Ecosystem Structure and Types

Ecosystem Structure and Types

An ecosystem is a functional unit made up of a community of organisms (the biotic component) together with the physical environment they live in (the abiotic component), linked by the flow of energy and the cycling of nutrients. Ecosystems are broadly divided into aquatic and terrestrial types.

Aquatic Ecosystems

Aquatic ecosystems are classified primarily by their water chemistry and water movement:

• Marine ecosystems — oceans and seas, characterised by high and fairly constant salinity, covering the largest area of Earth's surface and supporting distinctive marine flora and fauna adapted to a hypertonic, saline environment.
• Freshwater ecosystems — rivers, streams, ponds and lakes, with low dissolved salt content; these are further divided by water movement into:
  • Lentic ecosystems — standing or still water bodies, such as lakes, ponds and swamps, where water does not flow directionally.
  • Lotic ecosystems — flowing water bodies, such as rivers, streams and springs, characterised by continuous unidirectional water movement.
• Estuarine ecosystems — the zones where rivers meet the sea, marked by a fluctuating gradient of salinity as fresh water and salt water mix with the tides; estuaries are highly productive and support organisms specially adapted to tolerate wide and rapid salinity swings.

The Lake Ecosystem: A Model of Zonation

A lake, as a representative lentic ecosystem, is typically organised into three distinct horizontal/vertical zones based on light penetration and depth:

1. Littoral zone — the shallow, near-shore region where sunlight reaches all the way to the bottom, supporting rooted aquatic plants, algae and a rich diversity of bottom-dwelling and shore organisms.
2. Limnetic zone — the open-water surface region away from the shore, extending down to the depth light can still penetrate sufficiently for photosynthesis; it supports floating phytoplankton, zooplankton and free-swimming fish.
3. Profundal zone — the deep-water region below the limit of effective light penetration, where photosynthesis cannot occur; this zone depends on organic matter sinking down from above and is inhabited mainly by decomposers and organisms tolerant of low oxygen and darkness.

Thermal stratification describes how a lake's water forms distinct temperature layers across the seasons:

• In summer, the lake stratifies into a warm, well-mixed, oxygen-rich surface layer (the epilimnion), a middle transition layer with a sharp temperature drop (the thermocline), and a cold, denser bottom layer (the hypolimnion) that can become oxygen-depleted since it is cut off from surface mixing and from photosynthesising plankton.
• In winter, surface water cools and can even freeze, while, because water is densest at about 4°C, the bottom layer remains relatively warmer than the surface — producing an inverse stratification compared to summer. During the spring and autumn transitions, surface and bottom water temperatures converge and wind-driven mixing can turn over the entire water column, redistributing oxygen and nutrients throughout the lake (see figure: lake ecosystem diagram with zonation and thermal stratification).

Terrestrial Ecosystems …