Artificial habitats, especially artificial reefs, have been employed as biodiversity recovery and conservation solutions across many coastal regions. Artificial reefs can comprise well-designed sculptures or abandoned ships, oil rigs, deconstructed bridges and buildings, and even military vehicles such as tanks. Artificial reefs aimed at bivalves like mussels have also been successfully deployed to clean up water pollution in estuaries and coastal waters.

Can such an approach be brought to bear on the Mumbai and Thane coastal waters? In Mumbai, much debate is now focussed around the potential negative environmental impacts of the nearly 30-km coastal highway from Kandivali to Marine Lines, that is partly under construction and partly under litigation. And yet, Mumbai’s beaches and its coastal water are steeped in truckloads of trash and poor water quality.

The human impacts on climate, habitats, species and the air and water quality are complex inter-connected problems. This means there are never any simple solutions or silver bullets, but a portfolio of solutions which is needed.

Improving water quality

Water quality includes the physical and chemical as well as biological parameters. The Maharashtra Pollution Control Board recently released its detailed analysis of the State’s surface and groundwater water quality. As can be expected, the news is mostly bad with only a few spots showing improvements in water quality, measured as acidity and faecal coliform levels, dissolved oxygen, and biochemical oxygen demand. The Mumbai and Thane saline coastal waters have remained steady in the water quality for over a decade, but unfortunately have hovered within the ‘polluted’ grade of the Water Quality Index.

Whether or not the coastal highway will make water quality worse is being debated under the environmental impact assessment demands. But it is clear that solutions for improving the water quality are needed, irrespective of the outcome of that debate.

If one were to envision a bright future for Mumbai’s coastline, then it has to include clean beaches and coastal waters, where people congregate and do not hesitate to take a dip in the ocean. Natural solutions such as mussels and seaweed can go a long way in helping achieve this vision for the future.

Introducing bivalves

There are many benefits to designing artificial reefs to establish bivalves along the coast. Bivalve reefs tend to be biodiversity hotspots, which adds value to local artisanal fisheries and also makes for attractive sites for snorkelling, diving, and eco-tourism with underwater marine observatories. One can even imagine the line of eateries serving delicious mussel dishes along the coast, providing alternative livelihoods for fishermen.

Marine biodiversity can be greatly enhanced by combining the bivalve restoration with seaweed and seagrass, which can absorb nutrients and reduce eutrophication, revive oxygen levels and a multitude of habitats while also serving as a natural defence against cyclones and storm surges. Coastal erosion will also be reduced by the combination of reefs, seaweed and seagrass. As most solid structures in ocean waters tend to do, reefs can also be designed to be fish aggregating devices.

To accomplish this exalted water quality, natural solutions must play a big role. Sustaining existing mangroves and recovering lost ones will be critical. But novel and complementary approaches should include building extensive artificial reefs for bivalves which can filter up to 25 litres of water per day. That may not sound like much, but imagine the coastline bedecked with reefs supporting millions of mussels.

The most important thing to remember about filter-feeders such as bivalves is that they accumulate the toxins and metal in the water while filtering gallons of water each day. This bio-accumulation can render them dangerous for human health. However, up till the mussels are safe to eat, they can in fact be used as bio-indicators of pollution. Bio-accumulation makes them ideal for dissection, a way to continuously track pollution levels in the coastal waters.

Monitoring quality and certifying edibility of mussels is fairly straightforward, once water quality indicators reach safe standards. Considering that most of the meat and fish we consume these days carry some levels of pollution and plastics and require continuous monitoring as well, the ecosystem services and eventual bounty of delicious mussels should be enticing as a natural solution to coastal water pollution.

Bivalves are known as nutrient capacitors and the ‘intestines’ of coastal ecosystems because of their ability to ingest and release nutrients. Their lifespan can range from 2-100 years, which, along with their ability to withstand disturbances from terrestrial runoff, cyclones and waves, can make them a worthy investment as natural coastal water cleansers. Invasive species must be guarded against, since ballast waters and other marine activities can introduce diseases and non-native species which can have deleterious impacts on local species. Considering the growing population closer to the coasts and some of the inevitable development activities such as the coastal highway, environmental-friendly economic development must consider as many natural solutions as possible.

Artificial reefs and bivalves with seaweed and seagrass offer a well-tested natural solution with many benefits. Mumbai is an ideal test-bed for a pilot experiment in this natural solution. Replicating it along the extensive coastline of India can follow with the lessons learned in Mumbai coasts.

The writer is a professor of Atmospheric and Oceanic Science and Earth System Science at the University of Maryland. He is currently a visiting professor at IIT-Bombay

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