A preliminary study on bimodal gradient patterns of larval fish assemblage in a tropical estuarine-lagoon ecosystem:...

The tropical estuarine-lagoon ecosystem provides essential habitats that support a wide variety of fish species by serving as nurseries, refuges, and migration routes. However, the connectivity between these systems, the characteristics of larval fish assemblages, and their relationship with environmental factors were poorly understood. In the present study, we conducted the first investigation of the temporal and spatial patterns of larval fish assemblages in the Wanquan Estuarine-lagoon Ecosystem, Hainan Island, China. We collected larval fish from May 2021 to April 2022, with monthly sampling during the peak breeding season (May-August) and bimonthly sampling in the non-peak season. We recorded a total of 23,164 individuals belonging to 110 taxa. Spatial analysis using Generalized Additive Models uncovered distinctive bimodal gradient patterns: one that showed increased larval abundance and species diversity from upstream areas to the estuary outlet, and another that indicated an increase from the estuary to mangrove-rich lagoon areas at the confluences of the Longgun and Jiuqu Rivers. These bimodal gradients reflect how larval abundance and diversity vary along spatial gradient. Our findings revealed significant seasonal fluctuations in the assemblage structure, with peak abundance from June to August. Canonical Correspondence Analysis identified water temperature, dissolved oxygen, and mean monthly precipitation as the main environmental factors shaping species-level variation. Canonical Analysis of Principal Coordinates further showed that temperature, salinity, and depth significantly contributed to differences in community-level structure across larval assemblage groups. While our results highlight the potential role of habitat connectivity in shaping larval fish patterns, direct quantification of connectivity was not performed and remains a direction for future research. The observed spatial patterns and tributary-lagoon interactions suggest that functional connectivity may enhance larval retention and support nursery functions. We recommend that conservation efforts prioritize preserving these confluence areas and maintaining the spatial connectivity of diverse habitats to support fishery resources in tropical estuarine-lagoon ecosystems.