Submarine groundwater discharge and its biogeochemical and ecological impacts in coastal environment

Abstract

Submarine groundwater discharge (SGD), as a major component of the hydrological cycle, is an important pathway for the delivery of terrestrial solutes to coastal waters. With rapid urbanization, coastal ecosystems have been receiving increasingly high land-derived nutrient loadings transported through SGD and river discharge, leading to coastal eutrophication. Eutrophication provides a favorable nutrient environment for phytoplankton growth, as a result high SGD input may induce red tide occurrence. However, nutrients in coastal environments are supplied by multiple sources such as SGD, river discharge, precipitation, sediment release and seawater. This research aims to explore the role of SGD in influencing phytoplankton growth among all sources and explain how SGD influences phytoplankton biomass and its community structure through changing the physical and biogeochemical conditions of coastal water. This research integrates field observation, incubation experiment and long-term monitoring data analysis and Tolo Harbour, Hong Kong, is selected to carry out the case study. Tolo Harbour is a P limited environment and NH4+ is the dominant N species, so water sources delivering more NO3- and P may intensely influence the phytoplankton growth. Flux and nutrient loading of different water sources were firstly estimated using mass balance model. Total SGD flux was estimated to be a magnitude higher than the river discharge and precipitation. Sediment release attributes a similar level as total SGD, as calculated by 224Ra/228Th disequilibrium method. SGD and sediment release contributed the top most NO3- and P loadings. The contribution of NO3- from different water sources to surface harbour water in both wet and dry seasons was then qualified using stable isotopes with a Bayesian mixing model. Precipitation contribute the most to NO3- in surface harbour water in wet season and sediment release becomes the dominant source to the surface NO3- in the dry season. SGD is the second most important source supplying NO3- in both seasons. Chemical compositions of SGD are substantially altered by the physical and geochemical processes in coastal groundwater mixing zone before discharge to the sea. The observation in Ting Kok in Tolo Harbour shows that NO3- may be reduced by denitrification when groundwater flows through the beach aquifer, however NH4+ and P are mainly produced through remineralization. During transport to surface water, nutrients are strongly influenced by vertical mixing and stratification. In wet season, fresh water sources (precipitation, river and fresh SGD) supply nutrients with NH4+:NO3- ratio <1 to surface water and create intensive vertical stratification. In dry season, with the decline of fresh water sources and the ease of stratification, vertical mixing uplifts the nutrient with NH4+:NO3- ratio >1 released from benthic sediment, leading to a NH4+ dominant environment. Therefore, diatoms, as a NO3- specialist, is prone to be the major group in wet season, while dinoflagellates and other groups then become dominant in dry season. The results of an incubation experiment and long-term monitoring data analysis further supported this observation. The findings of this study will benefit predicting red tide occurrence and managing water quality effectively in Tolo Harbour and other coastal environments with similar conditions.