Impacts of super typhoon Mangkhut on the biogeochemical cycle in a mangrove beach aquifer

Abstract

Super Typhoon Mangkhut (MK) was a very powerful and catastrophic tropical cyclone that caused extensive damage in Hong Kong on September, 2018. The strong wind, heavy rain and storm surge produced by MK induced considerable changes in environmental setting in coastal areas. Submarine groundwater discharge (SGD) and the biogeochemical cycle in the coastal aquifer were greatly altered. Through a 4 weeks’ field investigation before and after the passage of MK, this study presented the changes of carbon and nutrients dynamics induced by MK in the mangrove beach aquifer in Tingkok, Hong Kong. As a tracer of groundwater, dissolved 224Ra activities in groundwater experienced a strong fluctuation during the passage of MK. 224Ra activities in nearshore seawater followed the change in groundwater with relatively low activities. High precipitations and strong storm surge increased the discharge of old fresh groundwater and the groundwater salinity in aquifer respectively, which led to the rise of 224Ra activities in the 1st week after MK. However, continuous precipitation diluted the ­224Ra concentration in the coastal discharge zone and induced the decline of 224Ra in the 2nd week after MK. The beach aquifer was flushed by strong storm surge and heavy precipitation leading to the decline of particulate and dissolved organic matters in groundwater. Following the great changes of hydrodynamics, biogeochemical reactions varied profoundly. With less organic matter supply, denitrification was suppressed and more O2 was used to nitrify NH4+ to NO3-. As a result, the groundwater was enriched in NO3- instead of NH4+ as usual. The increased fresh SGD with high NO3- concentration altered the nitrogen speciation of nearshore seawater. Phytoplankton in nearshore seawater grew with the increasing NO3- supply from groundwater after MK and its biomass reached the peak in the 3rd week after MK. This study is the first attempt to explore the processes in biogeochemical cycle in a beach aquifer under the effect of a super typhoon. The findings may contribute to the existing knowledge on the environmental impact of typhoon.