Freshwater ecosystems are a subset of Earth's aquatic ecosystems. They include lakes and ponds, rivers, streams, springs, and wetlands. They can be contrasted with marine ecosystems, which have a larger salt content. Freshwater habitats can be classified by different factors, including temperature, light penetration, and vegetation.
An ecological footprint is a popular metric that shows the amount of natural resources required to provide raw materials and food to sustain an individual or a country. The large number of chemicals released into the environment and their different physical and toxicological properties have stymied previous attempts at developing a chemical footprint.