Query: But as that upper layer warms up, the oxygen-rich waters are less likely to mix down into cooler layers of the ocean because the warm waters are less dense and do not sink as readily.

Relevant passages:
Sea <=SEP=> A sea is a large body of salt water that is surrounded in whole or in part by land . More broadly , `` the sea '' is the interconnected system of Earth 's salty , oceanic waters -- considered as one global ocean or as several principal oceanic divisions . The sea moderates Earth 's climate and has important roles in the water cycle , carbon cycle , and nitrogen cycle . Although the sea has been traveled and explored since prehistory , the modern scientific study of the sea -- oceanography -- dates broadly to the British Challenger expedition of the 1870s . The sea is conventionally divided into up to five large oceanic sections -- including the International Hydrographic Organization 's four named oceans ( the Atlantic , Pacific , Indian , and Arctic ) and the Southern Ocean ; smaller , second-order sections , such as the Mediterranean , are known as seas .   Owing to the present state of continental drift , the Northern Hemisphere is now fairly equally divided between land and sea ( a ratio of about 2:3 ) but the South is overwhelmingly oceanic ( 1:4.7 ) . Salinity in the open ocean is generally in a narrow band around 3.5 % by mass , although this can vary in more landlocked waters , near the mouths of large rivers , or at great depths . About 85 % of the solids in the open sea are sodium chloride . Deep-sea currents are produced by differences in salinity and temperature . Surface currents are formed by the friction of waves produced by the wind and by tides , the changes in local sea level produced by the gravity of the Moon and Sun . The direction of all of these is governed by surface and submarine land masses and by the rotation of the Earth ( the Coriolis effect ) .   Former changes in sea levels have left continental shelves , shallow areas in the sea close to land . These nutrient-rich waters teem with life , which provide humans with substantial supplies of food -- mainly fish , but also shellfish , mammals , and seaweed -- which are both harvested in the wild and farmed . The most diverse areas surround great tropical coral reefs . Whaling in the deep sea was once common but whales ' dwindling numbers prompted international conservation efforts and finally a moratorium on most commercial hunting . Oceanography has established that not all life is restricted to the sunlit surface waters : even under enormous depths and pressures , nutrients streaming from hydrothermal vents support their own unique ecosystem . Life may have started there and aquatic microbial mats are generally credited with the oxygenation of Earth 's atmosphere ; both plants and animals first evolved in the sea .   The sea is an essential aspect of human trade , travel , mineral extraction , and power generation . This has also made it essential to warfare and left major cities exposed to earthquakes and volcanoes from nearby faults ; powerful tsunami waves ; and hurricanes , typhoons , and cyclones produced in the tropics . This importance and duality has affected human culture , from early sea gods to the epic poetry of Homer to the changes induced by the Columbian Exchange , from burial at sea to Basho 's haikus to hyperrealist marine art , and inspiring music ranging from the shanties in The Complaynt of Scotland to Rimsky-Korsakov 's `` The Sea and Sinbad 's Ship '' to A-mei 's `` Listen to the Sea '' . It is the scene of leisure activities including swimming , diving , surfing , and sailing . However , population growth , industrialization , and intensive farming have all contributed to present-day marine pollution . Atmospheric carbon dioxide is being absorbed in increasing amounts , lowering its pH in a process known as ocean acidification . The shared nature of the sea has made overfishing an increasing problem .

Marine habitats <=SEP=> The marine environment supplies many kinds of habitats that support marine life . Marine life depends in some way on the saltwater that is in the sea ( the term marine comes from the Latin mare , meaning sea or ocean ) . A habitat is an ecological or environmental area inhabited by one or more living species .   Marine habitats can be divided into coastal and open ocean habitats . Coastal habitats are found in the area that extends from as far as the tide comes in on the shoreline out to the edge of the continental shelf . Most marine life is found in coastal habitats , even though the shelf area occupies only seven percent of the total ocean area . Open ocean habitats are found in the deep ocean beyond the edge of the continental shelf .   Alternatively , marine habitats can be divided into pelagic and demersal zones . Pelagic habitats are found near the surface or in the open water column , away from the bottom of the ocean . Demersal habitats are near or on the bottom of the ocean . An organism living in a pelagic habitat is said to be a pelagic organism , as in pelagic fish . Similarly , an organism living in a demersal habitat is said to be a demersal organism , as in demersal fish . Pelagic habitats are intrinsically shifting and ephemeral , depending on what ocean currents are doing .   Marine habitats can be modified by their inhabitants . Some marine organisms , like corals , kelp , mangroves and seagrasses , are ecosystem engineers which reshape the marine environment to the point where they create further habitat for other organisms .  __ TOC __

Anoxic event <=SEP=> Oceanic anoxic events or anoxic events ( anoxia conditions ) refer to intervals in the Earth 's past where portions of oceans become depleted in oxygen ( O2 ) at depths over a large geographic area . During some of these events , euxinia , waters that contained hydrogen sulfide , developed . Although anoxic events have not happened for millions of years , the geological record shows that they happened many times in the past . Anoxic events coincided with several mass extinctions and may have contributed to them . These mass extinctions include some that geobiologists use as time markers in biostratigraphic dating . Many geologists believe oceanic anoxic events are strongly linked to slowing of ocean circulation , climatic warming , and elevated levels of greenhouse gases . Researchers have proposed enhanced volcanism ( the release of CO2 ) as the `` central external trigger for euxinia '' .

Lake ecosystem <=SEP=> A lake ecosystem includes biotic ( living ) plants , animals and micro-organisms , as well as abiotic ( nonliving ) physical and chemical interactions .   Lake ecosystems are a prime example of lentic ecosystems . Lentic refers to stationary or relatively still water , from the Latin lentus , which means sluggish . Lentic waters range from ponds to lakes to wetlands , and much of this article applies to lentic ecosystems in general . Lentic ecosystems can be compared with lotic ecosystems , which involve flowing terrestrial waters such as rivers and streams . Together , these two fields form the more general study area of freshwater or aquatic ecology .   Lentic systems are diverse , ranging from a small , temporary rainwater pool a few inches deep to Lake Baikal , which has a maximum depth of 1740 m . The general distinction between pools/ponds and lakes is vague , but Brown states that ponds and pools have their entire bottom surfaces exposed to light , while lakes do not . In addition , some lakes become seasonally stratified ( discussed in more detail below . ) Ponds and pools have two regions : the pelagic open water zone , and the benthic zone , which comprises the bottom and shore regions . Since lakes have deep bottom regions not exposed to light , these systems have an additional zone , the profundal . These three areas can have very different abiotic conditions and , hence , host species that are specifically adapted to live there .