- Following the tropical Cyclone Ditwah, unusual sea-foam appeared along parts of Sri Lanka’s northern coast, a natural phenomenon caused by storm-driven turbulence and organic compounds released by plankton, not marine pollution, scientists say.
- Extreme rainfall from Ditwah released an extraordinary volume of freshwater into the ocean, and researchers estimate that nearly 10% of Sri Lanka’s average annual rainfall was received in a single day and rapidly drained to sea through the island’s river network.
- Flood-driven sediments and sudden changes in salinity may stress coral reefs and coastal ecosystems, but Sri Lanka lacks systematic sediment monitoring at river mouths, leaving scientists with limited data on downstream impacts.
- The floods also swept plastics, debris and nutrients into coastal waters, potentially intensifying plankton blooms and fish aggregations while increasing the risk of algal blooms, oxygen depletion and long-term marine pollution.
COLOMBO — Cyclone Ditwah, which claimed around 650 lives and left at least 200 people missing, cast a heavy pall of gloom over Sri Lanka from the moment it entered the island’s skies Nov. 28. By Dec. 2, the storm had finally moved on, leaving behind silence, debris and grief — but also, in some places, an unexpected calm.
Along stretches of the Jaffna coast in northern Sri Lanka, the returning sunshine revealed a strange and startling sight. Thick patches of snow-like white foam had gathered along the shoreline, astonishing residents who said they had never seen anything like it before. For children, especially, the sight felt surreal. Amid a nation still mourning its losses, they ran barefoot along the beach, laughing and playing with the foam, as if it were snow.
The fleeting scene offered a gentle contrast to the scale of devastation elsewhere, but it also raised questions about how the ocean responds to land-based disasters. In the aftermath of the floods, fears spread that polluted waters sweeping through industrial zones and sewage systems had contaminated the sea, and the foam was visible evidence of that pollution. Scientists, however, say the phenomenon was natural.
“Such sea foam can form naturally after powerful storms and strong winds, like those experienced in Jaffna during the northeast monsoon,” said Ganapathypillai Arulananthan, former director-general of the National Aquatic Resources Research and Development Agency (NARA). “Rough seas churn microscopic algae and plankton, releasing organic compounds that act like natural surfactants. When waves trap air in these compounds, thick and stable foam can accumulate along the shore.” While visually striking — and sometimes unsettling — the foam is often a natural oceanic response to extreme weather rather than a sign of contamination, he told Mongabay.
Water discharge of extraordinary scale
While Ditwah wreaked havoc in Sri Lanka’s hill country, the most impacted region from the recent cyclone, the enormous volumes of rain it triggered ultimately flowed downstream to the ocean.
Lakshman Galagedara, a professor of hydrology at the Grenfell Campus of Memorial University, estimates that on Nov. 28, when rainfall peaked due to Ditwah, Sri Lanka received nearly 13 billion cubic meters (459 billion cubic feet) of rainwater within 24 hours — about 10% of the country’s average annual rainfall within a single day.
Because heavy rains had already begun several days earlier, soils across much of the island were nearly saturated, causing most of the water to run off directly into rivers rather than getting absorbed into the ground and ultimately ending up in the ocean.
Galagedara calculates this phenomenon translated into a discharge rate of roughly 150,463 m3 (5.3 million ft3) per second.
“For comparison, the Amazon River — which has the highest discharge in the world — averages about 222,000 m3 [7.8 million ft3] per second,” he explained. “But when you consider catchment areas, Sri Lanka’s figure is disproportionate, as the Amazon drains a basin of about 7 million square kilometers [2.7 million square miles], whereas Sri Lanka’s total catchment area is only around 65,000 km2 [25,000 mi2].”
During Ditwah’s peak, Sri Lanka recorded an estimated 2.3 m3 (81 ft3) per second per km2 (0.4 mi2), compared with the Amazon’s 0.03 m3 (1.1 ft3) per km2, Galagedara noted. The extreme weather event highlights how extreme rainfall over a small island can generate disproportionately intense runoff, he said.
All of this excess water eventually reached the sea through Sri Lanka’s 103 rivers, including the Mahaweli, the island’s longest river, and others such as the Kelani, which meets the ocean near Colombo, the commercial capital. Satellite imagery captured during Ditwah’s peak shows vast volumes of freshwater entering coastal waters.
Sediments, salinity and coral stress
“The floodwaters carried colossal volumes of freshwater into the ocean, and this can have several impacts,” said Arjan Rajasuriya, a marine scientist. One major concern is sedimentation. “These sediments can smother corals. If corals remain covered for extended periods, they can die, sometimes through processes associated with bleaching,” he said.
Yet Sri Lanka lacks sediment gauging stations at river mouths, leaving scientists largely blind to how much material is being flushed into the sea during extreme weather events. Such stations are critical for quantifying erosion linked to landslides in the hill country, riverbed siltation that worsens flooding downstream and impacts on coastal ecosystems, fisheries and coral reefs, Rajasuriya noted. “Without this data, the true downstream cost of deforestation, hill-cutting and intense rainfall remains invisible.”
Rajasuriya added that sedimentation is not limited to extreme floods. Chronic soil erosion upstream means that reefs surviving today may already be those most tolerant of repeated sediment stress.
Freshwater influx itself can also stress corals. “We know that warm ocean currents can trigger coral bleaching, but sudden changes in salinity caused by heavy freshwater inflows can also stress corals and provoke bleaching responses,” said Nishan Perera, a marine biologist with the Blue Resources Trust. “Unfortunately, we don’t yet have enough research in Sri Lanka to fully understand these impacts,” he told Mongabay.
However, Sri Lanka’s eastern and northern coastal areas are getting fully blown away by the northeastern monsoon winds, so these will create powerful underwater water movements that would help to spread the large amount of freshwater and also thin out the sedimentation. Both Rajasuriya and Perera are hopeful that the natural processes themselves will help to reduce the impacts of Ditwah on coral habitats.
Waste swept from land to sea
Ditwah-induced floods also carried vast quantities of land-based waste into the Indian Ocean. As rivers overflowed, they swept plastics, polyethylene, construction debris and discarded consumer goods from towns and villages downstream. Much of this material is nonbiodegradable and can persist in the marine environment for decades, fragmenting into microplastics that add to an already growing pollution burden.
Ocean currents and wave action ensure that much of this debris does not remain offshore. Lighter plastics are repeatedly pushed back toward land, accumulating on beaches, coral reefs and mangroves. In Sri Lanka, where many coastal ecosystems are already under stress, this returning debris can smother turtle nesting sites, entangle marine life and degrade habitats that support fisheries and tourism.
According to Marine Environment Protection Authority chairman Samantha Gunasekara, more than 140 km (87 mi) of coastline have been affected by pollution linked to Ditwah, particularly near river mouths. Cleanup efforts, he said, will require thousands of human work hours.
Scientists warn that climate-driven extremes such as Cyclone Ditwah are likely to intensify this land-to-sea pollution pathway. Without effective solid waste management, flood-resilient infrastructure and rapid post-disaster cleanup, each major flood risks delivering another surge of plastics into the ocean.
Nutrient pulses and ocean productivity
Beyond sediment and debris, floods also deliver nutrients. “Cyclone Ditwah has likely delivered a massive pulse of land-derived nutrients into the coastal ocean,” said oceanographer Charitha Pattiaratchi of the University of Western Australia. “Floodwaters carry high loads of sediments, organic matter and nutrients such as nitrogen and phosphorus. This represents a sudden and large injection of nutrients to coastal waters, which can trigger extensive chlorophyll events.”
Such nutrient enrichment can drive short-term phytoplankton blooms, visible from satellites as intense chlorophyll concentrations. “While increased productivity might appear beneficial, these blooms can alter ecosystems, reduce water clarity and, in some cases, lead to oxygen depletion when the organic matter breaks down,” he said. The impacts can persist for weeks or months, depending on ocean circulation and weather conditions.
According to Arulananthan, the post-Ditwah fish advisories have indicated increased fish aggregations in some coastal areas, likely linked to enhanced plankton growth. However, nutrient-driven algal blooms can also become harmful, particularly in semi-enclosed bays and lagoons that trap water — a phenomenon previously observed along Sri Lanka’s eastern coast.
“What we are seeing is not just a land-based disaster,” Pattiaratchi said, “but a strong land-ocean connection. Understanding and monitoring these nutrient pulses is critical, as they have implications for fisheries, coral reefs and the overall health of the marine environment.”
Banner Image: Ditwah, a catastrophic tropical cyclone left a trail of destruction in 22 districts in Sri Lanka, with 650 reported deaths and over 200 people still missing. Image courtesy of the Department of Meteorology.
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