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
