The study area could hardly be more exposed to petroleum pollution. The Arabian Sea receives more than 700 tonnes of oil spill input annually, a burden driven by some of the world’s busiest international tanker routes connecting Middle Eastern oil producers with Japan via the Malacca Strait and with South Africa via the Mozambique Channel. The eastern Arabian Sea, bordering the Indian states of Gujarat, Maharashtra, Goa, Karnataka and Kerala, is also dotted with offshore oil installations, major ports, and shipping lanes where wrecks and accidents occur with troubling regularity. Operational discharges such as tank washings and cargo residues add a steady background of petroleum input, while natural seepage from subsurface reservoirs contributes the remainder. Globally, anthropogenic sources account for slightly more than half of the marine tar load, and the Arabian Sea sits at the intersection of nearly every major contributor.

Tarballs are the end product of a chemical and physical transformation sequence that begins the moment crude oil hits seawater. Lighter hydrocarbon fractions evaporate or dissolve within hours, while wave turbulence and wind shear whip the remaining oil into a water-in-oil emulsion that is foamy and stickier than the parent crude. Roughly half of a typical spill disperses through weathering within twenty-four hours, but the heavier asphaltene-rich residue persists, accumulating debris and fragmenting into small, dark aggregates that are hard and brittle on the outside and soft and semi-solid within. Because these aggregates are less dense than seawater, they float at the surface, riding currents and winds for weeks. The study reports that floating tarballs can remain in the marine environment for sixty to ninety days, ample time to travel hundreds of kilometres before washing onto beaches, mangroves and other sensitive coastal habitats.

The fieldwork behind the new findings was conducted as part of the Marine Ecosystem Dynamics of Eastern Arabian Sea programme, a national initiative of India’s Ministry of Earth Sciences. Researchers aboard the vessels ORV Sagar Kanya and FORV Sagar Sampada repeatedly occupied ten transects running from Cape Comorin in the south to Okha in the north, visually surveying the sea surface each month. Tarballs appeared only in May, September, October, November and December of 2018, and were collected at stations off Mumbai, Goa, Ratnagiri and Bhatkal using buckets and a towed bongo net. The observed tarballs ranged from 0.1 to 4 centimetres in diameter, with the largest specimens, between 2 and 4 centimetres, found off Mumbai in November and December. Notably, no tarballs were sighted during the peak summer monsoon months of June through August, when winds exceeded 9 metres per second and rough seas likely broke oil patches into fragments scattered over much wider areas.

The spatial pattern of sightings maps neatly onto the region’s reversing circulation. The West India Coastal Current flows poleward from November to February and equatorward from April to September, and the researchers used satellite-derived surface current data from NASA’s OSCAR dataset to show how these currents, combined with cross-shore winds, herd tarballs toward or away from the coast. In May, tarballs appeared near Mumbai under onshore-directed currents of less than 0.15 metres per second. By December, during the winter monsoon, tarballs were detected in open-ocean waters as far as 350 kilometres from the coast, caught in well-defined northward current pathways. Many of the tarballs carried goose barnacles of varying sizes, a biological clock of sorts: large barnacles off Bhatkal in September suggested prolonged residence at sea, while small ones off Mumbai indicated fresher arrivals. The authors argue that these sighting locations can be used to validate forward-tracking and backtracking models capable of pinpointing the original sources of the oil.

Chemical analysis revealed that the tarballs are far more than a nuisance for beachgoers. Using inductively coupled plasma optical emission spectrometry, the team measured ten metals and found strikingly elevated concentrations, particularly at nearshore stations off Mumbai, where zinc reached 2,039 parts per million, copper 2,514.8 ppm, nickel 2,293.7 ppm, cobalt 2,220.6 ppm, chromium 2,746.3 ppm, lead 1,396.3 ppm and cadmium as high as 1,354.0 ppm. Iron peaked at 20,052.5 ppm at a station between Ratnagiri and Mumbai, and magnesium reached 53,385.5 ppm off Mumbai. The authors attribute the higher nearshore values to land runoff and industrial activity, noting that the tarballs’ high surface area and hydrophobic chemistry allow them to adsorb additional trace metals from the water column as they drift through polluted coastal waters. The heavy metal burden is partly inherited from the asphaltene fraction of the parent crude oil, which acts as a natural chelating agent for elements such as nickel, vanadium, iron and copper.

The organic fraction of the tarballs proved equally alarming. Gas chromatography tandem mass spectrometry identified eleven polycyclic aromatic hydrocarbons, a class of fused-ring compounds listed by the United States Environmental Protection Agency as priority pollutants for their carcinogenic, mutagenic and toxic properties. Low molecular weight PAHs with two to three rings dominated at stations off Mumbai and Ratnagiri, reaching concentrations of 1,497 and 525 micrograms per gram respectively, a signature the researchers interpret as evidence of relatively fresh petrogenic input, since these lighter compounds degrade and volatilise readily. Four-ring PAHs dominated at four other stations, indicating older, more weathered material. Because low molecular weight PAHs are more water-soluble and bioavailable, freshly formed tarballs are actually more toxic than aged ones, and small tarballs can enter the food chain most easily, consumed directly by filter feeders including whales, sharks, fish and zooplankton.

To trace where the oil came from, the team applied diagnostic ratio analysis, comparing the relative abundance of anthracene to phenanthrene and fluoranthene to pyrene. These isomer pairs form at different rates depending on whether petroleum matured slowly at low temperature underground or was generated by high-temperature combustion. The ratios pointed overwhelmingly to a petrogenic origin, meaning the tarballs derive from crude oil or refined petroleum released through spills, shipping operations or natural seepage, with only a few samples showing values consistent with fossil fuel combustion, plausibly linked to the region’s heavy marine traffic. The authors caution that diagnostic ratios alone provide only a preliminary source assessment, and that definitive fingerprinting would require biomarker or isotopic techniques. Even so, the analysis confirms that the Eastern Arabian Sea is receiving continuous inputs of fresh petroleum rather than merely recycling old residues.

The ecological consequences extend well beyond chemical toxicity. Oil films at the sea surface can impede air-sea gas exchange, promoting temporary anoxia in a basin already notorious for its expanding oxygen minimum zone. Tarballs that beach can fuse with plastic debris to form composite aggregates called plastitar, a newly recognised sink for coastal plastic contamination. Following a shipwreck off Kochi in May 2025, oil contamination was detected in zooplankton, raising the prospect of petroleum residues moving up the food web into fish. Perhaps most unsettling for public health, previous research has found that disease-causing bacteria, including Vibrio vulnificus, occur on tarball surfaces at counts significantly higher than in surrounding sand and seawater, meaning tarballs may serve as rafts dispersing pathogens and non-indigenous species such as barnacles across entire ocean basins.

The study’s authors are candid about its limitations: sampling was opportunistic rather than standardized, no quantitative density measurements were possible, and some samples could not be retained for full chemical characterisation. They frame the work as a qualitative baseline and call for standardized monitoring, improved oil spill response and stronger regulation. With heavy metals known to cause renal failure, birth defects, and damage to the nervous, cardiovascular and respiratory systems, and with PAHs bioaccumulating and biomagnifying through marine food webs to reach human consumers, the stakes are high. The findings feed directly into United Nations Sustainable Development Goal 14 on conserving oceans, and they deliver a clear message: the monsoon that replenishes India’s west coast also delivers its oil pollution, and only sustained observation can reveal where it comes from and how to stop it.

Subject of Research: Spatial and temporal distribution of floating tarballs and their associated heavy metal and PAH pollution in the Eastern Arabian Sea

Article Title: Spatial and temporal distribution of floating tarballs and associated ecological pollution in the Eastern Arabian Sea

Article References: Shaju, S. S., Naseera, K., Ramu, C. V., Ardra, K. R., Kumar, V. A., & Gupta, G. V. M. (2026). Spatial and temporal distribution of floating tarballs and associated ecological pollution in the Eastern Arabian Sea. Discover Oceans, 3(1), Article 65. https://doi.org/10.1007/s44289-026-00180-y

Image Credits: AI Generated

DOI: 10.1007/s44289-026-00180-y

Keywords: tarballs, oil pollution, Eastern Arabian Sea, polycyclic aromatic hydrocarbons, heavy metals, monsoon currents, West India Coastal Current, marine ecotoxicology, goose barnacles, Vibrio, coastal pollution, SDG 14

The study, published in Discover Oceans by Madhurima Paul, Rashmi R. Nayak, and Arun Chakraborty of the Indian Institute of Technology Kharagpur, draws on 263 depth-resolved profiles collected between March 2015 and December 2020 by a biogeochemical Argo float (WMO ID 2902174) deployed by the Indian National Centre for Ocean Information Services. Every ten days, the float dove to 2000 meters and ascended, measuring temperature, salinity, dissolved oxygen, and chlorophyll-a fluorescence along the way. Because the float stayed within a compact box between roughly 70.3°E and 73.35°E and 13.2°N and 17.2°N, the researchers could build a clean, location-specific climatology of how plankton biomass is distributed through the water column across the entire annual cycle.

The headline finding is deceptively simple. Surface chlorophyll-a in the CEAS climbs through the southwest monsoon months, peaking at about 0.31 milligrams per cubic meter in August, then plummets sharply in September before surging back to roughly 0.36 milligrams per cubic meter in October. Satellite ocean-color data and the float measurements agree closely, with a correlation of 0.86, and the September dip and October rebound appeared in every single year of the record. That consistency rules out a one-off anomaly and marks the pattern as a genuine climatological signature of the region, one that neither the persistently upwelling southeastern Arabian Sea to the south nor the freshwater-dominated northern waters display in the same way.

Why does September fail? The answer, according to the study, lies in the physics of the retreating monsoon. During the peak monsoon, strong winds associated with the low-level Findlater Jet push wind stress to around 0.13 to 0.14 newtons per square meter, driving turbulent mixing and positive Ekman pumping velocities of 1.5 to 2.5 times ten to the minus six meters per second. This Ekman suction draws cold, salty, nutrient-rich water upward, feeding the phytoplankton and deepening the mixed layer to about 40 meters. In September, however, wind stress collapses to roughly 0.06 newtons per square meter, the wind stress curl weakens, and the Ekman pumping velocity drops sharply. Vertical mixing falters, the upward nutrient conveyor stalls, and the phytoplankton bloom runs out of fuel despite the water still being relatively cool.

The float data add a crucial vertical dimension that satellites simply cannot capture. During the spring intermonsoon from March to May, intense solar heating stratifies the upper ocean, shoaling the mixed layer to around 20 meters. Surface nutrients are rapidly exhausted, and near-surface chlorophyll falls below 0.1 milligrams per cubic meter. Yet below the mixed layer, chlorophyll thrives, reaching about 0.2 milligrams per cubic meter in a pronounced subsurface chlorophyll maximum between 30 and 50 meters, where light from the deeply penetrating euphotic zone, extending to 100 to 140 meters, meets residual nutrients. Integrated biomass during this period is modest at about 17 milligrams per square meter, but most of it sits beneath the layer that satellites would count.

This vertical seesaw between the mixed layer and the water beneath it is the study’s central structural insight. Correlation analysis shows that mixed layer depth is positively linked with chlorophyll in the near-surface and mixed layers (a correlation coefficient of 0.48) and strongly negatively correlated with chlorophyll below the mixed layer (minus 0.87). In other words, when monsoon winds deepen the mixed layer and pump nutrients upward, biomass concentrates near the surface and thins at depth; when the ocean calms and stratifies, the opposite occurs, and phytoplankton migrate downward to a depth where light and nutrients overlap. The subsurface chlorophyll maximum, which the authors note can account for nearly one-fifth of total euphotic-zone biomass basin-wide, is a hidden reservoir that satellite chlorophyll maps systematically miss.

The October rebound has its own mechanism, and it is not simply the mirror image of the September collapse. As the southwest monsoon winds die away, sea surface temperature climbs to a secondary peak of about 29.2 degrees Celsius, the mixed layer shoals to roughly 17 meters, and sea surface salinity drops sharply to about 35.4 practical salinity units. That freshening reflects the poleward reversal of the West India Coastal Current, which begins transporting low-salinity water from the Bay of Bengal along the coast, alongside increased discharge from rivers such as the Periyar, Vaigai, and Tambraparni. These freshwater inputs, combined with northward-flowing surface currents, deliver nutrients and displace nutrient-rich water northward, driving the sharp rise in near-surface chlorophyll, concentrated mainly in the top ten meters of the water column.

The study also highlights how salinity structure shapes the region’s biology in less obvious ways. A barrier layer, the salt-stratified zone between the isothermal layer depth and the mixed layer, thickens markedly during the winter monsoon months of December through February, insulating the mixed layer from cooler subsurface water and influencing sea surface temperature evolution. Meanwhile, the depth of the 20 degree Celsius isotherm, a standard proxy for thermocline depth, deepens during winter, a feature the authors link to heat trapped by the Findlater Jet during the previous summer and redistributed by westward-propagating downwelling Rossby waves. As spring approaches, these waves decay, the thermocline shoals, and the trapped heat is ventilated upward, resetting the upper-ocean structure for the next annual cycle.

Ecologically, the findings help explain why the CEAS sits in an awkward middle ground. To the south, the southeastern Arabian Sea’s vigorous upwelling supports large phytoplankton, rich fisheries, and planktivorous fish communities; to the north, winter convective mixing sustains an extended growing season. The CEAS, by contrast, experiences weaker winds, reduced upwelling, and a deeper thermocline, resulting in lower chlorophyll and integrated biomass during winter and a weaker fishery dominated by different trophic structures. The region also acts as a perennial source of carbon dioxide to the atmosphere in some seasons, while river-fed productivity during the southwest monsoon can temporarily tip it toward being a carbon sink, underscoring how sensitive its biogeochemical balance is to the timing of winds, currents, and runoff.

Perhaps the study’s most important message is methodological. A single float, profiling faithfully every ten days for six years, exposed a seasonal pattern invisible to two decades of satellite ocean color and decades of shipboard sampling. As climate change alters monsoon winds, river discharge, and ocean stratification, the biological response of regions like the CEAS will depend on fine-scale vertical processes that only autonomous biogeochemical sensors can resolve. The September dip and October rebound documented here are more than a regional curiosity; they are a demonstration that the ocean’s hidden interior still holds surprises, and that the robotic observers drifting silently through it are our best hope of catching them before the baseline itself begins to shift.

Subject of Research: Seasonal variability of chlorophyll-a and phytoplankton biomass in the central eastern Arabian Sea observed by biogeochemical Argo floats

Article Title: Variability of chlorophyll-a in the central part of the eastern Arabian Sea using biogeochemical Argo observations

Article References: Paul, M., Nayak, R. R., & Chakraborty, A. (2026). Variability of chlorophyll-a in the central part of the eastern Arabian Sea using biogeochemical Argo observations. Discover Oceans, 3(1), Article 15. https://doi.org/10.1007/s44289-026-00128-2

Image Credits: AI Generated

DOI: 10.1007/s44289-026-00128-2

Keywords: chlorophyll-a, Arabian Sea, BGC-Argo, phytoplankton, monsoon, upwelling, mixed layer depth, subsurface chlorophyll maximum, West India Coastal Current, ocean biogeochemistry, satellite ocean color, marine productivity