The Halo on the Reef

Satellite imagery provides new ways to see and monitor coral reefs. Dr Anjani Ganase tells us where we can see halos, what they mean and how they may help in on-going monitoring programmes for marine protected areas. 

Through the technology that provides satellite imagery of the earth, we can explore the planet from above. The surface of the earth is covered by impressive structures and patterns curated by man and nature: consider the meandering tributaries of the Colorado and Amazon Rivers, the Great Barrier Reef and cities like New York. The patterns observed from space have given great insights into landscape ecosystems, such as the arrangements of flora along mountainsides and the contraction of the polar ice caps. We are also able to see some distance below the surface of the sea.

Here’s what Dr Elizabeth Madin, assistant professor at the University of Hawai`i, observed in the lagoon of Heron Island, on the Great Barrier Reef. Shallow patches of coral reef stood out against the sand background, but surrounding each patch of reef was a distinct halo of what appeared to be very clean white sandy substrate. These perfect rings of clean sand encircling the reef patch transitioned to sand covered in macroalgae and seagrass. Seen from space, these circles would become known as the "grazing halos"; and would refer to heavily grazed, vegetation free sandy areas that surround patches of coral reefs, or run along the edge of a shallow reef flat. These grazing halos can be seen on shallow reefs around the world on Google Maps.

 

Patch reefs of Heron Island in the Southern Great Barrier Reef. Halos of freshly grazed sandy areas can be seen surrounding mounds of corals. Photo by Google Earth Pro

So why were these areas preferred grazing sites for herbivorous reef fish? Risk-averse grazing fish, such as Parrotfish and surgeonfish, seek shelter from predation within the physical structure of the patch reef. When they forage beyond the protection of reef, they tend to graze as close to the reef as possible. As the algae closest to the reef is eaten, and they gradually venture farther away from the reef, the cleared substrate forms the halo patterns. Grazing halos were observed in the 1960s and 1970s by ecologists in the Caribbean. They were created by reef fish and invertebrates moving out from the reef to grab bites from nearby seagrass along the edges of the fringing reefs and reef patches. In the US Virgin Islands, sea urchins would migrate from the reef into the sandy lagoon to graze on seagrass under the protection of night; they would start eating at the point closest to the reef.

Dr Madin has driven the rediscovery and renewed interest in the ecology of these halos. She investigated further to determine whether these halos provide clues to how well a coral reef ecosystem was functioning. Compared to the 1970s when the halos were first observed, present-day coral reefs around the world are severely compromised by humans, through over-exploitation of fish. Fishing continues down the food chain; after removing the large predators, sharks and groupers, smaller and smaller fish are targeted and populations get depleted. How is this reflected in the halo formations? Could the presence and the size of the halos be used as a simple indicator of a reef system with a healthy population of fish or one that is compromised and over-fished? What became obvious was that reefs that were not managed or protected and had lost fish life, especially herbivores, had fewer grazing halos along the edges of their patch reefs compared to reefs that had been protected for at least 40 years. However, on reef systems between these conditions, the patterns of halos seem to be more complicated. Also, less obvious are the patterns that would explain the changes in the size of the halos from place to place, and even over time. 

Imagine a reef with a healthy fish community going all the way up to the predators. Under this scenario, the grazing fish need to be diligent about their foraging behaviour away from the reef, as predators hunt along the margins of the reef. However, as (human) fishing intensities remove the large predators first, followed by the smaller predators, we would expect halos to expand in size as predation pressure subsides. When the fishing intensities persist and the herbivorous fish now become targeted, the depleted numbers of grazers may result in thinner grazing halos. Apart from the dynamics of the food web, we also consider the environmental conditions, the size and the composition of the reef and the amount of fish it can house, as these factors may also influence grazing halos.

Scientists and marine park managers have come to see the value in observing changes in the patterns of grazing halos on coral reefs. It is being recognised as a low cost tool for monitoring remote coral reefs, especially in large marine protected areas. Satellite imagery, which is becoming more accessible and affordable, can be used to determine the initial state of the reef before further investigations on site. Changes in halo formations over time can be monitored alongside fish and coral community assessments on site. 

In Tobago, patch reefs are not common except in the Buccoo Reef Marine Park, which had a few patch reef systems in Bon Accord Lagoon. Although there is speculation that the seagrass in Bon Accord Lagoon has been expanding as result of nutrient pollution and minimal grazing pressures, there are still patterns that can be observed from above. These patterns may give insight into the dynamics between the fish, coral and seagrass communities and the ecological state of the marine protected area.

 

Patch reefs and halos of Buccoo Reef are hard to find and may be symptomatic of a compromised coral reef community. Thick meadows of seagrass beds are observed in different shapes and proximities to the reef. What do you think drives the shapes of these seagrass communities? Photo by Google Earth Pro.

REFERENCES:

Atwood TB, Madin EMP, Harborne AR, Hammill E, Luiz OJ, Ollivier QR, Roelfsema CM, Macreadie PI and Lovelock CE (2018) Predators Shape Sedimentary Organic Carbon Storage in a Coral Reef Ecosystem. Front. Ecol. Evol. 6:110. doi: 10.3389/fevo.2018.00110

Madin E. Riddle of the reef halos. New Scientist. 2019 Apr 27;242 (3227):38-40.

Madin EMP, Precoda K, Harborne AR, Atwood TB, Roelfsema CM and Luiz OJ (2019) Multi-Trophic Species Interactions Shape Seascape-Scale Coral Reef Vegetation Patterns. Front. Ecol. Evol. 7:102. doi: 10.3389/fevo.2019.00102

Madin EMP, Harborne AR, Harmer AMT, Luiz OJ, Atwood TB, Sullivan BJ, Madin JS. 2019 Marine reserves shape seascapes on scales visible from space. Proc. R. Soc. B 286: 20190053. http://dx.doi.org/10.1098/rspb.2019.0053

Ogden JC, Brown RA, Salesky N. Grazing by the echinoid Diadema antillarum Philippi: formation of halos around West Indian patch reefs. Science. 1973 Nov 16;182(4113):715-7.

Randall JE. Grazing effect on sea grasses by herbivorous reef fishes in the West Indies. Ecology. 1965 May;46(3):255-60.