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Chapter 17
The Open Sea
Karleskint
Turner
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Key Concepts
• The open sea is a pelagic ecosystem, in
which the living components are plankton
and nekton.
• Plankton range widely in size, taxonomic
diversity, and life style.
• Phytoplankton are the primary producers
in open-ocean food web, and their
productivity is limited by the scarcity of
nutrients.
Key Concepts
• Bacteria provide a second base to openocean food webs, and they allow the
scarce nutrients to be efficiently recycled.
• Limited primary production and food webs
with several energy-wasting steps limit the
number of large animals the open ocean
can support.
Key Concepts
• Gelatinous plankton such as salps and
ctenophores play significant roles in openocean ecosystems because of their
efficient feeding mechanisms, reduction of
nutritional quality, and provision as prey
for specialist carnivores.
• Several structural features and behaviors
have evolved to keep afloat organisms
that are not strong swimmers.
Key Concepts
• Plankton display a number of interesting
adaptations that help them avoid
predation.
• Large zooplankton include jellyfish,
gastropod molluscs, and colonial pelagic
tunicates.
• Fishes, squids, and mammals make up
most of the nekton in the open sea.
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:
–
–
–
–
–
taxonomy
motility
size
life history
spatial distribution
Life in the Open Sea
• Classification of plankton (continued)
– taxonomic groups
•
•
•
•
seston: particles suspended in the sea, include:
tripton: non-living seston
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 (continued)
– motility
• akinetic: plankton that don’t move at all, e.g., viruses,
diatoms and forms
• 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 (continued)
– 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: femtoplankton, picoplankton,
mesoplankton, macroplankton, megaplankton
Life in the Open Sea
• Classification of plankton (continued)
– 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 (continued)
– 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 life 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:
•
•
•
•
•
•
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
• Lévy Walks = movement of predators (e.g.,
basking sharks, bigeye tuna, Atlantic cod,
leatherback sea turtles) in patchy, fractal
patterns
– leads to greater foraging success
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
(thecosome pteropods) or absent (gymnosome
pteropods) shell
• pteropod ooze: calcareous sediments formed from
shells of dead thecosome pteropods
• 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
• pyrosomes: close relatives of salps that produce
colonies made up of hundreds of individual animals
joined to form a hollow cylinder up to 14 m long
– occur worldwide but most commonly in tropical and
subtropical seas
Life in the Open Sea
• Megaplankton (continued)
– urochordates
• larvaceans – secrete mucus structures called
houses, entrap tiny plankton
– houses discarded several times a day because of
accumulation of fecal material that cannot be eliminated,
become homes for bacteria and end up as particles of
marine snow
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
• yellow-bellied sea snakes in tropical waters of Indian
and Pacific Oceans
– have no known enemies due to distasteful meat
• leatherback sea turtle – reduced shell, feeds on
gelatinous zooplankton, must return to land to lay
eggs
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
• Remaining 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
– none could live alone
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)
Toothed
whale
Baleen whale
Squid
Shark
Jellyfish
Sardines
Leatherback
turtle
Tuna
Tintinnids
Heterotrophic
nanoflagellates
Meso- and
Microzooplankton
Herring
Ctenophores
Phytoplankton
Dolphin
Dissolved
inorganic
nutrients
Free
heterotrophic
bacteria
Dissolved
organic
matter
Viruses
Particulate organic
matter
(to deeper waters)
Stepped Art
Figure 17- 17, p. 481
Large and medium
fishes, squid
Small fishes
Macrozooplankton
Nekton (fishes, squid, whales)
Mesozooplankton
Macrozooplankton (krill, shrimp)
Microzooplankton
Phytoplankton (nanoplankton and smaller)
Mesozooplankton
(copepods)
Large and medium
fishes, squid
Small fishes
Macrozooplankton
Microzooplankton
(foraminiferans, radiolarians)
Mesozooplankton
Microzooplankton
Nanoplankton
(cyanobacteria,
small diatoms)
Phytoplankton (nanoplankton and smaller)
Stepped Art
Figure 17- 18, p. 482
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
Ecology of the Open Sea
• Food webs in the open sea (continued)
– efficiency of open-ocean food webs
• pyramid of production = diagram that indicates the
rate at which new biomass is produced at successive
trophic levels
• standing crop = amount of biomass of organisms in a
given area at a given time
• standing crop of phytoplankton in open ocean might
be very small, giving a pyramid of production that is
partly inverted
• The wealth of the open sea resides in its
microscopic inhabitants