Abstract
Terrestrial surface waters and submarine ground water discharge (SGD) act as a source of dissolved substances for coastal systems. Solute fluxes of SGD depend on the ground water composition and the water-solid-microbe interactions close to the sediment-water interface. Thus, this study aims to characterize and evaluate the hydrogeochemical gradients developing in the fresh-salt water mixing zone of the Wismar Bay (WB), southern Baltic Sea, Germany. Sampling campaigns covering the WB, the fresh-salt water mixing zone at the beach of the WB shoreline, terrestrial surface and ground waters near the WB as well sediments pore water were carried out. In these different waters, the distribution of dissolved inorganic carbon, nutrients, major ions, trace elements, stable isotopes (H, O, C, S), and radium isotopes have been investigated. Enhanced concentrations of radium isotopes together with dissolved manganese, barium in the surface waters of the eastern WB indicated benthic-pelagic coupling via the exchange between pore water and the water column. Salinity, stable isotopes, and major ions in sediment pore water profiles identified the presence of fresh ground water below about 40 cmbsf in the central part of the bay. Geophysical acoustic techniques revealed the local impact of anthropogenic sediment excavation, which reduced the thickness of a sediment layer between the coastal aquifer and the bottom water, causing, therefore, a ground water upward flow close to the top sediments. The fresh impacted pore water stable isotope composition (δ18O, δ2H) plot close to the regional meteoric water line indicating a relatively modern ground water source. The calculated organic matter mineralization rates and the dissolved inorganic carbon sediment-water fluxes were much higher at the fresh impacted site when compared to other unimpacted sediments. Therefore, this study reveals that different fresh water sources contribute to the water balance of WB including a SGD source.
Highlights
Coastal regions are great of interest because they receive water and elements from the continents
The multi-tracer approach used in the present study demonstrated the impact of different fresh water sources on the Wismar Bay (WB), southern Baltic Sea, and in particular to prove the occurrence of submarine ground water discharge (SGD) at the center of the bay
The sediment cores retrieved from the central part of the bay together with the acoustic survey discovered fresh water discharge at the sea bottom in the central part of the bay
Summary
Coastal regions are great of interest because they receive water and elements from the continents. A combination of the enhanced material transport and high pelagic productivity in coastal regions lead to an intensive benthic biogeochemical element recycling (Jørgensen and Kasten, 2006) and steep pore water gradients of, e.g., dissolved carbon species, nutrients, and major and trace elements to surface waters. In contrast to rivers, where water discharge and their compositional variations are known in sufficient resolution and precision, the quantification of SGD entering coastal waters remains a challenge (e.g., Stieglitz et al, 2008; Rodellas et al, 2012; Douglas et al, 2020; Moosdorf et al, 2021). More studies are needed combining different techniques to understand the dynamics in SGD and its role for water and element transfer on different scales (Burnett et al, 2006; Taniguchi, et al, 2019; Moosdorf et al, 2021; Rocha et al, 2021)
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