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Is There Any Future for Coral Reefs?


Back in March, I took a look at the state of climate change and the tasks facing nations if we are to limit average warming to 1.5oC.  Those tasks were, and remain, demanding.  Now we are enduring the covid-19 pandemic, and action on climate change has slowed perceptibly, even as the economic collapse leads to bluer skies, and much lower rates of CO2 emissions.  Meanwhile the warming has continued.

I have a book in press, due out in early 2021.  In it I refer to the increasingly slim chance that we will have anything resembling 20th century coral reefs by the end of this century, so I remain personally very interested in how coral reefs are doing these days.  Will that slim chance have vanished to no chance at all before the book even hits the market?  Or will something unexpected have happened that makes it laughable that we would be concerned at all for the well-being of these iconic oceanic wonders?

Bleached Acropora on the Great Barrier Reef.  It has happened again in 2020.  Image © ARC Centre of Excellence for Coral Reef Studies.

The warming has continued to cause trouble from time to time at locations around the world as sea surface temperatures reach critical thresholds for coral reefs.  This year, there was serious bleaching again along the Great Barrier Reef (the third major bleaching event there since 2016) as well as in some other southwest Pacific locations.  The media, preoccupied with Covid-19, paid little attention and it is hard to find reports of bleaching other than on the GBR.

The Great Barrier Reef bleaching event, like the ones that preceded it, however, has been particularly well documented because of the science and monitoring resources able to be thrown at it and the media have responded accordingly.  Once more, scientists from James Cook University were able to conduct aerial surveys along the full 1500 km of the reef region during the second half of March, making 11 flights in 9 days, surveying 1036 reefs and scoring them for extent of bleaching.  They also did some in-water ground-truthing of the surveys.  Work was curtailed as the country shut down because of covid-19.  Later this year, the James Cook team will be diving many of these reefs to determine the extent of mortality.

Of the 1036 surveyed reefs, 39.8% exhibited little or no bleaching (less than 10% of corals bleached).  But 25.1% were severely affected (meaning at least 60% of the corals in shallow water were bleached.  The remaining 35% showed moderate bleaching (i.e 10% to 60% of corals bleached).  Compared to the bleaching in 2016, there are more unbleached or lightly bleached reefs and fewer severely bleached reefs, making this the second most extreme event since the first extensive bleaching of the GBR in 1998.  However, in contrast to both 2016 and 2017, bleaching in 2020 has impacted reefs throughout the length of the GBR province.  At the far southern end, I’m told by others that shallow reef flat corals at Heron Reef were severely bleached but that bleaching deeper on the slopes was lower (20% to 40% of corals) and more variable.  Shallow corals at nearby One Tree Reef have also been bleached.  I’ll be watching for the official report on the surveys of this bleaching event.

These three maps show each of the reefs surveyed during each of the last three bleaching events along the Great Barrier Reef.  Red circles reflect >60% of coral colonies bleached.  Green circles reflect <10% of coral colonies bleached.  The remaining 35% of reefs surveyed, scattered among the reds and greens, are omitted from the figure but had intermediate levels of bleaching (10 – 60%). Image © ARC Centre of Excellence for Coral Reef Studies.

The most notable thing about Great Barrier Reef bleachings is that of the five major ones that have occurred, only 1998 and 2016 occurred during el Niño years when water temperatures might be unusually warm.  The others (2002, 2017, 2020) have occurred in non-el Niño years, showing that global warming has now reached the point, at least in the south-west Pacific, that typical temperature fluctuations due to local weather (heat waves) are sufficient to trigger bleaching.  February 2020 had the warmest sea surface temperatures ever recorded in the GBR region.

In the Caribbean, records have always shown that the degradation of reefs, as measured in loss of live coral cover, had been well under way long before climate change was on the scene.  This is also true of the GBR – until 2016, climate change was a distant third to explosions in numbers of Crown-of-thorns seastars, and cyclones, as causes of coral loss – but while significant loss of live coral cover was evident in the 1970s on the GBR, that downward trend in coral cover began in the 1950s in the Caribbean.  Climate change became important there in the 1980s and subsequently, but, unlike the Pacific, the Caribbean has suffered from a number of pandemic diseases of corals and other major organisms.  Diseases, while important in the Pacific and elsewhere, have been particularly destructive of Caribbean reef systems. 

Rapid progression of Stony Coral Tissue Loss Disease is shown in this time series of photos off St. Thomas, USVI.  Photo © Sonora Meiling and Science News.

There is a growing suspicion among scientists who study such things that the serious diseases prevalent in the Caribbean are themselves a consequence of the long-term degradation of reefs by a multitude of human activities.  The problem for corals is now being exacerbated by the warming due to climate change.  Among the most destructive causes for reef integrity have been the unidentified pathogen that virtually wiped out Diadema sea urchins throughout the Caribbean in 1983, removing a major reef herbivore that has never really recovered.  This facilitated a shift towards a much more algae-dominant reefscape.  White Band Disease, which was largely responsible for the near disappearance of Acropora palmata and A. cervicornis during the late 1970s and the 1980s, continues to crop up.  And Stony Coral Tissue Loss Disease (usually just called SCTLD) is a new one currently ravishing populations of about 20 coral species in the northern Caribbean (many of these species are among the ones less likely to bleach as water warms).

SCTLD, first seen in Florida in 2014, has since been sighted in many locations throughout the northern Caribbean, including sites along the Mexican Yucatan but not Belize, and as far south in the eastern Caribbean as St. Kitts and Nevis, according to data compiled by the AGRRA monitoring program.

Two recent papers provide new information on the current threats to Caribbean reefs (both are open access).  Aaron Muñiz-Castillo and Ernesto Arias, both from Centro de Investigación y de Estudios Avanzados del I.P.N. (CINVESTAV), Mérida, Mexico, and five colleagues from Mexican and U.S. labs, published an article late in 2019 in Scientific Reports.  Their article deals with the spatial and temporal pattern of heat stress faced by coral reefs across the Caribbean, and therefore with the way warming is influencing different parts of the Caribbean in different ways and to different extents.  Katie Cramer of Arizona State University, and six, chiefly U.S.-based colleagues published an article in Science Advances on 22nd April that concerns the temporal pattern of loss of the two species of Acropora.  Their article is sure to generate discomfort in a number of places because of the putative causes they list. 

Muñiz-Castillo and colleagues used remote sensing to plot heat stress variation over the period from 1985 to 2017 at sites throughout the Caribbean.  They showed, as expected, that heat stress was overall greater in more equatorial regions and in more recent years, but they revealed significant variation among locations in these trends.  They showed that heat stress does not map simply according to the ecoregions recognized across the Caribbean and proposed a new set of heat stress regions to be used when considering effects of warming in this region.  Among the most stressed, and most rapidly warming, locations are central and eastern Venezuela, the Honduran and Nicaraguan Miskito Keys, and the more southern Lesser Antilles along with the western Venezuelan coast, Aruba and Curaçao.  At the other extreme are the northern Lesser Antilles, most of the Mesoamerican Reef, and most of the Greater Antilles, the eastern Bahamas and Florida.  These least stressed regions may serve as heat refugia for coral reef systems.

The main message for me from their article is that patterns of heat stress vary significantly across the Caribbean and that, by knowing the local pattern, it is possible to infer likely relative heat stress in the future among locations.  All coral reefs, even within the Caribbean, are not experiencing the same thermal stress as the planet warms, and there may be opportunities to use the differences among locations in managing reefs more effectively.  For example, it would be wise to look after those putative heat refugia.

Cramer’s paper is a historical look at the decline in abundance of the two Acropora species found in the Caribbean – very historical in that it includes Pleistocene data.  When Tom Goreau first described the structure of Caribbean reefs in 1959, he named two of the nine zones he recognized as the cervicornis zone and the palmata zone because those two species tended to be overwhelmingly common at those parts of the reef.  He described the typical occurrence of A. palmata as starting at the top of the reef slope in “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 this region, Acropora palmata is clearly dominant and exists as a nearly pure population except on the sides of surge channels…  [To depths of 5 – 6 meters] Acropora palmata is still the dominant coral, growing in large isolated heads that also are strongly oriented into the prevailing seas”.  Needless to say, very few people diving today have ever seen those serried ramparts that formed the upper portion of most Caribbean reefs; that species, along with A. cervicornis, is on the endangered species list.

The crashes of those two iconic species were under way or about to begin as Goreau completed his article.  Cramer and colleagues have used a variety of types of data, from many locations across the Caribbean, and extending from Pleistocene times to the present.  While White Band Disease, which caused widespread die-offs of both species during the 1980s, has long been identified as a major factor, these new data show clearly that the decline in A. palmata was well under way at the time Goreau was working in Jamaica.

The proportion of reef sites with the species present (gray line) or dominant (black line), Pleistocene to the present.  The onset of White Band disease, and the Diadema die-off are both marked in red.  Stars mark the first period with a significant decline in abundance from Pleistocene levels.  Note that A. palmata is characteristic of reef crest sites and A. cervicornis is characteristic of midslope sites.  Image © K. Cramer and Science Advances.

At reef crests, the proportion of sites dominated by A. palmata declined from 78% during the Pleistocene to 6% in 2011.  Even by the 1950s, palmata abundance was significantly lower than in the Pleistocene.  At midslope sites, the prevalence of A. cervicornis declined from 63% of sites in the Pleistocene to 12% of sites in the 1960s.  Its prevalence has declined to <1% of sites at the present time.  Clearly, White Band Disease, or even White Band Disease and climate change are not the whole story here. 

In their search for drivers of these trends, Cramer and colleagues argue that several local human stresses have played major roles before WBD and climate change began their involvement.  In particular, they point to water quality, but their analysis is hampered by the lack of water quality data for reef locations even today.  (A little bit like Covid-19 testing, if you don’t monitor water quality, you cannot be asked or ordered to clean things up.)  The only long-term water quality data for the Caribbean – data on clarity for Belize and Puerto Rico – reveal a trend to increasing turbidity between 1993 and 2012.  Cramer and colleagues argue convincingly (at least to me) that the long-term decline of reefs coincides not only with increasing use of fertilizers on agricultural land, but also with increasing use of synthetic herbicides and pesticides.  Many of these are known to interfere with coral reproduction through effects on fecundity, larval development, and settlement success.  As well, human settlements near coastlines deliver a stream of pharmaceuticals and other chemicals through wastewater that can enter the reef environment.  These also have seldom been tested for their impacts on reef organisms.

I vividly remember a tiny pilot experiment Chris Metcalfe and other colleagues of mine carried out in the Yucatan in December-January 2008-2009.  With the help of experienced cave divers from the region, they deployed passive samplers in a series of cenotes that were downstream from tourism developments along the Mexican Riviera and in one cenote upstream from all development.  Water in these cave systems was all flowing towards the coast and would be expected to percolate up at offshore sites along the Mesoamerican Barrier Reef.  (Such sites are readily visible to a diver on the shallow forereef as the brackish water slowly mixes with surrounding salt water.)  They showed easily measurable concentrations of a variety of anthropogenic chemicals ranging from PCBs and organochloride pesticides to pharmaceuticals and illicit drugs.  Few of these compounds have been tested for toxicity to reef organisms such as corals.  Their results demonstrated clearly that runoff from agriculture and sewage from the hotel developments were entering the ocean along that shore.  But without time series of data, it is not possible to be certain that such chemicals have increased in abundance in reef waters – not possible to be certain, but does anyone really doubt this has happened as tourism has grown over the years?

In addition to onshore pollution, overfishing is the other local impact of people on reefs that could have played a major role.  Overfishing appears to have been rampant at many Caribbean sites but again there are not the long-term data across this region that would be necessary to test any trends in this factor against Cramer’s coral abundance data.

I think the main message from Cramer’s article is that while we spend plenty of time talking about the very real, and growing, link between climate change, bleaching and reef decline, we need to remember those other human impacts, that are still playing a role, although more locally.  They were sufficient to commence the deterioration of reefs before climate was a problem and mostly they are still acting.  The second message is the reminder that the trajectory for coral reefs in the Caribbean has been particularly severe, with diseases playing a much larger role than elsewhere.  The prevalence of these diseases is almost certainly also influenced by the various human stresses acting on reefs.

Coral-dominated reefs such as this one are getting harder to find as the various human-caused stresses act against them.  With loss of coral, the structural complexity of the habitat, and the rich biota characteristic of coral reefs disappears also.  Image © Robert S. Steneck.

A coral reef exists as an exquisite balance between calcification processes and a variety of forces of destruction, physical and biological.  As human use of the coastline increases, and as direct use of the reef increases, a multitude of human-caused stresses act to shift this equilibrium.  Whether we facilitate algal growth through overfishing of herbivores, reduce coral survivorship or reproductive success through pollution or climate change, or simply disrupt reefs physically through our sometimes thoughtless manipulation of coastal lands, dredging of navigation channels, or construction of artificial islands, our actions shift that equilibrium in the direction of reef degradation and decline.  We are now at a point where we have increased our impacts on many reefs so far that they are disappearing before our eyes.  My suggestion that we will not have anything resembling the coral reefs of the 1960s by the end of this century is not in danger of becoming obsolete before my book emerges next spring.  I wish I could be more positive about the future.  I would love for us all to begin acting in ways that will prove me wrong!