One day in 2008 a strange wave washed in at Mandapam in the Gulf of Mannar, Tamil Nadu. In sharp contrast to the azure blue of the coral-filled waters of the Marine National Park, this wave was reported to be orange. This harbinger of death was not a result of chemical dyes, or a sudden spillage or leak from a passing vessel. It was evidence of an algal bloom. Also, it was a symptom of a dead zone.
Algal blooms are increasingly common phenomena caused by a sudden explosion in the population of algae; the marine world's most ubiquitous set of species. Algae are often single-celled organisms that are a natural part of the marine ecosystem. Being photosynthesisers, they start most marine food chains.
An increase in their population would usually result in more food for the rest of marine life but some algae release toxic chemicals while blooming. These toxins could kill other living organisms in the water, resulting in what is known as a dead zone. Dead zones could also arise in areas of very little oxygen, where hyper-populated algae consume most of the life giving gas. As a result, all other oxygen- dependent life either dies or has to leave.
Scientists across the world noted an increase in the number of dead zones since the 1970s. This led them to look for the cause of such harmful algal blooms. By mapping where these dead zones were turning up, they found these zones forming alongside coastlines with a high human population.
Human activity, therefore, was identified as an important cause. Researchers from the Smithsonian Tropical Research Institute, Panama identified another interesting relationship with the location of dead zones. They found that estuarine areas were relatively more prone to reduced oxygen levels or hypoxia conditions.
This means that not only is coastal human activity a threat, pollution brought by rivers, from inland areas, are also creating dead zones. Scientists realised that there was something familiar about this dead zone problem. An issue that was studied in the 1960s had a storyline that ran quite similar to this one. It was called eutrophication.
Excessive nutrients from untreated sewage, fertilisers and other waste are dumped into rivers and lakes, and these feed population explosions of plants in these aquatic bodies.
Scientists call this phenomenon eutrophication. Research now shows that the marine systems react to pollution in a manner similar to fresh water bodies, like lakes. Except, that instead of being covered by plants, like in lakes, algae take over the marine ecosystem.
The problem, unlike lakes, is that the long coastlines have several thousands, if not millions of sources of pollution and excess nutrients. The Millennium Ecosystem Assessment released by the United Nations in 2005 reported that the supply of nitrogen-containing compounds to the sea grew by 80 percent from 1860 to 1990. The coast, however, is not the only source of pollution. In his book Lawn People, Paul Robbins identifies that lawn care materials, including fertilisers and pesticides, being used in the American Midwest, were being washed down the Mississippi River and creating a dead zone off the coast of Louisiana, nearly 1,500 km away.
Dead zones are therefore a clear indicator of the land-sea connection. This means that waste disposal behaviour of inland cities like Bengaluru, New Delhi and Hyderabad are equally important to the occurrence or prevention of dead zones.
Dead zones in India
Most coastal cities in India release their waste and effluents into the sea. In most cases, this waste remains untreated. In newer parts of coastal cities, like Chennai or Mumbai, where the municipality is yet to install water and sewage lines, pipes from houses and buildings along the seashore run directly along the beach, releasing raw sewage at the high- tide line. This forms ideal concentrated sources of feed material for marine algae.
Harmful algal blooms have been reported in Indian waters since 1981, according to researchers from the National Institute of Oceanography (NIO). In a paper, published in Current Science in 2004, NIO researchers S R Bhat and Prabhu Matondkar identified several occurrences of algal blooms linked to human deaths, along the coast of Maharashtra, Karnataka, Kerala and Tamil Nadu, with unconfirmed reports from Goa.
However, there has been no confirmed scientific report of dead zones along India’s coastlines. This is more due to a lack of serious study than actual absence of dead zones. The researchers write, “(Indian) universities and research institutions have limited sea observation and laboratory facilities, and therefore explorations and studies were sporadic.”
Anecdotal evidence, however, is abounding. Fishermen and recreational divers in the Gulf of Mannar tell of patches of reef that no longer have any fish, where the corals are dying and being replaced by algae. Mandapam, the site of the wave in 2008, is situated in the Ramanathapuram district, which has a highly populated coast, and an agricultural hinterland — perfect conditions for sewage and fertiliser runoffs into the sea.
The hundreds of fishing villages and towns dotting the coastline have no sewage system to speak of. Additional pressures in the form of dredging and siltation due to development projects in the area, only exacerbates the problem.
Connecting the dots
Dead zones are forming in the shallow seas, from where we get most of our seafood. The shallow seas, particularly estuaries, are important areas for the breeding of both freshwater and marine fish. Researchers from Stanford University and other institutions based in California, USA, showed that oxygen depleted estuaries (dead zones) were no longer acting as nurseries for marine fish. Instead, fish like the sole (a flatfish) were undergoing severe declines due to the loss of their nurseries to dead zones.
Excessive nutrient content in sea water is also linked to overpopulation of species other than algae. For instance, some of the jellyfish blooms, which caused alarm in the late 1990s have been linked to eutrophication in the sea. Unfortunately, these zones are increasing rapidly.
A 2004 report by the UN Environment Programme identified 146 dead zones across the world, while four years later, scientists from the Virginia Institute of Marine Science, USA and the University of Gothenburg, Sweden reported more than double that number. Across the world’s coastlines, an area of shallow sea close to 1.5 times the area of Karnataka is now lifeless.
The good news is that some of these dead zones can be reversed. By stemming the source of the problem, i.e. correcting waste disposal mechanisms, we can bring patches of the sea back to life. Safe sewage and waste disposal is not only essential for our health, but also the health of the marine ecosystem and the fish that we eat. But recovery is an elaborate and long drawn out process. Help exists in the form of natural ecosystem engineers like oysters and other shellfish. If healthy oyster beds or shell fish reefs occur in the vicinity of a dead zone, they can help to protect species that need to take refuge.
The structural complexity of oyster beds or coral reefs also helps fish move to a different part of the water column that might have more oxygen, thereby enabling the fish to survive. For best results, dredging these reefs or harvesting these oyster beds should be avoided, because this can reduce their structural complexity, reducing the reef's capacity to help species in dead zones.