Freshwater ecosystems are fundamental to life on Earth, encompassing environments with low salt concentrations, typically less than one percent. These diverse systems include lakes, ponds, rivers, streams, and wetlands. Freshwater plays a global role in various ecological processes, from providing drinking water to influencing nutrient cycles.
Standing Water Bodies
Standing water bodies, or lentic ecosystems, feature still or slow-moving water, ranging from small pools to vast lakes. Lakes and ponds are primary sub-types, distinguished by depth and light penetration.
Lakes and ponds often display distinct zones and thermal stratification. The epilimnion, the uppermost layer, is warm, well-mixed, and rich in dissolved oxygen. Below it, the metalimnion (thermocline) is a transition zone with rapidly decreasing temperature. The deepest, coldest layer, the hypolimnion, may have lower oxygen levels.
Light penetration also defines zones: the littoral zone is the shallow area near the shore where light reaches the bottom, supporting rooted plants and diverse invertebrates. The limnetic zone is the open, sunlit water away from the shore, dominated by phytoplankton and zooplankton. In deeper lakes, the profundal zone lies below the limnetic zone where light is too limited for photosynthesis, supporting organisms that feed on decaying matter. Organisms in these systems range from algae and aquatic plants to various fish, amphibians, and invertebrates, adapted to specific light, temperature, and oxygen conditions.
Flowing Water Bodies
Flowing water bodies, or lotic ecosystems, are characterized by unidirectional water movement in rivers and streams. This continuous flow defines their physical, chemical, and biological properties. Lotic systems transport dissolved minerals and sediments, with water speed, temperature, and oxygen levels varying significantly along their course.
Flow rate dictates many aspects of these environments. Fast-moving sections, called riffles, are shallow and turbulent with high dissolved oxygen, often flowing over rocky substrates. Organisms in riffles have adaptations like streamlined bodies or attachment mechanisms to withstand the current. Runs are stretches of moderate current with continuous surfaces, deeper than riffles, while pools are deeper areas with slower currents where sediments tend to accumulate.
Oxygen levels are higher in lotic systems due to water turbulence, particularly in faster-flowing areas. The physical structure of the riverbed, including riffles, pools, and runs, creates diverse microhabitats. This variety supports a wide range of aquatic life, from fish to various invertebrates adapted to specific flow conditions.
Wetland Environments
Wetlands are distinct freshwater ecosystems defined by saturated soils, hydrophytic vegetation, and unique hydrological conditions. These areas serve as transitional zones between aquatic and terrestrial environments, often inundated or saturated. Wetlands are among the most productive ecosystems globally, supporting a high diversity of plant and animal species.
Common types of freshwater wetlands include marshes, swamps, and bogs/fens. Marshes are characterized by mineral soils and herbaceous vegetation like grasses, reeds, and rushes, often continually flooded from various sources and rich in nutrients. Swamps, in contrast, are forested wetlands dominated by woody plants such as trees and shrubs, with waterlogged soils. Bogs are acidic, peat-accumulating wetlands with limited nutrient availability, often covered by sphagnum moss and primarily fed by precipitation. Fens are also peat-forming but receive nutrients from groundwater, making them less acidic and supporting a more diverse plant community than bogs.
Wetlands perform ecological functions, including water purification by trapping sediments and retaining excess nutrients and pollutants. They also control floods by temporarily storing excess water and facilitating groundwater recharge. These environments act as biodiversity hotspots, providing habitats, breeding grounds, and food sources for a multitude of species, from microbes to mammals.