Surface and pore water sampling: Dissolved organic and inorganic carbon in the water column and pore water was sampled one hour after high tide when the water started to drain from the mangrove forest. High tides were monitored from the tidal station in Curaçao which is 76 km away from Bonaire, and had to be corrected by direct field observations due to a changing delay inside the bay depending on individual hydrodynamics of each channel and the tidal cycles. All water samples were taken within one hour after high tide. Surface water was retrieved from the upper 30 cm with a bucket and filtered through 0.7 μm GF/F filters (Whatman) into glass vials with rubber tabs – 24 ml vials for DOC and 12 ml vials for DIC. Before sampling all vials were washed and rinsed three times with MiliQ water. Then they were heated in an oven for 4 hours at 450 °C to remove any organic remains. The rubbers were rinsed for 24 hours in MiliQ water. Two blanks were prepared for each parameter with boiling drinking bottle water to measure possible carbon remains. Pore water samples were retrieved with a push point sampler. The push point sampler was inserted into the sediment along the channels into a depth of 50 cm. Samples were then collected by using a syringe. The vials for dissolved organic carbon were filled to ¾ leaving a headspace. Vials for dissolved inorganic carbon were carefully filled to the top avoiding any bubbles and thus CO₂ production. For each sample one replicate was taken. Samples were taken during the field campaign, placed into a cool box and transported to the marine laboratory within 2 hours for further processing. Dissolved organic carbon samples then were preserved with 0.2 ml HCl-solution. The HCl-solution was prepared with 2N HCl-solution which was diluted 1:5 with bottled water to reach a pH lower than 2. The acidified samples were carefully closed, shaken and stored upright in the fridge until further analysis. Dissolved organic carbon was measured using a Shimadzu Total Organic Carbon Analyzer (TOC-VCPH) using the non purgeable organic carbon (NPOC) method where the CO2 is removed via sparging with carrier gas. What remains is a solution of non-volatile organic carbon compounds. These can be oxidized to CO2 and detected via non-dispersive infrared (NDIR) method. Dissolved inorganic carbon samples were preserved with 0.2 ml of saturated mercury chloride (HgCl₂) solution using two needles without opening the vials again. The preserved samples were stored upright in the fridge until further analysis. Dissolved inorganic carbon was measured using a QuAAtro device. Samples are mixed with H₂O₂ to remove potential interference from sulphide ions and then acidified to pH <1, which forces all carbonate components into the CO₂ gas phase. The stream is led over a gas-permeable silicone membrane in a dialyzer, where the alkaline recipient stream absorbs the gas and discolours the indicator, phenolphthalein. DIC concentrations is then determined photo-metrically. Nitrogen is used for segmentation to avoid possible interference from CO₂ in the atmosphere. Dickson standards were used as reference material. Physical parameters oxygen, pH, temperature and conductance were measured in the surface water at each site using a handheld meter Multi 340i with three different micro sensors (TetraCon925 for conductivity, Sentix980 for pH and FDO 925 for oxygen). All sensors were calibrated before each sampling day. Sentix980 micro sensor was calibrated with two pH buffer solutions of 4.0 and 7.0. Calculating Total Alkalinity and pCO₂: We calculated Total Alkalinity (TA) and pCO₂ from DIC, pH, salinity and temperature measurements using carbonic acid dissociation constants of Millero (2006, doi:10.1016/j.marchem.2005.12.001) (based on those of Harned and Scholes, 1941 and Harned and Davis, 1943) and the CO₂ solubility from Weiss (1974, doi:10.1016/0304-4203(74)90015-2) as implemented in the CO2SYS program, version 01.05, written by Ernie Lewis.
