Chapter – III: (implemented April 2005) - Main Page

Reefs in flux: Most people picture reefs, and their associated fauna and flora, as vigorous, flourishing, and healthy ecosystems. Though, over the last decades, this prosperous image has taken a dramatic turn. Based on widespread opinion both among the lay and scientific communities, it seems an inescapable conclusion that the world’s reefs are changing and in the opinion of many, they are "declining".3.1 Since tropical reefs are usually found across the circum-equatorial belt, they often border countries with booming populations and low living standards. And that’s where the main concerns arise. The current world population is estimated to be greater than 6·E9, of whom more than 3·E9 live in coastal areas. Much of this population is located in tropical countries with surrounding fringing and offshore coral reefs. Population growth rates in some regions of the eastern Pacific are among the greatest in the world, with the rates of urbanization increasing even faster, making coral-reef management even more difficult. Due to persisting trends in urbanization, it is predicted that by the year 2055, these population numbers are set to more than double, with the greatest increasing population densities in tropical developing countries (see fig.3.1).3.2 As a result, global climate change and human pressures increasingly threaten coral reefs worldwide, particularly the various utilizations of reef-associated resources by an ever-increasing number of people having access, adds to the aggravated situation of reef-ecosystems.

Already 58% are threatened by human activities out of which 27% are at high risk.3.3 Since corals grow best in the warm, clear and oligotrophic (nutrient-poor) waters of tropical oceans,3.4 increased coastal as well as in-land development in tropical countries has led to eutrophicated and contaminated coastal environments in the form of increased level of nutrients, sedimentarion and heavy metal concentrations. But the decline in coastal water quality is just one of many factors making it harder for corals to thrive and persist. Most stressors have been shown to result in a range of direct impacts on coral reef organisms, in that the incidence of pathogenic infections among corals has dramatically increased - especially in the densely populated Caribbean Sea, which makes it a hot-spot for coral diseases.3.5 Similar increases have been observed among other coral reef organisms, such as sea urchins, gorgonians and coralline algae. Already since the mid-1970s did the scientific community realize that corals and other reef organisms are susceptible to diseases caused by opportunistic pathogens and parasites as well as conditions caused or aggravated by exposures to anthropogenic pollutants and habitat degradation.
The flourishing reef ecosystem seems incapable in coping with the current global changes. Numerous publications, documents, travel reports, etc. not only mirror this trend but also triggered an increased awareness as well as sensibility that something is happening at an alarming rate (fig.3.2b). The manifold varieties, the frequencies, and the intensity of anthropogenic influences have likewise soared in this period, triggering a process that brings about massive changes to reefs worldwide. There are hardly any reefs left that still do reflect an undisturbed constitution - on the contrary, almost every reef reveals in one way or the other effects that are the result of adverse impacts on both a local as well as on a global scale.3.8

Fig.3.0: Human's ecosystemic grip (80kB)

Fig.3.1: The demographic pressure (190kB) 3.6

Fig.3.2a: Global distribution of mangroves and coral reefs (230kB) 3.9

There can be no doubt that reefs are being exposed to ever-increasing stress levels. These are not only the mere result of developed nations that spread their influence to embark the entire planet, but include developing nations alike as they struggle to "catch up" with modern lifestyles.3.10 Human activities have and still affect tropical coral reef ecosystems far more than has been generally recognized. Our fossil-fuel driven society perturbs the long-term cycles; i.e. unsustainable extraction of fossil fuels, their combustion, and the corresponding release of enormous quantities of CO2 into the atmosphere. Along with already naturally existing sources, these processes convert "solidified" carbon from long-term storage in the Earth’s crustal rocks to gaseous CO2 in the atmosphere (fig.3.2c). At the same time, we are cutting, slash-burn vegetation, and expand agricultural practices. Hence, humans reduce bio-accumulated carbon in vegetation to atmospheric CO2, further reducing the removal-rate of atmospheric CO2 (to be stored in short-term deposits such as vegetation). By perturbing both long- and short-term cycles, human activities over the past 100 years have increased CO2 concentrations in the atmosphere more than reef growth increased CO2 in the past 150·E3 years.3.11 In this sense, reefs worldwide are hit twice, on the one hand they are damaged by increased radiative forcing and the resulting rise in sea-surface temperatures, while on the other the rise in CO2 directly influences the rates of CaCO3 production (compare Chapter-II, fig.2.11 & 2.12a).


Fig.3.2b: Documented and reported disturbances to coral reefs (70kB) 3.7a

Fig.3.2c: Fuel consumption since the 1890s (95kB) 3.7b

What is becoming abundantly clear is that recovery from stress (human or non) is affected by the combined and synergistic effects of duration and intensity of stressors acting on reef ecosystems. In order to better understand the implication of stress-induced responses, figure 3.3a depicts the various phases that result from external stimuli. Such a/biotic stressors affecting reef organisms include: increased solar radiation, fluctuating water salinities, predation, epiphytic growth, bacterial or viral infection, etc. This kind of interaction is commonly summarized as distress that tends to restrict the chance of survival; i.e. in which increasing physiological demands exerted upon the animal leads to an initial destabilization, followed by normalization and improved resistance or when destabilization is excessive, to chronic damage and eventually causes even premature death.


Fig.3.3a: Phasemodel of stress-responses (135kB) 3.12a

However, if impairment is merely temporary, the functional state is restored to its original level. If necessary, any damage incurred may be repaired in a phase known as regeneration. And again, if the limits of tolerance are exceeded and the adaptive capacity is overtaxed, premature damage or even death may result. Coral animals, entire reefs and even the planet’s ecosystems behaves more or less in this manner. Corals can die as a result of both natural occurrences and in/direct human activities. If coral cover is not replaced through the process of reproduction and recruitment, a reef will eventually degenerate. Once the coral population on a reef goes into decline, so will the rest of the community. Whether a reef is killed quickly by catastrophe or slowly by attrition (no population replenishment), the result is the same: the loss of the reef and all it has to offer.3.13 Most case histories of combined stresses suggest that reefs that are chronically disturbed by human influences, pollution, etc. and then affected by natural stress show very poor recovery following the second stress.
Since the interface between the surrounding seawater and the extremely thin living veneer of the coral animal is the MPSLayer (see chapter-II, fig.2.4 & 2.6), it is obvious that environmental conditions directly affect the chemical composition of this layer and therefore the fitness of the animal to grow, reproduce and to defend itself. Hence, it is not surprising that the chemical nature and quantity of mucus does fluctuate when corals are stressed, thus changing the MPSL environment, the microbial community residing on top of it and those enclosed therein. The MPSL is therefore a suitable indicator to monitor a coral’s state of health. Overall, the fitness of coral animals and in fact the resulting robustness of the entire reef is an important environmental indicator to monitor the state of health of the reef-ecosystem.

A/biotic Stressors mediated by human activities:
Since reefs are described as a temporal mosaic of communities in different stages (or recovery from various sources of disturbances), ecological theories suggest that the high diversity characterizing many coral reefs is actually maintained by intermediate disturbances. Such models have gained additional support from observations made during the last decade as it gave reef scientists the opportunity to study the effects of disturbance on coral reefs. Due to the dynamic equilibrium of coral reefs there is now general agreement among experts that neither the view that coral reefs are robust nor inherently fragile is true over all timescales. What became obvious over the last decades was a striking increase in the numbers of reports of coral mortality as a result of natural and anthropogenic perturbations. Increased awareness partially explains such increases; however, threats to reefs from escalating human disturbances during this period are the primary agents.3.14 Corals that suffer tissue injuries, immediately respond by secondary effects in the form of reproductive damages, reduced growth and a lowered ability to withstand stress, along with a shrunken hyperspace. Ultimately, such corals become increasingly less fit.

Under natural conditions, corals experience physical damage from a multitude of sources that include storm damage, sedimentation, emersion at low tide, competitive interactions, predation, and disease.3.15 Anthropogenically induced damages - be it physically or chemically – add to natural disturbances, thus pushing corals closer towards the distress phase (depicted in fig.3.3a). As a result the rising concerns over the decline of "reef health" have caused scientists to critically reevaluate what we do and do not know about reefs. The following categories serve as examples of how land- and water-based activities shift the tolerance limits of coral animals into a regime, which make it difficult for them to survive in the long-term. Since it is almost impossible to lessen the effect of natural disturbances (the effects of bioerosion and predation are dealt with in Chapter V), it is feasible to regulate anthropogenic disturbances to prevent further degradation of the coastal marine environment of the tropics.


Fig.3.3b: Coral Reef Resilience (70kB) 3.12b

Sedimentation: Coral reefs do much better along coasts occupied by dense vegetation, in particular mangrove forests backed with a vegetation belt that stretches further inland. Although corals can survive intensive short-term sediment loading, it becomes intolerable for lower-level events that last for extended periods of time. The Amazon River is such a chronic natural agent; the sediment plume discharged into the western tropical Atlantic creates a large gap in coral distribution thereby separating the corals of this region into two bio-geographic provinces.3.16 Actually, sedimentation gradually wears down the coral’s fitness causing it to metabolize faster than normal. The major sediment induced stress-events can be grouped into (1) smothering by blocking oxygen-carrying water, (2) abrasion, (3) shading that induces localized tissue bleaching due to starvation, and (4) inhibition of recruitment.3.17 Indeed, exposure to chronic sedimentation rates does reduce the life-force of the animal as it unnecessarily drains energy away from long-term purposes; e.g. energetic drainage in the form of excessive self-cleaning effort or increased tissue repair – an observation that was observed during dredging activities in Miami (FL, USA), Castle Harbour (Bermuda) and other sites, where sedimentation alone has triggered the expulsion of endosymbiotic algae, leading to tissue swelling and thus, rerouted energetic requirements of the coral animal toward the production of excess mucus.3.18 In fact intense coastal development in rain-battered environments (urban centers, ports, tourism) result in the removal of coastal vegetation and a change in flow patterns off the land, the amount of sediments and nutrients entering the water column tends to increase. The most obvious implications are stimulated algal overgrowth on inshore coral reefs and facilitating outbreaks of potential coral predators (e.g. Crown-of-Thorns starfish, Acanthaster planci);3.19 (the outbreaks of A.planci in the Red Sea - surrounded by deserts – originate from different forcing agents; i.e. those that promote increased larval survival, in particular due to eutrophicated waters – see below) In fact, depending on the type of human activity, rates of erosion can increase a 100-fold or more.3.20 Due to agricultural practises alone, the mean soil erosion rates in Asia alone averages 30-40t·ha-1·yr-1. In contrast, soil erosion rates in undisturbed forests range from 0.004 to 0.05 t·ha-1·yr-1.
As already stated in Chapter-I, natural stressors have significantly shaped the current coral distribution over the past 50·E6 years. Apart from natural and usually localized disasters, major changes in this finely tuned dynamic equilibrium coincided with the pandemic spread of modern agriculture, land management practices so widely applied since the onset of the industrial revolution, and most recently, impacts associated with increased visitation by tourists leading to anchor damage, diver damage, boat strikes and trampling.

Manmade adverse effects with their origins far inland are particularly true for smaller rivers running off steep slopes of high islands (fig.3.4). Such waterways have greater sediment yields (load per unit drainage basin area) than do larger rivers off the continents.3.21 A scientific study undertaken at Baicut Bay (Palawan island, Philippines) investigated the effects of land-based activities unraveled striking results:3.22 the gross revenue under the Stop Logging option (1) was more than double the gross revenue under the Continued Logging option (2). Although the gross revenues from logging under option 1 would be zero, fisheries and tourism - the more sustainable option 2 however, would generate larger and continuing profits.
It should be reminded though that adult massive corals are more resistant to sedimentation than branching or plating corals. The slow growth rates (and thus slower metabolism) of massive colonies more readily tolerate higher sediment loads. But for corals with a more fragile morphotype and in particular for the recruitment effort of any coral species, the total opposite is observed. Loads of carbonate silt not only act as a barrier and thereby preventing coral larvae from detecting the chemical signals of their preferred settlement substrata like coralline algae,3.24 they also severely affect or kill young colonies before they can even grow above the level of frequent sediment motion (compare mortality rates versus age of fig 2.2 in Chapter-II).


Fig.3.4a: Sedimentation (180kB) 3.23a

Fig.3.4b: Sedimentation (115kB) 3.23b

Approximately 50% of the global sediment input has its origins from the continental high islands of PNG, Philippines, and Indonesia, while some additional 25-30% originates from Southeast Asia, with the rest being shared among the remaining oceanic domains. In fact, the bulk of sediments are introduced into the areas with greatest diversity of coral-reef communities.3.25 This is particular worrisome as terrestrial runoff along tropical shorelines, as a result of agricultural, mining, or building activities led not only to major increases in the amount of sediments, but also introduced excessive nutrients, pesticides and herbicides into adjacent coastal water columns. These have a range of effects, including algal overgrowth and potential coral kills from the associated toxins.

Eutrophication: Since tropical reefs flourish in oligothrophic waters, eutrophication of reef waters - that usually goes hand in hand with sedimentation - is considered to be one of the main causes for their decline; i.e. increased nutrient load in the form of both as inorganic ammonium and phosphate as well as organic soluble and particulates in the form of phytoplankton and associated zooplankton alike (see fig.3.5a). Such increases in nutrients, organic matter, and plankton biomass open up a new front to which corals are incapable of defending themselves: eutrophication favors endolithic bioeroders and algal proliferation. Since most bioeroders are suspension or filter feeders, they easily outcompete the calcifier populations.3.26 The long-term consequences of nutrient influx become manifest by the resulting shift from a coral dominated reef to one in which algae and filter feeders such as sponges and soft corals prevail.3.27 In fact, the rates of photosynthesis in endosymbionts of soft corals (Octocorallia) are much lower compared to those in hard corals, and indeed insufficient to even cover the basic respiratory carbon demand (compare fig.2.4b – autotrophy versus heterotrophy, Chapter II). Such surprisingly low photosynthetic efficiency is also related to the ratio of the colony surface area to its volume (the greater the surface, the more energy can be retrieved via the endosymbionts - soft corals are "fleshy" animals, while stony corals form a relatively thin veneer, thus have a larger surface area to volume ratio, pushing them closer to autotrophy).3.28

Therefore, the photosynthetic efficiency of scleractinian is high, respiration quite low; the reverse is true for octocorallia, which explains why soft corals do much better in eutrophicated than in oligotrophic waters. In addition, high phosphate levels in the water easily shuts down the calcification mechanism (phosphate poisoning), in addition, higher nutrient levels favour (macro)-algae, which readily replace the coral-algae symbiosis for space and prevent larval settling.3.29
Since endosymbionts and coral animals are in a complex physiological balance (see fig.2.10b - Chapter-II), an increased nutrient input can cause rapid numerical increase of the dinoflagellates. Due to carbon limitation in the super-dense and multilayered algal population, the photosynthetic rate of the algae is significantly reduced. This leads to photoinhibition of the endosymbionts in the upper layer causing the colony to become darker and to absorb much more light.3.30 This overcrowding easily disrupts the physiological conditions of the coral, which may react by expelling the excess endosymbionts.3.31
Another fact working against the coral population in an eutrophic environment is the resistance to grazing of macroalgae. Eventually, under prolonged eutrophication pressure, filter-feeding animals outcompete even the plants and the remaining coral populations and become the dominant group of organisms.3.33 Higher levels of nutrient availability favor heavy infestation by infaunal borers such as Cliona spp., which will progressively destroy the remaining skeleton and remove any record of its existence. (see chapter V - Bioerosion).3.34


Fig.3.5a: Global Nitrogen outfall (90kB) 3.32a

Fig.3.5b: Sewage outfall (140kB) 3.32b

Currently, approximately four times as much sediment, nitrogen, and phosphorous enter the marine environment off the Queensland coast than before western-type agriculture began some 100 years ago (for a global trend, see fig.3.5a). Again, the bias toward nutrient enriched conditions work in favor of coral predators such as A.planci.3.35 In a field experiment on One Tree Island (GBR, AUS) in 1971-72, a small lagoonal patch reef 25m in diameter was subjected to a concentration of 20µM nitrogen over 8 months. Within this short period, a 25% increase in primary production and a 50-60% decrease in reef calcification has been noted, confirming the adverse effects on coral reef communities found in earlier observations.3.36
From 1972 to 1987 nutrient concentrations along the coast of Barbados (Lesser Antilles, eastern Caribbean) increased substantially, caused dramatic changes in community structure on affected reefs. Such changes include reduced coral species diversity (e.g. a reduction in skeletal growth in the massive coral Montastrea annularis, and a reduction in reproductive efforts of the finger coral Porites porites.) whereas abundances of macrophytic and filamentous algae at polluted sites soared.3.37a
Similar experiences have been made in Kaneohe Bay (Hawaii, USA) from 1963 to 1977, when secondary sewage was discharged into the bay from three outfalls amounting to a total peak flow of 19·E3 m3/day (see also fig.3.5b). Sewage enters reef systems as point sources from rivers and outfall pipes as well as non-point sources such as ground water or runoff from spilling sewers after heavy rain. Apart from the solid and liquid human waste fraction, which contains nutrients, bacteria and viruses, sewage also contains synthetic hormones (e.g. contraceptive pills, hormone replacement therapy, etc.), which also contribute to an increase in diseases associated with contamination of marine life from.3.37b Over the years, this massive nutrient input made the green bubble alga Dictyophsaeria cavernosa a major component of the community. By 1977 particle feeders were the dominant members (e.g. zoanthids, sponges, and barnacles). Only by 1979, when the sewage outfall was directed offshore, did the previously heterotrophic reef flats gradually swing back from filter feeder- to a macrophytic-dominated algal community (1982). Eventually, another three years later did the algal populations decline and coral recruits were observed over all the reef flats monitored.3.38

Since mariculture is a recent development to enhance productivity, deliberate input of nutrients proves to be particularly problematic. As with waste-water discharge, fish farming activity does also contribute to nutrient enrichment. As the experiences of the northern Gulf of Aqaba/Eilat showed, (fig. 3.6) phytoplankton populations experience dramatic growth rates (can double every 1-2 days) and are the prime substrate zooplankton feeds on (includes larval stages of corallivores), with fish ultimately preying upon these secondary consu. Thus, the nutrients that were passed on to higher trophic levels in the food chain are distributed over wide areas of the northern Gulf. Eventually, decaying organic matter accumulates in the depths and across the northern Gulf.


Fig.3.6: Mariculture (135kB) 3.39

However, it just does not stay there - during winter, cold surface waters sink to deeper layers creating vertical mixing, "pumping" nutrient-rich waters back to the surface, resulting in "algal blooms" and competitive exclusion of stony corals by algae. During the winter months of 1992, vertical mixing resulted in excessive eutrophication of the upper illuminated reef layers, generating wide-scale algal blooms and suffocating the corals of the northern Gulf. Eutrophication in the Gulf of Aqaba / Eilat has resulted in an increase of epizoic activity, especially of filamentous algae and caused a decrease in coral cover by 25%.3.40a Since such an additional nutrient input also boosts the larval and juvenile population densities of grazing organisms (e.g. corallivores such as A.planci – see Chapter-V, bioerosion) the once adult form are commonly observed as mass outbreaks causing entire reefs to be stripped of live coral coverage.3.40b

Unsustainable fishing & Mariculture: As outlined in Ch.1, fig.1.2, the trophic complexity of coral reefs exceeds that of pelagic systems, placing coral reefs among the most complex ecosystems on this planet.3.40c Coral growth is influenced at all levels within the reefs trophic network. Thus, by altering species composition of migratory fish species, which contribute to the dispersal of nutrients in coral-reef ecosystems, does influence coral growth.
Since there is a significant linear relationship between the number of fish species and the number of coral species among reefs, the fisheries of coral reefs these ecosystems are most vulnerable to over-exploitation and corals in particular are quite sensitive to harmful a/biotic stress-impacts.3.41 In fact, overexploitation of herbivorous / predatory fish jeopardizes the very existence of coral reefs as herbivores are indirectly responsible for the success of corals.3.42 In many so-called "threshold" countries of the tropics, the rates of urbanization are exceeding both population and economic growth. As a result, a large number of families concentrated in urban areas turn to fishing and gleaning of reefs as alternatives for salaried jobs that become increasingly scarce.3.43a
The combined effect of both excessive local subsistence fishers and the fishing industries put large pressures on fish stocks associated biota. Since fishes and invertebrates do have major influences on the integrity of coral-reef ecosystems, overfishing is not just restricted to a substantial change in species composition on a specific trophic level, but initiates major changes in ecosystem processes themselves. Overfishing does put additional strain to the system and will result in large-scale ecosystem-level effects.3.43b Ecosystem overfishing occurs when the removal of resource species – which are often keystone species - causes a substantial change in species composition of the underwater community and results in major ecosystem shifts (see fig.3.7b and fig.3.3b).3.43c

Obtaining information on catch, effort, and population of fishes is necessary for scientific fisheries management. However, fish-catch and harvest censuses of other species are prohibitively expensive. Under these conditions, it is illusory to demand some quantitative ideal, such as the maximum sustainable biological yield, maximum economic yield, or optimum yield so often featured in fisheries textbooks.3.43d Realistically only coarse estimates can be supplied – during the late 1990s, fisheries for the tropical Atlantic have been as high as 160-200 tons per km2 and 93-239 tons per km2 for the tropical Pacific.3.43e
Yet, some selected numbers are available; harvest from the reefs of American Samoa has been recorded to be as high as 26.6 tons per km2. But there was a 70% drop in CPUE (catch per unit effort) from 1979 to 1994, the number of fishes per hectare decreased by 75% from 1980 to 1994, and the relative abundances among Samoan species changed drastically, with a decrease in preferred species. Calculating the Jamaican trap-based fishing technique used to be 16·E3 tons per year (“conservative estimates”). But CPUE at Southeast Pedro Bank near Jamaica declined by 82% over a 15-year period (fig.3.7c). That the tropical Atlantic is an already heavily used and poorly managed area is further exemplified in the case of the Bermudas: fishing efforts there were considered “low”, but the total catch still dropped from a maximum of 678·E3 kg in 1986 to about 380·E3 in 1990. Estimates for the Western Pacific are likewise scarce - 16.5 tons per km2 and year in the case of the Philippines. However, in a 4 year study in which Philippine reef-fish populations were monitored, the numbers of adult fish declined by 80% and the number of species known to reach adulthood declined by 33%.3.43f And this shows the dilemma: as adult fish became harder to find, a greater proportion of the catch consists of juveniles – the so-called fishing down the food-web effect. As highlighted in fig 3.7a fishing down the food-web proofed to be disastrous to the complex reef ecosystem.
That overfishing of herbivorous fishes and triggerfish does substantially influence the decrease of living coral coverage was shown when repeated censuses were done along Jamaica’s northern coast; initially amounting to 70% in the 1970s it dropped to less than 5% in the early 1990s.3.43h


Fig.3.7a: Fecundity vs. body size in fish (160kB) 3.43g

Fig.3.7b: Herbivorous feeding pressure (140kB) 3.45

Fig.3.7c: Fisching effort (70kB) 3.43l

By harvesting the larger individuals of smaller, more rapidly maturing fishes, a greater portion of the breeding stock is taken than is perceived by the inexperienced fisherman (see fig.3.7a).3.43i Once the more prized species disappear the less desired species are targeted. In last-measure attempts and to obtain enough smaller fish to feed their families, fishermen turn to methods such as dynamite and muro-ami (cyanide fishing) that are especially destructive to the habitat and to the recruits of the fish population further reduce the already limited chance for the fish populations to recover (see below fig.3.8a-c).3.43j Reefs that have been treated in that way, reflect a drastically reduced living coral cover on the reef flat and in the lagoon by as much as 60%. The difficulty to operate with yields can be further underlined by data from reefs that where only “lightly fished” failed to sustain their promised yield.3.43k
Most scientists regard the dominance by the black sea urchin Diadema antillarum in Caribbean coral-reef communities as a rather "unique" condition, but an unilateral shift in favor of the urchin in the last half of the 20th century has been the consequence of unrestricted and overexploiting fishing practices on predators that feed on it (fig.3.7).3.44
It was not before the 1982 to 1984 period that the fragile dominance of this species became evident when a virus pandemic lead to the sudden loss of this sea urchin across the Caribbean. The high population densities of urchins that resulted from over-exploitation of predatory fish stocks do explain the stunning lethal efficiency of this viral strike. Since control factors that previously kept the urchin population in check have been removed, entire sections of the biocoenosis relied on a single species (i.e. D.antillarum). This further reduced the resilience of Caribbean reefs, thus hampering its capacity to absorb shocks. It just required a single viral species to turn an entire ecosystem up side down. With no other grazer to take the place of D.antillarum, macroalgae rapidly out-competed corals for space on reefs, and eventually dominate the substrata around Jaimaca and beyond.3.46

Since most fish stocks in tropical as well as temperate oceans are in major decline with an estimated 10% left in the “unexploited state”, a further removal of fish inventory will lead to a loss of functional groups (herbivores, predators, and keystone species) that are important in controlling other organisms within the ecosystem. In the short term, we cannot but expect widespread ecological changes.
It is not just the quantity of harvested fish that interferes with the well-being of entire ecosystems, it is also the way how it is done. The most aggressive of such non-sustainable fishing practices include blast fishing (use of dynamite, fig.3.8a) and cyanide fishing (the use of cyanide to stun fish, fig3.8b). Since most common food fish have air bladders (which give them buoyancy), a bomb thrown into the water ruptures their air bladders besides breaking bones. The injured and dead fish floating on the surface or sinking to the bottom are collected manually (see videoclip in fig.3.8c).
Cyanide fishing on the other hand is commonly employed to "harvest" live fish for the aquarium trade. This substance is one of the most lethal and rapidly acting poisons known to man. Among others, the two most notable comounds used are NaCN (sodium cyanide) and KCN (potassium cyanide). Generally, simple cyanic forms are more toxic than complex forms.3.47d Thus, a squeeze used to stun a selected species is already deadly for the myriads of minute species within the targeted area (see section pollution further below). In any case, both methods are widespread and have quite serious impacts on surrounding reef communities – their direct and indirect physical/chemical destruction of reefs and the impact affect the health of corals and associated organisms alike.
Unsustainable military use in the name of national security: Reef “dynamiting” on a larger scale in the name of national defense was conducted by several nations. The US however hit the jackpot of destructiveness – and did so even by acknowledging that such activities can only be classified as crime against humanity and the wider living system known as GAIA (World System Theory). Nuclear testing by the US alone produced an incomprehensible scale of reef destruction and contamination as a result of 82 nuclear detonations, particularly in the Marshall Islands District of the US-administered Trust Territory of the Pacific Islands, from 1946 to 1962.3.47e The "Mike Test" for example, conducted in 1952 at Enewetak vaporized the island of Elugelab and left a 70m deep, 1.9km wide crater and a deeply fractured reef platform. The subsequent "Koa Test" in 1958 caused the fractured reef next to Mike Crater to break away and plummet to the ocean depths. From 1977 to 1980 the US conducted a partial cleanup and rehabilitation of Enewetak at a cost of US $218 million.3.47f Work at Enewetak included removal of debris, derelict ships, piers, and other structures from the reefs in addition to burial of tons of radioactive material produced by 43 atomic and thermonuclear explosions. Cactus Crater on Runit Island, Enewetak, was formed by a single nuclear test in 1958. The crater was 10m deep and 120m across. The crater was filled with thousands of tons of radioactive material. When it became clear that the crater was too small to contain all waste, a mound was created and the top capped with a dome of 460mm thick reinforced concrete. Contamination on reef and island ecosystems at Enewetak and several other atolls is still pervasive. Even today, food grown in experimental plots still shows high levels of 137Cs.3.47g


Fig.3.8a: Blast-fishing (162kB) 3.47a

Fig.3.8b: Cyanide-fishing (134kB) 3.47b

Fig.3.8c: Blast-fishing video (1.5MB) 3.47c

Fig.3.8d: Thermonuclear Detonation (160kB) 3.47h

Pollution: With rising levels of industrialization and urbanization of the tropics, potentially harmful effluents easily make its way into adjacent coastal habitats. Solid waste such as plastic bags frequently choke turtles and fish, cover coral colonies and thereby block off sunlight required to keep them alive. Another form of problematic civilization-trash are discarded fishing nets ("ghost nets"), which continuously kill fish and other marine life long after they have been decommissioned. Even more problematic than the physical effects are chemicals dumped into the sea by ships or coastal developments. They lead to a build-up of compounds that poison corals and their associated organisms. As this is serious enough in the dry season, it poses a serious threat during the wet season. The combined effects of increased sediment load, nutrient input and toxins drastically decrease the stress-resistance of affected coral ecosystems. Common toxins in street runoff, including oil and petroleum products, cadmium from automobile tire wear, and miscellaneous heavy metals from industrial applications, including mining activities, highlight the need for sound watershed management. Agricultural pesticides and other chemicals easily bind to soil particles on land, but due to the slightly higher pH in seawater, they become rapidly dissolved when these particles reach the ocean. Chemical factors such as copper ions, cyanide, herbicides, pesticides and biological factors readily evoke the loss of algal pigments from symbiotic invertebrates.3.48

A particularly concerning example has been documented with Chlorpyrifos, a pesticide widely used on golf courses (fig.3.9). This chemical has been shown to negatively affect coral recruitment. Coral larvae are sensitive to chemical signals at levels that are well below detectable limits of current human technology. Experimental work has shown that crustose coralline algae easily take up this pesticide, and that coral larvae had statistically lower recruitment rates on substrates exposed to the chemical at concentrations even as low as 5ppb(!) than on untreated controls (i.e. 5 parts per 1·E9).

Fig.3.9: Fate of chemicals (145kB) 3.49

Thus, an environmental toxin that allows 100% survival of larvae over 96 hours, but prevents them from detecting an appropriate settlement cue and hence prevents recruitment, has the same overall effect on the reef as causing 100% mortality of the larvae.3.50 In similar fashion do minute concentrations result in a net photosynthetic reduction (100ng Irgarol/L aprox. 100ppm or 100 parts per 1·E6) of the herbicide Irgarol 1051, while concentrations exceeding 1µg/L after 2-8h exposure results in a complete blockage of photosynthesis at all irradiances. Already at environmentally relevant concentrations, this herbicide proofed to be a potent inhibitor of coral photosynthesis. Other compounds such as Diuron, an antifouling agent, is known to block the electron transport in PS-II. Chronic exposure to this substance will cause photo-inhibition even at much lower concentrations than observed with Irgarol.3.51 Corals respond to sodium cyanide with decreasing fluorescence, pointing at a reduction in the photochemical capacity. It also causes a long-term decrease in variable fluorescence of endosymbiotic algae. With the effect of NaCN on dark-adapted photosynthetic efficiency being light-dependent, exposure results in a series of responses similar to those seen during temperature-related bleaching (i.e. loss of their endosymbionts).
Unless coral colonies are not exposed to increased temperature, cupper-contamination for example of 11µg/L will not result in any harmful effects; however, this changes once the ambient temperature is raised by 4ºC. This shows that Cu will act as a stressor only when combined with another stressor.3.52 In addition, it is known that the behavior of pesticides and toxic substances can change over time and space as breakdown products are often found to be more stabile, while at the same time possess higher / lower toxicity, and biological activity. These are just few examples how corals and their associated biota are negatively affected by toxins.

Oil Pollution: The major problem though nowadays is chronic oil pollution (fig.3.10a). It not only damages coral communities by triggering higher overall mortality, but it also inhibits coral recruitment. Petroleum derivates harm the reproductive system of corals and result in a decrease in the number of breeding colonies as well as in the average number of ovaria per polyp (compare fig.3.16). It suppresses the viability of coral larvae and the number of planulae produced per colony as well. Oil pollution also alters the physical properties of the reef flat thereby suppressing settlement of coral larvae.3.53  
A practical example illustrates these effects; in 1986 at the refinery in Bahia Las Minas, a ruptured storage tank resulted in a major oil spill just east of the Caribbean entrance to the Panama Canal and introduced 8·E6 liters of crude oil onto nearby reefs. The immediate response of the affected coral colonies ranged from tissue bleaching as well as swelling and conspicuous production of mucus, breakdown products excerting a long-term distressing pressure (e.g. photoisomerization and methylation increases stability, toxicity, and biological activity of these contaminants). Indeed, corals affected by such spills showed decreases gonad size compared to colonies from unaffected reefs even years after the spill incident occurred. Oil pollution was found to abort the formation of viable larvae in brooding coral species. Thus, the long-term effects on heavily oiled reefs resulted in a net overall decrease in coral cover in the order of 76% at depths to 3m and by 56% at depths up to 6m. By then, estimated minimum times for recovery of the reef at this site were 10-20years on the assumption that no other events would further stress the coral populations.3.55 Fig.3.10b summarizes accidental oil spills since the early 1970s. Although the Mediterranean and the North Sea are amongst the areas worst hit, both the Caribbean and the Indo-Pacific are likewise sites of extensive oil pollution.


Fig.3.10a: Hydrocarbon Pollution (185kB) 3.54a  

Fig.3.10b: Global Oilspills (285kB) 3.54b

Salinity: As mentioned above, the introduction of huge amounts of fresh water, especially during the wet season is already a severe distressor for fringing reefs and exposed atolls. And still, most corals tolerate low salinities for a few hours or days, but salinity changes in combination with other stressors ultimately will lead to high coral mortality.3.56 Typically, coral reefs are limited to areas of reasonable salinity that fluctuates between 3.3-3.6%. Below these levels, carbonate build-ups are no longer achieved by scleractinian corals but are progressively dominated by vermitids, oysters, serpulids, and blue-green algae.3.57 Low salinity coupled with turbidity is the primary reasons why extensive coral reefs do not occur opposite the mouths of major rivers.3.58 On the other side, reefs do not generally occur above the 3.6% salinity range. Higher levels (>6%) create an environment in which only halophilic bacteria flourish.3.59
Recovery from an eutrophic to an oligothropic state in Kaneohe Bay (Hawaii) was well in place by 1988 when a freshwater inundation from a sever storm caused a significant reduction in salinity and resulted in considerable mortality of the recovering reef corals. However and under absence of other stressors, the rate of recovery in the years following the flood was quite rapid.3.60

Radiation: Due to both diffusion and absorption, light penetration into the water column decreases with depth. Light attenuation is also depended on the wavelength. Shorter wavelength UV radiation is attenuated most rapidly (the rate of attenuation decreasing throughout the UV region and into the visible).3.61a A similar trend is observable at the other end of the visible spectrum as red colors fade out quicker with depth than bluish hues – the further down one goes the more evident the blue-shift becomes – with red fading out quite quickly. As a result, photosynthesis and CaCO3 production drop rapidly under reduced light conditions.3.61b In order to cope with the reduced spectrum at greater depths, polymorphic corals change their shape from mounds in shallow water to plates at greater depths (fig.3.11a and fig.1.9b). The latter is more efficient for light gathering as it places all the photoreceptors along vertically facing surfaces, where light intensity is greatest. Fig.3.11b shows the radiation variability influenced by astronomical and stratospheric parameters (e.g. manmade CFCs). Ozone-related increases in UV radiation are consistently predicted for the Southern Oceans (with a net decrease of primary production in near-surface waters by a about 15%). However, the ozone hole is present for only about three months of the year, during the austral spring and is confined to the subpolar region. Yet still, the midday solar irradiance at the ocean surface at the equator in mid-January integrated over UV-b, UV-a and visible wavelengths (280-700nm) is approximately 524W/m2, in which UV-a radiation accounts for 11.7%, while visible light accounts for the remaining 87.8%. 3.62a


Fig.3.11a: Morphological adaptation to altered irradiance (250kB) 3.61c

Fig.3.11b: UV Irradiance (150kB) 3.61d

Although marine species living close to the surface have develop protective mechanisms to withstand the damaging effects of UV-radiation (see fig.2.7c) Chapter-II - the protective properties of ultraviolet (UV) absorbing compounds, the mycosporine-like amino acids, are well known for their role in screening out UV-b radiation), they provide only limited protection against the effects of the longer wavelengths of UV-a and blue light (UV-c is efficiently absorbed by the atmosphere and therefore is selectively removed from solar radiation before it reaches the ocean surface).
Indeed, reef zonation is in large part controlled by the ability of various corals to adapt to conditions of either very high or very low levels of light. Due to the ongoing changes in global climate, it is expected that the average low-latitude UV-b surface exposure will increase by 1-10%.3.62b Such increases are especially alarming as the short-wave (blue fraction) of the spectrum illuminates the euphotic zone of the oceans and is predominantly harvested with chlorophyll c pigments.
Since UV-exposure is largely an unavoidable, UV-induced changes affect the genome (DNA-damages are considered to be the primary lethal effect of UV radiation), and damage to other molecules (including proteins, RNA, and membrane-associated molecules resulting in inactivation of membrane transport functions). Furthermore, UV radiation damages also affect the photosynthetic apparatus of endosymbionts (in particular the photosystem-II energy transfer, thereby bleaching photosynthetic pigments).3.62c
Photoreactivation is one of the two dominant repair mechanisms for light-induced DNA damage, while the other repair mechanism is a light-independent process (dark reaction), also referred to as nucleotide excision repair. In this case, repair relies on the redundant information in the non-damaged complementary DNA strand to repair damage.3.62d
Thus, species living close to the surface have to develop protective mechanisms to withstand the damaging effects of UV-radiation. One such UV-protective molecule is mycosporine-like amino acid (MAA); it is a metabolite found in a variety of marine organisms. For this purpose, concentrations of MAAs of A.microphthalma tissues for example at 1m depth are significantly higher than those at 20 and 30m depths.3.62e MAAs are one of nature’s sunscreens, with 19 structurally distinct MAAs presently identified in marine organisms (the skeletal configuration of this molecule is shown in fig.2.7c, Chapter-II).
Experiments have shown that Symbiodinium cultured at high temperatures has lower concentrations of MAAs than cultures grown at lower, unstressful temperatures – indicating that MAA may be synthesized by the endosymbionts and forwarded to the coral host. Again, potential interactions of synergistically acting stress events can easily shift the balance from tolerance to severely unfavourable conditions. Severe stress causes premature death and when taking place on a large scale, this in turn results in a rapid decrease of CaCO3 accretion and in greater susceptibility of corals to bioerosion. Together, these factors can easily reduce the ability of a reef to keep up with rising sea-levels.

Climate change: Over the 4.7·E9 year lifetime, the luminosity of our sun has increased roughly 40% as part of the natural aging process of a star.3.63 As it is known from numerous sediment analysis’ across the globe, the climate throughout the last 3.5·E9 years was never completely unfavorable to life.3.64 Even the period some 600·E6 years ago, when the entire planet was covered in ice (Snowball Earth Theory) enabled autotrophic cyanobacteria and green algae to survive under meter thick ice.3.65 Since this corresponded with a drastic drop in photosynthesis, pCO2-levels eventually scratched the 10% mark causing the ice to retreat, and thereby enabling biological organisms again to respond by significantly reducing greenhouse gases in order to maintain a homeostatic equilibrium that is favorable for biota.
In fact 150 to 65·E6 years ago, atmospheric CO2 concentrations were about 5-10 times higher than current conditions (1.5-3%), causing polar regions to be ice-free with the high latitude belts enjoying temperate climatic conditions.3.66 Under such conditions, coral reefs stretched out further from the equatorial belt toward the poles. In our contemporary world however, coral reefs are generally restricted to sea-surface temperatures that oscillate between 18 and 36ºC (optimal range of 16-28ºC) confining their distribution to the tropics of Cancer and Capricorn. Since most coral exist near their upper thermal limits, even slight increases in seasurface water temperatures will have significant impacts on the distribution of corals in the affected area and ultimately bring about rapid changes in reef diversity and community structure.3.67 Concentrations of so-called greenhouse gases (such as carbon dioxide CO2) are much higher than at any other time in the last 400·E3 years.3.68 The effective residence times of CO2 is of the order of approx. 230. Many other greenhouse constituents like methane (CH4) have more potent greenhouse warming potentials (has a forcing factor 10 times that of CO2), but shorter residence times in the atmosphere. Overall, it is expected that the global surface temperature will increase over the next 25-50 years.3.69 So far, there is already extensive evidence of changes to the distribution, abundance and health of earth's terrestrial and aquatic ecosystems. As a result, species are again showing a "migrational" shift towards the poles, stationary ecosystems like coral reefs are experiencing increasingly stressful conditions with their associated populations declining as a result of the combined effects of climate change and other anthropogenic impacts.3.70a Several lines of evidence show that over the past few decades the tropical belt has indeed expanded.3.70b Indeed, poleward movement of large-scale atmospheric circulation systems, such as jet streams and storm tracks, could result in shifts in precipitation patterns affecting natural ecosystems, agriculture, and water resources. These far-reaching changes in the climate system include the oceans and biosphere may alter the Hadley circulation (see Ch.1, fig.1.1a) and in turn induce changes in the ocean circulation. These again will have important feedbacks on tropospheric climate, marine ecosystems (including fisheries) and biogeochemical cycles.3.70c

Rise in Sea-Surface Temperatures (SST): Since the beginning of instrumental records (around 1880), the global atmospheric temperature average has increased by 0.6±0.2°C, with the 1990s being the warmest decade on record.3.71 Enormous data collected over the past decades document a continuous increase in atmospheric greenhouse gases. Since the onset of the industrial revolution, anthropogenically emitted combustion products have been associated with the warming of the Earth. Although abrupt temperature changes are limited by the high specific heat capacity of water, the increase over the past 100 years has contributed to a rise in sea-surface temperatures in some regions by up to 3°C (fig.3.12a).3.72

More recent evidence indicate that most of the warming observed over the last 50 years is attributable to human activities.3.73a Rapid industrialization on a global scale and the wholesale cutting of forests that absorb part of the CO2 will further aggravate the already stressed greenhouse gas budget.3.73b As mentioned above, the relatively stenobiotic temperature tolerance window restricts any large-scale adaptability to wildly fluctuating sea-surface temperatures. Reef-building corals live within 1-2°C of their upper temperature limits (vary geographically with water temperature). Coral growth decreases rapidly once the water temperature falls below 20ºC, whereas a temperature increase beyond 30ºC for prolonged periods of time triggers massive bleaching that can bring about mortality across coral species of invested entire reefs (fig.3.12b). During the single episode of mass coral bleaching (e.g. 1997-98 El-Nino event, alarming losses of living corals of 10 to 16% across the Pacific have been observed. Reefs in the Indian Ocean, which locally experienced much higher SSTs than other sites, lost an estimated 46% of their living coral cover (fig.3.12b).3.75

As a direct result of the increasing level of greenhouse gases in the atmosphere and the associated warming of the lower troposphere along with high irradiance and calm weather conditions (as it frequently happens with ENSO-events, see fig.3.12d), sea-surface temperatures are likely to cross more often the upper thermal threshold of reef corals resulting in the collapse of entire stands of monospecific coral gardens (fig.3.12c, see also abiotically induced tissue bleaching, TBL - Chapter-IV).
An initial database of bleaching observations was compiled by UNEP-WCMC using a variety of sources is accessible via fig.3.12e. This dataset also includes observations indicating no bleaching. You can add your observations of coral bleaching (or no bleaching) via our Bleaching Report.3.74e


Fig.3.12a: Computed SST-Rise (90kB) 3.74a

Fig.3.12b: Computed SST-Rise (103kB) 3.74b

Fig.3.12c: ENSO over the Seychelles (105kB) 3.74c

Fig.3.12d: SST-animation of the 1998 ENSO-event(2MB) 3.74d

Fig.3.12e: Bleeching & SST ( 3.74e

Due to the immediate short-term feedback onto our planet and onto human kind, global climate change should be of uppermost concern. The predicted continuation coupled with moderate or ceasing ocean currents facilitate mass tissue bleaching and further aggravates this alarming trend. As the adaptive bleaching hypothesis suggests (see ABH - Chapter-II), lost endosymbionts can be readily required (substituted with clades more tolerant to temperature stress) if exposure to elevated SSTs is limited in time. Regular bleaching in short intervals on the other hand causes the steno-biotic corals to suffer from chronic damages, causing corals to definitely wnter the distress phase and induces death even at sub-lethal exposure.3.76

Carbonate mineral saturation and Sea-Level Rise: Limestones are clearly important reservoirs of atmospheric CO2. But, one should not forget that rising sea temperatures and sea levels, and decreasing calcium carbonate saturation are likewise coupled and thereby changing the conditions under which coral reefs have prospered for at least 400·E3 years. Since the atmospheric partial pressure of CO2 keeps increasing, carbonate saturation in ocean waters must decrease (compare fig2.11 - Chapter-II). With less carbonate available calcification-rates inevitably must slow down, lowering the ability of reefs to accrete aragonite skeleton and thus to keep up with rising sea levels.3.77

Indeed, there is no doubt that reef communities will be affected by sea-level rise more profoundly than others. Theoretically, periods of rising sea level should be particularly favorable for reef builders as there is more space for accumulation of reef limestone.3.78 Reef flats constrained by present sea levels in protected waters might be expected to show increased diversity and productivity with progressively higher sea levels (so-called catch-up reefs). On the other end, deep-lying reefs may slip past the Darwin point (where reef accretion matches reef erosion), causing such reefs to drown, no longer able to keep up with rising seal-levels (so-called give-up reefs – see fig.3.13), whereas yet others may be subject of greater physical wave stress.3.79a


Fig.3.13: Give-up & Catch-up reefs (95kB) 3.79b

Sea-level rise must be seen also from another perspective. As the anthropogenic stress load on coral reefs keep increasing, bioeroding organisms are opportunists taking advantage of such systemic changes. Already now, reefs in the eastern Pacific are being eroded away faster than they are being deposited, reducing the reef framework to unconsolidated sediment on a large scale.3.80 The combined effects of increased bioerosion, thermal stress and reduced carbonate saturation in sea water results in a net reduction in fitness and obviously is also responsible for the observed increase in coral diseases (see fig.3.2b). Hence, the frequency and velocity with which such changes keep interfering with the stress-tolerance of both the coral animal and the endosymbiont do not match with genetic and site-specific phenotypic expressions; it rather seems that the coral-algal symbiosis is simply not able to adapt that quickly).3.81

Storm events and Adaptation to Wave energy: While a heavy surge causes physical abrasion by moving sediment and thereby causing substantial damage to coral tissue, a heavy storm results in large-scale damage by fracture and as such does have an important role in determining the character of a reef (see fig.3.14). Thus, the major controlling agents are storm frequency and the regional distribution of total wave energy. Accordingly, reefs can be grouped into three different types:3.82a

  • Type-I regards areas of high prevailing wave energy coupled with moderate-to-frequent storm disturbance; these areas are characterized by coralline algal ridges and where thick-branching branching corals dominate, such as Acropora palmata in the Caribbean or A.robusta in the Indo-Pacific.
  • Type-II reefs regard areas in which high wave energy prevails but storms occurring less frequently. Hereby, nutrients and waste products are cycled through the system, exerting a primary control on the character of the shallow-water reef crest and the zonation along the front. So, when a severe event eventually does occur it fractures branching and semi-branching coral colonies and compacts them into piles upon which coralline algae can recruit (fig3.15).
  • And finally, type-III areas with low prevailing wave energy but frequent storm occurrences. These are sites with open pavements and scattered coral cover, facilitating grazing and thereby effectively reducing the success rate of coral recruitment.
  • Operating at most levels of organization, the mass of organic material in a coral-reef ecosystem is the result of opposing processes of production and decomposition. As a result of homeostasis (self-regulation), at the highest levels of biological integration (i.e. ecosystem), coral reefs exhibit relatively low net rates of biomass accretion or erosion (even though large imbalances may concurrently exist at lower levels of organization, e.g. with a population or organism). The fragility of reef-ecosystems becomes evident when major disturbances such as catastrophic storms or changes in water quality result in large deviations from modal flux rates, leading to rapid reef destruction or accretion (growth).3.82b This balance of production and decomposition of reef biomass is closely balanced (with net accumulation barely ahead of net reef loss); it regulates and influences ecosystem function of growth and decay.3.82c

    As coral colonies mature, so does their vulnerability to breakage and dislodgement increase. Rapidly growing tickets of branching corals ("weedy" species such as Acropora formosa) are especially vulnerable to cyclones, whereas slow-growing massive corals (such as Porites sp.) become vulnerable only in the aftermath of cyclones when bioerosive forces undermine the integrity of the skeletal framework. So here is the paradox. Although coral reefs are the most productive communities in the sea and have the ability to create the most massive structures in the world, the highest diversity of natural communities will be maintained only in areas suffering moderate disturbances on the scale of frequency and intensity (fig.3.14), an observation summarized in Connel’s intermediate-disturbance hypothesis.3.83
    Corals in fact are capable to adapt to many environmental conditions. Such adaptation is outlined by the "r and K" characteristics of the species involved. A K-selected coral population lives in a habitat that experiences very little random environmental fluctuation. As a consequence, a crowded population of fairly constant size is established. By contrast, an r-selected coral population lives in a habitat, which is either unpredictable in time or short lived. Such a population experiences benign periods or rapid population growth and are usually found among type-III reefs.3.85

    Between December 1982 and April 1983, no less than six cyclones hit French Polynesia. Damage due to sedimentation and mechanical breakage on the Tikeuah atoll varied from 50 to 100% and was a function of depth and coral morphology (highest among branching, lowest among massive species). Due to storm-induced waves, coral mortality ranged from 60 to 80% at depths between 12 and 30m, whereas below 35m and due to the destructive forces caused by coral rubble avalanches, 100% mortality was documented.3.86 On low-angle slopes damages to deeper sections of the outer reef was almost inexistent. Obviously the susceptibility of various coral species to storm damage is a function of age and "r versus K" characteristics.


    Fig.3.14: Connel’s intermediate-disturbance hypothesis (70kB) 3.84

    Fig.3.15a: Coral rubble (125kB) 3.87a

    Fig.3.15b: Framework regeneration (160kB) 3.87b

    Coral Regeneration:
    The tolerance limits outlined in the stress-phase-diagram of fig.3.3 determine whether coral animals (as with any living organism) are capable to deal with external stressors, to initiate repair mechanisms once damage has been inflicted or to subside and die of the pressure is permanent. The combined effects of abiotic and biotic stress factors, their type(s) and the intensities temporarily compress or even shift the optimal range along the environmental gradient of survivorship.
    The energy reserves and the energetic requirements provided (by the endosymbionts, via absorption of dissolved organic matter by bacteriovory and active predation; compare fig.2.10b - chapter-II) affect the rate and ability to regenerate after exposure to stress momenta. Physical damage, either as the amount of tissue lost or size of an inflicted wound, along with the ability for repair are delimited by the amount of fully functional tissue left intact within the colony and thereby dictates the coral’s ability to recuperate. Thus, tissue regeneration will be faster in larger colonies than in small ones (larger corals simply have greater energy and tissue reserves).3.88 Once the animal senses a lesion, specific amoeboid cells (wound or mesogleal cells) are the first cell-types that migrate toward the site of injury and are involved in the task of cleaning up cellular debris. Moreover, cells that are normally not phagocytic become activated thereby aiding in that process. As will be discussed in chapter-IV, it is likely that there is a reduction of amoebocytes engaged in tissue repair in disease-affected corals. Damaged tissue induces a shock-response to the coral animal. During the following stages (assuming that the colony survives) the site of repair is characterized by an immediate but temporary cessation of calcification. During this period, necrotic tissue surrounding the wound is sloughed off, while healthy tissue surrounding the wound stretches out to cover as much of the wound as possible (compare with the opposite reaction observed in SDR - Chapter IV). During that process, the cells involved not only do change their shape but also “thin out” in the attempt to cover the exposed skeletal elements. The duration of these processes does affect the ability of the coral to regrow tissue - in that the rate must be fast enough to compete with unwanted epibenthic settlers. It’s obvious that tissue stretching can only cover small lesions. Larger ones must be tackled by a different strategy, i.e. cell proliferation, and translocation, while torn epithelial tissues seal themselves off and form lip-like edges. In this case, the inter-connected canals of the coral’s gastrovascular system are used to transport gastrodermal cells (with or without healthy endosymbionts) toward the injured site. In either case does the animal respond by an overall increase in the endosymbiontic algal density surrounding the site of injury, thereby facilitating tissue regeneration.3.89

    The reproductive effort of polyps following damage to a colony depends on the polyp’s distance to as well as on the size of the damage inflicted. This suggests that there is a hierarchy in resource allocation, in that polyps closest to the damage invest more energy for repair than those distant from the lesion. This is particularly important as stem cells already committed to gametogenesis, for example continue to differentiate, while those cells which are still in the pluripotent stage are diverted toward repair.3.90
    A common exit strategy of modular organisms under "attack" involves asexual reproduction. The partly dead Fungia depicted in fig.3.17, (a solitary coral) develops acanthocauli, that apparently develop from the buried and damaged parent organism. Once the budding clone is developed, boring organisms break the disc and liberate the offsprings.

    Fig.3.16: Reproductive effort under Stress (95kB) 3.91a

    Fig.3.17: Reproductive effort under Stress (280kB) 3.91b

    The allocation of energy to tissue repair results in a decrease in the coral’s reproductive ability, especially among smaller-sized colonies (such as Goniastrea favulus), while older (larger) corals tend to be closer to their normal level of fecundity (compare video featuring the spawning YBD-affected coral, fig.2.15c, Chapter-II). For large-polyped forms though (such as Lobophyllia corymbosa), polyp size is more important than colony size – here damage of one polyp does not necessarily reduce fertility of the neighbouring polyp. Branching forms (like Pocillopora and Acropora) on the other hand may require 2-3 years to regain their reproductive potential, while severely damaged massive corals (i.e. Porites) may require 4 or even up to 7 years.3.92 While the above is valid for corals in their optimal tolerance range (best adapted to a given environmental setting), it is somewhat different for diseased corals. In fact, the modular nature of corals has a significant impact on the dynamics of the disease.3.93 As will be discussed in Chapter-IV, the progress of the disease may be gradual (slowly progressing band diseases) or suddenly (shut down reaction). While a disease may impact the whole animal in unitary organisms, modular organisms "may suffer only partial mortality" (fig.3.17). As with the mechanically induced damages it likewise comprises colony fecundity. Thus, the animal can temporarily re-allocate its resources toward asexual reproduction, thereby increasing its size (number of modules) exponentially. Hence, delayed reproduction does not necessarily delay population growth. On the contrary, once the colony is allowed to fully recover, the number of descendant progeny produced by a zygote may well be greater when there is exponential modular growth followed by a burst of reproduction. It must be said though that such immediate response strategies to a/biotically-induced damages are only successful if stressors act on a short-term basis. Unfortunately, the trends witnessed on a global scale are predominantly within the chronic regime, thus inducing long-term changes on a global scale.3.94


    Please continue with Coral Diseases - Chapter-IV (Documented Diseases and Syndromes)