Aquatic ecology and Biodiversity

Unit 5- Productivity

5.2.4. Relationship between composition light depth, critical depth and the depth mixing

Plant growth or net predation occurs when photosynthesis exceeds respiration the depth at which photosynthesis and respiration equal is called compensation depth (CD). It is the production of organic material by photosynthesis which exactly balances the breakdown of organic materials by respiration. Below the CD there is no net production even though photosynthesis still occurs; the CD varies with incident adiation and transparency. The area above the CD is aften referred to as euphotic zone with an average depth of 80 m (varied between-0-100m. depending upon the suspended particles matter load. From the Figure an area of ABCD indicates the amount of plant material used up in respiration is matched against the amount gained by photosynthesis (Area- ACE)
Note: If phytoplankton cells are mixed down below Dcritical, there can be no net photosynthesis. However as long as the depth of mixing is above the critical depth there can be net photosynthesis.
E.g., Euphotic depth –Gulf manner and Palk bay- 6m; West coast of India varied to be 50-60m and 90m. in the clear waters of Laccadive sea
The rate of photosynthesis varies in proportion to light intensity and there is a limit a light saturation by plant