Climate-induced shifts in precipitation and tidal extension likely alter the sources and timing of water and organic matter (OM) to coastal river networks. Temporal shifts in water sources associated with altered hydrologic flowpaths from urban stormwater management can introduce variable dissolved organic carbon (DOC) and nutrient loads. Using a combined isotope and chemical tracer approach, we asked how time-variable interactions among water source contributions influence DOC concentration and fluorescent dissolved organic matter (fDOM) composition in urban canals connected to the ocean. We evaluated the spatiotemporal variability of fDOM characteristics and water isotope composition in urban canal networks and quantified the relative contributions of marine water, rainwater, and groundwater using end member mixing analysis. Measurements of fDOM composition, DOC concentrations, ẟ18O and ẟ2H isotopic signatures, and chloride (Cl Abstract) were collected across diel and monthly time scales from 2018-2019 to represent a range of terrestrial and marine hydrologic flowpaths in three coastal urban canal systems of Miami, Florida (USA). Characteristics of fDOM were analyzed using excitation-emission matrix fluorescence spectroscopy. Combined isotopic and chemical tracers were used to characterize relative proportions of endmembers and the spatiotemporal extent of water column mixing.
Results/Conclusions
Synchronous monthly patterns of fDOM and DOC with changing water levels across canal sites indicated the influence of major hydrologic events – such as extreme high tide (> 1 m) and high-flow stormwater discharge – that connect canals to upstream sources of terrestrial DOM. Yet, concentrations of DOC were positively correlated with Cl Abstract (r = 0.48) and Fluorescence Index (FI, r = 0.35), suggesting that microbial-sourced, bioavailable fDOM is primarily supplied from the marine endmember. The two tracer mixing models including either ẟ18O and ẟ2H or ẟ18O and FI produced similar results and indicated groundwater contributions (up to 10% of total) at peak high tide during the height of the subtropical wet season (September - November). Marine water, precipitation, and groundwater contributions varied monthly and accounted for 55%, 43%, and 2% of canal water on average, respectively (n = 312). Increases in fDOM concentrations with increasing Cl Abstract in canals indicate that evaporation is an important control on DOM dynamics when not supplemented from shallow groundwater inputs. Overall, a combined tracer approach with dual water isotopes and fDOM characteristics reveals differences in short-term fluctuations in water source and fDOM composition that likely influence biogeochemical function in urban canals.