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Reactive Solute Transport Through Two Contrasting Subterranean Estuary Exit Sites in the Ría de Vigo (NW Iberian Peninsula)
Frontiers in Marine Science ( IF 3.7 ) Pub Date : 2021-01-25 , DOI: 10.3389/fmars.2021.626813
Elisa Calvo-Martin , Xosé Antón Álvarez-Salgado , Carlos Rocha , J. Severino Pino Ibánhez

Subterranean estuaries (STEs), where continental groundwaters and saltwaters meet, are zones of intense biogeochemical reactivity. As such, STEs significantly modify groundwater-borne nutrient fluxes to the coastal zone. Thus, evaluating their reactive role is crucial to anticipate impacts of submarine groundwater discharge (SGD) over coastal ecosystems. Here, we studied the nitrogen biogeochemistry of two STEs with contrasting wave-exposure and redox conditions in Panxón and Ladeira beaches (Ría de Vigo, NW Iberian Peninsula). Seasonal surveys were performed at the permanently saturated zone of both beaches during low tide in February, May, July, and October 2019. Sediment was sampled and porewater samples collected using push-pull piezometers. Salinity, 222Rn and 226Ra activities were used to trace water circulation inside each beach. Porewater nitrate, ammonium, nitrite and dissolved oxygen were used to evaluate the role of these STEs as reactive sinks or sources of inorganic nitrogen. Our results showed a marked seasonal variability of water circulation inside both beaches, with strong salinity gradients in February and May and weakened circulation in July and October. The presence of a gravel layer in Panxón beach completely altered the typical structure of STEs by increasing porewater transport and mixing through the beach interior. As a result, Panxón beach profiles were highly enriched in nitrate and oxygen. Conversely, suboxic, and anoxic conditions were prevalent in Ladeira beach during the study period, with ammonium being the prevailing inorganic nitrogen form. High nitrate concentrations occurred associated to the tidal circulation cell during February and May, being the only effective mechanism of sediment oxygenation in Ladeira beach. Although nitrate reduction and production were observed in both STEs, comparison with averaged conservative mixing porewater profiles showed that Ladeira beach acted as a net nitrogen sink whereas Panxón beach acted as a net nitrogen source. The presence of a gravel layer oxygenates the interior of Panxón beach, thus limiting nitrate reduction and promoting the amplification of groundwater-borne nitrogen fluxes to the coast.



中文翻译:

通过在维阿河(西北伊比利亚半岛)的两个不同的地下河口出口站点进行反应性溶质运移

大陆地下水和盐水汇合的地下河口(STE)是生物地球化学反应强烈的地区。因此,STEs显着改变了地下水向海岸带传播的养分通量。因此,评估它们的反应性作用对于预测海底地下水排放(SGD)对沿海生态系统的影响至关重要。在这里,我们研究了Panxón和Ladeira海滩(Ríade Vigo,西北伊比利亚半岛)的两种STE的氮生物地球化学,其波暴露和氧化还原条件相反。在2019年2月,5月,7月和2019年10月的退潮期间,在两个海滩的永久饱和区进行了季节性调查。使用推拉式压力计对沉积物进行采样并收集孔隙水样品。盐度,222 Rn和226Ra活动用于追踪每个海滩内的水循环。孔隙水硝酸盐,铵,亚硝酸盐和溶解氧被用来评估这些STE作为反应性汇或无机氮源的作用。我们的研究结果表明,两个海滩内水循环的季节变化明显,2月和5月的盐度梯度较大,7月和10月的水循环减弱。Panxón海滩中砾石层的存在通过增加孔隙水的输送和在海滩内部的混合,彻底改变了STE的典型结构。结果,Panxón海滩剖面富含硝酸盐和氧气。相反,在研究期间,Ladeira海滩普遍存在亚缺氧和缺氧条件,铵是主要的无机氮形式。在二月和五月期间,与潮汐循环池相关的硝酸盐浓度很高,这是拉德拉海滩上沉积物氧合作用的唯一有效机制。尽管在两种STE中都观察到硝酸盐的减少和产生,但与平均保守混合孔隙水剖面的比较表明,拉德拉海滩(Ladeira beach)充当净氮汇,而潘松海滩(panxónbeach)充当净氮源。砾石层的存在给Panxón海滩的内部充氧,从而限制了硝酸盐的还原并促进了地下水向海岸的氮通量的增加。与平均保守混合孔隙水剖面的比较表明,拉德拉海滩(Ladeira beach)充当净氮汇,而潘松海滩(panxónbeach)充当净氮源。砾石层的存在给Panxón海滩的内部充氧,从而限制了硝酸盐的还原并促进了地下水向海岸的氮通量的增加。与平均保守混合孔隙水剖面的比较表明,拉德拉海滩(Ladeira beach)充当净氮汇,而潘松海滩(panxónbeach)充当净氮源。砾石层的存在给Panxón海滩的内部充氧,从而限制了硝酸盐的还原并促进了地下水向海岸的氮通量的增加。

更新日期:2021-02-12
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