The aphotic zone is the portion of a lake or ocean where there is little or no sunlight. It is formally defined as the depths beyond which less than 1% of sunlight penetrates.

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Hydrothermal vents are fissures on the seabed from which geothermally heated water discharges. They are commonly found near volcanically active places, areas where tectonic plates are moving apart at mid-ocean ridges, ocean basins, and hotspots.

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Increased carbon dioxide in the atmosphere can lead to increased salinity in the ocean. As carbon dioxide levels in ocean water increase, this causes the formation of carbonic acid, lowering the pH of the ocean.

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The oceanic zone is the vast open part of the ocean where animals such as whales, sharks, and tuna live.

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An estuary is an area where a freshwater river or stream meets the ocean. When freshwater and seawater combine, the water becomes brackish. Estuaries form a transition zone between river environments and maritime environments.

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