Marine Biology Terms Starting With E
Marine Biology Glossary: E
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Epipelagic Zone
/ ep-ih-puh-LAJ-ik ZOHN / · Greek epi meaning upon and pelagos meaning sea
Epipelagic Zone is the uppermost layer of the open ocean, extending from the surface to approximately 200 meters depth, where sufficient sunlight penetrates to support photosynthesis and where most of the ocean's primary production originates.
Phytoplankton in this zone fix carbon through photosynthesis, forming the base of marine food webs that extend from microscopic zooplankton up through fish, seabirds, and whales. Light intensity drops to about 1 percent of surface levels at the base of this zone, marking the compensation depth below which photosynthesis can no longer keep pace with respiration. Seasonal mixing in temperate oceans delivers nutrients from deeper water each spring, triggering phytoplankton blooms that can double the standing crop of algae within days.
Yellowfin tuna (Thunnus albacares) and oceanic whitetip sharks (Carcharhinus longimanus) are among the large predators that patrol this zone, tracking concentrations of smaller fish and squid.
Spring phytoplankton blooms in the North Atlantic are large enough to be visible from satellites as swirling green and turquoise patches hundreds of kilometers across. These blooms can fix more than 1 gram of carbon per square meter per day at their peak, briefly making the open ocean as productive as a temperate forest.
The sunlit layer extends all the way to the seafloor. In most open-ocean regions, photosynthetically active radiation fades to levels too low to support net primary production within the upper 100 to 200 meters, leaving the vast majority of the ocean floor in permanent darkness.
Sargassum seaweed mats floating in the epipelagic zone of the Sargasso Sea create a unique open-ocean habitat that shelters juvenile loggerhead sea turtles (Caretta caretta) during their early years at sea. A single square meter of dense Sargassum mat can harbor more than 100 invertebrate species, including shrimp, crabs, and filefish that are found almost nowhere else in the open ocean.
Estuary
/ ES-choo-air-ee / · Latin aestuarium meaning tidal inlet
Estuary is a semi-enclosed coastal body of water where freshwater from rivers mixes with seawater, producing a salinity gradient that shifts with tides, seasons, and river flow.
Salinity in estuaries can change hourly as tides push saltwater inland and river discharge pushes it back out, forcing resident organisms to tolerate sharp salinity swings, rapid temperature shifts, and fluctuating oxygen levels on a daily basis. Cordgrass (Spartina alterniflora) stabilizes mudflats along the Atlantic coast of North America, trapping sediment and building the organic-rich soils that underpin estuarine food webs. Nutrient input from rivers fuels dense phytoplankton and algal growth, which in turn supports shellfish, juvenile fish, and shorebirds.
The Chesapeake Bay, the largest estuary in the United States, produces roughly 500 million pounds of seafood annually, illustrating how productive these transitional systems can be.
The Amazon River discharges so much freshwater into the Atlantic that measurable salinity depression extends more than 200 kilometers offshore, creating a zone oceanographers sometimes call the "river sea" that functions like an open-ocean estuary without enclosing landforms.
Ocean Animal Adaptations →Estuaries are simply polluted river mouths with no ecological value. Healthy estuaries rank among the most biologically productive systems on Earth, supporting more fish species per unit area than most open-ocean habitats.
The Chesapeake Bay supports blue crabs (Callinectes sapidus), eastern oysters, and striped bass across its roughly 11,600 square kilometers of open water. Salinity in the bay ranges from nearly fresh near the Susquehanna River mouth to about 25 parts per thousand near the Atlantic entrance, and each species occupies the salinity band that matches its tolerance.
Euphotic Zone
/ yoo-FOH-tik ZOHN / · Greek eu meaning good and photos meaning light
Euphotic Zone is the uppermost layer of the ocean, extending from the surface to the depth at which light intensity drops to about one percent of its surface value, where photosynthesis can exceed respiration and net primary production occurs.
Depth of the euphotic zone varies from as little as 20 meters in turbid coastal waters to more than 200 meters in the clearest open-ocean regions such as the subtropical gyres. Phytoplankton, including diatoms and cyanobacteria, convert dissolved carbon dioxide and nutrients into organic matter throughout this layer, generating roughly half of Earth’s oxygen. Zooplankton graze on phytoplankton within the euphotic zone during the day and migrate downward at night to avoid visual predators, a behavior called diel vertical migration.
Water clarity, dissolved organic matter, and phytoplankton density all attenuate light and shift the lower boundary of the zone seasonally.
In the ultra-clear waters of the Sargasso Sea, researchers have measured the euphotic zone extending to approximately 175 meters, nearly three times deeper than in productive coastal upwelling zones. That exceptional clarity exists partly because nutrient scarcity limits phytoplankton growth and keeps the water nearly particle-free.
The euphotic zone has a fixed depth of 200 meters throughout the ocean. Its lower boundary shifts constantly with water clarity, season, and the concentration of light-absorbing particles such as phytoplankton and suspended sediment.
During spring in the North Atlantic, diatom blooms can double their cell numbers every 24 hours within the euphotic zone, producing chlorophyll concentrations detectable by satellite sensors. These blooms can cover hundreds of thousands of square kilometers and form the base of food webs that ultimately support commercially important fish like Atlantic cod (Gadus morhua).