Our study was conducted in three coastal drainage basins of Miami, FL (USA). We collected monthly water samples at four locations along inland-to-coastal gradients of three canals that drained into Biscayne Bay (Wagner Creek, WC; Coral Gables, CG; Little River, LR; n = 12). Surface water grab samples were collected monthly from twelve canal locations during baseflow from June 2018 through May 2019 (n = 288). We collected water samples during periods no less than 24 h after a rainfall event to remove the direct influence of rainfall-driven effects on streamflow and first-flush. Surface grab water samples were collected within 3 h of each other on sampling days. In addition, three groundwater wells were sampled for baseline water quality during wet and dry seasons (n = 12). Prior to sampling, the groundwater wells were purged of at least three standing well volumes. All water samples were filtered in the field using Whatman 0.7-µm glass fiber filters (GF/F) that were pre-combusted at 500ºC and collected in acid-washed, amber HDPE bottles. Samples were transported to the laboratory on ice and stored at 4°C and analyzed within seven days of collection. All water samples were analyzed for dissolved organic carbon (DOC) on a Shimadzu TOC-V total organic carbon analyzer after acidification and purging to remove inorganic C. The net change in longitudinal DOC concentration (Δ DOC; mg L-1) in each canal was calculated as the difference between downstream and upstream concentrations. Hence, positive values indicate DOC net gain, whereas negative values indicate DOC loss, inclusive of in-stream biotic and abiotic processes and tidal mixing as part of net DOC changes. At each sampling location, we also measured water physicochemical parameters [temperature, dissolved oxygen (DO), conductivity, and pH with a YSI multiparameter handheld sonde (YSI Company, Yellow Springs, OH, USA).

Water chemistry parameters, including total phosphorus (TP; µg L-1), total Kjeldahl nitrogen (TKN; mg L-1), nitrate + nitrite-nitrogen (NOx-N; mg L-1), and chlorophyll a (chl-a) were collected and analyzed by the Miami-Dade County Department of Environmental Resources Management (DERM; Lietz, 1999) at the most upstream and downstream sites of this study. Water quality analyses were analyzed at DERM using EPA methods 365.1, 353.2, 351.4, and 445.0 for TP, TKN, NOx, and chl-a respectively (U.S. EPA, 2018). δ18O and δ2H isotope compositions of water samples were analyzed using a Los Gatos DLT laser isotope analyzer at Florida International University (FIU).

A Matlab script is also included that describes the Bayesian Monte Carlo mixing model used to compute fractional end-member contributions to the study canals.

This data supports the findings of the manuscript "Linking seasonal changes in organic matter composition and nutrients to shifting hydraulic gradients in coastal urban canals" submitted to WRR for review in July 2022.