Contrasting Radium-Derived Groundwater Exchange and Nutrient Lateral Fluxes in a Natural Mangrove Versus an Artificial Canal

Abstract

Artificial canals may function differently than the natural coastal wetlands, floodplains, and estuaries they often replace. Here, we assess the impact of canal estate development on saline groundwater exchange (tidal pumping) and associated nutrient fluxes. Time series observations of short-lived radium isotopes and dissolved nutrients were performed in a canal estate and a nearby mangrove creek in subtropical Australia. A mass balance model based on 223Ra (1.3 ± 0.4 and 3.4 ± 0.9 cm day−1 in the mangrove and canal, respectively) and 224Ra (2.8 ± 3.0 and 5.4 ± 4.6 cm day−1) revealed tidally driven groundwater exchange rates were ~ 2-fold greater in the canal. Lateral fluxes of total dissolved nitrogen (TDN) from the nearby estuary into the canal estate were comparable with the mangrove creek (8.4 and 9.1 mmol m−2 day−1 in the mangrove and canal, respectively). Groundwater flows into the canal released ~ 5-fold more TDN than the mangrove. As expected, mangroves appear to be more efficient at retaining groundwater-derived nitrogen than vegetation-stripped, sandy canals. Overall, this study demonstrates that land reclamation for canal estate development not only drives losses of ecosystem services, but also modifies groundwater and related nutrient exchange with coastal surface waters.