Pelagic communities

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Answers to STUDY BREAK Questions

Essentials 5th

Chapter 13


1. What distinguishes pelagic communities from benthic communities?
Pelagic organisms live suspended in seawater, while benthic organisms (which you will meet in the next chapter) live on or in the ocean bottom

2. How are plankton different from nekton? Into which category would most fishes fit?
The pelagic organisms that constitute plankton are as important as they are inconspicuous. The word is derived from the Greek word planktos, meaning “wandering.” . The plankton drift or swim weakly, going where the ocean goes, unable to move consistently against waves or current flow. The nekton are pelagic organisms that actively swim. Most fishes are nektonic as adults.

3. Why did I write that plankton is an “artificial category” of organisms?
The plankton contains many different plant-like species and virtually every major group of animals. Thus, the term plankton is not a collective natural category like mollusks or algae, which would imply an ancestral relationship between the organisms; instead it describes a basic ecological connection.

4. Are all plankters plants?1 All animals?
The plankton contains many different plant-like species and virtually every major group of animals. Members of the plankton community, informally referred to as plankters, can and do interact with one another: There is grazing, predation, parasitism, and competition among members of this dynamic group.

5. Are all members of the plankton community capable of being collected using nets?
Very small plankton can slip through a plankton net. Their capture and study requires concentration by centrifuge, or entrapment by a fine plankton filter through which water is drawn. The filter is later disassembled and the plankton studied in place. The smallest of plankton is trapped by specially made unglazed porcelain filters through which water is forced under very high pressure.

6. What is a “photosynthetic autotroph?” Can you give a non-marine example?

Photosynthetic autotrophs construct food molecules (usually the carbohydrate glucose) from water and carbon dioxide using light energy (the sun). Autotrophic plankton are generally called phytoplankton, a term derived from the Greek word phyton, meaning “plant.” A huge, nearly invisible mass of phytoplankton drifts within the sunlit surface layer of the world ocean.

The roses blooming in your garden are also photosynthetic autotrophs.

7. How are phytoplankton different from zooplankton? Which category represents autotrophs?
Phytoplanktonic organisms are photosynthetic autotrophs. Zooplankton are heterotrophic – that is, they consume autotrophic organisms (or organisms that consumed autotrophs).

8. Why was the activity of picoplankton overlooked until quite recently?
These organisms are often too small to be resolved by light microscopes and slip undetected through all but the finest filters. Their size, typically about 0.2 to 2 micrometers (4 to 40 millionths of an inch) across, is made up for by their abundance: an astonishing 100 million in every liter of seawater, at all depths and latitudes!

9. I wrote of an “black-market economy” in the microbial loop. Why isn’t this “economy” available to the typical consumers of phytoplankton?
The microbial loop is a complete microecosystem—a community operating on the smallest possible scale—that manufactures and consumes particulate and dissolved carbon in amounts almost beyond comprehension. They function as a sort of ecological black market below the “official economy” of the relatively huge diatoms and dinoflagellates. As if they weren’t busy enough, these heterotrophic bacteria also decompose organic material spilled into the water when phytoplankton are eaten by zooplankton, turn soluble organic materials released by zooplankton back into inorganic nutrients, and break down particulate organic matter into a dissolved form they can consume for their own growth. Biological oceanographers now believe that the greatest fraction of organic particles in the water column of the open ocean is composed of these metabolically active heterotrophic bacterial cells operating in this microbial loop (Figure 13.4). This “black market economy” is almost certainly as productive as the “official economy.” It is not available to fishes and other larger consumers because the small animals on which they prey are unable to separate these exceedingly small organisms from the surrounding water. Microconsumers simply utilize the carbon and shuttle the metabolic products back to the small cyanobacterial producers.

10. Which group of relatively large single-celled autotrophs dominates the phytoplankton ? Why are they important?
Apart from cyanobacteria, the most productive photosynthetic organisms in the plankton are the diatoms. Diatoms evolved comparatively recently, and began to dominate phytoplanktonic productivity in the Cretaceous period about 100 million years ago. Their abundance and photosynthetic efficiency increased the proportion of free oxygen in Earth’s atmosphere. More than 5,600 species of diatoms are known to exist. The larger species are barely visible to the unaided eye.

11. How is the covering (shell, or test) of a diatom different from that of a dinoflagellate?
A diatom’s consists of silica (SiO2), giving this heavy but beautiful covering the optical, physical, and chemical characteristics of glass—clearly an ideal protective window for a photosynthesizer. A dinoflagellate’s covering is often of a cellulose-like compound, neither completely transparent nor completely rigid.

12. Which planktonic organisms are usually responsible for HABs? Can a HAB event be harmful to people?
Some species of dinoflagellates can become so numerous that the water turns a rusty red as light reflects from the accessory pigments within each cell. These species are responsible for harmful algal blooms -- HABs (Figure 13.6b). During times of such rapid growth (usually in springtime), concentration of these microscopic organisms may briefly reach 6 million per liter (23 million per gallon)! At night, the huge numbers of dinoflagellates in a HAB (also called a “red tide”) can cause breaking waves to glow a bright blue, a phenomenon known as bioluminscence.

HABs can be dangerous because some dinoflagellate species synthesize potent toxins as byproducts of metabolism. Among the most effective poisons known, these toxins may affect nearby marine life or even humans. Some of the toxins are similar in chemical structure to the muscle relaxant curare, but are tens of times more powerful. Humans should avoid eating certain species of clams, mussels, and other filter feeders during summer months when toxin-producing dinoflagellates are abundant in the plankton. If shellfish from a particular area are unsafe, a state governmental agency will issue an advisory which may remain in effect for six weeks or more until the danger is past.

13. Why are the open tropical oceans essentially oceanic deserts?
The open tropical oceans have abundant sunlight and CO2 but are generally low in surface nutrients because the strong thermocline discourages the vertical mixing necessary to bring nutrients from the lower depths. The tropical oceans away from land are therefore oceanic deserts nearly devoid of visible (that is, non-cyanobacterial) plankton. The typical clarity of tropical water underscores this point. In most of the tropics productivity rarely exceeds 30 g C/m2/yr, and seasonal fluctuation in productivity is low.

14. If the polar oceans are lighted 24-hours a day in the local summer, why isn’t total annual productivity high?
At very high latitudes the low sun angle, reduced light penetration due to ice cover, and weeks or months of darkness in winter severely limit productivity. At the height of summer, however, 24-hour daylight, a lack of surface ice, and the presence of upwelled nutrients can lead to spectacular plankton blooms. The surface of some sheltered bays can look like tomato or split-pea soup because dinoflagellates and other plankton are so abundant. This bloom cannot last because nutrients are not quickly recycled and because the sun is above the critical angle for a few weeks at best. The short-lived summer peak does not compensate for the long, unproductive winter months.

15. Choosing between the polar, tropical, and temperate zones, which part of the ocean are the most productive over a year’s time?
With the tropics generally out of the running for reasons of nutrient deficiency and the north polar ocean suffering from slow nutrient turnover and low illumination, the overall productivity prize is left to the temperate and southern subpolar zones. Thanks to the dependable light and the moderate nutrient supply, annual production over temperate continental shelves and in southern subpolar ocean areas is the greatest of any open ocean area.

16. How are zooplankton different from phytoplankton?
Heterotrophic plankton—the planktonic organisms that eat the primary producers—are collectively called zooplankton. Phytoplankton are autotrophic (primary producers).

17. How are holoplankton different from meroplankton?
Most zooplankton spend their whole lives in the plankton community, so we call them holoplankton. But some planktonic animals are the juvenile stages of crabs, barnacles, clams, sea stars, and other organisms that will later adopt a benthic or nektonic lifestyle. These temporary visitors are meroplankton (Figure 13.11). Most animal groups are represented in the meroplankton; even the powerful tuna serves a brief planktonic apprenticeship. These useful categories can be applied to phytoplankton as well as zooplankton. Holoplanktonic organisms are by far the most numerous forms of both phytoplankton and zooplankton.

18. Why are krill important?
One of the ocean’s most important zooplankters is the pelagic arthropod known as krill (genus Euphausia), the keystone of the Antarctic ecosystem. This thumb-sized shrimplike crustacean mostly grazes on the abundant diatoms of the southern polar ocean. In turn, krill are eaten in tremendous numbers by seabirds, squids, fishes, and whales. Some 500 to 750 million metric tons (550 to 825 million tons) of krill inhabit the Antarctic Ocean, with the greatest concentrations in the productive upwelling currents of the Weddell Sea. Krill travel in great schools that can extend over several square miles, and collectively exceed the biomass of Earth’s entire human population! They behave more like schooling fish than planktonic crustaceans.

19. How is nekton different from plankton?
The pelagic organisms that constitute plankton are as important as they are inconspicuous. The word is derived from the Greek word planktos, meaning “wandering.” . The plankton drift or swim weakly, going where the ocean goes, unable to move consistently against waves or current flow. The nekton are pelagic organisms that actively swim. Most fishes are nektonic as adults.

20. How is an invertebrate different from a vertebrate?
Most nektonic animals are vertebrates (animals with backbones, such as fishes, reptiles, marine birds, and marine mammals), but a few representatives are invertebrates (animals without backbones, such as squid and nautiluses, and some species of shrimp-like arthropods).

21. Which organisms are the most abundant and successful nektonic invertebrates? Vertebrates?
Arthropoda, the animal category that includes the copepods (Figure 13.10), krill (Figure 13.12) lobsters, shrimp, crabs, and barnacles is the most successful animal group on Earth.

Fishes are the most abundant and successful vertebrates. There are more species of fishes, and more individuals, than species and individuals of all other vertebrates combined.

22. What adaptations contribute to the success of fishes?
Seawater may seem to be an ideal habitat, but living in it does present difficulties. These most successful vertebrates have structures and behaviors to cope. Among them are adaptations of movement, shape, and propulsion. Active fish usually have streamlined shapes that make their propulsive efforts more effective. A fish's resistance to movement, or drag, is determined by frontal area, body contour, and surface texture. A fish's forward thrust comes from the combined effort of body and fins. Muscles within slender flexible fish (such as eels) cause the body to undulate in S-shaped waves that pass down the body from head to tail in a snake-like motion. Advanced fishes have a relatively inflexible body, which undulates rapidly through a shorter distance, and a hinged scythe-like tail to couple muscular energy to the water. Maintenance of level is crucial to any swimming animal. The density of fish tissue is typically greater than that of the surrounding water, so fishes will sink unless their weight is offset by propulsive forces or by buoyant gas- or fat-filled bladders. Cartilaginous fishes have no swim bladders and must swim continuously to maintain their position in the water column. Bony fishes that appear to hover motionless in the water usually have well-developed swim bladders just below their spinal columns. The volume of gas in these structures provides enough buoyancy to offset the animal's weight. Gas exchange is also important: How can fish breathe underwater? Gas exchange, the process of bringing oxygen into the body and eliminating carbon dioxide (CO2), is essential to all animals. Fish take in water containing dissolved oxygen at the mouth, pump it past fine gill membranes, and exhaust it through rear-facing slots. The higher concentration of free oxygen dissolved in the water causes oxygen to diffuse through the gill membranes into the animal; the higher concentration of CO2 dissolved in the blood causes CO2 to diffuse through the gill membranes to the outside. The gill membranes themselves are arranged in thin filaments and plates efficiently packaged into a very small space. Feeding and defense is also critical to success. Competitive pressure among the large number of fish species has caused a wonderful variety of feeding and defense tactics to evolve. Sight is very important to most fishes, enabling them to see their prey or avoid being eaten. Even some deep-water fishes that live below the photic zone have excellent eyesight for seeing luminous cues from potential mates or meals. Hearing is also well developed, as is the ability to detect low-frequency vibrations with the lateral-line system. More subtle means of offense or defense depend on trickery -- looking like something you're not, or changing color to blend with the background. These kinds of cryptic coloration or camouflage may be active or passive. Schooling behavior is also useful -- about a quarter of all bony fish species exhibit schooling behavior at some time during their life cycle. I can personally attest to the effectiveness of schooling as a means of defense. On a few diving trips I've noticed a large moving mass just beyond the limit of clear visibility. Is it a fish school, or is it a single large animal? Many predators might not stay around long enough to find out!

23. What are the largest animals ever to have lived on Earth? From what are they thought to have evolved?
The 79 living species of cetaceans (whales) are thought to have evolved from an early line of ungulates -- hooved land mammals related to today's horses and sheep -- whose descendants spent more and more time in productive shallow waters searching for food.

24. How is a seal different from a sea lion? Which animal is featured in “seal shows” at oceanaria?
True seals have a smooth head with no external ear flaps, the external part of the ear having been sacrificed to further streamline the body. They are covered with a short coarse hair without soft underfur. Seals are graceful swimmers that pursue small fish, their usual prey, with powerful side-to-side strokes of their hind limbs. These rear appendages are partially fused and always point back from the hind end of the body, so they are of very little use for locomotion on land. The elephant seal, named for its long snout and large size, holds the diving depth record for all air-breathing vertebrates: 1,560 meters (5,120 feet).

Sea lions, familiar to many as the performers in "seal" shows, have hind limbs with a greater range of motion and thus are more mobile on land. They have a streamlined head with small external ears and a pelt with soft underfur; unlike seals, they use their front flippers for propulsion. Walruses are much larger than either seals or sea lions and may reach weights of 1,800 kilograms (2 tons).

1 Members of the plankton community, informally referred to as plankters, can and do interact with one another

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