The ocean is the body of salt water that covers ~70.8% of the Earth.[8]
In English, the term ocean also refers to any of the large bodies of
water into which the world ocean is conventionally divided.[9] Distinct
names are used to identify five different areas of the ocean: Pacific,
Atlantic, Indian, Antarctic/Southern, and Arctic.[10][11] The ocean
contains 97% of Earth's water[8] and is the primary component of the
Earth's hydrosphere, thus the ocean is essential to life on Earth. The
ocean influences climate and weather patterns, the carbon cycle, and the
water cycle by acting as a huge heat reservoir.
Oceanographers split the ocean into vertical and horizontal zones based
on physical and biological conditions. The pelagic zone is the open
ocean's water column from the surface to the ocean floor. The water
column is further divided into zones based on depth and the amount of
light present. The photic zone starts at the surface and is defined to be
"the depth at which light intensity is only 1% of the surface
value"[12]: 36 (approximately 200 m in the open ocean). This is the zone
where photosynthesis can occur. In this process plants and microscopic
algae (free floating phytoplankton) use light, water, carbon dioxide, and
nutrients to produce organic matter. As a result, the photic zone is the
most biodiverse and the source of the food supply which sustains most of
the ocean ecosystem. Ocean photosynthesis also produces half of the
oxygen in the Earth's atmosphere.[13] Light can only penetrate a few
hundred more meters; the rest of the deeper ocean is cold and dark (these
zones are called mesopelagic and aphotic zones). The continental shelf is
where the ocean meets dry land. It is more shallow, with a depth of a few
hundred meters or less. Human activity often has negative impacts on the
ecosystems within the continental shelf.