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

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 210 Pb 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 .