1. Chapter 17
The Open Sea

2. 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.

3. 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.

4. 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.

5. 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.

6. 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

7. 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

8. 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)

9. 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

10. 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

11. Figure 17-1 (macroplankton and nanoplankton) CLASSIFICATION OF PLANKTON BY SIZE.

12. 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

13. Figure 17-2 (b) PLANKTON.

14. 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

15. 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

16. 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

17. 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

18. Life in the Open Sea Megaplankton
most organisms classified as megaplankton are animals
cnidarian zooplankton
largest members of the plankton are jellyfishes

19. Figure 17-4 (a) JELLYFISH OF THE OPEN SEA.

20. Figure 17-4 (b) JELLYFISH OF THE OPEN SEA.

21. 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

22. 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

23. 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

24. Figure 17-6 PYROSOMES AND SALPS.

25. Life in the Open Sea Nekton invertebrates fish
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

26. Figure 17-8 TUNA.

27. Figure 17-9 OCEAN SUNFISH.

28. 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

29. 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

30. 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

31. Figure 17-13 (c) HOW OPEN-WATER ORGANISMS REMAIN AFLOAT.

32. 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

33. Figure 17-13 (a) HOW OPEN-WATER ORGANISMS REMAIN AFLOAT.

34. Figure 17-13 (b) HOW OPEN-WATER ORGANISMS REMAIN AFLOAT.

35. 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

36. Figure 17-14 COUNTERSHADING.

37. 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

38. 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

39. 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

40. 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)

41. Figure 17-17 OPEN-OCEAN FOOD WEB.

42. Stepped Art Figure 17- 17, p. 481 Dissolved inorganic nutrients Shark
Squid
Tuna
Herring
Ctenophores
Dolphin
Toothed
whale
Leatherback
turtle
Baleen whale
Jellyfish
Sardines
Tintinnids
Heterotrophic
nanoflagellates
Particulate organic
matter
Viruses
organic
Free
heterotrophic
bacteria
(to deeper waters)
Meso- and
Microzooplankton
Phytoplankton
Stepped Art
Figure , p. 481

43. Figure 17-18 (a) PLANKTONIC FOOD CHAIN.

44. Figure 17-18 (b) PLANKTONIC FOOD CHAIN.

45. Nekton (fishes, squid, whales) Large and medium fishes, squid
Small fishes
Macrozooplankton
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 , p. 482

46. 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

47. 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