Low dissolved organic carbon flux in small mountainous catchments of Taiwan

Stream dissolved organic carbon (DOC) is an important component of the global carbon cycle. The export ofDOC from land to the ocean is well quantified by examining large rivers, but this often excludes small mountainous rivers (SMRs), where DOC is primarily allochthonous and acts as a microbial energy source, shaping stream biogeochemical cycling. Revealing the temporal and spatial variation of DOC in SMRs is crucial for filling the missing piece of DOC export and understanding the role of DOC in stream ecology.Taiwan frequently experiences extreme weather events and earthquakes, thereby featuring deep river incisions, rapid uplift and erosion, and limited soil development. It represents an ideal model system for studying SMRs with high area-normalized material fluxes. Two catchments, the Gaoping and Beinan River systems, with high particulate organic carbon (POC) flux in Taiwan, were examined. The variation of DOCconcentrations was wider in the Gaoping River system (ranging from 0.07 to 8.85 mg/L with a mean of 0.66 mg/L) compared to the Beinan River system (ranging from 0.26 to 0.67 mg/L with a mean of 0.37 mg/L). However, the mean values in both systems are significantly lower than the global average. Despite a greater human impact in the lower reach of the Gaoping River as a result of the dense population, temporal variations were substantial at all sites, but the disparities between wet and dry seasons were notable at specific sites. Temperature appeared to be the primary factor controlling DOC concentrations during the non-typhoon period. During the typhoon event, the DOC concentrations were positively correlated with total suspended solids (TSS). By analyzing the temporal sequence, the variation in DOC concentration and TSS exhibited a clockwise hysteresis with the DOC max proceeding TSS max. This event contributed approximately 10% of the annual DOC flux in the catchment. Compared with the POC flux, the DOC flux derived from these two catchments is much lower, indicating a decoupling of transportation for particulate and dissolved materials and limited river metabolisms in SMR catchments.