Freshwater ecosystems are inland water bodies defined by their low concentrations of dissolved salts. These environments represent a relatively small but diverse portion of the planet’s aquatic habitats. Freshwater systems are categorized primarily based on water movement: still water (lentic systems) or continuous flow (lotic systems).
Lentic (Standing Water) Ecosystems
Lentic systems encompass bodies of standing water, including lakes, ponds, and reservoirs. The defining characteristic is the long water residence time without the disruption of a strong current. In deeper lentic bodies, the absence of flow facilitates thermal stratification, where temperature differences create distinct layers based on density.
During warmer months, thermal stratification creates a light, warm surface layer (epilimnion) above a denser, colder bottom layer (hypolimnion), separated by the thermocline. This stratification affects resource distribution; the upper layer is oxygen-rich due to photosynthesis, while the deeper layer can become oxygen-depleted. Lentic systems are divided into three main ecological zones: the littoral zone, the open water limnetic zone, and the deep benthic zone.
Organisms in lentic environments must adapt to these layered conditions, particularly the seasonal changes when the water column may mix completely during overturn events. Rooted aquatic plants thrive in the shallow, light-penetrated littoral zone, providing shelter and a food source for invertebrates. Free-floating organisms, such as plankton, are the primary producers in the limnetic zone, forming the base of the food web.
Lotic (Flowing Water) Ecosystems
Lotic systems are defined by their continuous movement of water, ranging from small creeks to large rivers. The persistent current is the dominant physical force, shaping the channel’s form. This constant turbulence causes a high rate of gas exchange with the atmosphere, resulting in a well-mixed water column with consistently high dissolved oxygen saturation.
The flow prevents permanent thermal stratification, leading to more uniform temperatures from the surface to the bottom. Nutrients and organic materials are rapidly transported downstream, meaning local food webs often rely heavily on organic matter from the surrounding terrestrial environment. Organisms inhabiting lotic systems have developed specific adaptations to anchor themselves against the current.
Many invertebrates, like caddisfly larvae, construct protective cases and cement themselves to rocks, while fish species have streamlined bodies and specialized fins for maintaining position. The physical structure of a lotic channel is characterized by alternating sections of riffles and pools. Riffles are shallow, fast-moving areas over coarse substrate, while pools are deeper, slower-moving sections where fine sediment accumulates.
Subtypes and Ecological Distinctions
The two main categories of freshwater ecosystems further subdivide into distinct subtypes that reflect a gradient of physical conditions. Lentic systems include deep, large, and seasonally stratified lakes, as well as smaller, shallower ponds and productive wetlands or swamps. Lotic systems are similarly varied, ranging from narrow, fast-flowing mountain streams and creeks to broad, meandering lowland rivers.
Ecological differences separate these subtypes beyond the simple presence or absence of flow. For instance, the high flow velocity in lotic systems typically keeps the water clear, resulting in low turbidity, which allows light to penetrate deep into the water column. Conversely, large, slow-moving lakes can experience high turbidity from suspended sediment or dense phytoplankton blooms, limiting light availability to deeper zones.
Temperature variability is also a distinction; fast-flowing, shallow streams are subject to immediate temperature fluctuations dictated by the surrounding air and canopy cover. Deep lakes, because of their large volume and stratification, maintain a more stable temperature in their lower layers throughout the year. These variations in light, temperature, and current create unique biodiversity structures, supporting communities adapted either to the stable, layered conditions of standing water or the dynamic, high-oxygen environment of flowing water.