Seasonality of Nutrient Flux and Biogeochemistry in an Intertidal Aquifer

Abstract

Biogeochemical reactions in coastal aquifers highly affect the nutrient flux associated with submarine groundwater discharge (SGD) to the ocean, which subsequently influences the oceanic environment and ecology. This study investigates a seasonal variation of SGD-derived nutrient flux to the ocean and nutrient biogeochemistry in an intertidal aquifer of Tolo Harbor, Hong Kong. The results show that the inventory of NOx− and PO43− in the intertidal aquifer has a clear seasonality with a large inventory in summer, a small inventory in spring, and a median inventory in autumn and winter, respectively. Differently, the inventory of NH4+ is large in winter and summer and small in spring and autumn, which results from the coupled effects of seasonal change of both production and removal of NH4+ in the aquifer. The SGD-derived nutrient (NH4+, NOx−, and PO43−) flux is the highest in summer (271.71, 24.86, and 116.66 mmol·day−1·(m coastline)−1) and is the lowest in spring (114.83, 1.70, and 20.26 mmol·day−1·(m coastline)−1). The majority of SGD-derived nutrient flux is supported by the local remineralization of organic matter along with seawater infiltration. In autumn, the recharge of seawater induced by tidal pumping significantly shifts the biogeochemical balance of nutrients and is the major source of groundwater nutrients in the intertidal aquifer. Among the various nutrient fluxes (SGD, river discharge, atmospheric deposition, and benthic sediment diffusion) to Tolo Harbor, SGD-derived PO43− flux is the second major source of seawater PO43− in addition to benthic sediment diffusion. The PO43− loading via SGD is of significance to the primary production in the phosphorus-limited environment in Tolo Harbor. After considering the natural attenuation of nutrients in a sandy/silty beach aquifer, this study suggests the overestimation of SGD-derived nutrient loading estimated previously that simply use average nutrient concentration of fresh SGD endmember and saline SGD endmember as the nutrient concentration of total SGD endmember. ©2018. American Geophysical Union. All Rights Reserved.