Why are the dominant primary producers in the ocean microscopic while the primary producers on land are usually much larger organisms?

In terrestrial ecosystems, primary producers constitute the great biomass, and they are often large such as trees. However, the primary producers in marine ecosystems is a direct contrast to the primary producers in land. The dominant primary producers in the ocean are the phytoplankton which are microscopic. Land primary producers are bigger because biomass is highly conserve like in woody plants. While the marine primary producers are involved in a lot of process either by consumers or reducers. As a result, they are smaller. 

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