Biogeochemistry of particulate organic matter from the Bay of Bengal as discernible from hydrolysable neutral carbohydrates and amino acids

Sinking particles from the deep northern, central and southern Bay of Bengal, suspended particulate matter (SPM) from the surface waters off the Ganges–Brahmaputra and sediments were analysed for the content and composition of hydrolysable carbohydrates (CHO) and amino acids (AA). CHO and AA made up 1–13% and 6.6–33% of total organic carbon (OC) in suspended and sinking particles, respectively, and 2.3–3.7% and 8.3–10.6% in sediments. Sinking fluxes were between 0.15–3.16 mg m−2 day−1 for CHO and 0.03–19.8 mg m−2 day−1 for AA following seasonal OC flux pattern. The export of AA and CHO was not linearly related to the opal and carbonate fluxes, which is attributed to variable transport mechanisms and sinking speed. The AA-based degradation index [Dauwe, B., Middelburg, J.J., Herman, P.M.J., Heip, C.H.R., Linking diagenetic alteration of amino acids and bulk organic matter reactivity. Limnol. Oceanogr. 44 (1999) 1809–1814] was used to establish a degradation sequence encompassing fresh SPM, sinking particles, river-derived SPM and sediments and to trace episodic lateral inputs of degraded material. CHO monomeric composition was source specific in plankton-dominated fresh SPM but was attenuated by degradation in sinking and sedimentary matter. Unlike AA, CHO composition was not suitable for a minute differentiation of individual samples by their reactivity. A 10–40-fold increase of CHO and AA concentration from river-derived SPM to sinking particles illustrated the dominant role of marine productivity for OM fluxes. The marine input is also visible in sediments indicating that monomeric similarities between river-derived SPM and sediments result rather from intense diagenetic transformation than from common source tissues.