also the lower the capacities for storage and buffering – particularly when supply rates fluctuate. Furthermore, nutrient concentrations on coral reefs are low, prokaryotes (bacteria) compete with eukaryotes (metazoans) to obtain these resources.1.16d In fact, detrital food webs dissipate the majority of reef primary production. 1.17

 


Fig.1.3a: Proposed recycling of coral mucus (140kB) 1.15a

 


Fig.1.3b: The Microbial Loop (90kB) 1.15b

Higher Trophic Levels: The sunlit back reef-surfaces not covered with corals and other sessile organisms sustain a herbivorous community - e.g. sponges, giant clams, etc. are coated with a layer of tiny filamentous green algae, rendering some shallow lagoon habitats net autotrophic for at least part of the year. The section of the coral-reef community facing the open ocean is so crowded and packed with sessile and motile life-forms, making this side of the reef highly efficient in further sweeping the already nutrient deprived incoming water masses of food; as a result it is referred to as the "wall-of-mouths" (fig.1.4).1.18
And still, the bulk of primary productivity within the reef is achieved by the endosymbionts of hermatypic corals, algae and photosynthetic bacteria (since the plant-animal symbiosis is the driving factor within the trophic relationships, it will be at the focus of the following sections). But it doesn’t stop here; in effect corals and other sessile organisms acquire necessary nutrients directly by harvesting microbes from the water column through mucus netting and indirectly via capture of protozoa that graze on bacteria. In fact, corals live within and depend on this soup of microbes - even though some of them are potentially pathogenic. Corals themselves gain up to 70% of their nitrogen budget from particulate sources, and bacteria from the water columns account for 30-45% of the incorporated particulate matter.1.19 Actually, grazing at that level – especially on algae - enhances primary productivity by keeping the individual algae in an “exponential growth phase”. This trimming feedback from herbivores makes sure that the algal assemblage stays stationary in the early successional stage; thereby not only recycling inorganic nutrients back to the benthic community, but thereby indirectly sustaining the success of corals. 1.20
As illustrated in fig.1.4, more and more of the production depends on the turnover (recycling) of nutrients within the system. Since the trophic pyramid for reef communities is steep-sided, the combined areal biomass of all heterotrophs (from bacteria to whale sharks) approximates that of the autotrophs. As a result, the body sizes of most reef organisms are small, while the interconnectivity among these organisms attains an extremely high level of complexity. With coral reef-food webs being so elaborated, compared to other marine ecosystems, the total ecological efficiencies of conversion of primary into higher heterotrophic levels (i.e. animal flesh) is very low. 1.21 And still, this poor efficiency results in one of the most spectacular and biodiverse habitats known on this planet.

Stepping back to obtain a broader picture of this complex ecosystem, it may become more obvious why coral reefs develop best under highly oligotrophic conditions. The secret to lush coral reef growth is not low nutrients per se but rather low nutrient levels in the water column and benthic nutrient pools, and high nutrient levels available to coral symbionts. These conditions can be achieved through tight cycling of nutrients with coral endosymbionts as the primary beneficiaries.1.22b Indeed, several experiments have demonstrated that corals grow faster, or are at least not directly compromised, under elevated levels of limiting nutrients.1.22c As outlined above, some of the nutrients available to coral reef organisms are regenerated from sedimentary and reef pore space nutrient pools. Even these, however, were initially imported into the reef system from the water column and adjacent benthic marine habitats by motile reef-associated animals. Recycled nutrients are deposited on the reef in the form of these animals' waste products. One can simplify this phenomenon by breaking it down into the maijor components.
First comes the fouling community itself, which we can think of as a "rug of mouths." The reef's mat of sessile invertebrates, including all of the corals (both with and without zooxanthellae), is perpetually engaged in filtering the waters that pass over it for organic matter, whether living (i.e., bacteria, phytoplankton, zooplankton - see also fig.1.3b) or even nonliving particulate organic matter (POM) e.g. detritus. The rug of mouths is best developed on reef surfaces most directly exposed to food-laden currents. Due to the shear gradient and boundary layer formation over the reef-water interface, exposure is enhanced by height in the water column.1.22cc This "reef effect" is the very essence of why a wave?resistant structure so quickly forms the foundation for a rich benthic community such as a coral reef.



Fig.1.4a: Sketch of Wall-of-Mouths (200kB) 1.22a

 


Fig.1.4b: Photo of Wall-of-Mouths (150kB) 1.22aa

Attracted by and sheltering amid the mat of fouling organisms are three guilds of motile animals, each of which contributes further to nutrient flow into the reef community: the "wall of mouths," "commuters," and "residents." The wall of mouths is composed mostly of zooplanktivorous fishes, plus a few motile filter-feeding crustaceans such as porcellanid crabs, holothurians, and ophiuroids. The wall of mouths extends outward from crevices and hide-holes on the reef surface up into the living, shimmering curtain of zooplanktivorous fishes. There are both a diurnal and a nocturnal wall of mouths, which differ in taxonomic composition and behavior. All of the diverse participants in the wall of mouths void most of their concentrated, nitrogen- and mineral-rich waste products directly over or onto the reef. For example, diurnal wall-of-mouths participants feed until dusk but continue to pass waste products for hours afterwards. Thus a good deal of the nutrients in their waste stream is probably released beneath the boundary layer, readily accessible to corals and their dinoflagellate symbionts.
If this relationship is generalizable, it could help to explain the great success of Stylophora, Pocillopora and tabulate acroporids as the premier early colonists of regenerating coral reefs. Such corals are strongly targeted for settlement by numerous species of damselfishes (e.g., Dascyllus aruanus, D. reticulates, D.trimaculatus, Pomacentrus molluccensis, Chromis viridis, C.atripectoralis), butterflyfishes (Chaetodon trifascialis, C.trifasciatus), acanthurids (Paracanthurus hepatus), and serranids (Pseudanthias spp.). The zooplanktivores form dense aggregations on these corals, sleeping among the closely spaced branchlets. The butterfly-fishes feed on coral mucus and bits of the polyps themselves without damaging the colonies, and numerous reef fishes feed on coral larvae and sperm-egg bundles, both processes furthering nutrient regeneration and recycling.1.22d The commuters are mostly fishes, though some crustaceans and regular echinoids could also function in this manner. Among the major commuters on both Indo-Pacific and Atlantic coral reefs are fishes of the families Haemulidae (grunts), Lutjanidae (snappers), Holocentridae (squirrelfishes), and Mullidae (goatfishes). These species mill about the reef by day, sometimes forming huge aggregations, and fan out across nearby seagrass beds and sand flats by night to hunt for the benthic invertebrates and fishes that comprise their prey base.1.22e
The Atlantic grunt-species (Haemulon flavolineatum), that a commuter can be a significant source of nutrient input for a coral (in this case elkhorn coral, Acropora palmata), that builds the commuter's resting habitat.1.22ee Large parrot fishes that move between coral reef and seagrass or mangrove habitats may function in a similar way, but on an opposite time cycle. Residents are motile species that never leave the coral reef environment but can simultaneously mediate spatial competition and nutrient cycling by removing one group of space occupants and fertilizing others with their refuse. Herbivores, invertebrate feeders, and piscivores all contribute to this process, but the outcome can vary widely as a consequence of the overall food web dynamics of the community. For example, large reef piscivores facilitate nutrient flow into corals by concentrating their nitrogen in daytime resting spots. By the same token, predators have indirect negative effects on corals by consuming resident herbivores, thus triggering atrophic cascade favoring macrophytic algae.1.22f
Nitrogen-fixing cyanobacteria are another taxon whose contribution to nutrient dynamics must be considered. Some authors found them to be a surprisingly minor factor.1.22g While others discovered that cyanobacteria seem to be in close association with the Caribbean coralliomorph Ricordia.1.22h This possibility has since been confirmed through the discovery of symbiotic cyanobacteria in hard corals. Cyanobacteria seem tp be important to nitrogen flux on coral reefs after all, but in a different way than previously suspected. Reef-building corals are, in part, their rich associated microflora. Thus it is essential to understand a corals' prokaryotic camp followers and inquilines if we wish to comprehend their state of wellness and functionality in a community context. Sometimes, the flip-floping property among alternative nutrient flow channels is on a hair trigger. On coral reefs, the switch settings correspond to alternate living fabrics: a rug of mouths on a living coral mound, or a carpet of seaweed on carbonate rock (the corals' dead skeletons). The amount of nutrients can sometimes matter, but it is more likely that the routing of these nutrients matters most. If the endosymbionts are on the receiving end, then symbiotic corals dominate the benthos. When the links to these endosymbionts are broken, phase transition from hard corals to fleshy algae becomes a likely event. In an intact, healthy coral reef system, organisms are passing nutrients to each other quickly and efficiently. Removing key species or functional groups, however, can cause the nutrient pools in reef rock and sediments to grow and ultimately bleed into near reef waters. Normally, the nutrients concentrated on a coral reef are sequestered in the living organisms themselves which, aside from corals, hail from about 30 other phyla and hundreds of thousands of species. If enough of those species that create nutrient closure on the reef are destroyed by some disturbance event, then nutrients from all of the importers will pool and adventitious weeds can prosper.1.22i
All these species are what Aldo Leopold was referring to when he talked about all the little "cogs and wheels" that you had better not lose.1.22j This somewhat mechanistic illustration are the pieces that an intelligent being would be careful to save because without them the system as a whole will not work. Herbivores are not the only cogs that could make the ultimate difference. Then there are corallivores, coral mucous?eaters, cleaners, sand sifters, microbes of myriad functions, and so on. When a coral reef shifts to fleshy algal pavement, the key may just be whatever species was, by chance, the last one keeping the degraded system functional.

Reef Topography & Zonation: Modern reefs are commonly envisaged in terms of the prolific, shallow water corals of the tropics. From the geologic record though, we known that reefs include also most other in-situ accumulations of skeletal calcium carbonate that rise from the seafloor, such as photosynthetic bacteria (stromatolites), encrusting corallinacean algae, banks of gastropods (vermitid snails) or polychaetes (serpulidae), molluscs (bivalves), just to name a few. Thus both biological as well as geological processes characterize and distinguish corals, oysters, and other biogenic reef formations from sand and rock reefs.

Biogenic reef formations are grouped into two separate processes that are different in terms of generation but often happening simultaneously. Limestone structures that are produced by biological activity attain a typical lens-like body are termed bioherm. These are in contrast to the biostrome, which are embedded structures of in-place carbonate skeletons (see fig.1.5). The latter originated almost exclusively from sedimentary processes and are bound together by sedentary organisms.



Fig.1.5: Bioherm vs. biostrom (225kB) 1.23

Over time, and under favorable conditions, the continuous accreting activity of carbonate precipitators shape the massive bank-like structure (bioherm) so familiar to most underwater enthusiast. The sequences that characterize the phases in reef evolution are sketched in fig.1.6. The smallest elements are coral knobs that rise just a few meters above the sandy substrate. These tiny structures about 5m in diameter and 3m in height and are characterized by inter-growths of corals, algae and associated organisms. Larger structures (often referred to as patch reefs) are for the most part aggregates of coral knobs. They are generally unzoned and emerge from depths up to 20m deep. Such patch reefs vary in shape from pinnacles, bommies, to walls, and micro-atolls that can even include small, enclosed sandy areas at their apex.1.24



Fig.1.6a: Reef structures (150kB) 1.25a

The structural evolution of a coral-reef can be grouped into four essential phases. A tiny living veneer of sedentary organisms (such as diatomea, ciliates, foraminifers, corallinaceae, Millepora, Pocilloporidae, etc.) form the solid and stable foundations on which reef systems can flourish (fig.1.6b). Secreting calcareous organisms constitute the encrusting components of a reef: e.g. rhodophyta (red algae), foraminifera, bryozoa, annelida, and mollusca. Acting as a binder, they cement enclosed sediments together and add additional strength to the reef. Together with the scleractinians as the major framework constituents (and some sponges), they enable rapid upward growth of the reef into shallower water. Soft-bodied organisms known as bafflers and binders like flexible gorgonians, alconyacea (soft corals), porifera (sponges), grasses, algae, etc. act as sediment traps by slowing the speed of moving currents above the reef.

 


Fig.1.6b: Framework
building towards Climax community (100kB) 1.25b

Based on the location within the oceanic setting the following four basic types of coral reefs can be characterized (fig.1.7|). Fringing reefs are located very close to shore, and because of water run off they are frequently exposed to high nutrients and excessive water turbidity. Barrier reefs are further from the shore with a "lagoon" separating the reef from the shore. Platform reefs are typical for mid-ocean locations, and at the edges of continental shelfs. And finally atolls are more or less circular reefs with a central lagoon and possibly small islands or coral cay.

It is theorised that each of these types of reefs corresponds to a differing age within the evolution of reefs. The youngest is the fringing reef, with the corals colonizing a shallow water area close to the land. With gradually rising sea levels or subsiding landmasses, reef structures that are able to catch-up grow upward. Eventually a shallow area with no coral growth will form behind the main reef, called a lagoon, forming a barrier reef. If these processes of rising sea-levels or subsiding landmasses persist - to cause the land to disappear below the water surface, then an atoll is formed.



Fig.1.7: Reef types and formation (65kB) 1.26

Reef Zonation: Coral reefs are critical for the stabilization of coastlines and establish the proper conditions for associated ecosystems such as sea grass and mangroves. Prevailing wave energy has been shown to exert a primary control on the character of the shallow-water reef crest not only by moving nutrients and waste products through the system, and thereby establishing a characteristic zonation (a continous gradient) across the entire reef profile.

The Intertidal Zone of many tropical localities worldwide (land-Sea interface) is occupied by dense tangles of mangrove trees. This salt-tolerant hearty bush produces numerous prop roots that account for a very rich and diverse habitat ranging from algae, sponges, marine invertebrates, nurseries for young shrimp and coral fishes. Seagrass beds are often seen in close vicinity to the shore. These angiosperms trap sediment in their roots and thus prevent cloudy water from settling on nearby coral reefs. Holothurioids and echinoids are prolific inhabitants and constantly rework the surface sediment. The very shallow inshore sandy zone of the Back Reef is often covered by seagrass. Because of the modification of wave forces across the reef crest, this is an environment of totally different physical processes, ecology and sediment characteristics.



Fig.1.8: Reef Zonation (190kB) 1.27

The organisms within this section must be able to withstand intense ultra violet radiation, desiccation, high salinities and elevated water temperature. Sediments and rubble from the reef crest are dumped into the back reef widening it through time. The sandy substrate consists of dead coral rubble and algal covered coral boulders. Atolls feature a typical sub-zone, the Lagoon that is occasionally also seen on fringing reefs, but absent in bank- and barrier reefs. Loosely situated micro-atolls (typically consisting of massive Porites sp.) are a common occurrence in lagoons. Branching corals of the genus Acropora, Porites, and Heliopora are another common sight in this section. Corals abundance increases only with greater distance to the intertidal zone. Because of the reduced wave action within the reef flat, brilliantly coloured living corals are to be found when they are kept free of sand and silt. Where the outer reef flat merges with the boulder zone, a rich-crypto-fauna can be found, ranging from bryozoans, ascidians, sponges, hydroids, tube worms, among others.1.28
The Reef Rock Rim or Reef Crest, often expose corals to extremely harsh conditions, particularly at low tide. The crest attenuates the incoming waves battering against the rim. The corals present in this zone respond by very short-fingered, encrusting or even massive structures capable to withstand the strong wave action (compare fig.1.9). Coralline algae and other encrusting biota are essential in this region as they act as binding cement. Just below the rim, branching hydrocorals, e.g. Millepora, and calcareous chlorophycaea (e.g. Halimeda) are of common occurrence. The Reef Front at 5-10 depth is a portion of the reef where environmental conditions are optimal for growth of many reef organisms. The result of this ideal environment is the development of large coral buttresses. Because this setting is so ideal for reef biota, it is characterized by the highest diversity of any sites on the reef. On the windward side of the Reef Slope, channels covered with sandy patches usually interrupt the more consolidated outer slope channels - the spur-and-groove structure (fig.1.8). While the spurs carry luxuriant developments of coral (huge spreading and branching species of the coral genus Acropora, and large heads of massive corals), the sandy rubble floors appear almost denuded. This structure provides a region where nutrients are "channelled" and transported into the reef flat area, enabling optimal extraction of the scarce amount suspended in the ocean water. Also prevalent within this zone are Reef Terraces. These are relatively flat surfaces of the reef, which extend from 10-15m water depth. Slim branching corals that are interspersed by larger head corals dominate this environment. At depths below 20m, the Deep Fore Reef Slope represents the offshore limit of the reef and is often referred to as "the Wall" where organisms cling to irregular rocky ledges.

Apart from the structural zonation, a reef’s morphology also depends on other growth-determining factors. Besides wave action, other important parameters include light, temperature, salinity, sedimentation. Zonation does also vary between islands and leeward/windward sides. Coarsely one can discriminate between the following depth windows (example of a Hawaiian reef):1.29c The light-dominating window is found between 0-6m, with moderate wave energy, resulting in a "Cauliflower coral" zone (Pocillopora meandrina). At depths between 6-13m light is less intense; occasional surges as a result of storm activity lead to the expression of the "Lobe coral" zone (Porites lobata). At 13-25m light becomes increasingly scarce, wave energy is limited, which is ideal for "Finger coral" (Porites compressa). Below 25m there is very little light, with almost no wave energy present, only downwelling currents predominate. It is usually characterized by sand zone, coral rubble with occasional coral cover (maybe thin or absent), such as "Plate coral" like Porites rus (see fig. 1.9a).
Because of the increased depth, light composition changes gradually, leaving only the shorter wavelengths of the spectrum to illuminate greater depths (blue light, see fig. 1.9). Corals have to take on expansive body forms designed to maximize the exposure to sunlight. In response to lowered light levels, corals adapt and grow as large plates. Some of the plates seen here are species, which form rounded heads in shallow water. Nonetheless, at a certain depth, the point at which the rate of reef accretion becomes less than the rate of reef subsidence and/or erosion is termed the Darwin Point (this point beyond which reef accretion is minimal to non-existent, is also known as the “drowning depth”).

 

Fig.1.9a: Idealized wave zonation (95kB) 1.29a

 


Fig.1.9b: Coral morphology vs. depth (105kB) 1.29b

Since net accretion below that point is so slow, reef biota gives way to a community of sponges and deep-water, non-reef-building corals (ascleractinia). The force of the waves and associated surges that batter against the reef structure, break some of the reef building corals weakened by boring organisms. Such coral debris are often found at such depths.

 

Please proceed with Chapter-II (Coral Anatomy, Morphology, Physiology, Nutrition and Coral Reproduction).