Horizontal zonation of ocean –
Different zones moving outward from the shore towards the open ocean
- Littoral zone – Also referred to as intertidal or coastal zone. Includes the area between high tide and low tide. During high tide, this zone remains covered with water for several hours. When low tide returns, it is exposed to air for many hours. Thus, organisms living in this region have adapted to both the conditions.
- Pelagic zone – It is the area from the low tide mark into the open ocean and always remains covered with ocean water. Includes the entire volume of water above the ocean floor. Divided into 2 zones – neretic zone above the continental shelf and oceanic zone above the ocean floor. Organisms in this region float and swim. The floaters either float in the water column or at the air-sea interface.
- Neretic zone – Includes the water column from the shore line to the edge of continental shelf and further overlies the continental shelf. Sunlight penetrates the entire water column. Extends upto a depth of only 600 feet. This environment is generally more productive than the open ocean because more nutrients are available. This part of the water column is also susceptible to wave action, turbulence and coastal currents.
- Oceanic zone –Region of the sea extending from the edge of continental shelf, over the continental slope and over ocean floor. Includes water column above the deep parts of the ocean away from land. The upper layer of water is sunlit and warm while the lower layer is characterized by darkness and tremendous pressure.
Ocean stratification
The ocean forms different layers as the water has different density throughout. Water with higher density sinks to the bottom while water with lower density floats on top. Temperature and salinity both affect density of the water. Low density water tends to be warmer and less saline while higher density water is generally cooler and more saline. Pressure also affects seawater density but only in the deeper parts of the ocean. It is important to know about layering since it affects ocean currents. Deep water currents are part of the global circulation of Earth’s oceans.
Water stratification occurs when water masses with different properties – salinity (halocline), oxygenation (chemocline), density (pycnocline) and temperature (thermocline) form layers that act as barriers to water mixing which could lead to anoxia or euxinia. These layers are normally arranged according to density.
The ocean has three primary layers. They are surface layer (or mixed layer), thermocline and the deep ocean. The surface layer is the topmost layer of the ocean. It is well stirred by the winds and other forces. It is the warmest layer due to heating from the sun. Thermocline lies between the warm surface layer and cold deep ocean. Its size varies based on latitude and season, but it rarely occurs deeper than 1000m. In this layer, temperature changes rapidly with depth. Water in the deep ocean is cold and dense. Temperature tend to remain relatively constant.
Pycnocline – This encompasses both halocline (salinity gradients) and thermocline (temperature gradients). It refers to the rapid change in density with depth. Because density is a function of temperature and salinity, the pycnocline is a function of thermocline and halocline. Since temperature tends to influence seawater density, the depth range and base of both pycnocline and thermocline often tend to be similar.
Water stratification also creates barrier to nutrient mixing between layers. This can affect the primary production in an area by limiting photosynthetic processes. When nutrient from the benthos cannot travel up into the photic zone, phytoplankton growth may be limited by nutrient availability. Reduced primary productivity also leads to lowered net productivity in waters.
Benthic Population
- Marine sediments range from the very shallow to the deepest trenches. The temperature of the sediment depends upon the proximity of geothermally active sites. Most microbes survive under high pressure, no light and at temperatures between 1-4°C.
- They not only survive on the ocean bed but also deep down in the sediments uptil a depth of at least 600m. AT these depths, microbes must be capable of tolerating high atmospheric pressures upto 1100 atm. Such microbes are called barophiles.
- There is something called as methane hydrates which are basically pools of trapped methane produced by methanogenic archaea. They accumulate in lattice-like cages of crystalline water 500m or further below the sediments in many regions of the world’s ocean.
- Many algae and protozoans floating in the photic zone possess Ca or Si containing cell walls. When these organisms die and sink to the bottom of the water body, these cell walls form deposits.
- Diatoms, radiolarians and silicoflagellates produce siliceous skeletons whereas foraminifera and coccolithophores generate calcareous skeletons. Such beds occur in various regions of sea forming coral reefs.
- Bacteria, especially cyanobacteria, participate in the precipitation of calcium carbonate and subsequent limestone formation. Microorganisms found in the benthic zone of the sea are also involved in the formation of petroleum deposits from accumulated and buried organic matter