Hydrogeochemical evidence for biotic over volcanic control in Lake Maninjau, Indonesia

Abstract

Lake Maninjau, a volcanic caldera lake in Indonesia, frequently experiences sulphide upwelling events that trigger periodic fish kills. Although these events are commonly attributed to eutrophication and organic matter enrichment, the potential contribution of residual volcanic activity remains unclear. This study re-evaluates the hydrogeochemical characteristics of Lake Maninjau to distinguish volcanic signatures from biotic sulphide generation. Field observations were conducted at four sites representing contrasting environments: a dome-proximal area with presumed volcanic influence (DM3), a eutrophic bay affected by intensive aquaculture (DM1), and two additional stations (DM2 and DM4) representing transitional and deep-water reference sites. Temperature, pH, total dissolved solids (TDS), and major anions (HCO₃⁻, Cl⁻, SO₄²⁻, HS⁻) were measured in the water column and porewater, while sedimentary sulphur and carbon contents were analysed to identify sulphide sources. The results showed no thermal or ionic anomalies near the dome-proximal area. Across all sites, the waters exhibited neutral to alkaline pH (6.5−8.9), low TDS (<0.11 g∙dm⁻³), and bicarbonate dominance, indicating a carbonate-buffered system rather than magmatic or hydrothermal influence. Hydrogen sulphide accumulated in the lower water column, reaching a maximum concentration of 0.401 mg∙dm⁻³. A strong positive correlation between sediment sulphur and carbon (Pearson’s r = 0.73, p = 0.01), together with positive intercepts in C–S plots, indicates that sulphide production is primarily driven by microbial sulphate reduction associated with organic matter degradation. These findings demonstrate that sulphide accumulation in Lake Maninjau is predominantly biotic, highlighting the role of internal biogeochemical processes rather than residual volcanic activity.