Bio-invasion and decadal changes in the trophic dynamics of a temporarily closed estuary: An Ecopath model from Veli-Akkulam Estuary, Kerala, India

Abstract

Temporarily closed estuarine ecosystems (TCEs) are ecologically sensitive environments increasingly impacted by anthropogenic pressures and biological invasions. This study employs a mass-balanced trophic model using the Ecopath with Ecosim (EwE) framework to evaluate long-term changes in the structure and function of the Veli-Akkulam Estuary, a small TCE on the southwest coast of India. Based on field data from 2022 to 2023, the model integrates 17 functional groups, including invasive alien species such as Oreochromis mossambicus, O. niloticus, and Pterygoplichthys pardalis. Model outputs were compared with historical assessments from 1993 to 2008–2010 to examine decadal-scale shifts in ecosystem properties and dynamics.

Key ecological indicators such as mean trophic level of the catch (2.69), transfer efficiency (7.69 %), and Finn's Cycling Index (2.64 %) point to a simplified, detritus-dominated food web with reduced energy cycling and declining trophic maturity. Biomass is concentrated at lower trophic levels, while apex predators and native piscivores have markedly declined. Mixed Trophic Impact analysis revealed strong negative interactions between invasive species, particularly O. mossambicus, and native fish groups, suggesting competitive displacement and altered trophic relationships. High niche overlap further supports the hypothesis of dietary competition between invasive and native taxa, contributing to trophic reorganization.

System-level metrics, including a relative ascendancy of 37.42% and system overhead of 62.58%, indicate reduced ecosystem organization and rising instability. These trends reflect a transition from a functionally complex system to one increasingly dominated by opportunistic and invasive species. The study underscores the utility of Ecopath with Ecosim-based modelling in diagnosing trophic degradation and informing adaptive management. This study provides the first decadal Ecopath model demonstrating invasion-driven trophic simplification in a temporarily closed tropical estuary, and offers a replicable framework for assessing cumulative ecological impacts in tropical estuarine systems impacted by invasive species.