Water column samples from seagrass incubations were collected into acid washed single use, sterile, 60 mL syringes (Thermo Fisher Scientific, USA) via water column ports on the PVC tubes. Water column samples from algae incubations were collected into acid washed 60 mL syringes after opening incubation bags on board. Water column samples were filtered immediately through a precombusted 45 mm diameter glass microfiber GF/F filter (Whatman, UK). For each sample, 30 mL were pushed through the filter for rinsing before starting collection. To determine the concentration of dissolved organic carbon (DOC) and total dissolved nitrogen (TDN), 15 mL sample were filtered into a precombusted 24 mL glass vial containing 60 µL 25% hydrochloric acid and the vial sealed with an acid washed teflon-lined cap. Another 10 mL of sample were filtered into a precombusted 12 mL glass vial and the vial sealed with an acid washed teflon-lined cap for colored dissolved organic matter (CDOM) and fluorescent dissolved organic matter (FDOM) analysis. Subsamples for DOC were frozen at -20°C and samples for CDOM/FDOM were stored at 4°C until analysis within two weeks. Optical FireSting O2 sensors (Pyroscience GmbH, Germany) were used to determine oxygen concentrations in water samples. Calibration of the sensors was achieved using MilliQ water. Oxygen concentrations were measured in 600 mL subsamples from water column within two hours of sample collection. For algae incubations, oxygen concentrations could also be determined in water from incubations without prior filtration. For DOC and TDN concentrations, the samples were analyzed using a Shimadzu TOC-VCPH-analyzer and an autoanalyzer (Aquakem 250). CDOM absorption was measured using a Shimadzu 2401PC spectrophotometer with 5 cm quartz cuvette over the spectral range from 200 to 800 nm with 1 nm resolution. Ultrapure water was used as the blank for all samples. Excitation-emission matrices (EEMs) of FDOM were measured with a Varian Cary Eclipse fluorometer (Agilent). Processing of the EEMs was done using the eemR package for R software. A blank sample of ultrapure water was subtracted from the EEMs, and the Rayleigh and Raman scattering bands were removed from the spectra after calibration. EEMs were calibrated by normalizing to the area under the Raman water scatter peak (excitation wavelength of 350 nm) of an ultrapure water sample run on the same session as the samples, and were corrected for inner filter effects with absorbance spectra.