Impact of sea mouth shift in shaping the depositional and ecological condition of the seaward region of Chilika Lagoon over the last ~200 years

Abstract

Chilika Lagoon, the largest brackish water lagoon in India and a Ramsar site, exhibits a highly dynamic ecology primarily governed by shifts in its sea mouth, which regulate freshwater–seawater exchange and influence sedimentation and faunal diversity. This study assessed ecological variability in the Chilika Lagoon over the last 200 years using a foraminifera record retrieved from a 72 cm sediment core recovered from the seaward region of the lagoon. The sediment deposition rate was ascertained using the ²¹⁰Pb value. Principal component analysis of foraminifera abundance data, added with cluster analysis, suggests the prevalence of four major ecological phases in the lagoon over the last 200 years. Between 1820 and 1940 ce, the lagoon exhibited a highly restricted ecosystem, dominated solely by Ammonia beccarii, due to limited seawater inflow caused by the progressive northward migration of the sea mouth. Between 1940 and 1975 ce, a moderately brackish environment prevailed, as evidenced by the increased abundance of agglutinated foraminifera taxa, which are indicative of suboxic bottom waters resulting from periodic tidal restriction and sediment accumulation. Between 1980 and 2000 ce, the lagoon’s sea mouth was nearly choked, and lagoon ecology was under threat, marked by the near absence of foraminifera, except for episodic seawater ingress linked to cyclonic events. The opening of the artificial sea mouth and its subsequent maintenance after 2000 ce improved the lagoon ecology, marked by an increase in foraminifera abundance and diversity. This is characterised by high salinity, low organic matter, and stable oxygen levels, as observed in modern conditions. Cyclones and stronger storms have played a significant role in leading to higher sea water ingress and periodically shifting the sea mouth before 2000 ce.

INTRODUCTION

Coastal lagoons are dynamic landforms that link mainland river systems to the open sea through multiple channels. It is typically separated from the ocean by extensive barrier spits punctuated by small tidal inlets (Pradhan et al., 2022). The morphology and hydrodynamics of lagoons are strongly influenced by the periodic opening and closing of tidal inlets, which play a pivotal role in maintaining ecological balance by regulating salinity regimes, sedimentation patterns, water circulation, and energy fluxes within the lagoon system (Chacón Abarca et al., 2021).

Chilika Lagoon, situated on the eastern coast of India, is Asia’s largest brackish water lagoon, recognised as one of the most geomorphologically and ecologically dynamic coastal systems (Mishra et al., 2022). This extensive shallow basin undergoes marked seasonal fluctuations in area and water volume, influenced by monsoonal freshwater input and marine tidal inflow (Barik et al., 2019; Das et al., 2024). Over the last century, Chilika has experienced substantial ecological and hydrological transformations, driven by natural forces and increasing anthropogenic pressures (Mishra et al., 2022; Sahu et al., 2014). The frequent opening and closing, as well as morphological instability, of its sea mouths have significantly altered tidal connectivity, disrupting salinity distributions and circulation patterns, which in turn have contributed to episodic variations in the lagoon ecosystem (Sahu et al., 2014). These environmental challenges have drawn considerable attention at the international level, resulting in Chilika’s designation as a Ramsar Wetland of International Importance in 1981 and its subsequent listing in the Montreux Record in 1993—a register of wetlands facing significant ecological threats—highlighting the urgent need for informed conservation efforts (Sarkar et al., 2012).

Microfaunal assemblages, particularly benthic foraminifera, have proven to be highly sensitive bioindicators of environmental changes in coastal lagoons (Nigam, 2005). Previous studies (Barik et al., 2019, Barik et al., 2022a; Das et al., 2024) have documented the impact of seasonal variations in salinity, wave action, and tidal regimes on the abundance and diversity of foraminifera in Chilika Lagoon. The distribution and composition of foraminifera provide significant insights into environmental parameters, including salinity variations, oxygen concentrations, sediment properties, and organic matter content, rendering them reliable proxies for reconstructing historical ecological conditions (Barbieri et al., 2019; Barik et al., 2019, 2022a). Hence, this study focuses on reconstructing ecosystem variability over the last two centuries and the role of sea mouth dynamics using benthic foraminifera assemblages.

STUDY AREA

Chilika Lagoon, located between 85°06′ and 85°35′ E and 19°28′ and 19°54′ N (Figure 1). This shallow lagoon with an average water depth of 2.5 m covers an area of over 1,100 km² during the monsoon. It shrinks to ~900 km² in the dry season, demonstrating significant seasonal and spatial variability (Barik et al., 2019). The northern portion of the lagoon receives considerable freshwater and sediments from the distributaries of the Mahanadi River system, including the Daya, Bhargavi, and Nuna Rivers (Figure 1). The lagoon is separated from the Bay of Bengal by a 65 km long barrier spit (Baral et al., 2023). Active sea inlets, shaped and altered by natural processes and human activities, maintain tidal exchange (Figure 1; Baral et al., 2023; Panigrahi et al., 2007). The saline water of the bay mainly influences the seaside region/the outer channel of the lagoon. The lagoon is globally recognised for its rich biodiversity that supports extensive seagrass meadows, over 200 species of fish, 263 species of holoplankton, large populations of migratory birds, and threatened species such as the Irrawaddy dolphin, diverse benthic foraminifera, ostracods and other microbial communities (Barik et al., 2022b; Ghosh et al., 2006; Srichandan & Rastogi, 2021). This complex ecology underscores the lagoon’s value as an ideal site for understanding cross-ecosystem interactions in this brackish wetland environment.

Figure 1.

Study area map showing Chilika lagoon in India, Chilika lagoon with its major tributaries, the core location, and the present mouth. The default topographic base map of the region is taken from ArcGIS 10.4.

METHODOLOGY

Sample collection

A 72 cm long gravity sediment core was collected from the seaside region of the Chilika Lagoon in May 2016 using a gravity corer from the L16 monitoring station of Barik et al. (2019) (henceforth L16 core; 85º24′12.3′′E, 19º42′7.6′′N, water depth ~1.8 m). The core was divided into two halves, which were further subsampled at 1 cm intervals. One-half of the sample was used for micropaleontological analysis, while the other half was preserved/used for sedimentological and geochemical studies.

Age model

The age model for the L-16 sediment core was established using a combination of ²¹⁰Pb-derived sedimentation rates and the ¹³⁷Cs activity peak at 41.5 cm, which served as a chronological marker for the 1963 nuclear fallout event (Aravind et al., 2024). Approximately 10 grams of freeze-dried sediment sample from depths of 15–17, 26–27, 32–33, 42–43, 54–55, and 68–69 cm were processed and analysed for ²¹⁰Pb activity using Mirion High Purity Germanium (HPGe) gamma spectrometry with a broad energy detector BE-6530 at the Sediment Laboratory of Nanyang Technological University, Singapore. All radionuclide activities were decay-corrected to 2016. Sediment ages were determined using the constant rate of supply (CRS_PW) model, implemented via the R package Serac (Bruel & Sabatier, 2020). The CRS model assumes a constant flux of excess ²¹⁰Pb to the sediment surface while allowing for temporal variation in sediment accumulation rates, following the approach described by Appleby and Oldfield (1978). The resulting age-depth model was cross-validated using the ¹³⁷Cs activity profile to ensure chronological accuracy.

Benthic foraminifera abundance and diversity calculation

The standard method described in Barik et al. (2019) is followed for micropaleontological analysis. Approximately 20 gm of freeze-dried samples were taken and wet sieved over a 63 µm sieve, and manually analysed under the stereo zoom microscope. Individual specimens of benthic foraminifera are identified up to genus and, wherever possible, up to species level following Loeblich and Tappan (1988) and Scott (2000). The number of foraminifera specimens in the samples is counted and converted to foraminifera specimens per gram of dry sediment. Samples with fewer than 15 specimens are considered foraminifera-free in statistical analysis. Furthermore, in each sample, the number of species (S) and species diversity, measured in terms of the Shannon diversity index H(S), were calculated using Shannon and Wiener’s equation (Shannon & Weaver, 1949).

$$\text{H(S)}={\displaystyle {\sum }_{i=1}^s{p}_{i}ln{p}_i}$$

(1)

Where S is the species richness (species present in the sample), p_i is the proportion of the ith species in the sample, and ln is the natural logarithm.

STATISTICAL ANALYSIS

Principal Component and Cluster Analysis

Principal component analysis (PCA) was carried out on selected data (i.e., major benthic foraminifera species, total calcareous foraminifera per gram of dry sediment, total agglutinated foraminifera per gram of dry sediment, Shannon diversity index, and coarse fraction of processed sediments, i.e., >63 µm) using the FactoMineR package in R software (Lê et al., 2008; RStudio Team, 2023) to identify the key factors influencing faunal distribution and sediment characteristics. Variables were grouped by k-means clustering to assess the ecological association. Furthermore, the samples (temporal data) were grouped using Q-mode cluster analysis, which was performed using Ward’s minimum linkage method in R software (RStudio Team, 2023).

RESULTS

Age model data interpretation

The age model data of the studied sediment core were developed using a combination of excess ²¹⁰Pb and ¹³⁷Cs activities. The excess ²¹⁰Pb activity was observed in three samples (from depths 42–43, 54–55, and 68–69 cm). In contrast, a distinct ¹³⁷Cs activity in one sample at 41.5 cm was assigned to 1963 ce, providing an independent chronological marker (Figure 2). Notably, the top three samples showed no detectable ²¹⁰Pb activity. The constant rate of supply (CRS) model was applied to the ²¹⁰Pb data to derive sediment accumulation rates and establish the age-depth relationship (Appleby & Oldfield, 1978). The surface is assumed to be present (year of core collection: 2016 ce) as the core site regularly receives sediment from the riverine influx and seawater intrusion. Based on this model, the 72 cm sediment core extends back to ~1825 ce, and the average sediment accumulation rate is estimated to be ~ 3.5 mm per year. However, the sediment accumulation profile is not uniform over time, and a distinct inflection in the age-depth curve is observed at 41 cm (Figure 2). The average time resolution between samples is ~3 years. Nonetheless, the agreement between the CRS-derived ages and the ¹³⁷Cs marker supports the robustness of the age model.

Figure 2.

Short-lived radionuclide measurements and age-depth model for the core (L16) of the Chilika Lagoon sediment. From left to right: core photo, ²¹⁰Pb_ex activities, ¹³⁷Cs activities, and the age model (CRS and CRS_pw).

Foraminifera variability over the last ~200 years

The coarse fraction (%) was high from 1820 to 1920 ce and decreased thereafter, except between 1970 and 1980 ce (Figure 3a). The total foraminifera per gram of dry sediment ranges from 2 to 120 #/gm (Figure 3b). The highest abundance of foraminifera per gram of dry sediment occurred between 1940 and 1978 ce, dominated by calcareous benthic foraminifera (Figure 3c). This was followed by a period of very low to negligible from 1978 to 2000 ce, and after which the benthic foraminifera abundance increased relatively (Figure 3c). Agglutinated foraminifera show some isolated peaks at ~1970 and 2005 ce (Figure 3d). The benthic foraminifera species diversity shows an increasing trend between 1840 and 1970 ce, with high-amplitude periodic fluctuation between 1970 and 2000 ce and a relatively stable species diversity trend after 2000 ce (Figure 3e). Among the foraminifera, calcareous foraminifera are most dominant (Figure 3f). Sixteen genera of benthic foraminifera have been identified, and prominent among them are Ammonia genus, with four different species (Ammonia beccarii (Figure 3g), Ammonia tepida (Figure 3h), Ammonia parkinsonian, and Ammonia sp.1), which are most abundant throughout the core. The other important calcareous genera are Quinquiloculina (Figure 3i), Furthermore, other calcareous foraminifera (%, Figure 3j), comprising Nonion spp., Elphidium spp., Hanzawaia spp., Astrorotalia spp., and Pararotalia spp. Among the agglutinated foraminifera, Textularia spp. (Figure 3k) is dominant along with some other genera that are grouped with other agglutinated foraminifera (%, Figure 3l), comprising Miliamina spp., Trochammina spp., Haphlophragmoides spp., Ammobaculites spp., Ammotium spp., Agglutinated sp.1, and Agglutinated sp. 2.

Figure 3.

Variation of (a) Processed coarse sediment fraction (>63 µm; %), (b) Total foraminifera per gram dry sediment, (c) Total calcareous foraminifera per gram dry sediment, (d) Total agglutinated foraminifera per gram dry sediment, (e) Shannon Diversity Index, (f) Total calcareous foraminifera (%), (g) Ammonia beccarii (%), (h) Ammonia tepida (%), (i) Quinquiloculina spp. (%), (j) Other calcareous foraminifera (%), (k) Textularia spp. (%), (l) Other agglutinated foraminifera (%), (m) Planktic foraminifera (%) over the last 200 years in L16 core. The blue region indicates restricted lagoon conditions; the green denotes the most ecologically dynamic phase.

Statistical data interpretation

The first two principal components (PC1 and PC2) in PCA accounted for 60.1% of the overall variance. The k-means clustering of variables suggests the existence of three ecosystems of foraminifera over the last 200 years (Figure 4). Group ‘A’ is characterised by Ammonia spp. and a higher coarse fraction (%), indicative of brackish water conditions with restricted seawater inflow into the lagoon. Group ‘B’ comprises total agglutinated foraminifera (TAF, #/gm), Textularia spp. (%), Moreover, other agglutinated taxa, reflecting a low-salinity environment with minimal marine influence or a stagnant water regime. Group ‘C’ comprises total calcareous foraminifera (TCF, #/gm), Quinqueloculina spp., other calcareous foraminifera, and species diversity, suggesting a high-salinity and productive marine-influenced environment.

Figure 4.

Principal Component Analysis of the major microfauna assemblages (Ammonia spp. (Am, %), Quinquiloculina spp. (Qu, %), other calcareous foraminifera (%), Textularia spp. (Tx, %), other agglutinated foraminifera (%)), total calcareous foraminifera per gram dry sediment (TCN, #/gm), total agglutinated foraminifera per gram dry sediment (TAN, #/gm), Shannon Diversity Index (H(S)), and coarse fraction (%).

The Q-mode cluster analysis categorised the samples into four significant clusters, each corresponding to specific ecological phases (Figure 5). Cluster I comprises the sample between 1826 and 1940 ce, Cluster II between 1980 and 2000 ce, Cluster III covers the isolated periods between 1943 and 2014 ce, and Cluster IV covers most of the sediments after 2000 ce and a few isolated periods between 1958 and 2015 ce.

Figure 5.

Q-mode cluster analysis of L16 core based on the microfaunal data.

PLATE 1.

1. Ammonia beccarii (spiral view); 2. Ammonia beccarii (umbilical view); 3. Ammonia tepida (spiral view); 4. Ammonia tepida (umbilical view); 5-6. Elphidium excavatum (side view); 7. Quinqueloculina seminulum (side view); 8. Quinqueloculina sp. (side view); 9-10. Haphlophragmoides sp. (side view); 11. Trochammina inflata (apertural view); 11. Trochammina inflata (side view); 13. Miliammina fusca (side view); 14. Textularia sp. (side view); 15. Ammobacculite sp. (side view); 16. Ammotium sp. (side view); 17. Ammotium sp. (side view). Scale: 200 µm.

DISCUSSION

The seaward region of the lagoon is influenced by wave action and experiences two distinct conditions seasonally, namely brackish and marine environments (Barik et al., 2019; Das et al., 2024). Over the last 200 years, the barrier spit and lagoon sea mouth have experienced repeated transformations (Mishra & Jena, 2014; Mishra et al., 2022; Panda et al., 2013). It is driven by large volumes of sediment carried by distributaries of the river Mahanadi and the wave and tide actions from the Bay of Bengal (Mishra & Jena, 2014; Mishra et al., 2022; Sahu et al., 2014). The occurrences of periodic cyclones also influence the sea mouth position and its width. The periodic closing, shifting, and formation of new inlets and freshwater cause a major impact on the lagoon’s hydrological dynamics, resulting in notable shifts in salinity levels and affecting the ecological balance (Gopikrishna et al., 2014; Pradhan et al., 2017; Sahu et al., 2014). The frequent changes in the lagoon’s hydrological dynamics directly affect the population dynamics, distribution, and diversity of benthic foraminifera over time (Nigam, 2005).

Cluster I, comprising periods of 1820–1940 ce (Figure 5), shows that the benthic foraminifera assemblage was predominantly comprised of stress-tolerant A. beccarii, an euryhaline and opportunistic species known for thriving in suboxic, organic-rich, and moderate salinity conditions (Murray, 1968). The ecology that prevailed is marked by group ‘A’ in PCA, which is characterised by a dominance of A. beccarii and low species diversity, reflecting restricted, low-salinity lagoonal conditions (Figure 4). A. beccarii abundance (Figure 3g) showed an opposite trend with both the Shannon diversity index and species richness (Figure 3e), underscoring the biological stress existing in the lagoon. Barik et al. (2019) explained that such an ecosystem develops in the lagoon’s interior region due to the development of brackish water conditions with moderate to high energy levels. Such a condition suggests significant sediment influx, low marine connectivity, and changing salinity levels due to the narrowing of the mouth, which limits seawater inflow and promotes the formation of a brackish environment during that period (Ghosh et al., 2006; Sundaravadivelu et al., 2019). Previous studies have also documented inlet narrowing and dredging between 1828 and 1900 ce (Mishra & Jena, 2014; Mishra et al., 2022; Panda et al., 2013), as well as a northward shift of the inlet near Arakhakuda between 1915 and 1940 ce (Annandale & Kemp, 1915). However, the occasional appearance of planktic foraminifera (Figure 3m) is evidence of seawater ingression that may be linked to the severe cyclones and storm surges reported during 1842, 1852, and 1909 ce in the Puri district (Chiitibabu et al., 2004; Dube et al., 2000).

From the early 1940s to the late 1990s ce, the lagoon’s seaside region experienced a phase of ecological dynamism, as evidenced by the intermixing of samples from period across Clusters II, III, and IV. Cluster III (Figure 5; period between 1940 and 1975) was marked by a significant increase in agglutinated foraminifera taxa such as Textularia spp., Miliammina spp., Haplophragmoides spp., and Trochammina spp. (Figure 3k and 3l) along with calcareous forms dominated by Ammonia tepida (Figure 3h) and the appearance of Elphidium spp. and Nonion spp. (Figure 3j). This assemblage indicates conditions of moderate brackishness, high organic matter input, and suboxic bottom waters with fine-grained sediments, typical of low-energy, reducing environments, which corresponds to group ‘B’ in PCA. Das et al. (2024) explained that such an ecosystem exists in the inner part of the seaside due to the low mixing of coastal waters and shallow water depths. This condition was likely driven by periodic restrictions on tidal inflow and sediment buildup due to recurrent mouth instability, which affected salinity gradients and sedimentation dynamics (Mishra & Jena, 2014; Panda et al., 2013; Tripati & Vora, 2005).

However, between 1958 and 1970 ce, the lagoon experienced a brief period of enhanced water exchange and oxygenation, as indicated by an increase in species diversity (Figure 3e) and a rise in the abundance of calcareous foraminifera (Figure 3f). Hence, samples from this interval are excluded from Cluster III (Figure 5). According to Mishra et al. (2022), this period corresponded with stable inlet conditions, marked by minimal shifting and maximum width of the lagoon mouth. Following the 1970–1975 ce period, environmental conditions deteriorated once again, with a resurgence of reducing conditions characterised by the dominance of agglutinated foraminifera, also represented from Cluster III. This decline is likely linked to the narrowing or near-complete closure of the inlet around 1968 ce, with conditions improving only after the formation of three new mouths caused by the 1972 cyclone (Sahu et al., 2014).

Cluster II corresponds to the period between 1980 and 1995 ce (Figure 5), characterised by pronounced ecological instability in the core site region. This interval is marked mainly by the near absence of foraminifera, with only sporadic occurrences observed (Figure 3). These intermittent appearances may be attributed to cyclone-induced marine ingress events reported during this time (Chittibabu et al., 2004; Dube et al., 2000), which temporarily enhanced salinity and hydrodynamic conditions. However, the overall absence of foraminifera assemblages during this period suggests persistently unfavourable environmental conditions, which may be related to reduced hydrodynamic flushing, leading to stagnant, oxygen-depleted bottom conditions that promote organic matter accumulation and result in anoxic sediment zones. These unfavourable conditions suppressed foraminifera diversity and abundance (Barik et al., 2019; Das et al., 2024; Manasa et al., 2016; Rana & Nigam, 2009). This prolonged ecological decline culminated in Chilika’s designation as a Ramsar site in 1981 and its inclusion in the Montreux Record in 1993, signifying international concern over its deteriorating ecological condition and the need for urgent restoration (Sahu et al., 2014).

The Cluster IV, which encompasses the brief periods between 1958 and 1970 and the post-2000 phase (Figure 5), is characterised by elevated species richness, higher abundances of calcareous foraminifera (Figure 3), and is consistent with well-flushed, oxygenated, and moderately to highly saline conditions. This interval reflects the period of ecological improvement characterised by enhanced salinity, increased hydrodynamic exchange, and overall habitat recovery. The assemblage represented by the group ‘C’ of the PCA (Figure 4) includes taxa such as Quinqueloculina spp., and other calcareous foraminifera (%), which are known to thrive in marine-influenced environments with high saline conditions, low organic matter content, and stable oxygen levels (Narciso-Mezones et al., 2025). These ecological signals suggest hydrological and geomorphological transitions, either through natural mouth realignments or anthropogenic interventions, which facilitated enhanced seawater inflow and sediment export, reversing the earlier phases of ecological stress. Notably, the most significant recovery occurred after the artificial mouth opening at Sipakuda in 2000 CE, an intervention that restored lagoon sea connectivity, increased tidal flushing, stabilised salinity gradients, and improved oxygen availability, thereby fostering a resurgence in benthic foraminifera productivity and biodiversity (Figure 3; Panda et al., 2013; Tripati & Vora, 2005). This period marks a critical phase in the ecological restoration of Chilika Lagoon, as evidenced by both biotic and physicochemical indicators.

Overall, the L16 core data suggest a shift from a low-diversity, stressed, restricted ecosystem during 1830–1920 ce to a mixed-diversity environment during 1940–2000 ce, followed by a richer and more productive environment after 2000 ce (Figure 3).

CONCLUSIONS

The dynamic nature of the Chilika Lagoon is primarily governed by the shifting, opening, and closing of the lagoon inlet, which exerts significant control over its ecological and hydrodynamic regime. These geomorphological changes have resulted in three distinct environmental phases. From 1820 to 1940 ce, the progressive northward migration of the mouth led to a highly restricted system, marked by limited seawater exchange. Between 1940 and 1995 ce, the lagoon entered an ecologically unstable phase characterised by alternating suboxic to anoxic conditions, fluctuating salinity levels, with episodic well-oxygenated intervals linked to mouth openings triggered by cyclonic or storm events. These fluctuations had a pronounced impact on the abundance and diversity of benthic foraminifera. Following the artificial opening of a new inlet in the 2000s, the lagoon experienced significant ecological recovery, characterised by improved tidal exchange, stabilised salinity, and the restoration of benthic microfaunal populations.