ZONATION IN AQUATIC BIOMES

Zonation in Aquatic Biomes

Many aquatic biomes are physically and chemically stratified (layered), vertically and horizontally, as illustrated for both a lake and a marine environment in Light is absorbed by the water itself and by photosynthetic organisms, so its intensity decreases rapidly with depth. Ecologists distinguish between the upper photic zone, where there is sufficient light for photosynthesis, and the lower aphotic zone, where little light penetrates. The photic and aphotic zones together make up the pelagic zone. Deep in the aphotic zone lies the abyssal zone, the part of the ocean 2,000–6,000m below the surface. At the bottom of all aquatic biomes, deep or shallow, is the benthic zone. Made up of sand and organic and inorganic sediments, the benthic zone is occupied by communities of organisms collectively called the benthos. A major source of food for many benthic species is dead organic matter called detritus, which “rains” down from the productive surface waters of the photic zone. Thermal energy from sunlight warms surface waters to whatever depth the sunlight penetrates, but the deeper waters remain quite cold. In the ocean and in most lakes, a narrow layer of abrupt temperature change called a thermocline separates the more uniformly warm upper layer from more  uniformly cold deeper waters. Lakes tend to be particularly layered with respect to temperature, especially during summer and winter, but many temperate lakes undergo a semiannual mixing of their waters as a result of changing temperature