Cyclone Yass Reshaped Copepod Communities in an Indian Estuary Within Weeks

When Cyclone Yass slammed into India’s east coast on 26 May 2021, it did more than flatten homes and flood villages along the Bay of Bengal. According to a new open-access study published in Discover Ecology, the category-1 storm also quietly rewired the microscopic foundation of the food web in the Muriganga estuary of West Bengal, shifting which tiny crustaceans dominate its brackish waters and erasing the natural spatial patchiness that ecologists had documented there for years. The research, led by Sourav Paul of the University of Calcutta together with Samya Karan and Bhaskar Deb Bhattacharya of the Estuarine and Coastal Studies Foundation, offers one of the most detailed before-and-after portraits yet of how a tropical cyclone reshuffles a zooplankton community in a tropical river-estuary.

The Muriganga estuary, part of the Indian Sundarbans delta, is a macro-tidal system fed by freshwater from the Ganges, known locally as the Hooghly River. Under stable conditions it remains mesohaline, meaning its salinity sits in a moderate range, and slightly alkaline with a pH above 8. The estuary had already endured a punishing sequence of storms: Cyclone Fani in 2019, then Bulbul and Amphan in 2020 and 2021. Paul and colleagues had anticipated this vulnerability. In 2019 they established a dedicated ‘Cyclone Ecology’ research program with three permanent sampling stations arranged along a north-south axis of the estuary, specifically so that when the next storm arrived, they would have baseline data in hand rather than scrambling to reconstruct it afterward.

That foresight proved decisive. Yass made landfall near Dhamra Port in Odisha with maximum sustained winds of 130 to 140 kilometers per hour, gusting to 155, before sweeping across the West Bengal coast and passing directly over the program’s sampling stations. The timing was brutal in more ways than one. The storm coincided with an astronomical high tide of 5.71 meters at nearby Sagar Island, and Yass drove a storm surge of 3.5 to 5 meters, with tidal waves rising 1 to 2 meters above the astronomical tide. Nearly 1,195 millimeters of rain fell in a short period, triggering massive inundation on both banks of the estuary. In the Bay of Bengal itself, sea surface temperature dropped by about 3 degrees Celsius, while salinity and water density rose by roughly 1 practical salinity unit and 2 kilograms per cubic meter respectively.

The team’s sampling design was deliberately asymmetric, reflecting the realities of fieldwork in a disaster zone. Pre-cyclone samples were collected on three occasions between November 2020 and April 2021, providing a solid baseline of the estuary’s copepod community. After Yass, the flooding destroyed roads to Namkhana, the town adjacent to the estuary, and the COVID-19 pandemic complicated the mobilization of scientific resources. Despite these obstacles, the researchers managed four post-cyclone sampling trips at six-day intervals between 4 and 22 June 2021. On each visit they towed a 200-micrometer plankton net fitted with a mechanical flowmeter from a dinghy boat after sunset on the high tide, sampling sub-surface water at depths of 0.3 to 0.5 meters. Water temperature, salinity and pH were recorded simultaneously with a multi-parameter probe, and copepods were preserved in 4 percent formalin for species-level identification under the microscope.

The physical fingerprint of the storm was unmistakable. Before Yass, salinity in the estuary ranged from 7.50 to 17.90; afterward it climbed to a sustained range of 15.90 to 20.80, transforming the system from mesohaline to polyhaline for weeks. This saltwater intrusion from the adjacent Bay of Bengal echoed the aftermath of Cyclone Aila in 2009, but stood in contrast to Fani, Bulbul and Amphan, none of which had shifted the estuary’s salinity regime in this way. Water temperature, by comparison, showed no drastic change, remaining between 29.10 and 31.20 degrees Celsius in the post-cyclone period, consistent with the typical seasonal behavior of a tropical estuary. The pH stayed slightly alkaline, likely a consequence of the persistent high salinity, a pattern previously observed after Cyclone Phailin pushed salt water toward the estuary mouth.

What happened to the copepods was more surprising. Previous cyclones hitting the Muriganga had consistently knocked down both species richness and total abundance, and similar declines have been reported after storms in estuaries across India, South Africa and Taiwan. After Yass, the opposite occurred. Species richness rose from a pre-cyclone range of 17 to 20 species to a post-cyclone range of 20 to 24. Shannon diversity climbed from 2.37 to 2.61 up to 2.79 to 3.00, while Simpson’s index and Pielou’s evenness both increased as well. Most strikingly, total copepod abundance more than doubled, from 307,044 individuals per cubic meter before the storm to 650,635 individuals per cubic meter afterward. The authors caution that the asymmetry in temporal replicates before and after the storm means these figures should be interpreted carefully, but the direction of change contradicts nearly every previous cyclone study in the region.

The identity of the dominant species changed too. Before Yass, the tiny calanoid copepod Bestiolina similis was the most abundant member of the community, contributing up to 20.52 percent of total abundance, with Paracalanus parvus and Acartiella tortaniformis as co-dominants. After the storm, Acartiella tortaniformis took over as the most abundant species, followed by the cyclopoid Oithona brevicornis and Paracalanus parvus. Relative abundances of Canthocalanus pauper, Temora turbinata, Corycaeus crassiusculus and various Oithona species all rose. At the family level, the Paracalanidae, which had made up 49 to 59 percent of the community before the storm, fell to 28 to 35 percent, while the Oithonidae surged from 3 to 8 percent up to 13 to 19 percent. Two species, Labidocera euchaeta and Oithona nana, appeared in post-cyclone samples despite being entirely absent before. Labidocera euchaeta is not a resident of the Muriganga at all; it is a marine intruder, and the polyhaline conditions created by the storm surge likely allowed it to thrive temporarily, much as marine migrant species had flooded into South Africa’s St. Lucia estuary after Cyclone Imboa.

The rise of the Oithonidae carries a plausible ecological explanation. Oithona species are omnivores that feed heavily on organic debris, dinoflagellates, diatoms and proto-zooplankton, and cyclones typically flush large quantities of organic matter into estuarine waters. The same pattern emerged after Cyclone Aila, when the relative abundance of Oithona brevicornis jumped from zero to 10.5 percent, and after Cyclone Hudhud struck Chilika Lagoon, where cyclopoid copepods also increased. In other words, Yass may not have simply scattered the community; it may have restructured the food web’s resource base in a way that favored debris-feeding cyclopoids over the calanoids that had dominated under stable conditions.

Perhaps the most conceptually important finding concerns spatial homogenization. Before the cyclone, multivariate analyses using non-metric multidimensional scaling, Bray-Curtis dissimilarity and PERMANOVA revealed significant station-to-station differences in copepod assemblages, with a Pseudo-F of 17.66 and a p-value of 0.005, indicating that each sampling site hosted a distinct community. After Yass, that spatial structure vanished entirely: the PERMANOVA returned a Pseudo-F of just 0.41 with a p-value of 0.95, and SIMPER analysis showed average within-period similarity jumping from 37.04 percent to 77.09 percent. The storm, through its massive flooding and mechanical forcing, appears to have blended the copepod community into a single homogeneous assemblage across the estuary, a short-term homogenization of the spatial niche also observed after Cyclone Amphan.

Beyond its findings about one estuary, the study delivers a pointed methodological message. The rapid recolonization and restructuring observed within days to weeks suggests that bi-weekly, monthly or seasonal sampling frequencies are too coarse to capture the short-term ecological dynamics that follow a cyclone. The authors recommend that sampling begin within the first week after landfall and continue at daily or weekly intervals for weeks or months, with the timeline calibrated to the estuary’s water residence time, the distance from the landfall site and the complexity of the copepod community. They even suggest deploying Lagrangian floats to record real-time changes as a cyclone passes. Because India has no institutional mechanism for regular estuarine monitoring, the researchers argue that the United Nations Ocean Decade, running from 2021 to 2030, should serve as the baseline for establishing such programs. With cyclone intensity increasing in the Bay of Bengal, the copepods of the Muriganga may be among the most sensitive early-warning indicators of how tropical estuaries will weather a stormier future.

Subject of Research: Short-term effects of tropical cyclone Yass on copepod community structure in the Muriganga estuary, India

Article Title: Short-term impact of cyclone Yass on the copepod community of Muriganga estuary of India

Article References: Paul, S., Karan, S., & Bhattacharya, B. D. (2026). Short-term impact of cyclone Yass on the copepod community of Muriganga estuary of India. Discover Ecology, 2(1), Article 7. https://doi.org/10.1007/s44396-026-00024-1

Image Credits: AI Generated

DOI: 10.1007/s44396-026-00024-1

Keywords: Cyclone Yass, copepods, Muriganga estuary, Indian Sundarbans, zooplankton, estuarine ecology, Bay of Bengal, tropical cyclones, Oithona brevicornis, salinity, biodiversity, cyclone ecology