Porewater-derived nutrient fluxes in a coastal aquifer (Shengsi Island, China) and its implication

Abstract

As an important component of the hydrological and biogeochemical cycle, porewater discharge represents a significant pathway for releasing chemical solutes into coastal zones, particularly in highly permeable aquifers. In this study, a 222Rn advection-diffusion model was used to estimate the porewater discharge in a coastal aquifer (Shengsi Island, East China Sea) during November 2015. Porewater discharge was estimated to range from 7.4 to 25.8 (mean: 12.9 ± 5.8) cm d−1. Furthermore, the estimated porewater-derived nutrient fluxes (dissolved inorganic nitrogen (DIN), phosphorus (DIP) and silicon (DSi)) (mol m−2 d−1) were (1.7 ± 1.4) × 10−2, (2.1 ± 1.1) × 10−4 and (1.5 ± 1.3) × 10−2, respectively. The Si/N ratio of coastal seawater at Shengsi Island was ∼0.83, which is close to that of porewater along the coastal aquifers of Shengsi Island (∼0.92) but higher than that of the Yangtze River Estuary (∼0.68). Thus, porewater-derived Si flux with a higher Si/N ratio may mitigate the outbreak of non-siliceous algae (i.e., Prorocentrum dentatum) in adjacent waters of Shengsi Island. By comparing the SGD-derived nutrient fluxes worldwide, this study suggests that Si flux with a higher Si/N ratio through porewater discharge (or SGD) may strongly influence the Si budget and cycling because such porewater/SGD-derived Si can compensate for the dwindling Si flux from riverine sources due to human activity (i.e., dam construction, reservoirs). Our results are expected to increase our understanding of not only biogenic elements of cycling processes but also eco-environment processes such as the occurrences of harmful algal blooms alone river-influenced coasts.