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Abstract Coastal ecosystems are rapidly shifting due to changes in hydrologic presses (e.g., sea‐level rise) and pulses (e.g., seasonal hydrology, disturbances, and restoration of degraded wetlands). Changing water levels and sources are master variables in coastal wetlands that can alter carbon concentrations, sources, processing, and export. Yet, how long‐term increases in water levels from marine and freshwater sources influence dissolved organic carbon (DOC) concentrations and dissolved organic matter (DOM) composition is uncertain. We quantified how long‐term changes in water levels are affecting DOC concentration (2001–2021) and DOM composition (2011–2021) differently across the Florida Everglades. DOC concentrations decreased with high water depths in peat marshes and increased with high water levels in marl marshes and across mangroves, and these relationships were reproduced in freshwater peat marshes and shrub mangroves. In the highly productive riverine mangroves, cross‐wavelet analysis highlighted variable relationships between DOC and water level were largely modulated by hurricane disturbances. By comparing relationships between water level and DOC concentrations with carbon sources from DOM fluorescence indices, we found that changing water sources between the dry and wet season shift DOM from algal to detrital sources in freshwater marshes, from detrital marsh to detrital mangrove sources in the brackish water ecotone, and from detrital mangrove to algal marine sources in downstream mangroves. As climate change and anthropogenic drivers continue to alter water levels in coastal wetlands, integrating spatial and temporal measurements of DOC concentrations and DOM compositions is essential to better constrain the transformation and export of carbon across these coastal ecosystems. 

Abstract The capacity for coastal river networks to transport and transform dissolved organic matter (DOM) is widely accepted. However, climate‐induced shifts in stormwater runoff and tidal extension alter fresh and marine water source contributions, associated DOM, and processing rates of nutrients entering coastal canals. We investigate how time‐variable interactions among coastal water source contributions influence the concentrations of dissolved organic carbon (DOC), nutrients, and DOM composition in urban canals. We quantified the spatiotemporal variability of DOM quality and nutrient concentrations to determine contributions of tidal marine water, rainwater, stormwater runoff, and groundwater to three coastal urban canals of Miami, Florida (USA). We created a Bayesian Monte Carlo mixing model using measurements of fluorescent DOM (fDOM), DOC concentrations, δ¹⁸O and δ²H isotopic signatures, and chloride (Cl⁻). Fractional contributions of groundwater averaged 17% in the dry season and 26% at peak high tide during the subtropical wet season (September–November). The canal‐to‐marine head difference (CMHD) was a primary driver of groundwater contributions to coastal urban canals and monthly patterns of fDOM/DOC. High tide (>1 m) and discharge events were found to connect canals to upstream sources of terrestrial DOM. Loading of terrestrially sourced DOC and DOM is pulsed to urban canals, shunted downstream and supplemented by microbially sourced DOM during the wet season at high tide. Overall, we demonstrate that a combined tracer approach with isotopes and fDOM can help identify groundwater contributions to coastal waterways and that autochthonous fDOM may prime the degradation of carbon or nutrients as the CMHD pushes inland.