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Chapter 17Chapter 17The Open SeaThe Open Sea

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 open-ocean 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 open-ocean 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., by-the-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 blue-green 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)

Dissolvedinorganicnutrients

Shark

Squid

Tuna

Herring Ctenophores

Dolphin

Toothedwhale

Leatherbackturtle

Baleen whale

JellyfishSardines

TintinnidsHeterotrophicnanoflagellates

Particulate organicmatter

Viruses

Dissolvedorganicmatter

Freeheterotrophic

bacteria

(to deeper waters)

Meso- andMicrozooplankton

Phytoplankton

Stepped Art

Figure 17- 17, p. 481

Macrozooplankton (krill, shrimp)

Nanoplankton(cyanobacteria,small diatoms)

Microzooplankton(foraminiferans, radiolarians)

Mesozooplankton (copepods)

Nekton (fishes, squid, whales)

Phytoplankton (nanoplankton and smaller)

Mesozooplankton

Macrozooplankton

Large and medium fishes, squid

Small fishes

Phytoplankton (nanoplankton and smaller)

Large and medium fishes, squid

Microzooplankton

Mesozooplankton

MacrozooplanktonSmall fishes

Microzooplankton

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