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Cumulative Effects, Synergistic Stressors, and the Decline of Coral Reefs


The Coral Reefs of the Caribbean

Dr. Thomas F. Goreau, who died in 1970 at just 45 years age, was a pioneer coral reef ecologist who began studying the reefs of Jamaica in 1951.  A faculty member at the University of the West Indies, he established the Discovery Bay Marine Lab, which still operates on the mid north coast of the island – one of the oldest research facilities in the Caribbean.  Many reef scientists got their start as graduate students working at Discovery Bay, and some of these have gone on to make great contributions to coral reef science in their own right.  One of these, Dr. Jeremy Jackson, just published a report on the decline of Caribbean coral reefs.  But I am getting ahead of myself.

In 1959, Tom Goreau published an article in the journal Ecology, titled “The ecology of Jamaican coral reefs, I. Species composition and zonation.”  It was illustrated with gorgeous underwater and aerial photographs that are unfortunately grainy messes in the pdf versions of his paper that I can now download from the web.  They are photos of another world in two very real senses – a world so very different to the high-and-dry world that most of us inhabit all our lives, and an underwater world totally unlike that at any location that now exists in the Caribbean.  It was a world of magnificent coral architecture stretching for miles along the coast of Jamaica.

Coral reefs grow in a more-or-less typical form  comprising at its simplest, a wide but very shallow reef flat with some living but lots of dead coral, a reef crest, the highest point facing the breaking waves, and a reef slope that may be divided into several zones covering different depths.  In the beginning, which for coral reef science means the mid-20th century, corals were so abundant on typical Caribbean reefs, that when people like Tom Goreau set out to describe reefs, they gave names to most zones in the structure of the reef based on the dominant coral species present.  Thus, Goreau recognizes the ‘reef flat’ or Zoanthus zone, the palmata zone, the buttress zone, the cervicornis zone and the annularis zone.  Goreau wrote of the palmata zone:

In the upper, or breaker, region of this zone, the Zoanthus overgrown reef flat abruptly gives way to a narrow zone which is populated almost exclusively by huge tree-like colonies of Acropora palmata that take the full force of the surf.  The great serried outliers of this coral are predominantly oriented in the direction of the prevailing seas which thus give the whole zone the characteristic appearance of a great jagged comb with irregular teeth. ….. In front of the breaker zone, the reef slopes gently downward to depths of between 5 and 6 meters. …..  Acropora palmata is still the dominant coral, growing in large isolated heads that also are strongly oriented into the prevailing seas.”

A palmata by sailn1-flickr

Imagine miles of reef front composed of a magnificent rampart of Elkhorn coral (Acropora palmata), a species now on the Endangered species list.  Goreau’s reefs no longer exist.  Photo © sailn1/flickr

Notice his phrasing – “huge tree-like colonies,” “great serried outliers.”  Elkhorn coral (palmata’s other name) can indeed be giant and tree-like, with stout branches as big around as a human arm or leg, soaring upwards before fanning out to create archways as much as one to two meters high, and sometimes spacious enough that a careful diver can squeeze in between.  Treelike indeed, and in great serried ranks stretching along the coast as a giant barrier protecting the shore from incoming waves.  Elsewhere in the article, Goreau refers to “the extensive proliferation of Acropora palmata that is so characteristic of the upper zones of West Indian reefs.”  He also talks about the buttress zone having its surfaces almost 90% covered by living corals, and refers to the “immense beds of staghorn coral, Acropora cervicornis” that typify the “cervicornis” zone.  From his words and his pictures comes a clear picture of a place dominated by colonies of living corals, a living architecture that provides the background to the lives of all those other creatures one expects to find on a coral reef.

Several of the scientists featured in Reef Reminiscences describe the Caribbean reefs they remembered, using similar phrases, and Judy Lang remembers her time at Discovery Bay as a young graduate student working with Tom Goreau.

Fast forward to 2014, and the new IUCN publication authored by Jeremy Jackson.  It begins by quoting well-known senior marine scientist, Sylvia Earle, who wrote in 1972:

Perhaps the most striking aspect of plant life on a coral reef is the general lack of it.  It seems anomalous to even the casual observer that tropical reefs, notable for their dazzling profusion of animal life, are almost devoid of conspicuous plants.

It then states, “Sylvia Earle’s early observations upon Caribbean reefs describe a forgotten world. Caribbean coral reefs have suffered massive losses of corals since the early 1980s due to a wide range of human impacts including explosive human population growth, overfishing, coastal pollution, global warming, and invasive species.  The consequences include widespread collapse of coral populations, increases in large seaweeds (macroalgae), outbreaks of coral bleaching and disease, and failure of corals to recover from natural disturbances such as hurricanes.”

Status and Trends Caribbean 2014

Cumulative Effects

Welcome to the world of cumulative effects.  That phrase refers to the fact that different stressors acting on an ecological system have a total effect that is larger than the effect of any single stressor.  For example, the impact of one hurricane on an otherwise healthy coral reef will be some degree of physical damage, the extent of which depends on storm intensity, tidal phase and direction of storm travel at the time of impact.  That damage can be repaired by regrowth of corals in following years.  A succession of hurricanes of similar strength, hitting the same reef over a several year span will cause greater damage, and might even hammer the reef to rubble – each successive storm will further damage an already damaged reef structure.  Similarly, if a reef is subject to pollution from coastal agriculture, damage will be caused, and a hurricane will further damage this reef.  In each case there are cumulative effects.

Sometimes the cumulative effects are no greater than the sum of the impacts of each of the stressors, but the stressors may also act synergistically, meaning that one affects the degree to which the other has impacts.  For example, the pollution may weaken the reef, making it more susceptible to the physical damage caused by storms than it would otherwise be.

Caribbean and all other coral reefs have been suffering cumulative effects of human activities for a long time.  Overfishing removes organisms with important ecological roles, such as the parrotfishes that graze algae and scrape away at dead and living corals to generate newly exposed, clean rock surfaces that may be particularly suitable settlement sites for new coral larvae.  Pollution from domestic and agricultural run-off adds nutrients that favor growth of algae that may in turn smother corals, shade them out and ultimately kill them.  The pollution may also make the corals more susceptible to diseases.  Destruction of reefs through inappropriate coastal development, fishing using traps or explosives, anchor damage, and coral mining for use in construction projects, may further damage already damaged reefs, or may happen at rates that exceed the reef’s capacity to repair itself through new coral growth.

The story of Caribbean coral reefs is a textbook example of cumulative effects in operation, and the depressing results are there for all to see.  Jackson and colleagues report the results of a careful analysis of the various monitoring data from across the Caribbean and from the early 1970s until now.  Despite differences among data sets in data quality or sampling methods that make an analysis much more difficult, they have been able to develop a reasonably precise estimate of the extent to which coral cover has declined, and to examine the different patterns of decline from place to place across the region.  Coral cover across the region, based on the most recent data available is now on average 14.3%.  Abundance of various fleshy algae (nearly absent in Goreau’s time) has grown to an average of 24% coverage of the reefs over the same period.  Coral cover has declined from an average of 34.8% during 1970-1983, to 19.1% during 1984-1998, and to 16.3% during 1999-2011.  The variation among locations is large, both in pattern of decline through time and in overall extent.

As the graphs below show, the pattern of decline was steep prior to 1984 for nine well-studied locations from the Dry Tortugas to Costa Rica including five sites within Jamaica, with little change after that time (Graph A).  The decline was almost linear across the full time period at five other locations in the Virgin Islands, Mexico and the northern Florida Keys (Graph B), and was essentially non-existent at seven locations, five along the southern boundary of the Caribbean and one each at the Flower Gardens Bank in the northern Gulf of Mexico and at Bermuda well to the north.  Clearly, the loss of coral cover has not been uniform across the Caribbean.

Status & Trends Fig 4 A-C

Status & Trends Fig 4 D resize

Figure 4 from the IUCN report, Status and Trends of Caribbean coral reefs, 1970-2012, showing in Graphs A, B, and C the different patterns of coral loss at 21 well-studied sites located at the sites shown across the Caribbean.  Image © IUCN.

The lack of consistency in pattern is not surprising.  The relative intensity of such stressors as overfishing, pollution, and physical destruction varies from place to place, and the reefs themselves are in different oceanographic settings – close inshore, near a polluting river mouth, bathed by clean oceanic water, facing or protected from prevailing winds.  The cumulative effects, not surprisingly, also differ.

The report evaluates the various stressors, and possible interactions among them.  For example, certain coral diseases have been of major importance in Caribbean reef decline, and have virtually wiped out Elkhorn and Staghorn coral (they are both on the US EPA Endangered Species List).  There is some scientific evidence suggesting synergy: diseases appear to have been facilitated by the exuberant growth of algae and/or by pollution.  This is also the first major report I have seen to make the case that in some places tourism has had significant negative effects on reefs.  Overfishing is particularly interesting, because it may be that the removal of important grazers, particularly the parrotfishes, has facilitated the growth of algae that in turn has contributed to the loss of coral through competition for space on the reef, and inhibition of settlement of new coral larvae.  The effects are not only cumulative, there are many of them and they seem to be interacting in complex ways.  Naturally, the chief cause of coral decline in the Caribbean is our growing numbers, and growing desire to live near, and make use of reef systems.

One result that is at first surprising is that, in the Caribbean, bleaching and subsequent coral death due to the warming of climate change has been of less importance than several other stressors.  There is no doubt that bleaching damage has been important at some locations, and scientists expect that bleaching, together with ocean acidification, is going to become a more important stressor over the next decades as we further warm up the climate, but up to now it has played only a small role in this part of the world.

Dumbing Down the Story

The IUCN report does a generally good job of reviewing the evidence for each of the stressors in turn, and makes the point clearly that Caribbean coral decline has been a complex process with multiple causes that vary a lot from place to place.  Unfortunately, when it came to writing the summary for the report, and even more so, when it came to writing the press release, the people at IUCN made the common mistake of dumbing down the results, focusing on only a couple of stressors, and suggesting (unless the words are read very carefully) that all we have to do is to begin protecting parrotfishes and coral reefs will recover.  Naturally, the media dumbed the message down even further – coral reefs are degrading because we have been overfishing parrotfishes.  And the climate change deniers grabbed the few sentences on climate change to trumpet that the worldwide decline of coral reefs had everything to do with overfishing parrotfishes, and nothing to do with CO2 pollution.

Global Coral Decline

So let me say it once more.  Cumulative effects are real.  The global decline of coral reefs is a classic example of the consequences of cumulative effects, and has different causes from place to place.  In the Caribbean, climate change induced coral bleaching has been one of the causes, but not a major one until now.  A number of other factors have been more important there until now.  These include the overharvesting of parrotfishes, and if we begin protecting parrotfishes that should be helpful.  Paradoxically, if the many marine protected areas (MPA) throughout the Caribbean had been properly managed, so that fish within their boundaries largely escaped fishing pressure, we would be able, today, to see the extent to which protecting parrotfishes could help reefs.  Instead, of course, most Caribbean MPAs are only really protected on paper, as are far too many MPAs around the world, and fish inside their borders suffer from overfishing just as do fish living outside their boundaries.

I’ve written before about the report in Science in 2012, in which Glenn De’ath and colleagues used a 27-year long data set to evaluate extent and causes of change in coral cover on the Great Barrier Reef.  Their results were equally as depressing as those in Jeremy Jackson’s IUCN report.  Cumulative effects were again in evidence, but the three main stressors were cyclones (hurricanes), outbreaks of the Crown-of-Thorns starfish which feeds on living coral, and bleaching due to climate change.  I earlier discussed how humans had a partial role in two and perhaps all three of these.

These relatively recent results for the Great Barrier Reef and the Caribbean represent the two best surveys of coral decline over large geographic regions.  There is nothing to my knowledge to suggest that most other regions of the world are not suffering similar patterns of coral decline, and I anticipate that cumulative effects are evident in every location.

There is one more thing to say before leaving this topic.  If we want to restore coral reefs, in the Caribbean or anywhere else, we have to take action to reduce the impacts of at least some of the stressors acting.  We have to take real action, not plan to take action, not pass legislation or write new regulations.  We have to actually take action to alter our behavior around reefs.  This is good news!  Because it means there are actions we can take that will make a difference.  It’s not rocket science, but it does take a real commitment.

We have a serious problem that can be mitigated if we want to do so, because most of the stressors act locally.  While it is going to require major international action to reduce CO2 pollution, better regulation of fishing, of pollution, of coastal development, and of tourism can all be done locally, on single stretches of coast, where the community and the government want to make a difference.  It’s a serious problem, but a very fixable problem!  But not fixable if we only pretend to act, or if we grab at one cause only and run around telling people that saving parrotfishes will save the reefs.  It is really time now to get real.  Do we really want to save some of the world’s coral reefs?

Adding to the Cumulative Effects

One thing we should not be doing is adding to the cumulative effects.  Unfortunately, in Australia right now that is precisely what government action seems destined to do.  Australians value their Great Barrier Reef.  It is well managed at considerable annual cost, and it generates far more than it costs in the tourism it drives year by year.  It is a UNESCO World Heritage Site, recognized around the world.  I thought it was in safe hands.

This was not always the case, and I describe in Our Dying Planet how Australians changed their attitude to the reef and to the environment in general during the mid-70’s, all because a tiny NGO called the Queensland Littoral Society produced a bumper sticker with the slogan, Save the Barrier Reef.  That slogan led ultimately to legislation banning oil prospecting over reef waters, and to the formation of the Great Barrier Reef Marine Park and the Federal Agency that manages it in close cooperation with the Queensland state government.  I have long claimed the Great Barrier Reef to be the best example of reef management in the world.  It has a strongly science-based management process.  Among notable achievements, it now has 33% of its area protected as no-take fishing exclusion zones, and managers have been able to work with agricultural interests to modify the farming practices along the length of Queensland to reduce fertilizer and pesticide run-off and restore water quality for reefs hundreds of kilometers away from the nearest sugar cane fields.  That kind of reach beyond the borders of the area of jurisdiction is very rare in the world of marine protection.


A bumper sticker which saved the Great Barrier Reef in the 1970s.  Image © AMCS

But Australia is also a nation rich in coal and other mineral deposits and resource exploitation and export is a major component of the economy.  A significant battle has been going on over the past few months because the ‘need’ to expand port facilities to export more and more coal to China has run up against the need to protect the Great Barrier Reef.  The change of government, late last year, bringing a climate change dismissing (if not denying) government to power in Canberra has tilted the playing field back in the direction of coal and away from conservation.  In unrelated legislative activities, Australia has just cancelled its carbon tax – a backward move that may be substantially more serious than Canada’s abrupt withdrawal from Kyoto several years ago.

I started noticing media reports at the start of 2014.  An article in the Sydney Morning Herald for 4th February reported that plans called for an increase in coal exports from 240 million tonnes in 2013 to 787 million tonnes in 2030.  To do this will require expanded and new shipping terminals along the Queensland coast – and that means inshore of the Great Barrier Reef.  A report released by the Australian Marine Conservation Society early in 2014, Dredging, Dumping, and the Great Barrier Reef, identified nine port construction or expansion projects being planned.  While the dredging planned at Cairns is in order to permit entry of larger cruise ships, the others all relate to the coal export industry.  They range from Cape York in the far north of the reef to Gladstone at its southern end – they could not be more widely distributed along its shoreward side if they had been explicitly designed to be so.  The report estimates a total of 83 million m3 (or 149 million tonnes) of material to be dredged and then dumped offshore, but still within the Great Barrier Reef region (there is lots of open space between and inshore of the reefs).  The Guardian commented on the situation on 7th May, drawing attention to the mining industry’s claim that coral decline has been caused by bleaching, Crown-of-Thorns, disease, and cyclones (quoting De’ath’s article), and inferring therefore that dredging does no damage (the logic of mining companies is sometimes a bit shaky).  Dredging planned for the future is not going to show up as a cause of coral decline in the past unless Australia exists in a really unusual universe!  The Guardian article also mentioned a study of effects on corals of smaller scale dredging at one of the ports, and the inadequacy of the methods used to assess possible damage – needless to say, the mining industry quite likes to reference these inconclusive data.

New Scientist for 26th May carried a short article by Dr. Jon Brodie of James Cook University that looked specifically at one port – Abbott Point, currently the most northerly coal port on the Queensland coast, located 25 km north of the town of Bowen, and slated for expansion requiring 5 million tonnes of sediment to be dredged and dumped further offshore (but still inside the reef).  Brodie notes that when the plan was announced, the Environment Minister, Greg Hunt, stated that the project would not reduce water quality over the reef.  This is because the approval requires that the impacts due to sedimentation by the dredged material be ‘offset’ by improvements to agricultural practice to reduce sediment run-off in two nearby rivers.  As Brodie noted, to fully offset the delivery of 5 million tonnes of sediment requires that the waters of those rivers must have their sediment loads cut by 5 million tonnes of sediment.  Unfortunately, they carry only 6 million tonnes a year, so cutting that by 5 million becomes “a tall order”.  I’d call the pretense that this is a solution a tall tale.

Recently, a post on coral-list by Dr. Terry Hughes of James Cook University alerted me to two recent articles from that institution that are relevant to the issue.  The first, by Joe Pollock and seven colleagues from James Cook University, Australian Institute of Marine Science, and other institutions in Western Australia and Queensland, was published in the open access journal, Plos One, on 16th July.  Their study, funded as part of the required environmental impact assessment process, was a detailed study of coral health and survivorship at different distances from a sediment plume caused by dredging for another resource development project, the giant Gorgon gas project on the Pilbara coast of Western Australia.

Fig 1 Pollock resize

Figure 1 from Pollock’s article showing the incidence of diseased (red) or compromised (green) corals at sites exposed to the sediment plume and at sites away from the plume.  Numbers indicate number of days exposure to sedimentation.  Image © Plos One

Dredging for that installation amounted to some 7.6 million tonnes of sediment over an 18 month period from May 2010 to November 2011.  Their results show clear impacts of the sediment plume on coral health.  Primarily, corals exposed to the sediment plume suffered greater incidence of a suite of coral diseases.  In addition, the prevalence of a number of other morbidity factors was increased.  Tissue necrosis was 57 times more likely at sites within the sediment plume, and bleaching, overgrowth by sponges, and changes in pigmentation also increased.  Their conclusions are quite explicit:

This study provides the first empirical evidence linking turbidity and sedimentation with elevated levels of coral disease and other indicators of compromised coral health in situ.  We found two-fold higher disease prevalence, largely driven by increases in [‘white syndrome’, a common disease], and six-fold higher levels of other compromised health indicators at high sediment plume exposure sites.  Since these in situ health assessments were conducted more than 18 months after commencement of dredging, it is likely that the most susceptible corals experienced complete mortality prior to surveys being undertaken.  Therefore, these prevalence figures likely underestimate the true impact of dredging-associated sedimentation and turbidity on coral health”.

The second study, by Kathryn Burns of James Cook University, was published in Estuarine, Coastal and Shelf Science, and directly concerned the sediments being dredged within the Great Barrier Reef.  She examined samples of the sediments, and of material collected from the sediment plume caused by dredging at Hay Point, off Mackay, Queensland.  Looking for PAHs (polynuclear aromatic hydrocarbons – some of the nastier chemicals out there) she found ample evidence that PAHs derived from coal dust were in the sediments being deposited from the plume well out to sea and towards the outer Great Barrier Reef.  Her conclusions were also explicit:

The data shown here demonstrate that the coastal sediments offshore of the Hay Point coal port are already contaminated with coal residues which exceed the ANZECC/ARMCANZ (2013) toxicity guidelines and approach toxicity values under the US EPA guidelines.  If the US EPA guidelines had included the biphenyl and dibenzothiophene series which are also present in samples then calculated toxicity values would have been higher…”

Putting this all together, we have evidence that extensive dredging is going to damage the Great Barrier Reef through impacts on water quality, and that turbidity, smothering due to sedimentation, and noxious chemicals all play a role.  But we also have evidence that the industry and the government want to expand those coal ports, and are willing to claim there will be no damage to the reef.  The Australian government is playing the same ‘economy’ and ‘jobs’ cards being played by the Harper government in pushing expansion of Canada’s tar sands industry.  And the science and conservation community are doing their best to be heard.

Let’s Close with Some Good News

On 24th May, we learned that the HSBC bank had followed the lead of Deutsche Bank, and decided not to fund the expansion of port facilities at Abbot Point, citing unacceptable social, environmental and financial risks to investors.  Put into plain English, neither bank wants to risk its reputation by investing in a project that will damage the Great Barrier Reef.

On 20th June, Reuters reported that the Dudgeon Bay project, a new port facility near Hay Point intended to handle 180 million tonnes of coal a year, had been put on hold indefinitely.  The developers claimed that lower than anticipated prices for coal meant that demand for export facilities would not be sufficient to justify the expense.  It’s also possible they were just getting cold feet.

I do not, for one moment, believe this battle is over yet.  A number of port expansions remain in play, and the Australian government has certainly not seen any reason to slow down the rush to dig up and export every thing of value from Australia …. Boy, that does sound a lot like Canada, doesn’t it!

However, there is one more tiny bit of good news.  That tiny NGO, the Queensland Littoral Society, with its simple slogan, Save the Barrier Reef, that really did save the Great Barrier Reef back in the 70’s…. it changed its name a few years ago, and is now the Australian Marine Conservation Society.  It’s the same people!  You can read about its history on its web-site while you download its report on dredging.  It is still out there, saving the barrier reef.  With that kind of long-term commitment and dedication, and with the efforts from the science community to get the truth out about likely impacts, there is a good chance the reef can be saved yet again.

Putting this news about the Great Barrier Reef together with the story of coral decline in the Caribbean, I feel strangely invigorated.  Even though the threats due to climate change and ocean acidification still hang over coral reefs worldwide, there are many other stressors acting, and most of these are local.  It is possible to make a real difference now, by acting on each of these local stressors, by cutting pollution and by saving parrotfishes, by curtailing unwise coastal development and regulating tourism firmly.  A good start might be simply to protect all those unprotected MPAs around the world, while speaking out forcibly about any new projects to add to the difficulties faced by coral reefs.  Cumulative effects require cumulative efforts to stem the tide and prevent us from killing the reefs.  They are worth caring about.

A palmata by seestjohn-com

Fish sheltering under Elkhorn coral, St. John, USVI.  Photo © Gerald Singer