Submarine groundwater discharge estimation in an urbanized embayment in Hong Kong via short-lived radium isotopes and its implication of nutrient loadings and primary production

Abstract

224Ra and 223Ra are adopted as tracers to qualify submarine groundwater discharge (SGD) in Tolo Harbor, a highly urbanized embayment in Hong Kong. Based on the sampling data, a two-layered radium mass balance model is used to estimate lateral SGD and bottom SGD. Total SGD is estimated to be 1.2–3.0 cm d−1, including lateral SGD of 5.7–7.9 cm d−1 and bottom SGD of 0.3–2.0 cm d−1. Fresh SGD is estimated to be (2.1–5.5) × 105 m3 d−1. Nutrient fluxes (mol d−1) from SGD are estimated to be (3–7.4) × 104 (dissolved inorganic nitrogen), (2.4–6.2) × 102 (dissolved inorganic phosphate) and (6.5–16) × 104 (dissolved silicate). Primary productivity is estimated to be (1.5–15) × 106 gC d−1, 2–53% of which is supported by SGD-induced phosphate fluxes. The study indicates that SGD is a significant source of nutrients to coastal waters and may cause an obvious increase of primary production. These findings must be considered in future coastal ecological management.