Freshwater makes up a small percentage of Earth's total water supply, but there are a variety of freshwater biomes (defined by climatic, physical, and biotic characteristics) because the speed of water flow and the productivity of the surrounding soil give these biomes different attributes. Freshwater biomes include lakes, rivers, and wetlands. The chemical makeup of a freshwater biome is linked with that of its surrounding soils and plant life. The close proximity of freshwater biomes to human development makes them especially vulnerable to pollution and overuse.Both freshwater and marine biomes are stratified into layers. Photosynthetic organisms need sunlight, so they are restricted to the shallow photic zone, the upper portion of an aquatic environment where light can penetrate. The benthic zone is comprised of the waters from near the surface down to the deepest depths, where little, if any, light reaches. The body of water may be shallow or deep. These zones hold different organisms based primarily on the temperature and sunlight needs of the organisms. Shallow waters tend to be warm, and deep waters tend to be cold. The deep, dark, cold regions of the ocean generally have fewer organisms than the shallow regions. In the spring and fall, lakes often experience a turnover, the seasonal mixing that redistributes oxygen and nutrients in a body of water. Oxygen from the shallow parts of the lake is brought to the deep parts, and nutrients from the deep are brought to the surface.
Freshwater and Saltwater Zones
Lakes, Rivers, and Streams
Process of Eutrophication
Wetlands and Estuaries
An estuary is the location where a river meets the ocean. It is a place where fresh and salt water mix depending on the tides. Salty ocean water moves into and out of estuaries regularly. Many organisms that reside in estuaries must be able to tolerate a wide range of salinities, for example, mangrove trees and blue crabs. Nutrients flowing in from the river contribute to the high productivity of estuaries. Grasses and phytoplankton are common. Many species of invertebrates, fish, and birds reside in this biome. This makes estuaries a popular feeding place for humans and marine predators alike. Similar to wetlands, these highly productive biomes are in danger because of overuse by humans and habitat loss through urban development.