Record coral cover doesn’t necessarily mean the Great Barrier Reef is in good health

Record coral cover doesn’t necessarily mean the Great Barrier Reef is in good health

Subject : Environment

Section: Biodiversity

Context: The highest levels of coral cover, within the past 36 years, has been recorded in the northern and central parts of Australia’s Great Barrier Reef (GBR).

Recent Findings-

• Coral cover in parts of the Great Barrier Reef is at a record high, according to new data from the Australian Institute of Marine Science.
• In the north of the reef, coral cover usually fluctuates between 20 percent and 30 percent. Currently, it’s at 36 percent, the highest level recorded since monitoring began more than three decades ago.
• But this doesn’t necessarily mean that the reef is in good health.
• High coral cover findings can be deceptive because they can result from only a few dominant species that grow rapidly after disturbance (such as mass bleaching).
• These same corals, however, are extremely susceptible to disturbance and are likely to die out within a few years.
• Without more information at the level of individual species, it is impossible to understand how much of the Great Barrier Reef has been lost, or recovered, since the last mass bleaching event.
• This level of coral recovery, after a disturbing decade that saw the reef endure six mass coral bleaching events, four severe tropical cyclones, active outbreaks of crown-of-thorns starfish, and water quality impacts following floods, raises the question- what’s going on?

What is a coral reef-

• Corals are invertebrate animals belonging to a large group of colourful and fascinating animals called Cnidaria. They are the largest living structures on the planet
• Each coral is called a polyp and thousands of such polyps live together to form a colony, which grow when polyps multiply to make copies of themselves.
• These polyps have microscopic algae called zooxanthellae living within their tissues. The corals and algae have a mutualistic relationship. The coral provides the zooxanthellae with the compounds necessary for photosynthesis.
• In return, the zooxanthellae supply the coral with organic products of photosynthesis, like carbohydrates, which are utilized by the coral polyps for the synthesis of their calcium carbonate skeletons.
• In addition to providing corals with essential nutrients, zooxanthellae are responsible for the unique and beautiful colors of corals.
• Coral reefs are created by millions of tiny polyps forming large carbonate structures. Coral reefs are the largest living structure on the planet, and the only living structure to be visible from space.

Corals are of two types — hard corals and soft corals.

Hard corals extract calcium carbonate from seawater to build hard, white coral exoskeletons. Hard corals are in a way the engineers of reef ecosystems and measuring the extent of hard coral is a widely-accepted metric for measuring the condition of coral reefs.

Soft corals attach themselves to such skeletons and older skeletons built by their ancestors. Soft corals also add their own skeletons to the hard structure over the years. These growing multiplying structures gradually form coral reef

What are Great barrier reefs-

• The Great Barrier Reef spans 2,300 kilometers along the eastern coast of Australia, comprising more than 3,000 individual reefs.
• It is an exceptionally diverse ecosystem that features more than 12,000 animal species, plus many thousand more species of plankton and marine flora.

Significance of Coral reefs:

• Coral reefs support over 25% of marine biodiversity even as they take up only 1% of the seafloor.
• The marine life supported by reefs further fuels global fishing industries. Besides, coral reef systems generate $2.7 trillion in annual economic value through goods and service trade and tourism.
• In Australia, the Barrier Reef, in pre-COVID times, generated $4.6 billion annually through tourism and employed over 60,000 people including divers and guides

Coral bleaching :

• Corals share a symbiotic relationship with single-celled algae called zooxanthellae. The algae prepares food for corals through photosynthesis and also gives them their vibrant colouration.
• When exposed to conditions like heat stress, pollution, or high levels of ocean acidity, the zooxanthellae start producing reactive oxygen species not beneficial to the corals. So, the corals kick out the colour-giving algae from their polyps, exposing their pale white exoskeleton and leading to coral starvation as corals cannot produce their own food. Bleached corals can survive depending on the levels of bleaching and the recovery of sea temperatures to normal levels.
• Severe bleaching and prolonged stress in the external environment can lead to coral death

Mass Bleaching

The concern is that in the past decade, mass bleaching events have become more closely spaced in time.

According to the Australian government’s scientists, 91% of the reefs it had surveyed in March were affected by bleaching

Threat to Great barrier reef-

• The reef has been teetering on the edge of receiving an “in-danger” listing from the World Heritage Committee. And it was recently described in the State of the Environment Report as being in a poor and deteriorating state.
• To protect the Great Barrier Reef, we need to routinely monitor and report on its condition. The Australian Institute of Marine Science’s long-term monitoring program has been collating and delivering this information since 1985.
• The latest report provides a robust and valuable synopsis of how coral cover has changed at 87 reefs across three sectors (north, central, and south) over the past 36 years.

The findings of the report-

• Overall, the long-term monitoring team found coral cover has increased on most reefs.
• The level of coral cover on reefs near Cape Grenville and Princess Charlotte Bay in the northern sector has recovered from bleaching, with two reefs having more than 75 percent cover.
• In the central sector, where the coral cover has historically been lower than in the north and south, coral cover is now at a region-wide high, at 33 percent.
• The southern sector has a dynamic coral cover record. In the late 1980s coral cover surpassed 40 percent, before dropping to a region-wide low of 12 percent in 2011 after Cyclone Hamish.
• The region is currently experiencing outbreaks of crown-of-thorns starfish. And yet, coral cover in this area is still relatively high at 34 percent.

Findings vs. Reality-

• In the Australian Institute of Marine Science’s report, reef recovery relates solely to an increase in coral cover, so let’s unpack this term.
  • Coral cover is a broad proxy metric that indicates habitat conditions. It’s relatively easy data to collect and report on and is the most widely used monitoring metric on coral reefs.
  • The finding of high coral cover may signify a reef in good condition and an increase in coral cover after disturbance may signify a recovering reef.
  • But in this instance, it’s more likely the reef is being dominated by only a few species, as the report states that branching and plating Acropora species have driven the recovery of coral cover.
• Acropora coral is renowned for its “boom and bust” life cycle.
• After disturbances such as a cyclone, Acropora species function as pioneers. They quickly recruit and colonize bare space, and the laterally growing plate-like species can rapidly cover large areas.
• Fast-growing Acropora corals tend to dominate during the early phase of recovery after disturbances such as the recent series of mass bleaching events.
• However, these same corals are often susceptible to wave damage, disease, or coral bleaching and tend to go bust within a few years.

The loopholes in the study-

• Inferring that a reef has been recovered by a person being towed behind a boat to obtain a rapid visual estimate of coral cover is like flying in a helicopter and saying a bushfire-hit forest has recovered because the canopy has grown back.
• It provides no information about diversity, or the abundance and health of other animals and plants that live in and among the trees, or coral.
• While there’s no data to prove or disprove it, it’s also probable that extinctions or local declines of coral-affiliated marine life, such as coral-eating fishes, crustaceans, and molluscs have also occurred.
• Without more information at the level of individual species, it is impossible to understand how much of the Great Barrier Reef has been lost, or recovered, since the last mass bleaching event.

Prospects of the report-

• The study, published last year, examined 44 years of coral distribution records around Jiigurru, Lizard Island, at the northern end of the Great Barrier Reef.
• It suggested that 28 of 368 species of hard coral recorded at that location haven’t been seen for at least a decade, and are at risk of local extinction.
• Lizard Island is one location where the coral cover has rapidly increased since the devastating 2016-17 bleaching event. Yet, there is still a real risk of local extinctions of coral species have occurred.
• Based on the coral cover data, it’s tempting to be optimistic. But given more frequent and severe heatwaves and cyclones are predicted in the future, it’s wise to be cautious about the reef’s perceived recovery or resilience.