Differences in N loading affect DOM dynamics during typhoon events in a forested mountainous catchment

Abstract

The dissolved organic matter (DOM) and nutrient dynamics in small mountainous rivers (SMRs) strongly depend on hydrologic conditions, and especially on extreme events. Here, we investigated the quantity and quality of DOM and inorganic nutrients during base-flow and typhoon events, in a chronically N-saturated mainstream and low N-loaded tributaries of a forested small mountainous reservoir catchment in Taiwan. Our results suggest that divergent transport mechanisms were triggered in the mainstream vs. tributaries during typhoons. The mainstream DON increased from 3.4 to 34.7% of the TDN pool with a static DOC:NO-N ratio and enhanced DOM freshness, signalling a N-enriched DOM transport. Conversely, DON decreased from 46 to 6% of the TDN pool in the tributaries and was coupled with a rapid increase of the DOC:NO-N ratio and humified DOM signals, suggesting the DON and DOC were passively and simultaneously transported. This study confirmed hydrology and spatial dimensions being the main drivers shaping the composition and concentration of DOM and inorganic nutrients in small mountainous catchments subject to hydrologic extremes. We highlighted that the dominant flow paths largely controlled the N-saturation status and DOM composition within each sub-catchment, the effect of land-use could therefore be obscured. Furthermore, N-saturation status and DOM composition are not only a result of hydrologic dynamics, but potential agents modifying the transport mechanism of solutes export from fluvial systems. We emphasize the importance of viewing elemental dynamics from the perspective of a terrestrial-aquatic continuum; and of taking hydrologic phases and individual catchment characteristics into account in water quality management.

Reply to Referee #1

“…6. In the C-Q relationship (the plots in the center of the figure), increasing streamflow enhances the DOC concentration but dilutes the DIC concentration, which confirms previous stud- ies (e.g., Jin et al, 2014;Battin et al, 2003;Wymore et al, 2017;Zhong et al, 2017). The tighter C-Q relationship for DIC than for DOC indicates that the mechanisms of DOC transport cannot solely be explained by discharge control, possibly because microbial decomposition also played an important role (Yeh et al, 2018).…”

“…In our study, the DIC concentration and flux were 1951 µM and 52.1 t C km −2 yr −1 . The DIC concentration was even as high as in the karst landscape (characterized by extraordinarily high DIC concentrations) of Wujiang (Zhong et al, 2017). In addition, high physical erosion rates, which expose fresh rocks, enhancing interaction with water, also provide conditions favorable for chemical weathering (Larsen et al, 2012(Larsen et al, , 2014Lyons et al, 2005).…”

“…This method calculates the difference between the total dissolved anions (TZ − = Cl − + 2SO 2− 4 + HCO − 3 + NO − 3 , in µeq L −1 ) and total dissolved cations (TZ + = Na 2+ , in µeq L −1 ). The difference is attributed to HCO − 3 (Misra, 2012;Zhong et al, 2017). To affirm the estimated DIC through [HCO − 3 ], we also determined the DIC of some samples through the NDIR method (OI Analitical ® Aurora 1030W TOC).…”

“…DIC / DOC is calculated from either concentration or yield, depending on data availability.2 The DOC and DIC concentrations were reversely calculated from fluxes; the details can be found inHuang et al (2012).3 The values were averages of the listed studies, but did not includeZhong et al (2017), due to the specificity of karst landscapes.4 The discharge (1572 mm yr −1 ) that we used is consistent with the GRDC dataset but about 10 times higher than the value reported byHuang et al (2012).5 The discharge during the sampling period was only one-third of the long-term average due to the ENSO effect. 6 and 7 indicate low-and high-flow conditions, respectively.…”

Unusual Roles of Discharge, Slope and SOC in DOC Transport in Small Mountainous Rivers, Taiwan

“…This hypothesis highlights the alternating nature of DOC export during rainstorms. In general, typhoons (alias of tropical cyclone in the West Pacific) usually bring torrential rainfall within 48–72 hr, resulting in massive substrate transport through rivers in Taiwan 18,19 . Lee, et al .…”