This term is used to express the capacity of oceans for the production of organic matter, also called ocean’s productivity
Marine planktons are referred to as the pasture of the sea because larger organisms either feed directly on them or on smaller aquatic animals that feed on the planktons.
The Antarctic Ocean is the most nutritious ocean among all the other oceans because of the mixing of waters of Atlantic, Pacific and Indian ocean. This mixing is brought about by the drifting currents at the bottom of the cold water away from the continental shift of Antarctica
The abundance of nutrients in surface water results in phytoplankton growth. A shrimp like crustacean called krill, feeds on phytoplanktons and in turn serves as food for other organisms. Thus, it serves as a link in food chain of this ecosystem between the phytoplanktonic life and higher forms of life. The ocean being much larger than the total productive land, its gross production largely exceeds that of land
Although, estuarine and coastal environments provide a less stable physical environment than oceans, its fertility is much higher due to the availability nutrients from its surrounding land
Microbial population of oceans are also affected by the phenomenon of nutrient upwelling wherein convectional currents bring nutrients up from the sediments to the surface resulting in a high productivity
Eutrophication
Lakes and ponds in temperate climates develop a thermal stratification due to lack of mixing of water as they are stagnant water bodies. The upper layer containing the warm water is called the epilimnion while the lower half containing colder water is known as hypolimnion. The two are separated by a buffer zone called thermocline.
Such a stratification acts as a barrier to nutrient and oxygen exchange in such water bodies. However, twice a year during the warming cycle of spring and cooling cycle of fall, this stratification is disrupted, bringing the nutrients up from the sediment, thereby creating currents. This phenomenon is known as nutrient upwelling.
Because of this phenomenon, the H2S gas and other anaerobic by-products that are brought upwards, kill fish and other aquatic life. This is accomplished by triggering profuse algal and cyanobacterial blooms called eutrophication.
These microorganisms produce a toxic neurotoxin called brevitoxin that gets accumulated within the digestive tracts of molluscs and causes paralytic shellfish poisoning in humans.
Moreover, this thick mat of biomass effectively shuts off O2 supply to the lakes and ponds. The O2 that is pre-mixed in the water gets depleted soon by the aerobic microbes that are capable of degrading organic matter in the water body. Consequently, many living organisms die including aquatic animals and plants.
The situation is exacerbated by the addition of effluents from industries, agriculture and domestic sewage that contain heavy loads of organic debris, heavy metals and nitrogen and phosphorus rich fertilizers, which help to enrich algae and cyanobacteria.
Biological Oxygen Demand (BOD)
It is defined as the amount of oxygen consumed by living microorganisms to degrade the organic matter present in a water body
It can be determined by taking a sample of water, aerating it thoroughly, placing it in a sealed bottle, incubating it for days and then determining the residual oxygen content of the water sample followed by comparing this with the oxygen content at the start of the experiment
It gives us a measure of the organic matter present in the water sample that can be consumed by microorganisms in it
