The role of tidal creeks in carbon processing and export to the freshwater tidal zone at the land-estuary boundary of tropical mangrove estuaries

Tidal creeks are conduits for tide-driven processing and exchange of water and dissolved inorganic carbon (DIC) between the open estuary and mangrove forests. Yet, the sources of DIC, and the role of tidal creeks in the cycling of DIC and its transport to the open water of mangrove estuaries is poorly understood. This study was conducted in a tidal creek in the freshwater tidal zone at the land-estuary boundary of a tropical mangrove estuary. We made 177 measurements of salinity and 80 measurements of geogenic solutes (silica, Ca, Mg), DIC components (partial pressure of CO 2 (pCO 2 ), alkalinity and total DIC) and stable isotopes of water (δ 18 O and δD) and DIC (δ 13 C DIC ). We aimed to determine the sources and the timing of water input into the tidal creek, how the different water sources control input of DIC, and how DIC is cycled through semidiurnal mixed tides and over a diel cycle. The salinity-δ 18 O relationship indicates two sources of water, while the relationships for salinity-DO, salinity-solute, and salinity-DIC implicated submarine groundwater discharge (SGD) as the source providing the DIC processed and transported to the open estuary. Normalized water level vs. salinity reveals that SGD sourced DIC dominates the water column during tidal ebbing and halfway through tidal flooding. In contrast, freshwater from the open estuary mainly dilutes the solutes and DIC from halfway during tidal flooding to peak high tide. The photocycle, heterotrophic and autotrophic processes are unable to explain the more than 5 times greater than atmospheric pCO 2 recorded during the daytime and at low tide. These findings demonstrate that SGD is a significant DIC source in tidal creeks at the land-estuary transition zone which indicate the important role of SGD in the cycling of carbon in the freshwater tidal zones of mangrove estuaries. • We measured 177 salinity and 80 DIC concentrations and δ 13 C for semidiurnal mixed tides over 24 h. • Salinity and DIC concentration peaks are decoupled from tidal trough and peaks by a ∼ 3.5 h lag. • Submarine groundwater discharge (SGD) is the source of salinity and DIC influx to the tidal creek. • Tides rather than the photocycle and autotrophic and heterotrophic processes control DIC cycling. • The tidal creek mixes and transport DIC from SGD to the open estuary and then to the marine system.