Raw water surface samples were collected at sites along the Neuse River (NR), Neuse River Estuary (NRE), and Pamlico Sound (PS) in North Carolina over a period of three months, between October and December 2016, following the passage of Hurricane Matthew on October 9, 2016. Sampling was conducted approximately weekly across the NR, NRE, and PS sites. Riverine samples were collected, in 1 L brown HDPE bottles, from bridge overpasses along the main-stem of the NR at USGS gaged locations. Estuarine and sound samples were collected, in 1 L opaque HDPE bottles and frozen for shipment to NCSU, by the Neuse River Estuary Modeling and Monitoring Project ("ModMon") located at the University of North Carolina at Chapel Hill's Institute of Marine Science (UNC-IMS) in Morehead City, NC. Collection bottles were cleansed in a detergent bath, rinsed profusely with Milli-Q ultrapure water, and left to air dry before sampling. We conducted additional sampling in an area of freshwater riparian wetlands between the last riverine site on the NR near Fort Barnwell, NC and first estuarine site on the NRE at the Street's Ferry Bridge crossing near Vanceboro, NC in March and October 2017.

Raw water samples were thawed at room temperature, and a known volume of water was filtered through pre-combusted (at 45C for 5 hours) 0.7 m Whatman glass fiber filters (GF/F) via vacuum. Prior to sample filtration, 150 mL of Milli-Q water was used to rinse each filter. The filtrate was collected into 60 mL polycarbonate bottles (detergent-washed and rinsed thoroughly with Milli-Q water) for optical analyses and 40 mL amber-tinted borosilicate glass vials (detergent-washed, rinsed thoroughly with Milli-Q water, and combusted at 45C for 5 hours) for dissolved organic carbon (DOC) concentration and stable carbon isotope (13C-DOC) analyses. Filtrate was stored at 4C until optical measurements were made, generally within 48 hours. Filtrate for DOC analysis was acidified to a pH of 2 with 85% phosphoric acid (H3PO4), then stored at 4C until measurements were completed. GF/F filters were stored at -20C until particulate organic carbon (POC) concentration and stable carbon isotopes (13C-POC) could be measured.

Absorbance measurements were made with a Varian Cary 300UV spectrophotometer in 1 cm quartz cuvettes, over a range of wavelengths (200-800 nm), and then blank corrected using Milli-Q ultrapure water. (Osburn and Morris, 2003)

Fluorescence was measured on a Varian Eclipse spectrofluorometer under the following conditions: excitation (Ex) wavelengths from 240 to 450 nm at 5 nm intervals, emission (Em) wavelengths from 300 to 600 nm at 2 nm intervals. The fluorescence data was concatenated into excitation-emission matrices (EEMs), which display qualitative information about OM within a system, such as the sources of organic matter (Coble, 2007).

Samples for DOC were immediately filtered through 0.2 um polyethersulfone filters into glass vials, capped with Teflon-lined septa and stored without headspace at 4C until analysis. DIC concentration ([DIC]) and delta-13C-DIC value was measured on a OI Analytical TOC analyzer (Doctor et al., 2008)

To quantify DOC concentration, [DOC], samples were first sparged with ultrapure Argon gas for 20 minutes to remove dissolved inorganic carbon (DIC). Sparged samples were analyzed on an OI Analytical 1030D Aurora total organic carbon analyzer, using wet chemical oxidation, coupled to a Thermo Delta V Plus isotope ratio mass spectrometer (IRMS) to determine stable isotope values, expressed as the standard delta notation, 13C-DOC (Osburn and St. Jean, 2007). [DOC] were blank-corrected for ultra-pure Milli-Q water, then calculated using a linear regression curve of known caffeine standards with concentrations from 1 to 20 mg C per L. 13C-DOC values were blank corrected and referenced to the Vienna Pee Dee Belemnite (VPDB) scale via a linear regression of six caffeine (IAEA-600, -27.7 +/- 0.04‰) and two sucrose (IAEA-C6, -10.8 +/- 0.03‰) International Atomic Energy Agency (IAEA) standards. Precision for [DOC] and 13C-DOC values were +/- 0.4‰ based on reproducibility and calibration to sucrose standard. Milli-Q blanks were ran every 10 samples for quality control and confidence of instrument.

Frozen GF/F filters were thawed, then dried at 60C for at least 24 hours in an oven to prepare for measurement of particulate carbon and nitrogen concentrations ([POC] and [PN] respectively) and the stable isotope of [POC], 13C-POC. Analysis was conducted on a Thermo Finnigan Flash Series 1112 Elemental Analyzer (EA), coupled to a Delta V IRMS via a Conflo III interface. Calibration curves were made using acetanilide, L-glutamic acid (USGS-40), caffeine (IAEA-600), oxalic acid (NIST-4990C), and sucrose (IAEA-C6) standards. Weighed standards (0.200-0.600 mg) were placed in low blank tin capsules (4 x 6mm), while dried GF/F filters were folded then placed in larger low blank tin capsules (10 x 12mm). Carbon and nitrogen peak locations on the EA, [POC], and [PN] were determined using acetanilide standards with percent content values of 71.09% and 10.36% for carbon and nitrogen, respectively. 13C-POC for each sample was determined by converting the measured value to the VPDB scale using a linear fit regression of the L-glutamic acid (-26.39‰), caffeine (-27.77‰), oxalic acid (-17.8‰), and sucrose (-10.8‰) standards. Similar to the precision for [DOC] and 13C-DOC, 13C-POC values were +/- 0.4‰ based on reproducibility and calibration to known standards.

Synoptic measurements were collected using a handheld YSI probe and include salinity, pH, temperature, dissolved oxygen concentration (DO), DO saturation (%DO), Total Dissolved Solids (TDS), and Specific Conductivity (Spec. Cond.).