## 1 Introduction
### \(\delta^{34}\)S in Terrestrial and Aquatic Ecosystems  
The main factors that influence the \(\delta^{34}\)S of plants and the food chain, in general, are [7; 8]:  
* The lithology;
* The chemical form of sulfur in organic and synthetic fertilizers and S application over time;
* The soil conditions (mineralization, adsorption/desorption), the soil type (i.e., salinity), and the distance from the coast region (sea spray effect);
* The dissolved sulfate contents of water;
* The atmospheric deposition of sulfuric gases that originate from pollution;
* The cultivation type and the cultivation history of the soil. Plants absorb sulfur in the form of sulfate from the atmosphere, including marine aerosols and the soil. The soil \(\delta^{34}\)S, originating from sedimentary rock, varies greatly with the source, ranging between \(-50\) and \(40\%\)[9]. Sulfate evaporites present \(\delta^{34}\)S values of \(+10\%\) and more [9; 10], while marine-derived sulfate ranges between \(+20\%\) and \(+21\%\). Sulfate from pyrite oxidation ranges between \(-30\%\) and \(+10\%\) (depending on the isotopic signature of the pyrite) [2]. Sulfate evaporites present \(\delta^{34}\)S values of \(+10\%\) and more [9; 10], and marine-derived sulfate ranges between \(+20\%\) and \(+21\%\). Sulfate from pyrite oxidation ranges between \(-30\%\) and \(+10\%\) (depending on the isotopic signature of the pyrite) [2]. Reduced sulfides (Fe\({}^{2+}\)S\({}_{2}\), S\({}^{2-}\)) are most depleted in \(\delta^{34}\)S and present values below \(+10\%\)[11]. Granites from Europe present \(\delta^{34}\)S values between \(-4\) and \(+9\%\)[12]. Various energy mineral resources (i.e., coal, oil, and gas) and non-energy sulfide ones have been measured to range from \(-40\) to \(+30\%\)[12; 13]. The \(\delta^{34}\)S value of the most common fertilizer is between \(-10\) and \(10\%\) (depending on each product) [14]. Sulfur isotope analysis of aquatic ecosystems, both marine and freshwater, depends on the quantity of sulfate present in water. This quantity is related to the \(\delta^{34}\)S value, which serves as the primary source for aquatic plants. In modern oceans, the sulfur isotope ratio is very constant at +20.3%. [15; 16; 17; 18], while \(\delta^{34}\)S values of freshwater environments are highly variable. The \(\delta^{34}\)S values measured in freshwater sources (rain, snow) range between 0 and +10% [19]. The \(\delta^{34}\)S values of sulfates from different riverine ecosystems fall mostly between \(-\)5 and +15% [12; 13; 14; 15; 16; 17; 18; 19; 20]. The \(\delta^{34}\)S values of organisms living within freshwater environments have been shown to vary between 4.6% and +13.0% [21]. This variation is due to differing geological sources of sulfur, as well as anaerobic bacteria residing within lakes and rivers, which can reduce sulfate ions to H\({}_{2}\)S. The \(\delta^{34}\)S values of fish living in estuarine environments vary significantly due to the highly variable \(\delta^{34}\)S values of freshwater aquatic plants. The \(\delta^{34}\)S values of estuarine fishes reflect the areas of the estuary where their food originated [22; 23]. Therefore, one particular advantage of sulfur isotope analysis is the possibility of differentiating freshwater from marine input. In addition, sulfur isotope analysis of terrestrial ecosystems depends on sea salt, as plants absorb sulfur in the form of sulfate from marine sulfur, which is transferred to terrestrial environments via sea spray and aerosols [2; 24; 25]. These marine aerosols are the main source of plant sulfur close to the coast and in soils whose parent rocks contain sulfur [26]. Most plants present a \(\delta^{34}\)S value close to that of soil (or fertilizer), because no significant isotopic fractionation occurs (1-2%) [2; 24; 25; 26] concerning their primary sulfate sources during plant uptake of sulfur and metabolism [21]. Considering the wide range of sulfur isotope values in terrestrial ecosystems (\(-\)50 to 40%) [9], the aforementioned \"minor\" fractionation is not significant.