El Niño‐Driven Dry Season Flushing Enhances Dissolved Organic Matter Export From a Subtropical Watershed

Abstract

The 2015/2016 super El Niño event resulted in a positive precipitation anomaly during dry seasons in the Jiulong River watershed, southeast China. Four years (2014-2017) of high frequency, in situ humic-like fluorescent DOM (FDOM H) data in the Jiulong Estuary were coupled with extrapolation to a freshwater end-member FDOM H concentration and river discharge data to estimate riverine FDOM H export. The wetter El Niño dry season was followed by lower FDOM H export during the subsequent wet season. Furthermore, in the dry season after El Niño reached its strongest phase, a 90-187% increase in FDOM H export occurred. If widespread, this pattern suggests El Niño events may enhance export of FDOM H from south China rivers in dry seasons resulting in seasonal and annual shunting of the terrestrial DOM export, modulating coastal carbon cycling. This study highlights the need to incorporate climate-driven regulation patterns on DOM transport across the land-ocean interface.

Drivers of Organic Molecular Signatures in the Amazon River

“…Though an El Niño event was not captured during this study, we expect DOM composition in the Amazon River sampled during an El Niño year to also differ from a non‐ENSO year with opposite molecular trends of what was observed during the La Niña year (Figures 4 and 5). For example, El Niño events have recently been shown to influence the amount of fluorescent DOM exported from a small subtropical watershed to the Pacific Ocean (Qu et al., 2020). Although the El Niño caused an increase in precipitation in this watershed due to geographical differences (the opposite of what occurs in the Amazon), the effects show that positive sea surface temperature anomalies in the Pacific Ocean can also influence riverine DOM export, and will likely also occur in other large watersheds, including the Amazon Basin.…”

Dissolved Organic Matter Dynamics in the Epipelagic Northwest Pacific Low‐Latitude Western Boundary Current System: Insights From Optical Analyses

“…Optical analyses of DOM (i.e., absorption and fluorescence) are widely applied to characterize the sources and transformation/transport dynamics of chromophoric DOM (CDOM) and fluorescent DOM (FDOM) in the global ocean (Jørgensen et al., 2011; Nelson et al., 2010; Yamashita et al., 2017). Absorption coefficients at different wavelengths (e.g., 254, 325, 350, and 443 nm) and fluorescence intensities at selected excitation/emission wavelength pairs are commonly used as quantitative indicators of DOM components (e.g., humic‐like peaks C and M [FDOM H ] and protein‐like peaks T and B) (Coble, 1996; Guo et al., 2007; Qu et al., 2020). Similarly, the spectral slope ( S 275–295 ), carbon‐normalized absorbance (e.g., SUVA 325 ) and fluorescence indices (e.g., HIX) are widely used as qualitative proxies (Hansen et al., 2016; Helms et al., 2008; Ohno, 2002).…”

Sources, spectral characteristics, and fluxes of dissolved organic matter in coastal groundwater and river water in western Guangdong, China

“…However, the linear correlations among DOC, CDOM, and FDOM in river water samples (Supplementary Figure S3) supported that their distributions were mainly regulated by the same process (i.e., the terrestrial input). This conclusion sounds to be reasonable because the high precipitation occurred during the sampling period (315 mm/month, https://www.ncei.noaa.gov/) which could intensify the soil erosion and induce an enrichment of terrigenous DOM in river water (Qu et al, 2020). On the other hand, the residence time of groundwater was generally longer than surface river water (Downing and Striegl, 2018), permitting a stronger microbial reworking of DOM and lower DOC inventory preserved in the aquifer (Figure 2A).…”

“…These allochthonous sources show large diversities in terms of the level, composition, export flux, and biogeochemical activity of DOM, resulting in distinct fates and profound implications for coastal ecosystems (Raymond and Spencer, 2015;Shen et al, 2015;Wang et al, 2018). Moreover, climate change (e.g., global warming, extreme climatic events) and human activities (e.g., damming, urbanization) have dramatically changed the composition, reactivity, and export flux of riverine (Liu et al, 2020;Qu et al, 2020Qu et al, , 2022 and groundwater DOM (Lipczynska-Kochany, 2018;McDonough et al, 2020a) delivered to sea. It is, therefore, crucial to determine the quantity, quality, and export flux of DOM at the dynamic land-sea interface, to provide a basis for a better understanding of the coastal biogeochemistry on a changing Earth.…”