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- The combined use of radio frequency-electromagnetic surveys and chemical and biological analyses to study the role of groundwater discharge into the Guadiana estuaryPublication . Carvalho Dill, A M M; Stigter, T. Y.; Brito, R.; Teodosio, Maria; Chícharo, LuísThis paper intends to show how the radio frequency-electromagnetic (RF-EM) method can be successfully used in coastal wetlands, onshore and in shallow water. It characterizes and reveals geological and hydrogeological singularities that are often hidden by recent sediments and allows target sampling and consequent detection of ecological particularities. RF-EM methods have traditionally been used to detect the presence of groundwater or the existence of subsurface contamination plumes. The great majority of such studies have been conducted on land and to a lesser extent on freshwater lakes but rarely in coastal waters. Previous research in Portugal has shown that despite the attenuation effect of salt water, very valuable information can be obtained with the use of electromagnetic methods, wherever resistivity contrasts exist. In the current case study, RF-EM surveys were carried out in the Estuary of the Guadiana River, in order to understand the importance of geological structures on the formation of the wetland and the influence of groundwater discharge on the existing ecosystem. Structures detected on land were also identified in the tidal channels. Freshwater fault springs were localized; their biota is characterized and the nutrients analysed. The results showed a clear increase in nitrate and silica concentration near the submarine springs, with impact on the planktonic communities, which is most evident in chlorophyll a. This is especially relevant during the end of summer and autumn when a potential limitation of these nutrients exists that can increase the occurrence of toxic blooms in the salt marshes and lower estuary.
- Effects of external nutrient sources and extreme weather events on the nutrient budget of a Southern European coastal lagoonPublication . Malta, Erik-jan; Stigter, Tibor Y.; Pacheco, André; Carvalho Dill, Amélia; Tavares, Diogo; Santos, RuiThe seasonal and annual nitrogen (N), phosphorus (P), and carbon (C) budgets of the mesotidal Ria Formosa lagoon, southern Portugal, were estimated to reveal the main inputs and outputs, the seasonal patterns, and how they may influence the ecological functioning of the system. The effects of extreme weather events such as long-lasting strong winds causing upwelling and strong rainfall were assessed. External nutrient inputs were quantified; ocean exchange was assessed in 24-h sampling campaigns, and final calculations were made using a hydrodynamic model of the lagoon. Rain and stream inputs were the main freshwater sources to the lagoon. However, wastewater treatment plant and groundwater discharges dominated nutrient input, together accounting for 98, 96, and 88 % of total C, N, and P input, respectively. Organic matter and nutrients were continuously exported to the ocean. This pattern was reversed following extreme events, such as strong winds in early summer that caused upwelling and after a period of heavy rainfall in late autumn. A principal component analysis (PCA) revealed that ammonium and organic N and C exchange were positively associated with temperature as opposed to pH and nitrate. These variables reflected mostly the benthic lagoon metabolism, whereas particulate P exchange was correlated to Chl a, indicating that this was more related to phytoplankton dynamics. The increase of stochastic events, as expected in climate change scenarios, may have strong effects on the ecological functioning of coastal lagoons, altering the C and nutrient budgets.