Marine Biology
Unit 10
EPIPELAGIC ZONE
Unit summary & Lecture notes
Summary
This unit deals with the surface layer, or epipelagic zone, of the marine
environment. A discussion of the most important organisms found in the epipelagic
includes phytoplankton, zooplankton, and nekton. Important adaptations of epipelagic
organisms such as buoyancy mechanisms, adaptations for finding food or evading
predators, and mechanisms for fast swimming are addressed. We will then explore
food webs and the basic trophic structure of the epipelagic zone. Finally, we will
examine the physical forces that cause the global disturbance known as El Niňo.
Introduction
The epipelagic zone is a layer of the ocean from the surface to a depth of about
200 m. It is divided into the neritic province, which lies over the continental shelf, and
the oceanic province, which lies beyond the shelf. The epipelagic is similar to the photic
zone, the layer from the surface to the depth where light limits photosynthesis.
I.
Epipelagic Organisms
The epipelagic zone contains organisms that range from primary producers such
as phytoplankton to large predatory fish and marine mammals.
a. Phytoplankton are the most abundant marine organisms in the epipelagic.
They are usually single-celled or simple chains of cells. Phytoplankton are
the primary producers of the marine ecosystems, performing over 95% of
the photosynthesis in the ocean.
i.
Diatoms are extremely important in terms of photosynthetic
capabilities. They are common in temperate and polar regions
and other nutrient-rich waters. They are abundant both near
the coast and in the open ocean.
ii.
Dinoflagellates are important in both coastal and oceanic
waters, but they tend to prefer warm water areas. In the
tropics they replace the diatoms as the most abundant
members of the phytoplankton. Given nutrients in sufficient
quantities, dinoflagellates may bloom causing what are known
as “red tides.” These red tides can be extremely hazardous to
marine organisms as well as humans who feed on these
organisms.
iii.
Cyanobacteria are the most abundant type of phytoplankton,
and are the dominant primary producers in the ocean.
iv.
Protists such as coccolithophorids, and silicoflagellates are
common throughout the oceans and add greatly to the net
primary productivity in nutrient-poor areas of the oceans.
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b. Zooplankton mainly consists of herbivorous organisms that feed directly
on the phytoplankton. Zooplankton (including invertebrate larvae) are the
most important herbivores in the epipelagic.
i.
Protozoan zooplankton such as flagellates, ciliates,
foraminiferans, and radiolarians make up the majority of the
single-celled zooplankton. They are extremely important to
the overall trophic structure of the epipelagic because they eat
the most numerous of the phytoplankton: the pico- and
nanoplankton.
ii.
Copepods, krill, and other crustaceans dominate the net
zooplankton. Copepods are the main herbivores in the
epipelagic and are by far the most abundant group of
zooplankton. Copepods are probably the most numerous
group of animals on Earth.
iii.
Jellyfishes, comb jellies, and siphonophores can be quite
large, but are still considered zooplankton due to their weak
swimming abilities. All are carnivorous, and many eat small
fishes as well as other zooplankton.
c. Nekton consists of the large, strong swimming organisms such as fish,
marine mammals, and squids.
i.
Planktivorous nekton eats plankton. These include small fish
such as herring, sardines, and anchovies. They also include
the world’s largest fishes, the whale shark and basking shark.
The largest nekton of all, baleen whales, also eat plankton,
mostly krill.
ii.
Most species of nekton eat other nekton rather than plankton.
Nearly all the nekton are predators. Epipelagic food web.
II.
Epipelagic Adaptations
a. Organisms must avoid sinking to stay in the epipelagic. This is achieved
by increasing water resistance and increasing buoyancy.
i.
The higher the surface area of the organism, the higher the
resistance and slower the organism sinks. Planktonic
organism achieved this by long spines or projections.
ii.
Epipelagic organisms can increase buoyancy by storing lipids
in their tissues and utilizing gas-filled bladders or floats.
b. Epipelagic organisms have well-developed sense organs, especially
vision, the lateral lines and hearing of fishes, and echolocation in
cetaceans.
i.
Because light is plentiful in the epipelagic, most zooplankton
and nekton have well-developed eyes.
ii.
Fishes have another remote sensing system: the lateral line.
The lateral line is sensitive to vibrations in the water.
iii.
Most predatory fish have well-developed hearing and are
strongly attracted to splashes on the surface.
iv.
Dolphins and some other cetaceans use echolocation.
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c. Typical epipelagic nekton have, besides sharp eyesight, countershading,
streamlined bodies, well-developed and efficient muscles, and high,
relatively narrow tails.
i.
Countershading is where an organism’s dorsal surface is dark
and the ventral surface is white or silver. Countershading
organisms blend in against the background whether they are
viewed from above or below.
ii.
Practically all epipelagic nekton have streamlined bodies that
make swimming easier and more efficient. Their bodies are
sleek and compact.
iii.
Epipelagic nekton are also firm and muscular. Most of the
muscle in an epipelagic fish is red muscle. This type of
muscle is suited for long, sustained movement.
iv.
Fins on an epipelagic fish tend to be stiff instead of flimsy and
weak. This provides the fish maneuverability and lift at high
speed.
v.
The tuna is a swimming machine; the perfect fish.
d. Vertical migrating zooplankton stay below the photic zone during the day.
At night they migrate to the surface to feed.
III.
Epipelagic Food Webs
Epipelagic food chains and webs tend to be long and complex because they
contain many species and many epipelagic animals feed at different trophic
levels. Seasonal cycles of primary productivity at different latitudes.
IV.
El Niňo
El Niňo, short for El Niňo – Southern Oscillation or ENSO, is a reversal of normal
winds, currents, and upwelling effects in the southern Pacific Ocean.
a. Around Christmas, about every four years.
b. Normal conditions in south Pacific are: high pressure over Tahiti, and low
pressure over Australia. Equatorial divergence (upwelling), so H20 builds
up in western Pacific due to equatorial currents.
c. Southern oscillation promotes low pressure over Tahiti, and high pressure
over Australia. Thus, weaker trade winds, and reversal of these winds.
d. Storms start shifting to the East. Warmer western Pacific waters start
moving to the eastern Pacific.
e. Sea level rises in central Pacific, and moves east towards South America.
f. Surface temperatures rise off coast of South America and coastal
upwelling is covered up and suppressed. Fewer nutrients available for
organisms. Less fish and less marine mammals.
g. Rainfall increases which causes flooding in South America. The Gulf of
Mexico gets colder north Atlantic air.
h. Warm water moves north along coast of South America, crossing the
equator, and displacing Humboldt Current along Baja and southern
California. Increased rainfall along western U.S.
i. Indonesia, Australia, Africa, eastern S.A. experience severe drought.
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