A freshwater biome encompasses aquatic environments such as lakes, rivers, ponds, and wetlands, characterized by water with a low salt concentration, typically less than one percent. The “climate” within these environments refers to the unique physical and chemical conditions present in the water itself. These conditions are fundamental to sustaining diverse life and shape the aquatic habitat beyond just air temperature.
Defining Freshwater Climate
Water temperature is a primary factor in freshwater climate, varying with depth and seasons. Warmer water holds less dissolved oxygen, affecting aquatic organism respiration, while cooler water holds more. Temperature also influences the metabolic rates of aquatic life.
Light penetration is another factor, as sunlight is essential for photosynthesis by aquatic plants and algae. Turbidity, or cloudiness from suspended particles, limits light, defining zones like the euphotic zone where photosynthesis occurs. In deeper, aphotic zones, where light is insufficient, life relies on sinking organic matter.
Dissolved oxygen (DO) levels are crucial for the survival of most aquatic life, including fish and invertebrates. Oxygen enters the water from the atmosphere and as a byproduct of photosynthesis by aquatic plants. Higher water temperatures and the decomposition of organic matter can reduce dissolved oxygen.
Water movement, including currents in rivers and waves in lakes, aids mixing and oxygenation. This helps distribute dissolved gases and nutrients throughout the water column. Chemical composition, including pH and nutrient concentrations like nitrogen and phosphorus, also characterizes freshwater environments.
How Freshwater Climate Varies
Internal climate factors vary across freshwater biome types. In lakes and ponds, thermal stratification occurs in warm periods, forming distinct layers based on temperature and density. The warmest, least dense layer is the epilimnion; the middle layer with rapid temperature change is the metalimnion (or thermocline); and the colder, denser bottom layer is the hypolimnion. This layering can reduce oxygen in deeper waters, but seasonal turnover events in spring and fall mix these layers, redistributing oxygen and nutrients.
Rivers and streams are characterized by continuous water flow, which maintains more uniform temperatures throughout the water column. Constant turbulence also contributes to higher dissolved oxygen levels than in stagnant water. Near the source, water is colder and clearer with minimal silt. Downstream, the water becomes wider, slower, and carries more sediment.
Wetlands, such as marshes, swamps, and bogs, have unique conditions due to slow-moving or stagnant water. These environments can develop anoxic (low-oxygen) conditions, especially in soils, due to high organic matter. Water temperature profiles also support specialized life adapted to these conditions.
External Climatic Influences
External atmospheric conditions influence the internal climate of freshwater biomes. Air temperature directly affects water temperature, influencing evaporation and ice cover formation. Shallow waters are sensitive to air temperature increases, which can reduce habitat for cold-water species.
Precipitation, such as rain and snow, impacts water volume and flow rates. Changes in precipitation patterns can lead to increased nutrient runoff from surrounding land, affecting water quality. Excessive rainfall can also cause flooding, altering water levels and disrupting ecosystems.
Wind drives surface currents and mixes water layers in lakes and ponds. This aids gas exchange, such as oxygen, between the water and atmosphere. Wind can also influence evaporation rates from the surface.
Sunlight, or solar radiation, is a primary energy source for freshwater ecosystems. It directly influences water temperature and drives primary production via photosynthesis by aquatic plants and algae. Changes in solar radiation can affect algal growth timing and food web dynamics.