Limnological Review

Ancient irrigation tanks are a defining component of Sri Lanka’s freshwater landscape, but their CO₂ flux dynamics remain poorly characterized and are underrepresented in carbon databases. This study quantified hourly daytime CO₂ fluxes and examined associations between CO₂ flux and environmental variables in Thalangama Tank (TG) and Huruluwewa Tank (HW), which are situated in contrasting hydroclimatic zones. An automated floating chamber was deployed monthly in each tank between January 2025 and January 2026 to capture the monsoon seasons, while water quality parameters and hydro-meteorological data were monitored simultaneously. TG, a small and shallow tank located in the Wet Zone, showed predominantly positive daytime CO₂ fluxes across the sampled campaigns, and no significant seasonal dynamics were observed. In contrast, HW, a large but shallow Dry Zone tank, showed both daytime CO₂ uptake and emission, and daytime CO₂ flux varied significantly among seasons. Daytime net emission was observed during the first inter-monsoon and northeast monsoon periods, while CO₂ uptake was observed during the southwest monsoon and second inter-monsoon periods. At HW, daytime CO₂ flux was strongly associated with the ionic gradient, moderated by strong hydrometeorological forcing. Conversely, CO₂ flux in TG was weakly related to the ionic gradient but associated with the metabolic balance and rainfall variation. These contrasting patterns suggest that hydrometeorological and physicochemical conditions are important factors associated with daytime CO₂ exchange in tropical irrigation tanks. Also emphasize the potential for significant temporal variability in CO₂ flux among tropical artificial freshwater systems.

Regional background concentrations of 10 priority potential pollutants (Hg, Cd, Zn, Pb, Cu, Cr, Co, Ni, Fe, and Mn) were established for the first time in the water and bottom sediments of 18 lakes in the Central Baraba Lowland. Comparison of natural and technogenic limnosystems using three normalization standards (regional background concentrations (RBC), upper continental crust (UCC), and Post-Archean Australian shale (PAAS)) showed that the choice of reference fundamentally influences enrichment/depletion interpretations. Concentration coefficients for Mg, Si, Sr, Pb, and Mn are standard-independent (“stable”), whereas those for Cd, Sb, Cr, Cu, and Hg are standard-dependent (“unstable”) and require explicit specification of the reference. Despite similar bulk sediment geochemistry, spheroidal and irregular Si–Fe slag particles found in technogenic limnosystems provide direct mineralogical evidence of aerotechnogenic input. Alkaline, bicarbonate-sodium waters act as an effective geochemical barrier, sequestering elements in the solid phase and supporting self-purification. Sapropels meet Class I of State Standard GOST R 54000-2010 for all elements except Zn (Class II), confirming their resource potential under proper quality control. Future work should expand the list of analytics and include all components (water, sediments, biota, and catchment soils) to build a comprehensive model of limnosystem functioning under anthropogenic load.

Monopterus albus (Asian swamp eel) is a commercially important, air-breathing freshwater fish harvested throughout East and Southeast Asia. As a protogynous species, it begins life as a female, passes through a transitional phase, and ultimately becomes male, making sexual-state classification difficult and potentially affecting management in capture fisheries and aquaculture. We examined sex–size relationships in market-purchased, reportedly wild-caught M. albus from northern and northeastern Thailand using gonadal histology to determine sexual state, together with body length (BL), body mass (BM), and a separate otolith-aged sample. Among 199 histologically examined individuals, no females were observed above approximately 70 cm BL or 270 g BM. Across much of the intermediate size range, female, transitional, and male individuals overlapped in size. Transitional individuals reached 78 cm and 424 g, showing that the upper values represent female-exclusion boundaries rather than thresholds identifying males. No male was smaller than 55 cm BL or 200 g BM, although sampling below these values was sparse. Otolith-derived age estimates from a separate sample of 70 eels provided local size-at-age context but could not establish age at sex transition because sexual state was not determined in those individuals. Comparisons with published studies showed substantial regional variation in size–sex relationships, indicating that size-based criteria developed in one region may not transfer reliably elsewhere. The boundaries identified here provide practical guidance for fisheries monitoring and broodstock selection in northern and northeastern Thailand while emphasizing the need for locally validated criteria elsewhere.