# Heavy Metals in Post-Exploitation Reservoirs--The Bagry Lake Case Study (Poland)
## 1 Introduction  
The presence of heavy metals in lakes is related to chemical characteristics of anthropogenic substances and minerals found in sediments and the lake surroundings. Nearly every metal, if present at sufficiently high concentrations, may be toxic for plants and organisms living in the lake as well as for humans. Particularly dangerous are heavy metals since they persist in the environment and tend to accumulate in organisms; some of them may be very toxic [1]. If they are present in high concentrations, heavy metals can disturb life functions both in individual organisms and the entire ecosystem. In the aquatic environment, they may affect the number and structure of aquatic invertebratecommunities [2; 3; 4] and cause morphological [5], physiological, neurological and genetic changes [6; 7; 8; 9]. At the same time, some metals such as iron, copper, zinc, calcium and magnesium are a common and necessary component of plants and animals. Some others such as, e.g., arsenic, chromium, cobalt, manganese, molybdenum, nickel, selenium and vanadium may be vital for organisms in the environment, but their levels should be kept very low. On the other hand, metals such as mercury, lead and cadmium are not essential for organisms and are classified as highly toxic [10]. There are many different methods for assessing heavy metal toxicity. These may be indicators of the impact of metal on health, e.g., Average Daily Intake from Soil Ingestion, Inhalation and Dermal Absorption [11]. Toxicity indexes are also used to describe toxicity [12]. In order to analytically determine the metal concentration in the fractions of sediments with rose geochemical properties, metals are extracted using solutions containing various acids and, if it is necessary, other substances such as reducers, hydrogen peroxide and acetic acid salts (Tessier method) [13] or using BCR extraction [14]. None of these methods guarantees identical results of the metal concentration in four factions: F-1--an exchangeable fraction in which metals are adsorbed mainly by clay or exist as carbonate compounds, F-2--related to Fe-Mn oxyhydroxides, F-3--related to metals associated with organic matter and sulfides and F-4--residual part. The work abandoned the analytical determination of metal concentration in individual fractions. A new physical model for determining metal concentration in two fractions, mineral and organic, was proposed. Tessier and BCR methods allow chemical classification of the presence of metals in the settlement. However, they do not explain the relationships between metals and live organic matter. For example, iron or manganese bacteria can oxidize these metals. Oxidized and unoxidized forms of these metals accumulate in a biofilm produced by bacteria. Oxidation products can accumulate in biofilm as well as in intracellular organelles. Substances can penetrate the cell wall, followed by the bacterial cell membrane, and accumulate in cytoplasm. Biofilm [15; 16] is a structure produced by microorganisms consisting of proteins, polysaccharides, nucleic acids, lipid substances and other produced by these microorganisms. Biofilm has adsorption properties in relating to mineral matter, for example, metal [16]. Metals in the form of minerals can be associated with organic matter and occur in a biofilm, in the interior of the cell or on the surface of the cell wall or cell membrane which can bind metals using with specific polysaccharides, proteins and lipids. In the case of plants, metal transport to the interior of the cell can occur through diffusion, endocytosis or reaction with protein conveyors. Interestingly, some metals commonly considered toxic may be necessary in cellular metabolism. For example, in the marine diatoms of Thalassioira weissflogii, cadmium is necessary in the activation of the enzyme carbonate anhydrase participating in photosynthetic processes. Additionally, for other algae it can be an essential element in photosynthetic processes [17].