Despite of the huge surface area of oceans, the primary productivity of oceans is low. The reason could be

Factors which affect the productivity are:

• Photosynthetic capacity producers
• Solar energy (Light intensity and duration)
• Availability of nutrients (eg. O₂, CO₂, minerals)
• Water availability
• Topography

The limiting factors of the productivity in the oceans are:

• The amount of light decreases with the increasing depth of the ocean
• Deficiency of nitrogen (an important mineral in the growth of plants) in oceans 
• High salinity in oceans
• No substratum to support the plants

The productivity of oceans is less than 2 tonnes/hectare/year. Although oceans make up 70% of the earth’s surface, it contributes to only 55 billion tonnes out of a total of 170 billion tonnes by the biosphere.