DOM was collected in Doboy Sound off the southeastern U.S. in July and October 2014. Six 20 L carboys were filled with water and wrapped in black plastic. Three were processed immediately, while the remaining three were returned to Doboy Sound for a 24 h dark incubation before processing by an identical protocol. This experimental scheme was carried out twice during each sampling event, once at high tide (HT) and once at low tide (LT). Immediately after collection, samples were filtered (using 0.7 μm Whatman GF/F filters pre-combusted at 450°C for 5 h and pre-washed 0.2 μm Pall Supor membrane filters), and aliquots were stored frozen for DOC analysis. Filtrates were acidified to pH 2, and DOM was isolated using solid phase extraction (SPE) cartridges (Agilent Bond Elut PPL) as in Dittmar et al. (2008).

The molecular composition of the DOM extracts (200 mg C L-1 in methanol) was analyzed using a 9.4T Fourier transform ion cyclotron resonance mass spectrometer (FT-ICR MS) with electrospray ionization (ESI; negative mode) at the National ICR Users’ Facility at the National High Magnetic Field Laboratory (NHMFL, Florida State University, Tallahassee, FL).

Assignment of molecular formulae was performed by Kendrick mass defect analysis (Wu et al., 2004) with PetroOrg software (Corilo, 2015) considering a maximum mass error of 0.5 ppm and using the criteria described by Rossel et al. (Rossel et al., 2013).