The study of aquatic ecosystems, environments predominantly defined by water, requires a systematic method for organization due to the immense diversity of habitats globally. Ecologists classify these systems to better understand the distribution of species, manage natural resources, and assess the overall health of the environment. This classification allows scientists to compare different water bodies and accurately predict how they might respond to natural changes or human impact.
Primary Division by Salinity
The fundamental criterion ecologists use to categorize aquatic environments is salinity, the concentration of dissolved salts in the water. This factor determines the physiological adaptations required for organisms to survive, creating three broad categories. Marine ecosystems, the largest group, have a high salt content, typically averaging around 35 parts per thousand (ppt), and include oceans, seas, and coral reefs.
Freshwater ecosystems are in direct contrast, having very low salt concentrations, generally less than 1 ppt. This category encompasses lakes, rivers, ponds, and streams, which are critical for terrestrial life and human consumption. The third, intermediate category is brackish water, found where freshwater and marine systems mix, such as in estuaries and salt marshes. Salinity in these zones is highly variable, fluctuating between 5 and 35 ppt depending on tidal cycles and river flow.
Categorizing Freshwater Ecosystems
Freshwater classification centers on water movement, which dictates the habitat’s physical and chemical conditions. Ecologists divide these systems into two major types: lotic and lentic. Lotic systems are characterized by constantly flowing water, such as rivers, creeks, and streams, where the entire water mass moves in a definite direction.
The flow in lotic systems creates a longitudinal gradation in temperature, oxygen, and nutrient levels from the source to the mouth. Habitats can be subdivided into swift-moving sections called rapids and deeper, slower-moving areas known as pools. The constant mixing of water generally results in high dissolved oxygen content compared to still bodies.
Lentic ecosystems are composed of standing or relatively still water, including lakes, ponds, and reservoirs. The water residence time in a lentic system can be very long, sometimes up to ten years. Deeper lentic water bodies often develop thermal stratification, where density differences cause the water to separate into distinct layers during warmer months.
Thermal stratification results in the warmest, least dense epilimnion floating on the colder hypolimnion, separated by the thermocline. Lakes are also zoned by depth: the shallow, well-lit littoral zone is near the edge, the open water column is the pelagic zone, and the deep bottom is the profundal zone. Wetlands, including marshes and swamps, represent a distinct category where the soil is saturated or inundated for a significant portion of the year.
Categorizing Marine Ecosystems
Marine ecosystem classification is complex, relying on nested systems that account for depth, light penetration, and proximity to the shore. The major division is between the pelagic zone (the water column) and the benthic zone (the ocean floor and substrates). Horizontally, the pelagic zone is divided into the neritic province (above the continental shelf) and the oceanic province (the vast open sea).
Vertical classification of the pelagic zone is based on light penetration. It starts with the photic or euphotic zone, extending to about 200 meters, where sunlight allows photosynthesis. Below this is the aphotic zone, where light is insufficient for plant life. The aphotic zone is subdivided by depth, beginning with the mesopelagic zone, or “twilight zone,” from 200 to 1,000 meters.
Pelagic Depth Zones
The aphotic zone continues deeper with the following classifications:
- Bathypelagic zone (1,000 to 4,000 meters)
- Abyssopelagic zone (4,000 to 6,000 meters)
- Hadalpelagic zone, which encompasses the water in the deepest ocean trenches, extending below 6,000 meters.
Benthic Depth Zones
The benthic zone is similarly classified by depth:
- Sublittoral zone, covering the continental shelf.
- Bathyal zone, along the continental slope.
- Abyssal zone, across the deep plains.
- Hadal zone, in the trenches.
An additional horizontal classification is the intertidal zone, the shoreline area between the high and low tide marks, which is subject to constant exposure and submersion.