We identified a barely noticed contributor, submarine groundwater discharge (SGD), to acidification of a coastal fringing reef system in Sanya Bay in the South China Sea based on time-series observations of Ra isotopes and carbonate system parameters. This coastal system was characterized by strong diel changes throughout the spring to neap tidal cycle of dissolved inorganic carbon (DIC), total alkalinity, partial pressure of CO2 (pCO2) and pH, in the ranges of 1851-2131 μmol kg(-1), 2182-2271 μmol kg(-1), 290-888 μatm and 7.72-8.15, respectively. Interestingly, the diurnal amplitudes of these parameters decreased from spring to neap tides, governed by both tidal pumping and biological activities. In ebb stages during the spring tide, we observed the lowest salinities along with the highest DIC, pCO2 and Ra isotopes, and the lowest pH and aragonite saturation state. These observations were consistent with a concurrent SGD rate up to 25 and 44 cm d(-1), quantified using Darcy's law and (226)Ra, during the spring tide ebb, but negligible at flood tides. Such tidal-driven SGD of low pH waters is another significant contributor to coastal acidification, posing additional stress on coastal coral systems, which would be even more susceptible in future scenarios under higher atmospheric CO2.
Abstract:To quantify the contribution of submarine groundwater discharge (SGD) to the nutrient budget in tropical embayments, naturally occurring radium isotopes ( 223 Ra, 224 Ra, 226 Ra, and 228 Ra) 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 224 Ra/ 228 Ra, 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 226 Ra and 228 Ra, SGD was calculated to be 2.79 ± 1.39-5.07 ± 2.67 × 10 6 m 3 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 × 10 5 mol d −1 for oxidized inorganic nitrogen, 4.64 ± 2.74-8.42 ± 5.19 × 10 3 mol d −1 for phosphate, and 6.63 ± 3.29-12.0 ± 6.34 × 10 5 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.
Abstract. To investigate variation in nitrite, nitrate, phosphate, and silicate in a spring-neap tide in a coral reef system influenced by groundwater discharge, we carried out a timeseries observation of these nutrients and 228 Ra, a tracer of groundwater discharge, in the Luhuitou fringing reef at Sanya Bay in the South China Sea. The maximum 228 Ra, 45.3 dpm 100 L −1 , appeared at low tide and the minimum, 14.0 dpm 100 L −1 , appeared during a flood tide in the spring tide. The activity of 228 Ra was significantly correlated with water depth and salinity in the spring-neap tide, reflecting the tidal-pumping feature of groundwater discharge. Concentrations of all nutrients exhibited strong diurnal variation, with a maximum in the amplitude of the diel change for nitrite, nitrate, phosphate, and silicate in the spring tide of 0.46, 1.54, 0.12, and 2.68 µM, respectively. Nitrate and phosphate were negatively correlated with water depth during the spring tide but showed no correlation during the neap tide. Nitrite was positively correlated with water depth in the spring and neap tide due to mixing of nitrite-depleted groundwater and nitrite-rich offshore seawater. They were also significantly correlated with salinity (R 2 ≥ 0.9 and P < 0.05) at the ebb flow of the spring tide, negative for nitrate and phosphate and positive for nitrite, indicating the mixing of nitritedepleted, nitrate-and phosphate-rich less saline groundwater and nitrite-rich, nitrate-and phosphate-depleted saline offshore seawater. We quantified variation in oxidized nitrogen (NO x ) and phosphate contributed by biological processes based on deviations from mixing lines of these nutrients. During both the spring and neap tide biologically contributed NO x and phosphate were significantly correlated with regression slopes of 4.60 (R 2 = 0.16) in the spring tide and 13.4 (R 2 = 0.75) in the neap tide, similar to the composition of these nutrients in the water column, 5.43 (R 2 = 0.27) and 14.2 (R 2 = 0.76), respectively. This similarity indicates that the composition of nutrients in the water column of the reef system was closely related with biological processes during both tidal periods, but the biological influence appeared to be less dominant, as inferred from the less significant correlations (R 2 = 0.16) during the spring tide when groundwater discharge was more prominent. Thus, the variability of nutrients in the coral reef system was regulated mainly by biological uptake and release in a spring-neap tide and impacted by mixing of tidally driven groundwater and offshore seawater during spring tide.
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