## 1 Introduction  
Heavy metals are naturally present in waterbodies in dissolved, colloidal, and particulate phases. Some of them, for example copper (Cu) and zinc (Zn), are essential for living organisms when occurring at trace levels. Overex to heavy metals, however, negatively affects human health and the environment [1]. These harmful effects include the formation of metal complexes with proteins, inactivating enzymatic systems, which compromises the course of biogeochemical cycles [2; 3; 4]. In aquatic ecosystems, heavy metal pollution may severely affect all organisms along the trophic levels, from microorganisms to higher organisms, including birds [5; 6; 7; 8; 9; 10; 11]. Human activities from ancient times to the present have multiplied environmental concentrations of Cu, lead (Pb), and Zn in all regions of the world [12; 13]. Importantly, biosphere pollution by heavy metals has increased drastically since the beginning of the industrial revolution [14; 15; 16]. In contrast to organic pollutants, for example, heavy metals are nondegradable. Their long persistence, tendency to bioaccumulate, and their toxicity have made them a target of intensive research over the last few decades [17; 18; 19; 20; 21; 22; 23; 24]. Based on these features, heavy metals cause toxic effects at locations distant from the pollution source. In aquatic environments, they are poorly soluble in water and, therefore, are ultimately adsorbed into the sediments. For this reason, heavy metal concentrations in water are sometimes below detection limits and many studies have focused on sediment analyses [22; 23; 25; 26; 27; 28]. Heavy metals in sediments may bioaccumulate to benthic organisms, thus entering the food chain [29; 30]. Recent studies have also highlighted the positive correlation between the concentration of heavy metals and antibiotic resistance genes in aquatic environments [31; 32; 33]. The main anthropogenic sources of heavy metals in waters are urban and industrial effluents, agricultural fertilizers, and mining [34; 35]. Moreover, heavy metals can enter waterbodies after atmospheric transport and subsequent deposition [36]. Fertilizers not only provide macronutrients and micronutrients (e.g., Cu and Zn) but are also often contaminated with other heavy metals, such as cadmium (Cd) and Pb [37; 38]. The European Union has listed Cd and Pb, together with mercury (Hg), as priority substances in the field of water policy in Directive 2008/105/CE [39]. However, there is no unified European legislation regarding sediment quality criteria [40]. A better understanding of heavy metal bioavailability requires considering chemical interactions [41]. Mineral components and organic matter in sediments and soils are the main reservoir of heavy metals in aquatic environments and farming areas [27]. Heavy metals in sediments are most often associated with clays, iron and manganese (Fe-Mn) oxides, and organic matter due to ion exchange, complexation, and chemical adsorption [42]. Many studies have linked pollution sources with total concentrations of heavy metals in sediments and soils around the world [25; 43; 44; 45; 46; 47; 48; 49; 50; 51]. Although nondegradable, heavy metals show different levels of mobility in soils and sediments, mainly depending on their chemical speciation. This, in turn, is highly dependent on several factors, including pH, redox potential, organic matter content, and ion exchange processes [28]. In contaminated anoxic sediments, for instance, the reoxidation of metal sulfide species can lead to significant releases of soluble species into the aquatic phase and therefore increase the bioavailability of the respective metal [52]. Despite the different chemical forms in which heavy metals can occur in sediments, the total concentration is the most commonly used parameter in assessment studies [53]. Such complexity prompted numerous studies to focus on the mobility in different geochemical phases in sediments by applying a sequential extraction before measurement [47; 54; 55; 56; 57].