Drivers of dissolved inorganic carbon dynamics in tropical streams: insights from the Munnar Critical Zone Observatory, Western Ghats, India

Abstract

This study explores the primary drivers influencing dissolved inorganic carbon (DIC) dynamics in the intermittent streams of the Munnar Critical Zone Observatory in the Western Ghats, India. Seasonal sampling was conducted in seven small hilltop streams, their downstream large streams, and groundwater from surrounding areas. Stream waters were oxygenated (dissolved oxygen percentage = 30–136), favouring aerobic remineralisation of organic matter within the streams. DIC concentrations varied seasonally, ranging from 6 to 2838 µM in small streams and 44 to 2165 µM in large streams, with HCO₃⁻ identified as the dominant DIC species. In intermittent streams, DIC levels were primarily regulated by carbonate rock weathering, as indicated by ΔDIC/ΔCa²⁺ ratios, with evidence for enhanced weathering during the post-monsoon period. DIC concentrations were largely independent of stream size. Analysis of concentration-discharge relationships revealed chemostatic behaviour during the monsoon and chemodynamic patterns during the dry season. Isotopic analysis (δ¹³C-DIC) in large streams during the post-monsoon period highlighted significant DIC inputs from aerobic organic matter respiration, driven by terrestrial C₃ plant material and sewage. These findings underscore the complex interplay of hydrological, geochemical, and biogeochemical processes governing DIC dynamics in tropical mountainous catchments. Continuous, high-resolution monitoring is recommended to further elucidate the factors controlling stream water DIC in these sensitive ecosystems.