Marine debris is causing Mumbai’s corals to bleach

Coral bleaching is caused due to the loss of algae from coral colonies on account of exposure to pollutants

Mumbai: A new study published by researchers at the National Institute of Oceanography (NIO) Goa demonstrates that coral colonies off Mumbai’s coast have undergone patrial bleaching due to exposure to marine debris.

A picture of the rhizangiidae stony coral. In 2020, the BMC translocated coral colonies such as these, spread across Worli and Haji Ali, while reclaiming part of the shoreline for the Coastal Road project. (Abhishek Jamalabad)

This is the first such acacemic study that examines coral colonies off Mumbai’s coastline, which is ranked as among the world’s most polluted. Marine debris may refer to pollutants from the city that enters the sea, such as plastics and other forms of solid waste, or naturally occurring materials like driftwood.

Coral bleaching is caused due to the loss of algae from coral colonies on account of exposure to pollutants, changing salinity levels or rising temperatures, and as such is an important indication of the health of the ocean’s ecosystem.

NIO researchers identified 445 live coral colonies in the intertidal zone just off Navy Nagar in South Mumbai, and surveyed them over November and December 2020. They found 22% of the identified colonies to be in direct physical contact with marine debris. Of that proportion, 9% showed signs of partial bleaching, the researchers found.

Coral bleaching was recently detected in Australia’s Great Barrier Reef due to high ocean temperatures — the previous bleaching caused in 2020 damaged two-thirds of the reef. In India, corals are also given the highest protection under India’s wildlife laws.

Shaunak Modi, director of the Coastal Conservation Foundation, who documents the city’s intertidal biodiversity said, “This paper is one of the most important ones on the subject to come from Mumbai. Corals are protected under Schedule I of the Wildlife Protection Act (1972), so any extent of bleaching calls for corrective measures. Unfortunately, there is a dearth of academic research on the subject which can help drive protective action, so in that sense this kind of research is very valuable. ”

The study

In the study conducted in Navy Nagar – located at the extreme south promontory of the city – NIO researchers spent two months conducting an extensive field survey at five locations in a delineated study area. A total of 445 coral colonies were identified and examined for occurrence of coral-marine debris contact, signs of bleaching and damage due to sedimentation.

“We found that approximately 9.21% (n = 41) of live coral colonies were in direct contact with marine debris. Among the corals interacting with marine debris, 21.95% (n = 9) of colonies showed signs of partial bleaching or paling, representing 2.02% of the total colonies of the study area. Our results also found that an average of 38.2% (n = 170) of the live corals seemed to be negatively impacted by sedimentation,” the study noted.

While there has been considerable research on coral ecosystems globally, and even in India (in the Gulf of Mannar, Andaman and Nicobar Islands, Lakshadweep islands and the Gulf of Kutch), there has been a dearth of studies on the patchier distribution of reef-forming corals along the eastern coast of the Arabian Sea.

“This leaves out a huge gap in our understanding of their current ecological status and potential threats to their wellbeing, one of which has proven to be marine debris,” said one of the researchers associated with the study, who wished to remain anonymous as he didn’t have permission to speak to the media.

Coral bleaching is how corals lose their vibrant colours, which are imparted by microscopic algae called zooxanthellae. These share a symbiotic relationship with the coral, which provides the algae with an optimal environment for photosynthesis. The algae produce oxygen and nutrients which are shared with the host. This relationship is typical to nearly all hermatypic (or reef-building) corals.

When placed under stressful conditions, such as exposure to pollutants, changing salinity levels or rising temperatures in the form of marine heat waves, the coral expels its algae and loses colour. Not all bleached corals die, but they do become prone to mortality and disease.

“This also severely diminishes the ability of coral to perform other ecological services for fish and microorganisms. They are both animals and a habitat for other animals, in isolated fragments as much as in large reefs,” Modi explained.

A February 2020 study conducted in the Gulf of Mannar, for example, found live corals to be the dominant substrate for marine debris, which was composed largely of non-biodegradable plastic. About 3.28 percent of surveyed corals were in contact with debris, of which 47.56% were fragmented and 34% were found with tissue loss, it noted.

During NIO’s study in Navy Nagar, researchers encountered a total of 481 macro-marine debris items in the study area, which comprised 15 transects ((or delineated portions of the larger study area which are surveyed as part of the study) of 20 square metres (sqm) each, or 300 sqm of the intertidal shore in all. The average density of marine debris in these transects was found to be 1.60 items per sqm, which is higher than the global mean of 1 item per sqm.

“Plastic materials were the most abundant of all debris registered in the study area, comprising 91.27% of the total debris, while other components were glass (3%), metal (2%), and fabrics (2.5%),” the study noted.

“The average density of plastic items was estimated to be about 1.46 items per sqm, while sanitary waste, including sanitary pads and diapers (0.42% of total marine debris) and processed woods (0.42% of total marine debris) were also found in small amounts. Collected plastic debris was further sorted into bottles, bottle caps, bags, wrappers, fishing nets, fragments and tubes, out of which “plastic bags alone contributed 46.99% of the overall composition of plastic debris, followed by wrappers (23.49%), and fragments (11.85%).”

Polluted coastline

These findings, independent of their impact on a specific species, point to the larger crisis of plastic pollutants on Mumbai’s coast, which typically enter the ocean through drainage channels and rivers, along with other pollutants like sewage and industrial waste. NIO points to “public negligence, intentional and unintentional discards of solid waste in combination with inadequate municipal waste management” as the primary driver for this crisis.

“Debris can become entangled in coral, causing injury or dislodging it. It has also been proven that debris can introduce a range of harmful microbia to coral reefs, triggering disease. Over time, debris disintegrates into microplastics, which can be ingested by the coral and move up the food chain,” said the researcher quoted above.

Researchers also pointed out that Mumbai’s beaches are considered to be among the most polluted in the world – which has been demonstrated in the study using global indices – which exacerbates their concern over the fate of the city’s coral biodiversity.

Experts agree. “The area at Navy Nagar from the shore to Prong’s Island is called Prong’s Reef, and at one point there was likely a much higher prevalence of corals in the area which has now dwindled. But corals are present on nearly every rocky shore in the city, and this paper sets a precedent to study and protect what is left. With the kind of development we are going to see, it’s important to have a set of baseline studies which inform how we track their growth or decline from here,” said Modi.

“The most dominant coral taxon found in this study was Pseudosiderastrea tayamai, classified in the near-threatened category according to the IUCN red list, which implies an urgent need for active conservation and management,” the study also notes, adding that the rapid growth of Mumbai as a city has not been supported by the implementation of adequate waste management. It suggests a “detailed mapping and understanding of the sources and nature of waste material”, which is essential to prevent and mitigate the impact of marine debris.