Surface water samples for these experiments were collected from West Bay of the Neuse River Estuary, North Carolina USA (34°55'32.42" N, 76°21'54.25" W). The microcosms were 60 liters. The experiments were conducted in biological duplicates under three light treatment incubations: 12-hour light-dark cycle of photosynthetically active radiation (PAR), 12-hour light-dark cycle of UV-B radiation, or darkness. Samples were collected from the microcosms in duplicate every few days for over one month to examine how light and the resulting microbial activity altered the DOC pool over time.

For each 60-liter microcosm, water was sampled daily to weekly at the University of North Carolina over the month-long (September 2021) experiment or two-month-long experiment (April 2022). Water was sampled from the microcosms using a peristaltic pump (Masterflex) under gentle (75%) pressure through in-line 3-micron and 0.2-micron, 47-millimeter (mm) polycarbonate filters (Millipore Sigma) which were flushed with 250 milliliters (mL) of Milli-Q water. All tubing, filter holders and 60-mL HDPE collection bottles (Fisher Scientific) were acid-washed in 10% (v/v) HCl for six hours and triple rinsed with Milli-Q water. The collection bottles were twice rinsed with c.a. 10 mL of 0.2 micron-filtered sample water then filled to 40 mL and immediately placed in the freezer (-20 degrees Celsius). At the conclusion of the experiment (after two months), the samples were sent to the University of Georgia where they were run on a Shimadzu TOC-VCPH analyzer as in Letourneau and Medeiros (2019) which was calibrated with Consensus Reference Material as in Hansell (2005).

Instruments and Materials:
L/S Peristaltic Pump (Masterflex EW-07557-10) and 1/4 inch Masterflex L/S Platinum-Cured Silicone Tubing: Estuarine water sampled from microcosm tank at 75% speed.

Millipore SX0004700 Polypropylene Swinnex Filter Holder (47 mm) used in-line with the tubing to pump sample water from the microcosm tank through a 3.0 micron and 0.2 micron polycarbonate filter (Millipore Sigma, 47 mm).

Hydrochloric acid (36% w/w, ThermoFisher) was diluted to 10% (v/v) with Milli-Q water for acid washing procedures.

60-mL HDPE dark bottle (Fisher Scientific, 029235B): 0.2 micron filtered water was collected into these acid-washed and rinsed bottles and stored at -20 degrees Celsius until analysis.

Shimadzu TOC-VCPH analyzer: process samples for bulk DOC concentration, calibrated using Consensus Reference Material.