Seagrass meadows are critical marine ecosystems that provide numerous ecological and economic benefits. The diversity and distribution of seagrasses are influenced by various regulatory factors, including benthic light availability, sediment profile, wave exposure, water temperature, and nutrient dynamics (Dennison and Alberte, 1987; Duarte, 1991; Malmer and Grip, 1994; Greve and Binzer, 2004). These factors play a crucial role in the growth and persistence of seagrass populations, as seagrasses rely on optimal light for photosynthesis, suitable sediments for proliferation, and nutrients for growth (Duarte, 2002). Resultant declines in seagrass populations are reported due to light reduction (Short and Wyllie-Echeverria, 1996), altered nutrient dynamics (Lee and Dunton, 2000), and wave action (Japar Sidik et al., 2018). Impacts of wave exposure are more prominently seen in intertidal zones, which are at the transition of land and sea. Only species able to cope with the heavy tidal fluctuations and associated parameters can thrive in these littoral zones, thus restricting the upslope distribution of seagrasses (Hemminga and Duarte, 2000). Variations in water temperature, whether seasonal or due to climate change (marine heat waves), are also known to influence seagrass photosynthesis and productivity (Barber and Behrens, 1985; Seddon and Cheshire, 2001; Díaz-Almela et al., 2009; Garrabou et al., 2022). Nevertheless, seagrasses exhibit wide-ranging acclimation strategies as a response to changes in their natural environment (Vermaat et al., 1998; Lee et al., 2007). An experimental study in the Philippines demonstrated that seagrass species have a varied tolerance to enhanced siltation and altered light irradiance in the water column (Bach et al., 1998; Terrados et al., 1998). Besides, the shift in turbidity levels resulted in local variation in seagrass diversity, growth, and depth limits. Accordingly, global research provides substantial evidence that natural variability in the environment influences seagrass ecosystems at local and regional scales.

Additionally, unprecedented human-induced stressors catalyze the natural factors impacting seagrasses (Orth et al., 2006). The rise in anthropogenic footprints in the seagrass meadows is majorly attributed to dredging, marina construction, destructive fishing practices, land reclamation, nutrient and sediment loading, turbidity, and habitat alteration (Short et al., 2007). As a result, subsequent seagrass declines are reported worldwide (Waycott et al., 2009; Gunderson et al., 2016). Unfortunately, the degradation and loss of seagrass habitats have significant implications for the environment and society. With global recognition as a valuable marine ecosystem, seagrass shares a multifaceted relationship with its environment. Seagrass meadows provide essential services by maintaining the health of adjacent habitats such as coral reefs and mangroves (Green and Short, 2003; Short et al., 2007). They also serve as nursery grounds for commercially important fish and invertebrates, supporting fisheries and contributing to local economies (Hemminga and Duarte, 2000; Unsworth et al., 2019; Berkström et al., 2020; United Nations Environment Programme [UNEP], 2020). Furthermore, seagrasses are crucial for the survival of megaherbivores like dugongs, as they constitute their primary food source (Short et al., 2007). Under the socioeconomic context, seagrasses provide economical services higher than terrestrial habitats (Costanza et al., 2014) and support local communities’ livelihood and well-being (Mtwana Nordlund et al., 2016; Unsworth et al., 2019). Thus, seagrass losses or deterioration significantly impact these valuable services offered (Duarte, 2002; Orth et al., 2006). For instance, Unsworth (2007) reported a reduction in fish and invertebrate stocks in the Wakatobi Marine National Park, Indonesia, in response to seagrass degradation.

Similar seagrass trajectories are predicted to accelerate, particularly in developing countries, since these regions critically lack legislative measures to protect seagrasses (Duarte, 2002). Sadly, seagrass management has received little attention in the tropics than the adjacent coral reefs (Mumby et al., 2006). Although positive human interventions have reported an optimistic reversing of seagrass losses (Elliott et al., 2007; de los Santos et al., 2019; Tan et al., 2020), such initiatives, along with in situ seagrass management are yet substantially lacking in the tropics, including Indian waters. The widely accepted global seagrass distribution model classifies India into a diverse Indo-Pacific bioregion (Short et al., 2007). Of which the ecologically rich Andaman and Nicobar Islands (ANI) form the country’s second most varied seagrass habitats (Thangaradjou and Bhatt, 2018). Seagrass meadows in ANI are crucial to supporting the dugong’s remnant insular population (D’Souza et al., 2015). A recent study highlighted the importance of resource-rich seagrass beds for aggregating dugongs, possibly for calf protection (Gole et al., 2023). Despite such crucial ecological significance of seagrass meadows, there is a lack of information on their spatiotemporal changes, threats, and ecological and economic impacts in ANI. A report by Paulose et al. (2013) outlines the large-scale seagrass denudation in ANI (~1619 ha) caused by the Sumatra-Andaman earthquake and the 2004 Indian Ocean Tsunami. The severe deposition of marine debris from this disturbance has further altered the species distribution trends (Thangaradjou et al., 2010a). In addition, ANI is also a geologically vulnerable region, with reports of ~ 486 earthquakes (Richter scale >4; United States Geological Survey, 2023 ¹ ) and 25 cyclonic storms/depressions (Indian Meteorological Department, 2023 ² ) that have hit the islands’ coastline in the recent times (2018 to 2022). Unfortunately, the extent of seagrass loss to these threats remains uncertain due to sparse and discontinuous spatial seagrass assessments in the Islands.

For effective seagrass management, the accuracy of spatial studies and robust ecological knowledge is of utmost importance. Detailed information on species’ checklists, distribution ranges, depth limits, and, most importantly, regulatory factors governing distribution and growth is essential for translating seagrass science to management and conservation (Unsworth, 2007; International Seagrass Biology Workshop 13, 2013 ³ ; McKenzie et al., 2020). A few significant challenges to studying seagrasses in ANI are the inaccessibility of sites to researchers, funds, and logistics required to perform marine surveys. Despite this, prior studies have given valuable baselines on species distribution, natural history, descriptive habitat preferences, and spatial extent of seagrasses in ANI (Jagtap, 1991; Jagtap, 1992; Das, 1996; Thangaradjou et al., 2010a; Thangaradjou et al., 2010b; Paulose et al., 2013; D’Souza et al., 2015; Ragavan et al., 2016; Savurirajan et al., 2018). However, a data gap is still reflected in many of the management crucial aspects of the region. Published literature largely lacks precise spatial information (locality) of meadows, hindering spatiotemporal comparisons like other global initiatives (Sudo et al., 2021). Despite three decades of seagrass exploration and research, only limited studies have a quantified seagrass-environment relationship (Savurirajan et al., 2018). Likewise, recent reports of new species’ local distribution and range (Savurirajan et al., 2015; Gole et al., 2022) indicate more seagrass exploration to upgrade species checklist and distribution trends. As most of the seagrass meadows in ANI are recovering from the impact of the 2004 tsunami, it is essential to update existing information on the species composition, distribution, and factors that determine the existing trends. To address these knowledge gaps, this study aimed to investigate the spatial diversity and distribution patterns of seagrass meadows along the accessible coastline of ANI. Contrary to prior spatial assessments, our work instead focused on studying the complex interactions of seagrass species with their natural environment. Thus, we also aimed to understand the environmental regulatory factors that influence seagrass occurrence and distribution locally. By conducting a comprehensive assessment of seagrass ecology, this study will complement the existing knowledge on seagrasses and collectively contribute to developing effective management and conservation strategies for seagrass habitats in ANI.

The spatial diversity, distribution, and growth of seagrasses in the explored meadows are primarily determined by water depth, habitat heterogeneity, and substrate availability. Our findings reveal significant variations in species richness between intertidal/shallow and deepwater meadows, with the former habitats supporting higher species (10) than the latter (4). All 11 observed species were found up to a depth of 5 m, either in mono-species or mixed meadows. Beyond this depth, the species composition shifted toward exclusive dominance of Halophila spp. and Halodule spp. Complex, with Halophila ovalis and H. decipiens marking the deepest recorded seagrasses from our study area (ANI), reaching depths of 21 m. While the general depth limits for Indo-tropical seagrasses are commonly less than 10 m (Short et al., 2007), our findings, along with regional studies from Hervey Bay and Cape York (Queensland), the Red Sea, and the Mediterranean Sea, indicate region-specific deepwater trends (Den Hartog, 1971; Lee Long, 1996; Short et al., 2007). Halophila spp. has been recorded from depths of 50 m in the tropical Atlantic region, 70 m in the Red Sea (Short et al., 2007), and 90 m in the Coral Sea (Den Hartog, 1971) highlighting the species’ adaptability to deeper regimes. Additionally, H. decipiens has been reported from 58 m in the Great Barrier Reef (Lee Long, 1996), while H. ovalis is commonly observed beyond 35 m (Coles et al., 2000). The deepest recorded seagrass sample to date is a one-time dredged sample of H. stipulacea from 145 m (Lipkin et al., 2003; Short et al., 2007). Our findings are thus, consistent with global evidence, supporting the notion that Halophila spp. and Halodule spp. are adaptive to depths.

Although our observed depth-scaled distribution trends are consistent with previous local and regional assessments from ANI and mainland India (Das, 1996; Jagtap, 1996; Jagtap, 1998; D’Souza et al., 2015),we report some stark contrast for few species’ regional colonization depths. For instance, Jagtap et al. (2003) reported Thalassia hemprichii and Cymodocea serrulata from a depth range of 0-10 m in Southeast India, as opposed to restricted littoral distribution in our study (with one-time observations from 2.5 m and 5 m, respectively). Similarly, the compensation depth of Halodule uninervis, one of the deepest species from our sites (15 m), was 3 m from Tamil Nadu (Jagtap et al., 2003). We speculate these differences to be driven by local variation in the physical environment of the two regions. Palk Bay and the Gulf of Mannar in Tamil Nadu have a shallow continental shelf, a vast seagrass expanse (often 5 to 10 km from the shore), and a tidal amplitude of around 0.3 to 0.5 m (Geevarghese et al., 2018). In contrast, in ANI habitat discontinuity, narrow shelf and sharp depth slopes (a few meters from the shore), results in irregularity in seagrass distribution (Das, 1996; Geevarghese et al., 2018). Considering the limited physical variation in Tamil Nadu, water depth likely plays a lesser role as a regulatory variable in shaping seagrass species distribution. Nevertheless, in ANI, we propose that water depth favors certain adaptive species over others, thus influencing the colonization depths of seagrasses (Lee et al., 2007; Short et al., 2011).

In our study, furthermore, we suggest that water depth not only regulates the species distribution ranges, but also the plant growth. Overall, seagrass coverage, shoot densities, and community biomass declined in deep waters (critical limit between ~ 2 to 5 m) consistent with reports from other geographic regions such as the Southeast coast, insular Lakshadweep (Jagtap, 1996; Jagtap, 1998), north-western Cuba (Buesa, 1975), and from the Mediterranean Sea (Drew, 1978). As light attenuates strongly in deeper waters, the observed variations are a possible outcome of differential traits of seagrasses to adapt to depths (Lee et al., 2007; Short et al., 2011; Minguito-Frutos et al., 2023). Seagrass depth penetration and growth are directly influenced by light availability (Zieman and Wetzel, 1980; Duarte, 1991), owing to which conducive growth environment declines in the subtidal regions (Dennison and Alberte, 1987). Within similar environmental conditions, species-specific variation in colonization depths is a response to range of factors such as plant architecture, growth strategies, physiological responses, and acclimation potential (Dennison et al., 1993; Alcoverro et al., 2001; Greve and Binzer, 2004; Bité et al., 2007; Silva et al., 2013; Kilminster et al., 2015; Schubert et al., 2018; Tuya et al., 2019). A synthesis of the seagrass-water depth relationship indicates that seagrasses restrict their rhizome development in deeper regimes, with subsequent growth reduction (Duarte, 1991). This ability further is highly pronounced in pioneer, small-sized species with fast recolonization rates allowing them to cope up in deeper regimes. For instance, Halophila spp. and Halodule spp. can more efficiently regulate their respiratory demands in depths than large-sized seagrass species (Fourqurean et al., 1995). Therefore, deep water colonization of Halophila and Halodule species complex in our study could be accounted for higher acclimation potentials at greater depths (Minguito-Frutos et al., 2023). Since light attenuation is not the only predictor of depth distributions (Koch, 2001), in way forward, it would be critical to understand the fine-scale mechanisms which allow dominance of one species over the other across depth regimes.

Additionally, seagrass distribution is a product of habitat heterogeneity and substratum suitability from the sampled meadows. Heterogeneous habitats in the islands support a higher species richness (Das, 1996; Savurirajan et al., 2018). Our study reveals that variabilities in substratum types and habitat heterogeneity allow different species to dominate various spatial niches. Meadows with less substratum variability, such as those found in Little Andaman, Burmanallah, Laxmanpur, and Natural Bridge, had a vast extent but supported lower species richness. Our investigation of sediment texture affinities of seagrass species partially agrees with a previous study in the region by Savurirajan et al. (2018), which also reported high densities of H. ovalis in sand and clayey substrates. However, we observed T. hemprichii, E. acoroides, and Halodule pinifolia occupying coarse sand, fine sand, and silt, respectively, contrary to their preference for clayey sand in the previous study. It is important to note that the study by Savurirajan et al. (2018) focused on intertidal waters (< 2 m), while our study spanned depths up to 21 m. Therefore, the differences in spatial scale and sample sizes may contribute to the variations in results between the two studies. Additionally, we suggest that habitat and substratum profiles change from intertidal to deeper waters, limiting direct comparisons. In addition, the absence of H. decipiens from the intertidal regions is in line with the same report from South Andaman (Savurirajan et al., 2018). Yet, contrary to this study, we report C. serrulata and Syringodium isoetifolium from the littoral zones (0.2 to 0.4 m) of Ritchie’s archipelago and Little Andaman, updating the species’ distribution ranges.

Lastly, despite the vast spatial scale of our study, we did not observe H. ovata from the ANI’s seagrass checklist (12 species). Genus Halophila, with high taxonomic diversity, overlapping morphology, and phenotypic plasticity at local scales (Japar Sidik et al., 2010), has often led to species misidentification and systematic ambiguity (Fortes et al., 2018). The species was last reported in 2010 from ANI (Thangaradjou et al., 2010a; Thangaradjou et al., 2010b). A recent study (Ragavan et al., 2016) argued that H. ovata is a misidentified H. minor from all previous assessments in ANI. A similar report from Southeast Asia pointed to taxonomic discrepancies within Halophila spp., where the morphological resemblance between H. ovata and H. minor was “compounding” and has led to species misidentification (Fortes et al., 2018). Since all seagrass assessments in ANI have relied only on morphological traits for species identification, including our study, the possibility of misidentification cannot be ruled out. Thus, we suggest a more robust approach for species identification using molecular traits to resolve present inconsistencies.

Our study has provided updated information on the spatial distribution, habitat suitability, and depth ranges of seagrass meadows in the Andaman and Nicobar Islands (ANI). By conducting a comprehensive assessment across a broad spatial scale and depth gradients, we have filled significant data gaps in previously data-deficient regions of ANI. Our findings highlight key regulatory factors, such as water depth, influence seagrass growth, and species distribution. The observed differences in seagrass distribution along depth gradients can be attributed to various factors, including variations in covariates associated with water depth, species’ adaptability to different depth regimes, and habitat heterogeneity and substratum availability, which also change with depth. Any alterations in the benthic habitat profile of these meadows are likely to impact species distribution patterns and overall performance. Our study suggests that Halodule spp. and Halophila spp. are generalist species, capable of occupying suitable habitats across different depth regimes, while larger-sized species like Enhalus acoroides, Thalassia hemprichii, and Cymodocea spp. exhibit specialist characteristics. The presence of seagrasses in deepwater regions of Ritchie’s archipelago further confirms the availability of conducive growth environments in deeper regimes. Replicating similar investigations in other island groups would provide valuable insights to complement our findings. In summary, the outcomes of this study will have significant implications for the management and conservation of seagrass habitats in the region. The findings will provide valuable baseline information for policymakers, conservationists, and stakeholders and contribute to the broader scientific understanding of seagrass ecosystems and their responses to environmental changes.

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

SG: conceptualization, data curation, sample processing, formal analysis, drafted the manuscript, SP: data curation, sample processing, NP: formal analysis, critical inputs in manuscript review, HD: methodology designing, supervision, manuscript review, SK and JJ: funding acquisition, methodology designing, resources, supervision, manuscript review. All authors contributed to the article and approved the submitted version.