Marine Biology

Introduction

All heterotrophic life in the aquatic systems depend on the production of organic matter by primary producers. In the near-surface waters, the primary producers are green plants which convert carbon dioxide and water into organic matter using sunlight as the energy source (photosynthesis). In simplified form, the photosynthetic reaction is written as follows:

        6CO₂ + 6H₂O + sunlight → C₆H₁₂O₆ (carbohydrate) + 6O₂

In this reaction, carbon dioxide and water in the presence of sunlight  are converted to simple sugars and oxygen. The energy required for metabolic activity is derived by reversing this reaction (respiration), i.e., oxygen and sugar react to release energy, carbon dioxide and water. In the case of the primary producers (also referred to as autotrophs), if photosynthesis exceeds respiration there is a net gain in biomass. While photosynthesis is the primary pathway used to create organic matter, the required energy can also be obtained through chemical reactions (chemosynthesis). In the ocean, this pathway (chemosynthesis) occurs at deep sea hydrothermal vents where primary producers obtain their energy through the oxidation of hydrogen sulfide, released by the hydrothermal solutions, to sulphur and sulphate.

There are several types of productivity . Primary productivity is the conversion of inorganic compounds into organic compounds. Gross primary productivity is the total amount of organic material synthesized during photosynthesis or chemosynthesis. Net primary productivity is the difference between the gross productivity and the amount of organic material used during respiration.

Net productivity = Gross productivity - Respiration

Primary productivity can be determined in a number of ways.

The various methods include light and dark bottle method, ¹⁴C- method, through chlorophyll-a estimation, plankton biomass estimation, etc. There are direct and indirect methods of estimating primary productivity. Among all the methods, light and dark bottles are simplest and commonest one. The light and dark bottle metyhod was propounded by Gaarder and Gran (1929).This is an indirect method of estimating primary production and  in which the amount of liberation of oxygen during the process of photosynthesis is measured and productivity is calculated. ¹⁴C - method is a direct way of estimating primary production, In this method, the amount of carbon  assimilated during the process of photosynthesis  is measured and calculated the productivity. This method was first propunded by Steeman Nielsen (1952).