Export of particulate organic carbon and nitrogen to the northern Indian Ocean by the monsoonal rivers of India

Abstract

This study analyses particulate organic carbon (POC) and particulate nitrogen (PN) export from Indian monsoonal rivers to the north Indian Ocean. Indian monsoonal rivers export approximately 1.2 Tg yr^-1 (1Tg = 10¹² g) of POC and 0.14 Tg yr^-1 of PN, with about two-thirds entering the Bay of Bengal (0.8 and 0.1 Tg yr^-1, respectively) and the remaining reaches to the Arabian Sea (0.4 and 0.04 Tg yr^-1, respectively). Remarkably, just four rivers from northwest India's black soil-dominated regions contribute about half of the total POC and PN exports (0.64 and 0.06 Tg yr^-1, respectively). This is due to substantial erosion in these catchments, resulting in suspended matter concentrations averaging 596 ± 252 mg L^-1, significantly higher than catchments dominated by red sandy, red loamy and alluvial soils (54 ± 56 mg l^-1). In contrast, rivers originating from catchments with heavy precipitation, a tropical wet climate, red loamy soils (with peaty and marshy characteristics), rich tropical wet evergreen and moist deciduous forests, and higher soil organic carbon content yield more POC and PN (1704 ± 383 kgC km^-2 yr^-1 and 261 ± 56 kgN km^-2 yr^-1, respectively) than the other rivers of India (951 ± 508 kgC km^-2 yr^-1 and 120 ± 57 kgN km^-2 yr^-1, respectively). These findings stress that the export flux and yield of POC and PN from the Indian monsoonal rivers are primarily influenced by the interplay of hydrological, lithological, environmental, and climatic conditions within the catchment, rather than river size. Moreover, this study highlights the significant impact of incorporating POC data from medium-sized rivers worldwide, as it reveals that yield is independent of river size. This calls for a re-evaluation of global POC export fluxes, taking into account hydrological, lithological, environmental, and climatic factors.

Keywords: Arabian Sea; Bay of Bengal; Export flux; Indian monsoonal rivers; Particulate nitrogen; Particulate organic carbon.