Salinity levels refer to the concentration of salt in water, typically measured in parts per thousand (ppt). In oceans, salinity averages around 35 ppt, while freshwater sources like rivers and lakes have much lower salinity, often less than 1 ppt. Salinity affects water density, which influences ocean currents and marine life. Changes in salinity can occur due to factors such as evaporation, precipitation, and river inflow. For example, areas with high evaporation rates, like the Dead Sea, have much higher salinity levels. Understanding salinity is crucial for studying ecosystems and managing water resources effectively.