Plastic dominates marine debris in city mangroves: Study
A recent study by researchers at the Central Institute of Fisheries Education (CIFE) has found that plastics dominate the composition of marine debris present in the city’s mangrove forests, comprising 62% of all surface debris items (by number) and 43 percent (by weight).
The study — published in July this year in international peer-reviewed Elsevier journal ‘Science of the Total Environment’ — demonstrates how mangrove swamps are more polluted than city beaches, acting as traps that retain debris and rubbish flowing in from the city; and attaches empirical data to an issue that environmentalists and scientists have long been concerned about.
“The structural features of the mangrove trees let them hold the sea debris during the high tide and retain it for months to years. The accumulated debris shows a negative impact on the mangrove ecosystem,” the authors state. Trapped debris is a potential risk to mangroves as it creates a barrier over their canopy, blocking out solar radiation needed for photosynthesis.
The debris gets entangled within the flora, and masks their pneumatophores (aerial roots) which could lead to oxygen deprivation in the trees and also reducing available space for sapling growth, the researchers posit. Other impacts associated with debris pollution include impact on fauna, such as wader birds and marine line. “The plastic debris after accumulating in sediments impacts the mangrove crabs and also creates an anaerobic condition in the sediment,” the study claims.
Environmentalists who reviewed the study on HT’s request said that its findings are no surprise, but clarified that it is confined only to quantification of debris. ““The paper does not present evidence of mangrove growth being hampered because of plastic or other debris. Mangrove cover in the state is actually expanding at a good pace. The paper does not propose any solutions, such as installing nets in drainage outlets to keep the plastic from reaching mangroves. It suggests regular clean-ups, which will be an endless exercise if we don’t stop pollution at its source,” said Stalin D, director of NGO Vanashakti, which is fighting two separate PILs relating to marine debris in city drains and mangrove conservation.
The CIFE study uses data gathered from 20 ‘transect surveys’ conducted between September 2019 and March 2020, at six locations including Versova, Gorai, Bandra Bandstand, Sewri, Vikhroli and Elephanta Island. Researchers surveyed a total area of 800 sq mtrs and 35 mangrove trees, collecting 3526 debris items weighing 367.9 kg. These were later classified into ‘surficial’ debris and ‘trapped’ debris, based on where they were collected from. The 35 selected trees were all located at Bandstand and alone yielded 1224 items (88kg in weight) of ‘trapped’ debris.
Though the locations showed an “irregular pattern in the distribution of debris”, Versova and Gorai emerged as the most polluted areas, while Elephanta Island was the least. In all locations, plastic dominated the composition and distribution of materials. “By weight and number, plastics contributed highest to the collected surficial debris were about 43% and 62%, respectively. In the collected surficial plastic debris, plastic carry bags (19.2%) contributed more, followed by food wrappers (15.9%),” the study states.
Other plastics researchers found in the surface debris included personal care products (8.03%), plastic rope/small net pieces (7.08%), bottle or container caps (6.40%), buoys & floats (5.22%), styrofoam cups (3.95%), beverage bottles (2.86%) others. The majority of collected plastic (31.28% by weight), however, could not be identified and were classified as others, indicating that these materials had remained in the environment and degraded over a period of time.
The share of plastics was followed by metal, glass, rubber, processed lumber, cloth and other miscellaneous items, in that order. Further analysis of the 3,526 debris items collected revealed that “recreational/shoreline activities” were the primary source of pollution for city mangroves, contributing 54 percent of all the debris surveyed (including the bulk of plastic carry bags, food wrappers, plastic cups, bottles, plastic caps and similar waste).
Researchers also observed that, in terms of weight, the abundance of plastics in mangroves was much more than their abundance in beach plastics, as demonstrated by an earlier 2013 study on marine litter in Mumbai, by the National Aquatic Resources Research and Development Agency in Sri Lanka. Compared with similar studies conducted around the work, researchers also note that the mean abundance of surface debris (by area) in Mumbai is seven times higher than the mangrove swamps of the São Vicente Estuary in Brazil.
In terms of trapped debris, the study shows that there is a direct relationship between the quantity of debris and the height of a mangrove’s pneumatophores (aerial roots), which physically trap the material. “Larger trees accumulate debris (primarily plastics) in their trunk and branches, which stays in the tree for years and creates a new environment that acts as a shelter for various insects… more debris was found where the height and number of pneumatophores were more,” the paper says.
Hindustan Times spoke with two of the study’s researchers, including the corresponding author, who declined to comment citing official protocol. A senior official with the MPCB, too, declined to comment and asked that the matter be directed to the forest department’s mangrove cell. Independent experts, on the other hand, said the study merits further, impact-focused research. “This is a unique study that highlights the vulnerability of mangrove habitat for pollution accumulation... Long-term observations are necessary to reveal the actual impact on the associated biota, especially the edible fishes and molluscs. There is potential harm not just to marine life but also to human populations that consume fisheries products from mangrove mudflats.”
“Mangrove Foundation would like to take up further studies as suggested in this paper. We are in talks with many NGOs working on mangrove clean-ups and are finalising SOPs for clean-ups going forward. Mangrove Foundation will provide necessary assistance to stakeholders for such activities,” said Virendra Tiwari, APCCF, mangrove cell.
Also, the independent expert quote at the end needs to be attributed to Dr Baban Ingole, former scientist with the National Institute of Oceanography.