If 2015 broke all global temperature records, it is unlikely to hold on to that dubious achievement for long. We are currently in the thrall of the hottest pre-monsoon since we have had accurate data, and 2016 will, almost certainly, redefine the charts. For many ecosystems, this is going to be a banner year as species and assemblages struggle to cope with the suite of abnormal conditions that come in the wake of these increased temperatures. And for tropical coral reefs, 2016 is going to mark a watershed unlike any before. Already, large swathes of the Great Barrier Reef have succumbed to the largest coral bleaching event ever recorded, and many reefs in the Pacific are dead in the wake of increased sea surface temperatures. It is disconcerting to note that it is some of the most remote and healthy patches that seem to be directly in the line of fire. In our own backwaters, ocean temperatures are soaring, and we have begun to see early signs of corals turning pale and dying. The full extent of this damage will only become clear by the end of the year.
What is certain is that the way tropical coral reefs respond to these temperature conditions will set them on trajectories that may be very difficult to reverse. In other tropical reefs that have already succumbed to these catastrophic changes, piecing the ecosystem back together is proving much more difficult than it was to take it apart. The scars that this year will leave on coral reefs will stay with us for decades – if they heal at all. And while the rapid unravelling of these spectacular ecosystems would be an ecological tragedy, the consequences of this for the millions of marginalized people who depend either directly or indirectly on the reef for their day-to-day existence would be nothing short of disastrous. The cruel mismatch of global forces and local influences will be felt most acutely by the coastal communities least equipped to deal with these changes. Clearly, a lot rides on understanding the responses of coral reefs to global catastrophic events. Yet reefs are complex systems, and they respond in complex ways. It will require us to grapple with that complexity before we can understand and manage them with any degree of success.
Coral reef responses to climate change
Corals are Pre-Cambrian invertebrates whose success as the primary structuring agents of their ecosystems is hinged on the strong symbiotic association they have developed with unicellular algae called zooxanthellae. Housed in the soft tissue of the coral, these algae photosynthesize light and produce sugars and other compounds that constitute the bulk of the nutritive requirements for the host coral and help remove toxic wastes from the coral tissues. This interaction has allowed corals to successfully colonize warm, shallow and clear waters across the tropics, forming extensive reef structures of aragonite that hug coastlines, fringe islands or form ancient atolls over submerged volcanoes. The astoundingly complex architecture of the reef is home to a host of plants and animals, making reefs among the most diverse ecosystems on the planet.
The relationship between coral and its zooxanthellae is a fussy one; under conditions of stress (including elevated temperatures), the coral can no longer sustain the algae and they are expelled into the water column. This results in the corals losing their characteristic colours, bleaching completely white. If waters cool down fast enough corals can sometimes regain the zooxanthallae and recover, but if sea temperatures keep rising, corals remain bleached, becoming physically weaker and more susceptible to disease, eventually dying.
While many reefs typically show some amount of bleaching as summer heats up, in El Niño years, sea surface temperatures rise way beyond seasonal averages, causing widespread coral mortality. Simply put, an El Niño develops when Pacific trade winds fail and the warm waters that are normally restricted to the west coast of South America spread across the oceans, raising ocean temperatures way above normal levels. One of the consequences of manmade climate change is that El Niño cycles are becoming increasingly unpredictable, frequent and intense. In India, at least two major El Niños have affected our reefs – in 1998 and in 2010 – resulting in mortalities of coral that ranged from 20% to nearly 90% in some regions. With this huge die-off of coral, the entire assemblage of species dependent on coral and the complex architecture it forms also declined considerably.
The 2016 El Niño is gearing up to be even more intense than these earlier events but it is far from certain how Indian reefs will stand up to this onslaught. Part of the reason for this uncertainty is that corals themselves have differential susceptibilities to increased temperatures and some corals are better equipped to deal with high temperatures than others. It turns out that the symbiotic zooxanthellae are not a single taxonomic unit and have several separate clades, some of which are able to weather stress much better. In the run up to an ocean warming event, it has been demonstrated that some corals have a remarkable ability to adjust the composition of their zooxanthallae, replacing more susceptible clades with hardier ones.
This in turn appears to be dependent on local environmental conditions: corals in enclosed environments (where water temperatures usually vary considerably through the year) appear better adapted than corals in environments where temperatures don’t vary as much. In mountainous areas, the slopes themselves may protectively shade reefs making them cooler and less prone to ultraviolet radiation. High concentrations of suspended sediments and decomposed litter from mangroves – which in normal circumstances can be a serious problem for reefs – can also provide some protection against ultraviolet radiation, reducing the impacts of bleaching. Local current systems and cool upwellings may also serve to make some areas naturally cooler, thus reducing the impact of sea surface temperature increases. Apart from all these physiological, geographical and oceanographic factors, local weather conditions – cloudy days, rainfall, and high wave conditions – can all act together to influence how corals on reefs respond to ocean warming events.
Given the constellation of factors that together determine reef responses, the best we can do is to make considered guesses based on what we know of our reef systems. The systems we know best are the oceanic islands, where we have been working over the last two decades. The Andaman and Nicobar Islands are characterized by some of the most diverse reef systems in India. They form extensive fringing reefs which line the coasts of most of these islands. In addition, the Andaman and Nicobar Islands have a large submerged barrier reef, of which very little is known. A combination of high current flows between the labyrinth of islands, high sediment levels and organic content, and mountainous shading provides some amount of natural protection to these reef systems. While the 1998 El Niño devastated many other reef areas in India and the world, the Andaman and Nicobar reefs escaped relatively unscathed.
However, the 2010 bleaching affected several reefs across the archipelago leaving large areas dead – from which they are slowly recovering. In sharp contrast, the oceanic atolls of the Lakshadweep have none of these locally protecting mechanisms. Being low-lying islands surrounded by clear, nutrient-poor waters, these reefs declined dramatically after the 1998 event. It took at least a decade for these reefs to recover, only to be affected once more by the 2010 temperature anomaly. Recovery from this second event has been limping at best, and our most recent surveys in April 2016 is showing that even this little recovery is likely to be set back once again if anomalous temperatures do not abate. Thus, while the jury is still out on the Andaman and Nicobar reefs, the Lakshadweep reefs look primed for another mass die-off that it can barely afford.
What factors drive reef recovery after these events?
Amidst this rather grim picture, what hope do we have for these reefs recovering? From what we are discovering from the reefs of the Lakshadweep and the Andaman and Nicobar, the ability of reefs to recover depends on a coming together of several factors. All recovery is going to be essentially limited by larval supply – baby corals that get carried around in the water column to colonize the dead reef. To have an adequate pool of recruits requires that at least a few healthy and reproductively viable adult coral patches (a refuge of sorts) survive the bleaching and that these patches produce large enough numbers of larvae that can swim and colonize dead reef patches. Even when larvae are not limiting, finding a good substrate to settle on can be an enormous challenge. Dead reefs can get very rapidly taken over by macroalgae or thick turf, both of which can inhibit and outcompete any newly settling larvae. Herbivore fish and invertebrates such as the urchins play a critical role here: their constant browsing and scraping mow down these algae, ensuring that substrates are kept clean for these larvae to settle upon. After they settle these corals still have to grow, and we are finding that the stability of the underlying substrate can make all the difference in determining how easily they take hold on the reef. Coral larvae settling on unstable substrates may grow for a few years but can then be lost to a single storm event. As the coral grows the population of other coral-destroying species like crown-of-thorns starfish can also limit reef recovery. Keeping these species in check requires a healthy population of predatory fish. This then is the recipe for a relatively quick recovery: a large dose of coral larvae, settling on structurally stable, algal-free substrates with low corallivores or coral eroders. Getting these factors to line up would ensure that our reef systems are as prepared as they can be to deal with the inevitable surprises that climate change is going to throw their way.
This is where local management can make all the difference. Ensuring that fish populations (both herbivores and top predators) remain healthy depends a lot on local levels of exploitation and regulation; making sure that local fishers do not drive fish communities to decline is a critical part of maximizing reef resilience. Herbivore numbers in both the Andaman and Nicobar as well as the Lakshadweep islands are still relatively high: in both systems a targeted reef fishery has not yet begun, focusing on parrotfish and surgeonfish – the key herbivores in these reefs. Hook-and-line fishing in the Andamans however is affecting populations of long-lived predators like groupers and sharks, and this could have unforeseen consequences for the species they normally keep in check. From what we know, many of these long-lived predator species are highly dependent on structurally stable reefs, making them particularly vulnerable to even low levels of fishing pressure especially when reefs are changing so rapidly.
Climate change is now the new villain on the block, on whom we can conveniently pin all environmental crimes. A key prerequisite to any meaningful management intervention is to be able to disentangle the impacts of climate change from the rest. Easier said than done; managing coral reef ecosystems is a complex challenge with a unique, unsteady balance of science, politics, and economics. Millions of marginalized, economically backward people of the world depend on these ecosystems to eke out a living. Hence, solutions demand a nuanced approach that can balance the needs of these millions, while addressing global and local threats.
However, we are not the only ones going through a reef crisis; reefs of several countries have been affected by multiple catastrophes in the recent past. A 2009 study predicted the local extinction of many reefs in the next 50 years. This could seriously hinder the ecosystem services they provide us with. However, a few recent studies suggest that understanding and managing local processes e.g. local hydrodynamics, ecological and physical factors, fishing pressure, could play an important role in facilitating the recovery of coral reefs. Studies have shown that some coral species have learned to adapt, switching to algal types that can tolerate warmer waters.
Although global warming may respect no management boundaries, this does not necessarily mean that management can do nothing about it. While international climate politics and negotiations are crucial in addressing issues of human-made global warming, we need to invest as much, if not more, effort and resources in understanding local processes and dynamics. More than two decades of observation and research is showing that the capacity of reefs to resist and subsequently recover from these events can be strongly influenced by local management decisions. From both island systems, we are learning that global climate change makes it MORE important, not less, to pay attention to local anthropogenic factors. It is this nuanced understanding of local factors that can give us a way beyond the easy nothing-can be-done, attitude. Although we may not have a complete understanding of how to live with the global change, we do know that local actions make a difference to reef ecosystems as a whole and they do so across the entire spectrum of local human impacts and oceanographic conditions where reefs occur. By recognizing that coral reefs and our societies are inherently coupled, we can evolve better strategies to manage them, that are ecologically sound, as well as socially and economically equitable.
Photographs: Rohan Arthur, Vardhan Patankar and Naveen Namboothri