Sample collection: The Sargasso Sea and coastal Rhode Island (USA) samples were collected from 30-liter Niskin bottles secured to a CTD rosette. Samples were collected during the EN589 cruise aboard the R/V Endeavor from September 16 to October 15, 2016. Dr. David Kieber was the chief scientist for this cruise. Rolling Deck to Repository (R2R) data for EN589 can be found at http://www.rvdata.us/catalog/EN589

Samples from a Mo'orea coral reef and Pacific Ocean station (located ~2 kilometers beyond the coral reef) were collected in 2-liter opaque high-density polypropylene bottles aboard a small research boat moored at the Gump Research station. Sampling was between April 6 and April 24, 2018. Details regarding sampling should be directed to Dr. David Kieber.

Sample storage: Seawater samples from the surface were collected in 30-liter (L) Niskin bottles attached to a CTD rosette during the 2016 R/V Endeavor (EN589) Atlantic cruise or in pre-cleaned opaque polypropylene bottles by hand during the 2018 Mo'orea field trip. Following collection, water samples were gravity filtered through precleaned 0.2-micrometer (µm) pore-size POLYCAP 75 AS Nylon filters into precleaned 4 L Qorpak glass bottles. All Qorpak bottles were then filled with minimum headspace (ca. 5 milliliters (mL)), sealed tightly, and stored in the dark at 4 degrees Celsius until use for irradiation experiments.

Apparent quantum yield(AQY) method: A monochromatic irradiation system was used to determine wavelength- and temperature-dependent AQYs for the photochemical production of acrylate in 0.2 µm-filtered seawater at selected wavelengths from 290 to 400 nanometers (nm). See Zhu and Kieber (2018) for detailed setups. Briefly, for each AQY experiment, 25 mL of 0.2-μm filtered, air-saturated seawater was pipetted into a 5-centimeter (cm) pathlength, rectangular quartz cell and sealed with a Teflon-lined screw-top cap with no headspace. The filled quartz cell was placed in the sample chamber and the sample temperature inside the cell was controlled by a recirculating glycol-water bath. Parallel dark controls were obtained by blocking the incoming radiation. All irradiated samples and dark controls were continuously stirred during the experiment. For each water sample, triplicate irradiations were conducted at each wavelength, except for the duplicate irradiations at 360 nm. The photon flux in each AQY experiment was determined using nitrite actinometry (Jankowski et al. 1999) in a separate 5 cm quartz cell. The photon flux in the quartz cell was calculated using the procedure outlined in Kieber et al. (2007).