To measure CO₂ we used a CO₂ sampling data logger from the company CO2Meter, Inc. (https://www.co2meter.com/). The model was a CM-0002 gas detector handheld which has a measurement range of 0 – 10.000 ppm (0-1%) CO₂ with an accuracy of ± 30ppm ± 3%. The handheld is using the non-dispersive infrared (NDIR) method to determine CO₂ concentrations. Before each measurement the CO₂ data logger were calibrated to outdoor air which was assumed to have a CO₂ concentration of 400 ppm. At each sampling plot at low tide three CO2-Meters were positioned adjacent to each other but not covering pneumatophores or crab holes to avoid extra respiration not from the sediment-air interface.A chamber with 305 ml of volume was installed on the sediment and shaded to maintain temperature inside the chamber steady. Furthermore the chamber was darkened to prevent any respiration of ecosystems (Reco). Inside the chamber a HOBO temperature logger was positioned, with a logging interval of one minute. After setting up the instruments for measurements we waited between approximately 10 minutes until we started the logging. This was because the logging needed to start after any sediment disturbance has occurred. Each measurement took between 17 and 25 minutes. Data was considered to be obtained under steady state conditions if the temperature did not change more than 1.5 °C (Chojnicki et al., 2009).

CO₂ flux density [mg CO₂-C m² hˉ¹] (FCO₂) was calculated using the following equation from Chojnicki et al., 2009:FCO₂ = kCO₂ *(273 * Tairˉ¹) * (V * Aˉ¹) * (dc * dtˉ¹)…where kCO₂ is the gas-constant at 273.15K = 0.536 [μg C μlˉ¹], Tair represents the air temperature in the chamber [K], V is the chamber volume [l], A denotes the collar area [m²] and dc * dtˉ¹ is the concentration change [ml/l] over time [h] inside the chamber.