All organisms that live in the ocean are subject to the physical factors of the underwater environment. Some of the more important factors that affect marine (ocean) organisms are light levels, nutrients (chemicals required for growth), temperature, salinity (concentration of salt in the water), and pressure. In general, conditions in the ocean are more stable than those on land.

Light

The amount of light in a certain location controls the growth of the single-celled marine algae called phytoplankton. Phytoplankton are the base of the marine food chain, meaning they are the food for other organisms, who then are the food for higher organisms and so forth. These plants convert sunlight and water into the carbohydrates (sugars) they feed on in a process called photosynthesis. Unlike land, where plants generally live on surfaces, in the ocean, light travels through the water allowing phytoplankton...