Who, What, When, Where, Why, and How of
The Open Ocean
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Who inhabits the open ocean?
What defines the open ocean?
When do organisms live in the open ocean?
Where (which oceans) can you find open
ocean?
• Why is the open ocean a necessary
ecosystem?
• How does the open ocean interact with other
marine ecosystems?
Chapter 17
The Open Sea
Karleskint
Turner
Small
Regions of the Open Sea
• The open ocean lies beyond the neritic zone
• Vertical zonation depends on penetration of
sufficient sunlight to support photosynthesis
– photic zone: receives enough light for phytoplankton to
survive
• can extend to a depth of 200 meters (660 ft) in clear tropical
waters
• Epipelagic zone (corresponds to the photic zone):
the location of pelagic animals in the upper 200 m
of the ocean
• Aphotic zone: light rapidly disappears until the
environment is totally dark
Life in the Open Sea
• Two groups of organisms inhabit the oceanic zone:
plankton and nekton
– based on productivity, biomass, abundance and
diversity, plankton far outweighs nekton in open ocean
• Classification of plankton
• Plankton can be classified into logical groups
based on:
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taxonomy
motility
size
life history
spatial distribution
Life in the Open Sea
• Classification of plankton
– Classifying them into different groups:
• phytoplankton: primary producers
• zooplankton: heterotrophic eukaryotic microbes that
float in the currents
• bacterioplankton: archaeans and bacteria
• viriplankton: free viruses (the most abundant plankton
of all)
Life in the Open Sea
• Classification of plankton
– Classifying based on motility
• akinetic: plankton that don’t move at all, e.g., viruses,
diatoms
• kinetic: plankton that can move (include majority of
plankton)
– kinetic plankton move by use of flagella, jet propulsion,
undulation, swimming appendages
Life in the Open Sea
• Classification of plankton
– Classifying based on size
• original scheme (based on visibilty and collection
method):
– macroplankton – visible to the naked eye
– microplankton – caught with standard plankton net
– nanoplankton – concentrated by centrifugation
• newer classifications:
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Femtoplankton – less than 0.2µm
Picoplankton – 0.2-2.0µm
Mesoplankton – 2.0µm-20mm
Macroplankton
megaplankton
Life in the Open Sea
• Classification of plankton
– Classifying based on life history
• holoplankton: organisms that are planktonic
throughout their lives, e.g., microbes, arrowworms,
salps, siphonophores, comb jellies, copepods, krill
• meroplankton: planktonic larvae that will grow into
non-planktonic organisms
– in open ocean would include larvae of nektonic fish and
squid
– in coastal waters would also include larvae of benthic
invertebrates
Life in the Open Sea
• Classification of plankton
– Classifying based on spatial distribution
• neritic: distinguished by presence of meroplankton
and diverse diatoms
• oceanic: less diverse in diatoms and invertebrate
meroplankton; more salps, larvaceans,
arrowworms and sea butterflies
• neuston: plankton that live close to the water’s
surface
• pleuston: plankton which break the surface of the
water with their gas bladders or bubbles, e.g., bythe-wind sailor
Life in the Open Sea
• Patchiness in the open sea
– plankton occur in patches (localized
aggregations), often around upwellings
– patchiness can be caused by:
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upwelling
localized variations in sea surface conditions
vertical mixing
downwelling
waters of different densities coming together
grazing by zooplankton
Life in the Open Sea
• Patchiness in the open sea (continued)
– micropatchiness occurs throughout the photic
zone when marine microbes become attached
to particles of organic matter, esp. marine snow
• marine snow: strands of mucus secreted by
zooplankton that form translucent, cob-webby
aggregates
Life in the Open Sea
• Plankton migrations
– many open-ocean zooplankton migrate daily
from the surface to nearly 1.6 km deep
• provides access to phytoplankton in the photic
zone
• reduces predation by plankton-eating fishes in the
epipelagic zone
– deep scattering layer: a mixed group of
migratory zooplankton and fishes that are
densely packed
• can give sonar false image of a solid surface
hanging in mid-water
Life in the Open Sea
• Megaplankton
– most organisms classified as megaplankton
are animals
– cnidarian zooplankton
• largest members of the plankton are jellyfishes
Life in the Open Sea
• Megaplankton
– molluscan zooplankton
• pteropods (sea butterflies)
have a foot with 2 large
wing-like projections and a
greatly reduced or absent
shell
• purple sea snails produce
bubble rafts
• some species of
nudibranchs
Life in the Open Sea
• Megaplankton
(continued)
– urochordates
• salps have
barreled shaped
bodies opened
at both ends
– composed of
95% water,
hence grow and
reproduce
rapidly
Life in the Open Sea
• Megaplankton
(continued)
– urochordates
• larvaceans – only group
of pelagic tunicates
Life in the Open Sea
• Nekton
– invertebrates
• squids: reign supreme in open ocean as formidable
predators
– fish
• billfish: species with an enlongated upper jaw (bill)
and no teeth (e.g., marlin, sailfish, swordfish)
• tuna: most wide-ranging of open ocean fishes, lack
swim bladder – must swim constantly
• ocean sunfish: feed on large zooplankton, especially
jellyfish, have few natural predators
• sharks: most efficient predators of open ocean
• manta rays: have labial flaps which channel small fish
and plankton into their mouths
Life in the Open Sea
• Nekton (continued)
– reptiles
• Include sea snakes and sea turtles
Life in the Open Sea
• Nekton (continued)
– birds and mammals
• penguins of Southern Ocean
• whales
– baleen whales filter krill, pteropods and fish
– toothed whales feed on squid and fish
Survival in the Open Sea
• How do organisms remain afloat?
– swimming methods
• flagella, cilia, and jet propulsion
– dinoflagellates, coccolithophores, silicoflagellates, and bluegreen bacteria swim with flagella
– tintinnids, ciliates, and larvae use cilia
– jellyfish, siphonophores, salps, and squid use jet propulsion
• appendages
– appendicular swimmers: organisms that use appendages to
swim (e.g., copepods, pteropods)
• undulations of the body
– e.g., arrowworms, larvaceans, worms, fish, whales
Survival in the Open Sea
• Remaining afloat (continued)
– reduction of sinking rates
• frictional drag: can be increased by decreasing
volume, flattening the body or increasing body length
– adaptations that increase friction do not prevent organisms
from sinking, they merely slow the process
• buoyancy: increased by storage of oils, increasing
water content of the body, exchange of ions, and use
of gas spaces
Survival in the Open Sea
• Avoiding predation
– due to lack of accessible refuges in open ocean,
pelagic organisms have evolved a variety of
adaptations to avoid predation
– benefit of being less conspicuous
• countershading: having dorsal surfaces that are dark
blue, gray or green and ventral surfaces that are
silvery or white
• many planktonic species are nearly transparent
Survival in the Open Sea
• Avoiding predation
– safety in numbers
• animals such as siphonophores (e.g., Portuguese
man-of-war) increase chances of survival by
forming colonies
– looks like single individual
– made up of thousands of individuals
Ecology of the Open Sea
• Open sea is a pelagic ecosystem—one in which
the inhabitants live in the water column
– basis of food chain is many species of small
phytoplankton
• Small, primary producing organisms have a
relatively high surface area
– allows them to absorb more nutrients from surrounding
seawater
• Majority of herbivores in open ocean are
zooplankton which supply food for nekton
Ecology of the Open Sea
• Productivity
– all higher forms of life rely on plankton
– water near the surface receives plenty of
sunlight, but few nutrients from land or the sea
bottom (except in rare areas of upwelling)
– phytoplankton productivity is low in tropical
waters
• arrangement of water in layers with little circulation
between prevents nutrients from being brought from
the sea bottom
• low phytoplankton numbers support even fewer
numbers of zooplankton
Ecology of the Open Sea
• Food webs in the open sea
– basis of food webs in open sea is formed by
phytoplankton and heterotrophic bacteria
– dissolved and particulate organic matter
• phytoplankton release photosynthetic products as DOM into
surrounding seawater
• heterotrophic bacteria recycle DOM as they eat it and then are
eaten by nanoflagellates
• bacterial loop: process in which bacteria metabolize DOM and
return it to the water in an inorganic form available to
phytoplankton
• lysis of bacteria by viruses releases DOM and particulate
organic matter (POM)
Ecology of the Open Sea
• Food webs in the open sea (continued)
– efficiency of open-ocean food webs
• conversion of biomass from one level to the next is
surprisingly efficient
• entire phytoplankton or bacterial production may be
consumed daily by next trophic level
• conversion rates (food to biomass) may be high
• food webs may have food chains with 5-6 links
• few large animals are supported away from upwelling
areas because of limited rate of primary production
and declining conversion efficiency along the food
chain