The dynamics and scale of anthropogenically induced environmental changes in the Indian Himalayas remain largely unexplored, presenting a complex and enigmatic challenge. To tackle this, we investigated the pollution, eutrophication and fire history of the last century by analyzing the concentration, compositional variation, and temporal trends in polycyclic aromatic hydrocarbons (PAHs), monosaccharide anhydrides, fecal biomarkers and other aliphatic hydrocarbons (n-alkanes, acyclic isoprenoids and botryococcenes) from a lake sediment core in the Central Himalayas at 2084 m asl. The study revealed that the concentration of total PAHs increased steadily with a compositional change after 1970, paralleling an increase in socio-economic activities in the region. The PAHs diagnostic ratios revealed that pyrogenic sources accounted for the majority of the sedimentary PAHs and the high correlation between PAHs and fecal stanols (r = 0.84, p < 0.05) indicated the active role of humans in the catchment. The total concentrations of botryococcenes (C_31-34) and n-alkanes showed an increasing trend in eutrophication with greatest values recorded during the 1980s, coinciding with a compositional change between long (nC_26-36), mid (nC_21-25) and short chain (<nC₂₁) n-alkanes, depicting concurrent environmental change.

Moreover, based on the toxic equivalency factor of PAHs relative to benzo(a)pyrene and sediment quality guidelines (Canadian Council of Ministers of the Environment 2010), an increase in sediment toxicity was observed, surpassing thresholds for aquatic life protection, posing potential risks to both the lake ecosystem and human health.

The presence of monosaccharide anhydrides in the sediment core suggests a link to regional forest fires. These fires potentially contribute to the observed eutrophication, altering the lacustrine environment significantly. This hypothesis, if validated, could provide critical insights into the intertwined effects of wildfires and anthropogenic activities on lacustrine ecosystems. As the inaugural study of its kind from the Indian subcontinent, this research is pivotal in interpreting molecular signals in lacustrine records from this region.