Estimation of submarine groundwater discharge and associated nutrient fluxes in Tolo Harbour, Hong Kong

Abstract

Tolo Harbour, located in the northeastern part of Hong Kong's New Territories, China, has a high frequency of algal blooms and red tides. An attempt was made to first quantify the submarine groundwater discharge (SGD) into Tolo Harbour using 226Ra, and then to estimate the nutrient fluxes into the Harbour by this pathway. The total SGD was estimated to be 8.28 × 106 m3 d− 1, while the fresh submarine groundwater discharge (FSGD) was estimated to be 2.31 × 105 m3 d− 1. This showed that a large amount of SGD was contributed by recirculated seawater rather than fresh groundwater in the Harbour. Using the SGD and groundwater nutrient information around Tolo Harbour, the nutrient loading through SGD was estimated to be 1.1 × 106 mol d− 1 for DIN, 1.4 × 104 mol d− 1 for PO43 −–P and 1.4 × 106 mol d− 1 for SiO2–Si, which was much more significant than its counterpart through the river discharge. Despite uncertainties in the estimation, the nutrient loading to Tolo Harbour by SGD is clearly significant. Thus, the current efforts for management of red tides in Tolo Harbour have to be reviewed and control of groundwater contamination is obviously required.