Aquatic ecology and Biodiversity

Unit 6- Biogeochemical cycles

6.1. Biogeochemical cycles
All organisms require a variety of organic and inorganic nutrients for their survival. These chemicals enter into the biotic components from the environment. Hence, these chemicals tend to circulate in a definite path from the organisms to the environment and vice versa. This cycling of chemical elements from the biotic to environment and back to the living organisms is called biogeochemicals. Thus the biogeochemical cycles are the pathways in which chemicals circulate through the biotic and abiotic components of an ecosystem. As these chemicals are essential to the living organisms as food for their survival and for metabolic life processes and these cycles are also known as nutrient cycles.

Fig. In a typical biogeochemical cycle, the minerals or inorganic elements required for the growth and development of living organisms circulate from the non-living to the living and back to the non-living components of the ecosystem (Source Flint, M.L and Gouveia, P. 2001). The biogeochemical cycles superimposed on the various components of the ecosystem; movement of materials in cyclic manner, and that of energy in unidirectional i.e. non-cyclic manner.
Biogeochemical cycles have two phases, namely the biotic phase and the abiotic phase. In biotic phase, the chemicals flow through the living components, whereas in the abiotic phase, the chemicals flow through the non-living components of the ecosystem. In the biotic phase of the nutrient cycle, the chemicals flow through a path or links of various levels of the food chains. The biogeochemicals are of two types viz., gaseous cycles and sedimentary cycles or mineral cycles. In a gaseous cycle, elements move through the atmosphere. For gases, main reservoirs are the atmosphere and the ocean. In a sedimentary cycle, elements move from land to water to sediment. For the mineral elements, main reservoirs are the soil and sedimentary rocks i.e. Earth’s crust.
• Gaseous cycles: These include the oxygen cycle, carbon cycle and nitrogen cycle.
• Sedimentary cycles: These include phosphorous and sulphur cycles.