Coastal water quality assessment and groundwater transport in a subtropical mangrove swamp in Daya Bay, China.

Coastal water quality assessment is challenging due to the complex hydrological environment in mangrove swamps. Such assessment requires a good understanding of swamp hydrology and potential solute reactions. In this study, we investigated the concentration variations of a suite of major elements (Na+, K+, Ca2+, Mg2+, SO42-, HCO3- and Cl-), nutrients such as dissolved inorganic phosphorous (DIP) and silicate (DSi), and heavy metals (Cu, Zn, As, Hg, Cd, Pb and Cr) along a typical mangrove transect in Daya Bay, China. The transect comprises a tidal creek, a mudflat and two mangrove zones. The major elements exhibited various degrees of dilution and loss, such as losses of K+ due to uptake by mangrove trees and of SO42- due to microbial sulfate reduction. Numerical simulations of groundwater flow showed that the inland fresh groundwater can continuously discharge through the landward mangrove zone, where high concentrations of DIP and DSi occurred. However, the middle mangrove zone served as a sink of DIP and DSi due to the weak hydrodynamic environment. The spatial distribution of heavy metal concentrations showed that only Cu pollution occurred in the creek zone. The Metal Pollution Index (MPI) was developed to compare the enrichment capability of heavy metals at the different sampling sites. Results showed that the intertidal groundwater has the strongest heavy metal enrichment capability, followed by sea water, inland groundwater and river water. Within the intertidal zone, the creek zone has the highest MPI, followed by the mangrove zone and the mudflat zone. The local water circulation occurred around the creek should contribute the high MPI in the creek zone.

Nitrogen fate in a subtropical mangrove swamp: Potential association with seawater-groundwater exchange.

Coastal mangrove swamps play an important role in nutrient cycling at the land-ocean boundary. However, little is known about the role of periodic seawater-groundwater exchange in the nitrogen cycling processes. Seawater-groundwater exchange rates and inorganic nitrogen concentrations were investigated along a shore-perpendicular intertidal transect in Daya Bay, China. The intertidal transect comprises three hydrologic subzones (tidal creek, mangrove and bare mudflat zones), each with different physicochemical characteristics. Salinity and hydraulic head measurements taken along the transect were used to estimate the exchange rates between seawater and groundwater over a spring-neap tidal cycle. Results showed that the maximum seawater-groundwater exchange occurred within the tidal creek zone, which facilitated high-oxygen seawater infiltration and subsequent nitrification. In contrast, the lowest exchange rate found in the mangrove zone caused over-loading of organic matter and longer groundwater residence times. This created an anoxic environment conducive to nitrogen loss through the anammox and denitrification processes. Potential oxidation rates of ammonia and nitrite were measured by the rapid and high-throughput method and rates of denitrification and anammox were measured by the modified membrane inlet mass spectrometry (MIMS) with isotope pairing, respectively. In the whole transect, denitrification accounted for 90% of the total nitrogen loss, and anammox accounted for the remaining 10%. The average nitrogen removal rate was about 2.07g per day per cubic meter of mangrove sediments.