The food chain of aquatic species living in the world’s single largest mangrove forest, the Sundarbans, has been severely disrupted by the Sela River oil spill, a study report says.
The study report conducted by Khulna University (KU) said the presence of primary food producers like phytoplankton and zooplankton had decreased sharply in the contaminated water.
Under normal conditions, 47 species of phytoplankton flourish in the Sundarbans. A typical litre of water normally contains 226–456 units of phytoplankton. According to the study, only 18 phytoplankton species were recorded in the oil slick with just 24–67 units per litre of contaminated water.
Furthermore, whereas 8 species of zooplankton with an abundance of 53–77 units per litre were recorded in the Sundarbans under normal conditions, just 2 species were found in contaminated areas. Their abundance was very poor at just 6-10 units per litre of contaminated water.
Benthos species took a severe hit as well with the normal presence of 34 species whittled down to just seven species in oil contaminated areas.
“The diminished levels of phytoplankton, zooplankton and benthos in the study area indicate a decrease in diversity and abundance due to oil contamination and has obviously affected the food chain,” the study’s team leader Prof Abdullah Harun Chowdhury said.
The December 9 Sela River oil spill was caused by a cargo ship slamming into an oil tanker while the two were plying a waterway route inside a dolphin sanctuary.
It poured an estimated 350,000 litres [75,000 gallons] of furnace oil into waterways inside the forest reserve, spreading across an area of several hundred square kilometres and contaminating the land and water of the tidal mangrove forest.
Due to tidal systems, the oil spill has reached small creeks, canals and the forest floor.
Thick black furnace oil has been deposited on the soil of intertidal zones, forest floors, plants, leaves, stems, floating fruits, roots and pneumatophores [the breathing roots of mangrove plants].
Much of the surface of the rivers, creeks and canals of the south-eastern part of the Sundarbans was covered by the thick oil slick.
A 10-member research team of the Environmental Science Department of Khulna University conducted the study in 15 different oil contaminated areas of the Sundarbans from December 11 to 25.
The 1,200 square kilometre study area also collected samples from Gharilal, Jorshing and Kalagachia in the West Sundarbans, which were not affected by the oil spill.
The team found evidence of serious oil contamination in affected areas, exceeding the international oil contamination standard limit of 10 milligrams per litre.
The study titled “Impact of Oil Spillage on the Environment of Sundarbans in Bangladesh” noted a near total absence of eggs and hatchlings of common species of fish, crabs and snails even though they were common in the study area.
Bird species like the Masked Finfoot, Ban Morog, and other local and migratory birds were also conspicuous by their absence in the affected areas, the study team noted.
Prof Harun, a faculty member of the KU environmental science department, said the status of flora and fauna indicate that primary producers, consumers, some aquatic animals and some plants and their habitats had already been affected by the oil pollution.
Crocodiles were not able to prepare hatchling houses in these areas to lay and hatch their eggs, he added.
The study said the banks of the rivers, canals and creeks of the Sundarbans are a natural habitat for crocodiles to prepare hatchling houses in the months of April and May.
But before the heavy rainy season in July and August there is little possibility of reducing the oil pollution in crocodile hatchling habitats.
As a result crocodiles may migrate and face territorial conflict, ultimately affecting the crocodile population, the study said.
The tidal forest is rich in natural resources and boasts a floral and faunal diversity of 66 species of plants, more than 200 species of fish, 42 mammals, 234 birds, 51 reptiles, 8 amphibians and many invertebrates.
Around 500,000 people directly and indirectly depend on the Sundarbans for their livelihood.
