A freshwater biome is an aquatic ecosystem defined by its minimal salt concentration (typically less than one part per thousand), encompassing lakes, ponds, rivers, streams, and wetlands. While geography determines atmospheric weather, the biome’s internal “climate” is governed by the water’s physical characteristics. This internal climate is defined by three abiotic factors: temperature, water flow speed, and light availability. These factors dictate the distribution of nutrients, dissolved gases like oxygen, and the types of organisms present.
Defining Characteristics of Still Water Biomes
In still water biomes, known as lentic systems, the primary climatic driver is temperature, which creates stable thermal stratification. Freshwater reaches its maximum density at approximately 4 degrees Celsius, causing warmer, less dense water to float atop colder, denser water during summer. Deep, temperate lakes develop three distinct thermal zones when stratified.
The uppermost layer, the epilimnion, is warm and constantly mixed by wind, keeping its temperature uniform and dissolved oxygen levels high. Below this is the metalimnion (or thermocline), a transitional middle layer where temperature rapidly decreases with depth. This steep temperature gradient acts as a physical barrier, preventing the upper and lower layers from mixing throughout the summer.
The deepest and coldest layer is the hypolimnion, which remains near 4 degrees Celsius year-round in deep lakes. This bottom layer receives no oxygen from the surface, meaning the oxygen supply here can become depleted by the decomposition of organic matter.
This strong thermal structure breaks down twice a year during periods of seasonal turnover, when the surface water temperature matches the bottom water, allowing wind to mix the entire water column. During the autumn overturn, the cooling surface water sinks, redistributing oxygen to the deep hypolimnion and bringing trapped nutrients from the bottom sediments up to the surface.
A similar process occurs during the spring overturn after the ice melts, ensuring the periodic renewal of oxygen and nutrients throughout the lake. The size and depth of a lake directly influence layering stability; shallower ponds may not stratify, leading to uniform temperature and oxygen conditions.
Defining Characteristics of Moving Water Biomes
The internal climate of moving water biomes (lotic systems) is dominated by the unidirectional flow of the current. This continuous movement from a source downstream to a mouth shapes the physical environment and prevents the stable thermal stratification seen in lakes, resulting in a uniform temperature profile.
The constant turbulence and mixing of the water with the atmosphere ensures that lotic systems maintain a high concentration of dissolved oxygen, often near saturation levels. This high oxygen content supports a diverse community of aerobic organisms, such as trout, which require well-oxygenated water.
However, the speed of the current dictates the composition of the stream bed, or substrate, ranging from large, stable rocks in fast-moving upstream areas to fine, shifting silt and sand in slower downstream sections.
Conditions in a river change predictably along its length, a pattern described by the river continuum concept. Headwaters are often cold, shaded, and high in oxygen, with energy input primarily coming from surrounding terrestrial plant debris. As the river widens and slows down toward the mouth, it becomes warmer and more turbid with suspended sediment.
This downstream transition results in lower overall dissolved oxygen and a greater reliance on internal primary production from algae and phytoplankton, especially in slower-moving sections.
The Influence of Light and Depth
Light penetration and water depth define distinct zones integral to the freshwater biome’s internal climate. Sunlight availability fundamentally controls primary production, determining where photosynthesis occurs and influencing the distribution of life.
The photic (or euphotic) zone is the surface layer where enough light penetrates to support net photosynthesis. This zone is defined as the depth where light intensity is at least one percent of the surface level.
Below this sunlit layer is the aphotic zone, where light levels are too low for photosynthesis. These deep waters must rely on organic matter sinking from the photic zone for their energy.
The depth of the photic zone is highly variable and depends on the water’s clarity. High turbidity from sediment or algae significantly reduces light penetration. In smaller, shallow ponds, the photic zone may extend all the way to the bottom, but in deep lakes, the aphotic zone can be expansive.
Within a lake, the photic zone is divided into two areas: the littoral and limnetic zones.
The littoral zone is the shallow area near the shore where light reaches the bottom, allowing rooted aquatic plants to grow. This proximity to the shore and the presence of macrophytes create a highly productive and structurally complex habitat.
Beyond the littoral zone lies the limnetic zone, which is the open-water surface area of the lake. The limnetic zone’s biological community is dominated by floating organisms, such as phytoplankton and zooplankton, which thrive in the well-lit, open water.