Hydrological diversity and biological patterns of tropical karst marine lakes in Bai Tu Long Bay (Vietnam): initial observations across a gradient of connectivity

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Abstract
This study presents the first targeted survey of six marine lakes located on karst islands of the Van Don Archipelago (Bai Tu Long Bay, northern Vietnam). The aim of the study was to describe the morphometric, hydrological, and biological characteristics of these lakes and to evaluate differences associated with their degree of connectivity to the sea. Field investigations revealed a gradient of hydrological conditions ranging from tidal lagoons with direct seawater exchange to highly isolated anchialine lakes connected to the sea only through subterranean pathways. Lakes with direct marine connections exhibited relatively homogeneous salinity and temperature profiles, lacked persistent stratification, and supported communities dominated by typical coastal marine species. In contrast, more isolated lakes showed pronounced vertical stratification, increasing salinity with depth, and distinct biological assemblages adapted to brackish and stratified conditions. The most isolated lake displayed meromictic characteristics, including a warm stratified water column, a hydrogen sulfide-rich bottom layer, and a sharp decline in macrofaunal abundance in deeper waters. The results indicate that hydrological connectivity is the primary factor controlling environmental conditions and biological community structure in the marine lakes of the Van Don Archipelago. These lakes encompass a wide range of ecological states despite their close geographic proximity and represent valuable natural systems for future studies of isolation, stratification, and biodiversity in tropical coastal ecosystems.

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