A snapshot on vertical variability of dissolved organic matter in the epilagic zone of the eastern Indian Ocean.

Marine dissolved organic matter (DOM) plays an important role in aquatic ecosystems and is an essential reservoir of organic carbon in the marine carbon cycle. In this study, seawater DOM samples from 33 stations were collected in spring 2022 (April to May, 20 stations) and autumn 2020 (October to November, 13 stations) to better characterize and compare DOM variability within 200 m water layer in the eastern Indian Ocean (EIO). Hydrological parameters, nutrients and spectroscopic indices were calculated and evaluated for two cruises. In addition, excitation emission matrix spectroscopy combined with parallel factor analysis (EEM-PARAFAC) was used to directly analyse seawater DOM samples. The sources and processes of DOM in the EIO were assessed by fluorescence index (FI), freshness index (β/α), Biological index (BIX), and humification index (HIX). Three fluorescent components were identified in DOM samples from two cruises, including: C1 (tryptophan- and tyrosine-like), C2 (marine and/or terrestrial humic-like), and C3 (terrestrial humic-like). The components of C1 accounted for 39.45 % ± 8.79 %, C2 for 33.05 % ± 6.42 %, and C3 for 27.20 % ± 4.47 % within 200 m water layer. The intensity of the DOM fluorescence seems to varied due to seasonal differences. In particular, the source of the DOM fraction varied at <100 m layer, which may also be related to the structure of the microbial community. Further, there is a strong correlation between the depth of seawater and hydrographic parameters, fluorescence indices and fluorescence components. Nutrients (nitrate, dissolved inorganic phosphate, and dissolved silicate) and humic-like fractions are more likely to accumulate in the deeper layers of the ocean. Thus, these results provide some data support for the variability of DOM fractions on a vertical scale in the EIO.