Research trajectory and long-term trends of dissolved organic carbon in freshwater systems

Dissolved organic carbon (DOC) is a major and dynamic carbon pool regulating carbon cycling in freshwater systems. Over the past two decades, research on freshwater DOC has moved beyond simple concentration monitoring to examine its sources, molecular composition, degradation potential, and interactions with climate change. Lake studies increasingly focus on DOC processing, long-term storage, and its role in greenhouse gas production, while river studies emphasize DOC mobilization, transport, and connectivity between terrestrial landscapes, inland waters, and downstream coastal environments. Long-term observations reveal increases in DOC concentrations in lakes and rivers, with mean growth rates of 0.042 and 0.015 mg L^-1 per year, respectively. These trends are driven by multiple factors, including recovery from acid deposition, climate warming, extreme precipitation, and intensified human activities such as land-use change and wastewater discharge. Rising DOC concentrations lead to water browning, degraded water quality, biodiversity loss, and increased greenhouse gas emissions, with consequent socioeconomic impacts, particularly in fisheries and tourism. Addressing these challenges requires integrated strategies combining source control, long-term monitoring, and ecosystem restoration. Future research should prioritize global spatiotemporal dynamics of DOC concentration and composition by integrating field observations, remote sensing, and modeling to better understand and mitigate its ecological and socioeconomic impacts.