Geography of the Atlantic Ocean
The Atlantic Ocean is the second largest ocean in the world, covering one-fifth of the earths surface. It is made up of a variety of different ecosystems that support an extensive range of marine fauna. The Atlantic Ocean lies between the Arctic and the Antarctic, with the North Atlantic stretching between North America, Europe, and the Arctic Ocean, and the South Atlantic stretching between South America, Africa and the Southern Ocean around Antarctica. The average depth is 14,000 feet, and is 28,231 feet at its deepest point; Milwaukee Deep in the Puerto Rico Trench. The Mid-Atlantic Ridge runs the entire length of the Atlantic from north to south. It is made up of a series of underwater ridges, troughs and valleys, the same as a mountain range on the land.
Marine Food Webs
The fauna of the Atlantic Ocean comprises of a wide variety of marine animal species from microscopic zooplankton, arthropods and crustaceans, to fish, reptiles, seabirds, and marine mammals, including gigantic whales. The diversity and abundance of animal species found in the Atlantic is determined to a large degree by factors such as food availability (productivity), currents and temperature, with certain species being limited to certain zones of the ocean. At the base of all marine food chains are microscopic plant cells, known as phytoplankton, that flourish in nutrient rich waters and form the basis of complex marine food webs. The phytoplankton are fed on by zooplankton, tiny animals at the next level of the food chain, who in turn are fed on by small fish and filter feeding animals such as whales. The smaller fish are fed on by larger fish, which are fed on by marine predators such as sharks, seabirds, and marine mammals.
How Ocean Currents Affect Species Composition & Distribution in the Atlantic
The Atlantic has a wide range of temperatures, depending on the currents in force. Both these factors have a huge influence on the productivity of the ocean, and the species of marine fauna that it is able to support.
In areas that experience strong off-shore winds, upwelling occurs, bringing with it nutrient-rich water. The offshore winds blow the warmer surface waters out to sea, and this is replaced by cold water from the deep, which is high in nutrients. The nutrient-rich water that rises from the ocean depths, supports a high rate of productivity, enabling phytoplankton to bloom, which in turn supports vast numbers of fish, seabirds and marine mammals. Areas that experience upwelling in the Atlantic Ocean include the west coast of Africa, the Grand Banks of Newfoundland and the waters off the Icelandic coast, the Benguela upwelling system off southern Africa, and the Humboldt upwelling system off South America. These areas support fish in abundance and are important breeding grounds for other marine species such as seals and seabirds, and are also important for fisheries. Whales generally are restricted to the cold waters of the South Atlantic and Southern Ocean as this is where krill, their primary food source, is plentiful. However, many whale species, migrate northwards to breeding grounds in warmer waters, where some, such as the southern right whales, calve and suckle their young in the sheltered coves of protected bays.
While the cold waters are highly productive and support large numbers of invertebrates, fish, seabirds and marine mammals, the cool waters may limit biodiversity as many marine species cannot survive in cold temperatures. Warm tropical currents on the other hand, are low in nutrients and therefore not as productive as cold temperate waters. However, the warm waters support a greater variety of species as many organisms that cannot survive in cold waters flourish in tropical seas and associated habitats such as tropical coral reefs. In the Atlantic, coral reefs are found mostly in the tropical Caribbean, but these reefs do not match the diversity of the marine life experienced in the Pacific Ocean. In addition to tropical fish, invertebrates such as sea sponges, sea anemones, crabs and mollusks, together with sea turtles and marine mammals, such as dolphins and manatees, are found in the warmer tropical areas of the Atlantic Ocean, where the Gulf Stream carries warm water from the Carribbean and the Gulf of Mexico in a northerly direction up along the east coast of North America, flowing westwards across to the British Isles.
Like any marine ecosystem, the Atlantic Ocean is divided into the following zones: intertidal, benthic and pelagic, with the pelagic zone being divided further according to depth. The fauna of the Atlantic Ocean is distributed amongst the various zones according to the requirements of the species concerned, including their tolerance of light, temperature and pressure extremes.
Fauna of the Intertidal Zone
The intertidal zone is the area along the coast between the high water mark and the low water mark. It includes rocky shores, sandy beaches, and estuarine mud flats. Marine animals that live in these zones, particularly the sessile organisms, need to be specially adapted to cope with extreme variations in temperature, salinity and moisture as the tide advances and retreats, and they also need to be able to withstand the force of strong currents, large waves and rough seas. Fauna of the rocky intertidal zone, usually have some mechanism to enable them to stay put when conditions get rough. Sea urchins and starfish that inhabit rocky shore biomes are equipped with tube feet that not only allow them to cling onto the rock surface, but also enables them to move freely about, without being washed away. Anemones have a pedal disc that it uses for attachment to rocks or kelp, and mussels have a ventral muscular foot and fibrous byssal threads (or byssus) that attach the mussel to the rock surface. To prevent desiccation at low tide, sessile organisms either have a tough exoskeleton (starfish), spines (sea urchin), shell (mussels), or have some other mechanism to prevent dehydration. The sea anemone has a soft body, but sticky secretions allow sand particles to adhere to it to prevent desiccation.
Fauna of the Benthic Zone
The benthic zone is the area of the ocean floor extending from the shallows of the intertidal zone to the deep ocean trenches of the abyssal zone. The benthic zone is home to bottom living animals, known as benthos, that dwell on the seabed. The animals that live in this zone are often scavengers that feed on detritus that falls from above, or are well camouflaged predators that lie and wait for their unsuspecting prey. Fish such as skates, rays, sole and plaice are adapted to living on the ocean floor by having a flat body form with both eyes situated on the dorsal surface, and they are well camouflaged allowing them to blend in well with the sandy sea floor habitat.
Fauna of the Pelagic Zone
The pelagic zone is the area of the open ocean, which is home to the majority of marine life including phytoplankton, zooplankton and nekton (free swimming marine organisms). Most of these animals live in the surface waters, where food is abundant, visibility is good, and the temperature is not too cold.
The epipelagic zone extends from the surface to a depth of 660 feet (200 meters). The epipelagic zone receives sunlight, allowing phytoplankton to photosynthesize and flourish forming the basis of an extensive marine food web supporting many higher species, including large numbers of schooling prey fish such as sardines and anchovy. These in turn, are important for the survival of other marine species including predatory fish like the Atlantic salmon of the north, Atlantic bluefin tuna, and small sharks. Seabirds such as gannets, cormorants, and the Humboldt and African penguins are found in the Southern Atlantic Ocean; together with seals, and the largest predator in the ocean, the great white shark, a top predator that will feed opportunistically on whatever whets its appetite.
The mesopelagic zone extends from the epipelagic zone to a depth of 3280 feet (1000 meters) where sunlight cannot penetrate. This zone is dark and cold, and is home to predatory fish such as sharks, which generally have poor vision, but are equipped with finely tuned sensory organs to compensate. These include a keen sense of smell and hearing, combined with extra sensory devices such as the lateral line, and ampullae of Lorenzini, a unique electroreceptive device that enables them to detect prey by picking up the electric field emitted by all living organisms. Sharks are also able to reduce their metabolism to conserve energy in cold waters. The mesopelagic zone is also home to filter feeding fish that migrate upwards to feed on plankton in the upper layer at night, when they are less conspicuous to predators, and migrate back down to the safety of the dark depths by day.
The bathypelagic zone extends from the mesopelagic zone down to a depth of 13,000 feet (4000 meters), it is extremely dark and cold, with low oxygen levels, making it very difficult for animals to survive here unless they are specially adapted to survive these inhospitable conditions. Lantern fish are specially adapted to life in these dark depths by having large eyes that enable them to gather light in order to see in the dimly lit conditions, and photophores distributed over the surface of their body, which enable them to produce light through the chemical process of bioluminescence. This light is thought to aid in attracting prey on which it feeds.
The abyssopelagic or abyssal zone extends from the bathypelagic zone to the depths of the deep oceanic trenches reaching the ocean floor. The conditions are similar to the bathypelagic zone, but the pressure is extreme, and temperatures can reach near freezing, with the exception of thermal vents, where sea water is forced down fissures in the sea floor, where it is heated by molten magma in the Earth’s mantle, causing extremely hot water, rich in minerals, to escape, heating up the surrounding water considerably. Thermal vents in the deep troughs of the Mid-Atlantic Ridge provide warm water, rich in minerals that allows animals, such as mussels, tube-worms, crabs and eelpout fish to survive. These animals have adapted to cope with the extremely high water pressure experienced at these depths. In the absence of light, bacteria are able to utilize the minerals in the water as an energy source to produce organic matter through a process of chemosynthesis, similar to the process of photosynthesis. The bacteria not only provide a direct food source to filter feeders, but also form symbiotic relationships with tube worms, mussels and clams. Residing in the body of their host they receive processed minerals from the host, and in return, they provide the host with organic material produced through chemosynthesis.
There are many different living organisms that make up the biota of the Atlantic Ocean. The distribution and abundance of fauna is affected primarily by abiotic factors, such as light, dissolved oxygen content, pressure and temperature, which ultimately influence the biotic factors that affect their distribution, food availabiltiy being the most important factor.
Marine Biology: Mytilus edulis (the common or edible mussel), https://www.lifesciences.napier.ac.uk/teaching/mb/mbmussel.html
Harris, Tom, “How Sharks Work: Shark Senses”, https://animals.howstuffworks.com/fish/shark3.htm
Sea and Sky: Creatures of the Deep Sea - Lantern Fish, https://www.seasky.org/deep-sea/lanternfish.html
Marine Biology: Deep-sea Benthic Systems - Deep sea Vent Communities, https://www.lifesciences.napier.ac.uk/teaching/mb/MBdeep02.html#TOP
Encyclopedia Britannica: https://www.britannica.com/EBchecked/topic/41191/Atlantic-Ocean/33295/Temperature
National Geographic: https://environment.nationalgeographic.com/environment/habitats/deep-sea-vents/
World Info Zone: https://worldinfozone.com/oceans.php?country=Atlantic
Marine Food Web: © Hans Hillewaert [CC-BY-SA-3.0], via Wikimedia Commons
Upwelling: Modified by D. Reed from image by J. Wallace and S. Vogel, El Niño and Climate Prediction. Image courtesy of Sanctuary Quest 2002, NOAA/OER [Public domain], via Wikimedia Commons
Rocky Shore Intertidal Zone: NOAA [Public Domain], via Wikimedia Commons
Pelagic Zone: Finlay McWalter [Public Domain], via Wikimedia Commons