- A study investigated the spatial distribution of seagrass in Palk Bay, a dugong habitat, in Tamil Nadu.
- Research identified areas of healthy seagrass beds, which are critical marine habitats, and other areas with no vegetation with a potential for restoration.
- The study, using acoustics and ground-truthing techniques, highlights the utility of the combined approach to map seagrass beds and the potential to scale up seagrass restoration.
A new study maps the spatial distribution and status of seagrass beds along Manora’s coast in Tamil Nadu, a known dugong habitat, using acoustics and ground-truthing techniques, underscoring the potential for seagrass restoration.
Seagrasses are marine flowering plants that form meadows in shallow, warm waters. They are critical habitats for marine species like dugongs, turtles, fish, and invertebrates, as well as feeding grounds and nurseries for others. They also serve as powerful carbon sinks, capturing and storing significant amounts of carbon, thus, aiding in climate change mitigation.
Seagrass ecosystems are increasingly threatened by anthropogenic activities, climate change, and natural disasters, putting the marine life that depends on them at risk. In recent years, however, the global movement to conserve these vital ecosystems has gained significant momentum.
With that in focus, a team of researchers conducted a small-scale study to map and assess the spatial distribution and restoration potential of seagrass beds in the Palk Bay region which hosts the Dugong Conservation Reserve (DCR). Established in 2022, the reserve, spread over approximately 500 sq. km. protects the endangered dugong (Dugong dugon) or “sea cow”, a species that subsists entirely on seagrasses. The DCR is India’s first protected area dedicated to dugongs. The research was conducted by a team from OMCAR Foundation (Organization for Marine Conservation, Awareness and Research), Palk Bay, Tamil Nadu and Central Marine Fisheries Research Institute (CMFRI), Karwar, Karnataka.
Large meadows of seagrass are found in the Gulf of Mannar and Palk Bay. As per a 2022 estimate by the National Centre for Sustainable Coastal Management, the seagrass ecosystem in India spans 516.59 sq. km.; its CO2 sequestration rate is up to 434.9 tonnes/sq. km. per year with an annual net CO2 sink of 0.75 million tonnes for an area of 517 sq. km.
Called biological sentinels for their ability to be indicators of ecosystem health, they are also considered ecological engineers as they modify and stabilise the ecosystem. Seagrasses are reported to provide 28 ecosystem services and several of these services vary across genera and bioregions.

Mapping a critical seagrass habitat
The study was conducted approximately two kilometres off the Manora coast in Tamil Nadu and covered an area of 500 hectares (five sq. km.) in a known dugong habitat, with multiple documented sightings by local fishers and others.
The acoustic survey revealed that 46% (2.3 sq. km.) of the surveyed area was covered by seagrasses and seaweeds, while the remaining 54% (2.7 sq. km.) was open seafloor. The depth of the sampled areas ranged from 1.88 to 3.76 metres.
The dominant seagrass species identified was Cymodocea serrulata, commonly known as serrated ribbon seagrass. Other seagrass species found in the area include Syringodium isoetifolium (noodle seagrass) and Halophila ovalis, all of which make up the diet of dugongs.
Traces of these seagrass species have been found in the guts of dugongs along the Tamil Nadu and Gujarat coasts according to a separate study done by the Wildlife Institute of India (WII) in 2022. Senior scientist Anant Pande of the Marine Programme at Wildlife Conservation Society-India, who was a part of the WII study, explains, “Gut content analysis on stranded dugongs along the Tamil Nadu coast showed the presence of both algae and seagrass. Fragments of Cymodocea spp. were dominant in the gut content, followed by Halophila spp., Syringodium spp., and Halodule spp. These results could have been affected by the fact that softer seagrasses like Halophila spp. would have already been digested by the time the samples were obtained from the stranded dugongs.”
Referring to the current study, director of OMCAR Foundation Balaji Vedharajan says, “The expanse of the seagrass beds here was unknown, and mapping it was important since the area is part of the Dugong Conservation Reserve. We assumed the seagrass cover would be around 25-30% or so, but we got a combined seagrass and seaweed cover of 46%, so that is good.”
Vedharajan, who was a part of the study, says the exact reasons for the open patches are yet unclear, adding: “It could be due to fishing disturbances or something else; we just need more time to understand the reasons. But there are some open spaces that have been degraded or uprooted by small, local trawlers referred to as country boat dragnets.”
The Thanjavur District Forest Office plans to launch efforts to restore the degraded seagrass beds in the region, making this mapping study a crucial baseline to guide their efforts.

Another study by WII, published in 2024 on the habitat suitability of dugongs highlights the importance of some parts of Palk Bay as a critical dugong habitat. Pande who took part in the study says, “The WII team had conducted a habitat suitability and risk assessment survey of the larger area, and north Palk Bay including the Manora coast emerged as an important zone. The entire area also has large patches of seagrass that are intact and that explains why dugongs frequent this part of Palk Bay more.”
Technology navigates challenges
The team used BioSonics MX Aquatic Habitat Mapping instrument/ Echo Sounder System – an echo sounder that can map and acquire data about submerged vegetation and substrates. Mounted on the side of a survey boat, the instrument emitted acoustic pulses that reflected off the seafloor and vegetation, capturing an imagery of the returned signals on a linked laptop. A Differential Global Positioning System (DGPS) was used to record precise geographic coordinates.
To validate the acoustic data, ground-truthing techniques – the process of directly verifying the accuracy of the field data – were carried out at 21 randomly selected sites. A camera attached to a PVC frame was dropped at the selected sites to capture visuals of the seafloor to confirm the presence or absence of seagrasses. Eight of the 21 sites had seagrasses, corroborating the acoustic results. A Van Veen grab – a manually operated instrument used to “grab” samples – was used to collect seagrass specimens for species identification and canopy height measurement.

Overcoming challenges
Explaining the challenges involved in executing the recent study, Vedharajan points to the turbidity of the water as a major hurdle. “Nearly half the bottom area of the Dugong Conservation Reserve is not visible by drone or through satellite imagery due to the turbidity of the water. The Cauvery drainage basin is close to the Thanjavur district and brings sediments with it. The water is turbid for most of the year.”
There were other challenges, too, which the authors believe can be addressed through more robust studies and sampling. A variable bottom substrate (seagrass, seaweed, sand, etc.) made the interpretation of the acoustic data difficult, introducing a margin of error in the findings. Replicating this study across a much larger area would also require more logistical support, Vedharajan says.
While this study was conducted over a small area, it highlighted the utility of acoustic remote sensing to map seagrass areas, and the relevance of ground-truthing.
The authors underscore the potential for targeted restoration efforts in the 270 hectares (2.7 sq km) of non-vegetated seafloor. They recommend long-term, eco-friendly restoration techniques coupled with stricter enforcement of regulations to prevent destructive activities such as damaging fishing practices, especially in seagrass-covered areas.
Restoration of any habitat requires rigorous work and the process is rarely straightforward, requiring a well-rounded approach, says scientist Elrika D’Souza, of Oceans and Coasts Programme at Nature Conservation Foundation, adding, “Seagrass meadows are neglected ecosystems and their value is underestimated. While they may provide habitat and food for several threatened species, other functions are overlooked. Seagrass conservation cannot merely focus on restoration through planting, one also needs to focus on the threats that have led to their decline.”
Adopting and refining insights from multiple studies like this one, in coastal areas could contribute towards larger efforts to map and restore degraded seagrass beds, preserve existing habitats, and ultimately, conserve marine biodiversity. However, there is a need for more research to fill in critical gaps before on-ground action can yield results.
Read more: Newly declared dugong conservation reserve will help protect marine biodiversity
Banner image: Representative image of Syringodium isoetifolium seagrass, which was found off the coast of Manora, Tamil Nadu, and are part of dugongs’ diet. Image by Philippe Bourjon via Wikimedia Commons (CC BY-SA 3.0).