Groundwater-surface water exchanges and associated nutrient fluxes in Dan'ao Estuary, Daya Bay, China

Based on field measurements from two typical intertidal transects at Dan'ao Estuary, groundwater-surface water and associated nutrient exchanges are quantified. Both groundwater discharge rate (39.1 ± 7.0 cm d−1) and surface water inflow rate (7.7 ± 1.4 cm d−1) at the upstream mangrove swamp transect are much higher than those (1.6 ± 0.3 and 2.1 ± 0.4 cm d−1, respectively) at the downstream bare flat one. This large difference leads to their contrasting nitrogen forms. Much higher water exchange rates at the upstream transect generate much higher net dissolved inorganic nitrogen (DIN, including NH4+, NO2-, and NO3-), dissolved inorganic phosphorus (DIP), and dissolved silicate (DSi) fluxes (− 160.3 ± 39.2, − 14.6 ± 2.7, − 38.6 ± 7.0 mmol m−2 d−1, respectively) than those (2.9 ± 0.9, − 0.08 ± 0.03, 1.1 ± 0.4 mmol m−2 d−1, respectively) at the downstream one. The mangrove swamp at the upstream transect discharges substantial groundwater and associated nutrients to the estuary. The net nutrient loads by water exchange in this estuary can reach 23.5-78.7% of those by local river discharge.