Persistent Eutrophication in a Tropical Endorheic Lake Driven by Sediment–Water Interactions

Eutrophication in tropical endorheic lakes often persists despite reductions in external nutrient inputs, indicating an important role of internal nutrient loading. However, integrated evidence linking thermal stratification, sediment characteristics, and sediment-derived nutrient release in tropical endorheic lakes remains limited. This study investigated the mechanisms contributing to eutrophication in Lake Batur, a tropical endorheic volcanic lake in Indonesia, through seasonal water-column observations, sediment porewater profiling, diffusive nutrient flux analysis, and sediment characterization. Seasonal observations showed thermal stratification accompanied by hypoxic to anoxic bottom waters, while sediment-derived nutrient flux was dominated by ammonium and phosphate under reducing conditions. Sediment characterization at the representative sampling site revealed mineral assemblages dominated by biogenic silica, aluminosilicate clays, carbonates, and iron-bearing phases that may influence nutrient mobility under low-oxygen conditions. The results indicate strong coupling between thermal stratification, hypolimnetic oxygen depletion, and sediment–water interactions, suggesting that internal loading contributes to maintaining eutrophic conditions in Lake Batur. The endorheic nature of the lake likely enhances nutrient retention because of limited hydrological flushing and prolonged nutrient residence times. These findings improve understanding of eutrophication processes in tropical endorheic volcanic lakes and highlight the importance of considering sediment-derived internal loading together with external nutrient reduction in lake restoration strategies.