Net subterranean estuarine export fluxes of dissolved inorganic C, N, P, Si, and total alkalinity into the Jiulong River estuary, China

To evaluate geochemical impacts of the subterranean estuary (STE) on the Jiulong River estuary, China, we estimated seasonal fluxes of subterranean water discharge into the estuary based on the mass balance of radium isotopes and net subterranean export fluxes of dissolved inorganic C (DIC), N (DIN), Si (DSi), soluble reactive phosphorus (SRP), and total alkalinity (TA). Based on 226Ra data, the subterranean discharge (in 107 m3 d−1) was estimated to be 0.29–0.60 in the spring, 0.69–1.44 in the summer, 0.45–0.93 in the fall, and 0.26–0.54 in the winter. This was equivalent to 8–19% of the concomitant river discharge. The net spatially integrated material fluxes from the STE into the estuary were equivalent up to 45–110% of the concomitant riverine fluxes for DIC and TA, around 14–32% for DSi and 7–19% for DIN, and negligible for SRP. Paradoxically, the mixing lines along the salinity gradient revealed no apparent additions of these species. These additions are not revealed because the STE is a relatively small spatially-averaged source (at most 11% of the total input at steady state) that spreads throughout the estuary as a non-point source in contrast to the major point sources of the river and the ocean for the estuary and a true open ocean endmember is likely lacking. Greater water flushing in the summer might dilute the STE effect on the mixing lines even more. The great spatial variation in salinity in the estuary introduced the major uncertainty in our estimates of the flushing time, which further affected the estimate of the subterranean discharge and associated material fluxes. Additionally, the great spatial variation in the STE endmember caused the relatively large ranges in these flux estimates. Despite apparent conservative mixing of DIC, DIN, and DSi in estuaries, net subterranean exports must be taken into account in evaluating geochemical impacts of estuarine exports on shelf waters.