The concentration of lead Pb considered toxic to aquatic organisms in freshwater ecosystems can vary depending on the species and their sensitivity to lead exposure. However, a general guideline provided by the United States Environmental Protection Agency EPA suggests that lead concentrations should not exceed 3.2 g/L micrograms per liter in freshwater to protect aquatic life.Several factors can influence the bioavailability of lead in freshwater ecosystems, including:1. Water chemistry: The presence of other ions, such as calcium, magnesium, and sodium, can affect lead's solubility and bioavailability. High concentrations of these ions can reduce lead toxicity by competing for binding sites on aquatic organisms.2. pH: The acidity or alkalinity of water can impact lead's solubility and speciation. In general, lead is more soluble and bioavailable in acidic conditions low pH and less soluble in alkaline conditions high pH .3. Dissolved organic matter DOM : DOM can bind to lead, forming complexes that reduce its bioavailability to aquatic organisms. High levels of DOM can, therefore, decrease lead toxicity in freshwater ecosystems.4. Temperature: Temperature can affect the rate of chemical reactions and the solubility of lead in water. Higher temperatures can increase lead's solubility and bioavailability, potentially leading to increased toxicity.5. Sediment characteristics: Lead can adsorb onto sediment particles, reducing its bioavailability in the water column. The type and composition of sediment, as well as the presence of other contaminants, can influence lead's adsorption and desorption processes.6. Biological factors: The sensitivity of aquatic organisms to lead exposure can vary depending on their life stage, feeding habits, and other physiological factors. Some species may be more vulnerable to lead toxicity than others, and bioaccumulation can occur in organisms higher up the food chain.In summary, the concentration of lead considered toxic to aquatic organisms in freshwater ecosystems is generally around 3.2 g/L, but this can vary depending on the species and environmental factors. The bioavailability of lead in these systems is influenced by water chemistry, pH, dissolved organic matter, temperature, sediment characteristics, and biological factors.