Significance of Submarine Groundwater Discharge in Nutrient Budgets in Tropical Sanya Bay, China

Guizhi Wang,Shuling Wang,Wenping Jing,Zhangyong Wang

doi:10.3390/su10020380

Abstract

To quantify the contribution of submarine groundwater discharge (SGD) to the nutrient budget in tropical embayments, naturally occurring radium isotopes (223Ra, 224Ra, 226Ra, and 228Ra) were investigated as SGD tracers in Sanya Bay, China. Higher activities of radium were present along the north coast and near the Sanya River estuary. Using the activity ratio of 224Ra/228Ra, the apparent water age in Sanya Bay was estimated to be 0–13.2 days, with an average of 7.2 ± 3.2 days. Based on the mass balance of 226Ra and 228Ra, SGD was calculated to be 2.79 ± 1.39–5.07 ± 2.67 × 106 m3 d−1 (or 4.3 ± 2.1–7.8 ± 4.1 cm d−1). SGD associated dissolved inorganic nutrient fluxes into Sanya Bay were estimated to be 3.94 ± 2.00–7.15 ± 3.85 × 105 mol d−1 for oxidized inorganic nitrogen, 4.64 ± 2.74–8.42 ± 5.19 × 103 mol d−1 for phosphate, and 6.63 ± 3.29–12.0 ± 6.34 × 105 mol d−1 for silicate. The estuarine nutrient flux from the Sanya River was a few times smaller than the phosphate flux via SGD and at least an order of magnitude smaller than the oxidized inorganic nitrogen and silicate fluxes carried by SGD. SGD was also more important than atmospheric deposition and nitrogen fixation in the nutrient budget. Our results demonstrate that SGD compensated for at least 15% phosphate, 90% oxidized inorganic nitrogen, and 60% silicate of the nutrients sink in Sanya Bay.

Highlights

Coastal waters are prone to deterioration under a global context of climate change and changes in ocean and land-source forces
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The activity ratio of 224Ra/228Ra ranged 0.42–1.48 in Sanya Bay with the maximum occurring at Station P1 outside the Sanya River estuary, with higher values in the north and northeast of the bay (Figure 5a), indicating sources of radium from the coastline

Summary

Introduction

Coastal waters are prone to deterioration under a global context of climate change and changes in ocean and land-source forces. Acidification and hypoxia occur in coastal waters induced by upwelling [1,2,3,4,5] and coastal eutrophication and consequent hypoxia are observed due to increasing terrestrial nutrient loadings from catchment areas [6]. These massive nutrient inputs often stimulate phytoplankton growth in coastal waters and result in the conversion of inorganic carbon to organic carbon [6], which is an important step in the marine carbon cycle. The short-lived radium isotopes, Sustainability 2018, 10, 380; doi:10.3390/su10020380 www.mdpi.com/journal/sustainability

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Conclusion