The impact of submarine groundwater discharge on nutrient budget—a case in a typical mariculture bay of the Luoyuan bay, Fujian

Submarine groundwater discharge (SGD) has been widely recognized as an important pathway for nutrients transport from coastal aquifers to the sea, which plays an important role in coastal material flux, biogeochemical cycle and ecological environment. In recent years, with the development of mariculture, the impact of SGD on the aquaculture ecosystem has attracted much attention. In this paper, ²²³Ra and ²²⁴Ra are adopted as tracers to quantify SGD in Luoyuan bay, a coastal aquaculture bay in Fujian. The results shows that the radium activities of ²²³Ra (58.1±40.2) dpm/(100 L)(n=9) and ²²⁴Ra (1637±1118) dpm/(100 L)(n=9) in groundwater is higher than those of ²²³Ra (4.49±1.35) dpm/(100 L)(n=15) and ²²⁴Ra (65.5±15.8) dpm/(100 L)(n=15) in the surface water. Assuming steady state and using a radium mass balance model, the SGD rate in Luoyuan bay was estimated to be as (1.91±2.16)×10⁷ m³/d and the discharge rate is 0.068～0.101 m³/m²/d. The fluxes of dissolved inorganic nitrogen (DIN), dissolved inorganic phosphorus (DIP) and dissolved silicon (DSi) carried by SGD were calculated to be (3.23±3.65)×10⁶, (6.68±7.55)×10⁴, (3.58±4.04)×10⁶ mol/d, accounting for 79.6%, 84.8% and 75.4% of the total input. Therefore, the nutrient fluxes by SGD contribute significantly to the nutrient budget and nutrient structure as well as coastal aquaculture system.