Marine biology is the scientific study of the biology of marine life, organisms that inhabit the sea. Given that in biology many phyla, families and genera have some species that live in the sea and others that live on land, marine biology classifies species based on the environment rather than on taxonomy.
A large proportion of all life on Earth lives in the ocean. The exact size of this "large proportion" is unknown, since many ocean species remain undiscovered. The ocean is a complex three-dimensional world,Oceanographic and Bathymetric Features Marine Conservation Institute. Uploaded 18 September 2013. Covering approximately 71% of the Earth's surface. The habitats studied in marine biology include everything from the tiny layers of surface water in which organisms and abiotic items may be trapped in surface tension between the ocean and atmosphere, to the depths of the oceanic trenches, sometimes 10,000 meters or more beneath the surface of the ocean.
Specific habitats include estuaries, coral reefs, kelp forests, seagrass meadows, the surrounds of seamounts and thermal vents, tidepools, muddy, sandy and rocky bottoms, and the open ocean (pelagic) zone, where solid objects are rare and the surface of the water is the only visible boundary. The organisms studied range from microscopic phytoplankton and zooplankton to huge cetaceans (whales) in length. Marine ecology is the study of how marine organisms interact with each other and the environment.
Marine life supports numerous ecosystem services, providing food, medicine, and raw materials, in addition to helping to support recreation and tourism all over the world. Additionally, marine organisms contribute significantly to the oxygen cycle, and are involved in the regulation of the Earth's climate. Shorelines are in part shaped and protected by marine life, and some marine organisms even help create new land.
Many species are economically important to humans, including both finfish and shellfish. It is also becoming understood that the well-being of marine organisms and other organisms are linked in fundamental ways. The human body of knowledge regarding the relationship between life in the sea and important cycles is rapidly growing, with discoveries being made nearly every day. These cycles include those of matter (such as the carbon cycle) and of air (such as Earth's respiration, and movement of energy through ecosystems including the ocean). Large areas beneath the ocean surface remain effectively unexplored.
Biological oceanography
Marine biology can be contrasted with biological oceanography. Marine life is a field of study both in marine biology and in biological oceanography. Biological oceanography is the study of how organisms affect and are affected by the physics, chemistry, and geology of the oceanographic system. Biological oceanography mostly focuses on the microorganisms within the ocean, looking at how they are affected by their environment and how that affects larger marine creatures and their ecosystem.Lalli, Carol M., and Timothy R. Parsons. "Introduction." Biological Oceanography: An Introduction. First Edition ed. Tarrytown, New York: Pergamon, 1993. 7–21. Print.
Biological oceanography is similar to marine biology, but it studies ocean life from a different perspective. Biological oceanography takes a bottom-up approach in terms of the food web, while marine biology studies the ocean from a top-down perspective. Biological oceanography mainly focuses on the ecosystem of the ocean with an emphasis on plankton: their diversity (morphology, nutritional sources, motility, and metabolism); their productivity and how that plays a role in the global carbon cycle; and their distribution (predation and life cycle). Biological oceanography also investigates the role of microbes in food webs, and how humans impact the ecosystems in the oceans.
Marine habitats
Marine habitats can be divided into coastal and open ocean habitats. Coastal habitats are found in the area that extends from the shoreline 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 habitats. Pelagic habitats are found near the surface or in the open water column, away from the bottom of the ocean and affected by ocean currents, while demersal habitats are near or on the bottom. Marine habitats can be modified by their inhabitants. Some marine organisms, like corals, kelp, and sea grasses, are ecosystem engineers which reshape the marine environment to the point where they create further habitat for other organisms.
Intertidal and near shore
Intertidal zoness, the areas that are close to the shore, are constantly being exposed and covered by the ocean's tides. A huge array of life can be found within this zone. Shore habitats span from the upper intertidal zones to the area where land vegetation takes prominence. It can be underwater anywhere from daily to very infrequently. Many species here are scavengers, living off of sea life that is washed up on the shore. Many land animals also make much use of the shore and intertidal habitats. A subgroup of organisms in this habitat bores and grinds exposed rock through the process of bioerosion.
Estuaries
Estuaries are also near shore and influenced by the tides. An estuary is a partially enclosed coastal body of water with one or more rivers or streams flowing into it and with a free connection to the open sea. Estuaries form a transition zone between freshwater river environments and saltwater maritime environments. They are subject both to marine influences—such as tides, waves, and the influx of saline water—and to riverine influences—such as flows of fresh water and sediment. The shifting flows of both seawater and fresh water provide high levels of nutrients both in the water column and in sediment, making estuaries among the most productive natural habitats in the world.
Reefs
Reefs comprise some of the densest and most diverse habitats in the world. The best-known types of reefs are tropical coral reefs, which exist in most tropical waters; however, reefs can also exist in cold water. Reefs are built up by corals and other calcium-depositing animals, usually on top of a rocky outcrop on the ocean floor. Reefs can also grow on other surfaces, which has made it possible to create artificial reefs. Coral reefs also support a huge community of life, including the corals themselves, their symbiotic zooxanthellae, tropical fish, and many other organisms.
Much attention in marine biology is focused on coral reefs and the El Niño weather phenomenon. In 1998, coral reefs experienced the most severe mass bleaching events on record, when vast expanses of reefs across the world died because sea surface temperaturess rose well above normal.NOAA (1998) Record-breaking coral bleaching occurred in the tropics this year. National Oceanic and Atmospheric Administration, Press release (October 23, 1998).ICRS (1998) Statement on Global Coral Bleaching in 1997-1998. International Coral Reef Society, October 15, 1998. Some reefs are recovering, but scientists say that between 50% and 70% of the world's coral reefs are now endangered and predict that global warming could exacerbate this trend.Bryant, D., Burke, L., McManus, J., et al. (1998) "Reefs at risk: a map-based indicator of threats to the world's coral reefs". World Resources Institute, Washington, D.C.Wilkinson, C. R., and Buddemeier, R. W. (1994) "Global Climate Change and Coral Reefs: Implications for People and Reefs". Report of the UNEP-IOC-ASPEI-IUCN Global Task Team on the Implications of Climate Change on Coral Reefs. IUCN, Gland, Switzerland.
Open ocean
The open ocean is relatively unproductive because of a lack of nutrients, yet because it is so vast, in total it produces the most primary productivity. The open ocean is separated into different zones, and the different zones each have different ecologies. Zones which vary according to their depth include the epipelagic, mesopelagic, bathypelagic, abyssopelagic, and hadopelagic zones. Zones that vary by the amount of light they receive include the photic and aphotic zones. Much of the aphotic zone's energy is supplied by the open ocean in the form of detritus.
Deep sea and trenches
The deepest recorded oceanic trench measured to date is the Mariana Trench, near the Philippines, in the Pacific Ocean at . At such depths, water pressure is extreme and there is no sunlight, but some life still exists. A white flatfish, a shrimp, and a jellyfish were seen by the crew of the bathyscaphe Trieste when it dove to the bottom in 1960, which led to scientific debate surrounding the likelihood of bony fish surviving in such deep waters. General scientific consensus has discredited the possible viewing of a flatfish at such depths.Polmar, Norman; Mathers, Lee J. (January 2020). "The First Deepest Dive". U.S. Naval Institute Proceedings. 146/1/1, 403.Seven Miles Down: The Story of The Bathyscaph Trieste. , Rolex Deep Sea Special, January 2006.
In general, the deep sea is considered to start at the aphotic zone, the point where sunlight loses its power of transference through the water. Many life forms that live at these depths can create their own light known as bio-luminescence. Marine life also flourishes around seamounts that rise from the depths, where fish and other sea life congregate to spawn and feed. Hydrothermal vents along the mid-ocean ridge spreading centers act as oases, as do their opposites, cold seeps. Such places support unique biomes, and many new microbes and other lifeforms have been discovered at these locations. There is still much more to learn about the deeper parts of the ocean.
Marine life
In biology, many phyla, families, and genera have some species that live in the sea and others that live on land. Marine biology classifies species based on their environment rather than their taxonomy. For this reason, marine biology encompasses not only organisms that live only in a marine environment, but also other organisms whose lives revolve around the sea.
Microscopic life
As inhabitants of the largest environment on Earth, microbial marine systems drive changes in every global system. Microbes are responsible for virtually all photosynthesis that occurs in the ocean, as well as the cycling of carbon, nitrogen, phosphorus, and other nutrients and trace elements.
Microscopic life undersea is incredibly diverse and still poorly understood. For example, the role of viruses in marine ecosystems is barely being explored even at the beginning of the 21st century.
The role of phytoplankton is better understood due to their critical position as the most numerous primary producers on Earth. Phytoplankton are categorized into cyanobacteria (also called blue-green algae/bacteria), various types of algae (red, green, brown, and yellow-green), diatoms, dinoflagellates, euglenoids, coccolithophorids, cryptomonads, chrysophytes, chlorophytes, prasinophytes, and silicoflagellates.
Zooplankton tend to be somewhat larger, and not all are microscopic. Many Protozoa are zooplankton, including dinoflagellates, zooflagellates, foraminiferans, and radiolarians. Some of these (such as dinoflagellates) are also phytoplankton; the distinction between plants and animals often breaks down in very small organisms. Other zooplankton include cnidarians, ctenophores, chaetognaths, molluscs, arthropods, urochordates, and annelids such as polychaetes. Many larger animals begin their life as zooplankton before they become large enough to take their familiar forms. Two examples are fish larvaee and sea stars (also called starfish).
Plants and algae