Hydrological seasonality and nutrient stoichiometry control dissolved organic matter characterization in a headwater...

Dissolved organic matter (DOM) is a diverse and highly complex mixture of organic macromolecules, and thus plays a central role in aquatic ecosystems. However, responses of components and sources of DOM to hydrological processes and trophic levels (nutrient stoichiometric ratios) are poorly understood, particularly in monsoonal headwater streams of Asia that are vulnerable to catchment physical characteristics. In this study, the excitation - emission matrix florescence spectroscopy coupled with parallel factor analysis (EEM-PARAFAC) was used to explore the DOM characters in a headwater stream, where seasonal rainfalls and nutrient levels vary largely. The EEM-PARAFAC modelling identified one autochthonous protein-like fluorescence substance (C1) and two allochthonous fulvic- and humic-like fluorescence compounds (C2 and C3). The allochthonous compounds dominated the overall DOM signal in the headwaters. The hydrological seasonality coupled with nutrients was key in modulating headwater DOM sources and components. Seasonal rainfall events contributed more allochthonous terrestrial-derived DOM flushing into river waters, resulting in higher fulvic- and humic-like organic matter (C2 + C3) in the wet season. In the dry season, longer water residence time accompanying with higher C:P stoichiometric ratio was responsible for higher autochthonous microbial- and plant-derived DOM (tryptophan and tyrosine fractions), also reflected by higher C1, biological index (BIX) and freshness index (β:α). In-stream microbial metabolism of labile DOM fractions largely contributed to autochthonous DOM and partial pressure CO₂ increase in the headwater stream. Our findings indicate that quality and quantity of DOM in headwater streams play a crucial role in downstream carbon cycle. Furthermore, the evidence combined from PARAFAC components, pCO₂ and spectral slope clearly highlights the importance of microbial metabolism of carbon in lotic systems, especially during a dry season with increased residence time.