Twenty-Dollar Sediment Trap Catches Microfossil Journeys Along Bay of Bengal Coast

On a windswept intertidal flat at Junput, where the Pichaboni River meets the Bay of Bengal, a team of Indian researchers has shown that some of the most revealing instruments in ocean science do not need to be expensive. In a pilot study published in Discover Oceans, Arkaprava Mandal and Punyasloke Bhadury of IISER Kolkata describe a custom-built sediment trap costing roughly twenty US dollars per unit, designed specifically for the violent, sediment-choked shallows of the northeast coastal Bay of Bengal. Their results, though drawn from a modest recovery of just three traps out of eighteen, offer a rare window into how microscopic shells are shuffled, resuspended and redeposited in one of the most energetic coastal environments on Earth.

Benthic foraminifera are single-celled organisms that build intricate calcareous or agglutinated shells, known as tests, which persist in sediments long after the organisms die. Because different species tolerate different salinities, substrates and organic matter levels, their assemblages are widely used as proxies in palaeoecology and sedimentology. Increasingly, researchers have recognised a second role: as tracers of sediment transport itself. When dead tests are lifted from the seabed and carried laterally by currents, they carry information about where sediment has come from and how it moved. The problem is that in shallow, tide-swept settings, conventional sediment traps designed for calm offshore waters are quickly buried, displaced or destroyed.

The northeast Bay of Bengal is an extreme case. The Ganga–Brahmaputra–Meghna river system delivers more than one billion tonnes of sediment annually, of which roughly 0.7 billion tonnes reaches the bay, supplemented by peninsular rivers such as the Mahanadi, Godavari and Krishna. This relentless input reshapes coastal hydrodynamics, salinity gradients and sediment texture, all of which influence where foraminiferal assemblages live and how their tests are preserved. Previous sediment trap studies in the region focused on deep offshore particle fluxes and biogeochemical cycling, leaving shallow intertidal sediment reworking essentially unobserved with in-situ tools.

The trap the researchers designed is deliberately simple. A cylindrical body, 14 centimetres in diameter and 20 centimetres tall, was fabricated from durable 304H alloy steel and mounted on a base plate measuring 24 by 22 centimetres, giving the assembled unit a mass of about two kilograms, more than half of it in the base. Multiple evenly distributed inlet openings, each covered with a 250-micrometre mesh grid, allow water and suspended particles to enter while excluding large debris and dampening turbulent mixing. The small diameter was chosen to reduce hydrodynamic drag, and the openings were positioned roughly five centimetres above the sediment surface to capture recently resuspended material while minimising direct bedload entry. The bases were partially buried in the substrate for stability, with no tethers or stakes.

Eighteen traps were deployed across the Junput intertidal zone for 60 days, from November to December 2022, spanning the relatively calm post-monsoon season when the East India Coastal Current flows southward along the shelf. Deployment sites represented distinct micro-niches: sheltered spots near the estuarine bank, mid-intertidal flats and areas adjacent to small boat activity. The realities of the environment were unforgiving. Despite preliminary field tests that had achieved retrieval success above 70 percent, only three traps, numbers 1, 2 and 10, were recovered at their recorded GPS positions after the full deployment. The remaining fifteen were most likely displaced by strong tidal currents and wave action or buried beneath mobile sediments; no broken components were ever found.

The three recovered traps nevertheless yielded rich material. Trap 1, near the mouth of the Pichaboni River, collected about 154 grams of sediment, Trap 2 about 191 grams and Trap 10, on the adjacent open beach, about 187 grams. Grain-size analysis showed sandy silt dominating at 43 to 54 percent of the trapped material, with fine silt and clay making up roughly 9 to 11 percent. Total organic carbon was remarkably consistent across traps at about 6.4 milligrams per gram, indicating that sedimentation was dominated by lithogenic particles rather than organic-rich material. Nitric acid-extractable elemental analysis detected terrigenous-associated metals including iron, magnesium, manganese, zinc and copper, while calcium remained comparatively low, most plausibly because carbonate particles were diluted by the flood of siliciclastic sediment from freshwater discharge.

The biological signal was striking. The traps contained between 80 and 110 foraminiferal tests per 10 cubic centimetres of sediment, representing eight taxa from five families, with the assemblage dominated by Asterorotalia trispinosa and Ammonia sp. 1. Rose Bengal staining, applied within six hours of retrieval and maintained for two weeks, revealed that every single test was dead at the time of deposition, with no stained cytoplasm in any specimen. This is the study’s central clue: the shells were not a living community but a post-mortem assemblage, resuspended from the seabed and transported before settling into the traps. The scarcity of visibly degraded tests suggests transport and burial were rapid enough to limit chemical dissolution.

The taxonomic composition points to active sediment reworking across the nearshore zone. Ammonia species tolerate wide salinity variation and thrive in estuarine muddy substrates, while Quinqueloculina species favour shallow, high-energy marine settings. Their co-occurrence, alongside Bolivina, a recognised shelf indicator, in traps positioned seaward of those habitats, implies resuspension from shallow-shelf sediments and landward transport by tidal currents. Even more telling was the presence of planktonic Globigerinoides tests, organisms that live in open-ocean surface waters, at abundances of 10 to 20 tests per 10 cubic centimetres. Their arrival in an intertidal trap can only be explained by lateral advection and coastal mixing from offshore settings, or by resuspension from surficial sediments before coastward transport. Broken tests, most abundant in the open-beach trap, further attest to the physical battering of transport. Comparison with short sediment cores from the same flats, which held only 20 to 30 tests per 10 cubic centimetres dominated by Ammonia and Quinqueloculina, confirmed that the traps preferentially captured recently moved material rather than the long-term background accumulation.

The authors are careful about the limits of their pilot deployment. Without concurrent measurements of current velocity, wave characteristics or bed shear stress, the inferred transport mechanisms remain plausible interpretations grounded in the recovered evidence and the established hydrodynamics of the region, not direct observations. The 17 percent recovery rate underscores how difficult intertidal trap work is, and future versions of the design will need stronger anchoring, enhanced base stabilisation, internal flow baffles to reduce turbulence and possibly integrated current sensors. Longer deployments across multiple seasons, expanded geochemical work including major oxide and carbonate analyses, and hydrodynamic monitoring or numerical modelling are all flagged as necessary next steps. Yet the study’s core achievement stands: a trap costing less than twenty dollars, reusable and simple to fabricate, successfully captured both mineral particles and foraminiferal tests in an environment that defeats conventional instruments, with foraminiferal recoveries exceeding those reported from many coastal and shelf deployments. For resource-limited coastal nations facing shifting hydrological conditions, that combination of cheapness and capability could prove transformative, turning sediment transport from an inferred process into a directly measured one, one microscopic shell at a time.

Subject of Research: Benthic foraminiferal sediment transport and sedimentation in the high-energy intertidal northeast coastal Bay of Bengal using a low-cost custom sediment trap

Article Title: Benthic foraminiferal sedimentation in the northeast coastal Bay of Bengal using a custom designed sediment trap

Article References: Benthic foraminiferal sedimentation in the northeast coastal Bay of Bengal using a custom designed sediment trap. (n.d.). https://doi.org/10.1007/s44289-026-00181-x

Image Credits: AI Generated

DOI: 10.1007/s44289-026-00181-x

Keywords: benthic foraminifera, sediment trap, Bay of Bengal, intertidal zone, sediment transport, resuspension, Rose Bengal staining, coastal sedimentation, Ammonia, Globigerinoides, low-cost instrumentation, biogenic particles