Introduction to Ocean Sciences

Chapter 17: Ocean Ecosystems

Guide to Reading and Learning

You have read about the general characteristics of the dominant types of marine ecosystems in other chapters and hopefully have noticed that there are many references made to special circumstances that can make the ecology of some specific restricted areas of the oceans different from areas that are similar to them. You may have also noticed that these exceptional limited ecosystems appear to be different because of one or a small number of physical or chemical characteristics of the environment that are different in these locations. If you have noticed this, you will already understand that the biology of any specific ocean ecosystem is the product of a delicate balance of many geological, physical, and chemical parameters of the environment, each of which can vary on many time and space scales. Also, you may have realized that only a small and subtle change in one or more of these parameters can dramatically alter the environment available for life. In this chapter, we will follow up on this concept and provide just a very few briefly described illustrations. The endless variation of species adaptations to their environment is one of the things that fascinate me and many others about the oceans. No matter how much we know about the oceans, there are undoubtedly still many unique and wonderful ecosystems or ecological interactions that we have not yet discovered, some of which may be as alien to us as life on Mars would be.

The chapter starts by introducing the concept of ecological niches and the effects of environmental variables and competition on species distributions. It then provides a number of examples of limited marine ecosystems and a brief description of each ecosystem to illustrate the relationship between the special characteristics of the environment and the species that live in these ecosystems.

The ecosystems we have chosen to visit are coral reefs, kelp forests, rocky intertidal zones, the Sargasso Sea, polar regions, the region of the open-ocean water column where the sun’s light never reaches, and hydrothermal vents. Why have we chosen these ecosystems? First, coral reefs are one of the best examples of how associations between species are as common in the oceans as competition. Without the coral-zooxanthellae association, there would not be the abundant and diverse ecosystems of coral reefs as we now know them. Many of us have grown up watching videos of coral reef ecosystems with their multicolored and sometimes bizarrely shaped inhabitants. Some of us, including my wife and contributing author Elaine have developed such a love of these wonderful ecosystems that we have devoted a good part of our lives to visiting coral reefs and documenting them in images that we hope will give others the same enjoyment. We are not alone in this. In fact, it is almost certainly because of the fascination of millions with coral reefs depicted in images, videos, and aquariums that humans are as aware as they are today of the impacts that we have on the oceans and that they have on us.

After we visit coral reefs in Chapter 17, we visit two special ecosystems that are found on many coastlines of North America, Europe, and elsewhere in close proximity to where large concentrations of humans live. Thus, these are places that are local to many of us and are relatively easily visited, something I urge you to do if you can, whether on an organized field trip or just on your own. Your newfound knowledge, especially if it is supplemented by local guidebooks, local coastal park rangers, and guides, will allow you to enjoy your time at the coast in a wholly different way than before. If you do not live near a kelp forest or rocky coastline but have other marine or estuarine natural areas nearby, you can still experience the same enjoyment of knowledge gained from this text by visiting these ecosystems after doing a little research on your own to get readily available local information. We are not able to review every regional or local marine ecosystem but perhaps your instructor will also choose to add a discussion of coasts and ecosystems near you.

Next we examine the unique adaptation of the biological world found in the Sargasso Sea. All the plant and animal life in the Sargasso Sea is adapted to exist in a surface layer of water that is perhaps even more starved of nutrients than coral reefs. Unlike in coral reef ecosystems, organisms in the Sargasso Sea cannot attach to the seafloor because this seafloor lies kilometers below. This surface layer ecosystem floats far from land unseen and unappreciated by all but a few sailors who pass through the region. It’s almost unknown existence reflects well on the limited extent of human knowledge of the oceans.

After the Sargasso Sea, we move on to the much more familiar ecosystems of the polar regions, known to most of us primarily because of their populations of whales, seals and other marine mammals, polar bears, penguins, and more. Unfortunately, these ecosystems and their abundant life are acknowledged by most marine scientists to be the ecosystems that are most vulnerable to the effects of global climate change and seriously at risk in the immediate future.

From the well known ecosystems of the polar regions, we descend to the perpetual darkness of the deep oceans and learn a little of the life that exists without light and where temperatures are low and food is always scarce. Marine scientists have not yet visited or adequately sampled more than an incredibly tiny fraction of the massive volume of this realm. Many species that live beyond the sun’s light have unusual adaptations that make some of them appear completely alien to us.

Finally, in this chapter, we review a little of what has been learned about ecosystems based on chemosynthesis, especially hydrothermal vents that were completely unknown to humans until 1977. Discovery of hydrothermal vents and the profusion of species that depend on food produced by chemosynthesis completely revolutionized marine sciences and has led to major changes and advances in understanding the origin of life on the Earth. These changes continue. Hydrothermal vents and other chemosynthetic environments unknown before the 1970s are now being found in many locations and there is a growing realization that chemosynthesis may be comparable in importance to photosynthesis as the engine of primary production that supports life on the Earth.

Chapter 17 Essential to Know 

Critical Concepts used in this chapter

CC.9, CC.14, CC.16, CC.17, CC.18

17.1 Communities and Niches

  • All species have a survival niche, which is the range of environmental conditions in which they can survive, and a more restricted fundamental niche, which is the range of environmental conditions in which the species can both survive and reproduce. Many factors define the niche including, for example, salinity, temperature, turbidity, light intensity, and substrate grain size.
  • Species may occupy the entire extent of the ocean environment that corresponds to their fundamental niche or may be excluded from parts of that niche by competition from species with overlapping niches.

17.2 Coral Reefs

  • Coral reefs form only in areas where water temperatures never fall below about 18oC, salinity is relatively invariable, and turbidity and suspended sediment concentrations are low.

Environmental Requirements for Coral Reef Formation

  • Reef-building corals have a symbiotic relationship with dinoflagellate species called zooxanthellae. Because the zooxanthellae are photosynthesizers reef-building corals can only grow in depths that are within the photic zone.
  • Corals can clear away a certain amount of suspended matter that is deposited on them but are damaged if suspended sediment concentrations are too high. Reef building corals are also intolerant of low salinity and low temperatures. Therefore, coral reefs do not grow near river mouths or any other large suspended sediment sources and grow only between about 23.5oN and 23.5oS.

Factors Affecting Coral Reef Productivity

  • Productivity on coral reefs is much higher than in the surrounding ocean because the coral-zooxanthellae symbiosis recycles nutrients, the reefs create some upwelling by creating eddies in currents that flow past them, and most organisms in the reef ecosystem reside permanently on the reef and so do not export their nutrients.

Primary Producers in the Coral Reef Community

  • Much of the primary production in most coral reef communities is performed by benthic algae, whose coral hard parts form parts of the reef.

Coral Reef Niches and Topography

  • In part, reef-building corals’ niche is defined by the need for light to fuel photosynthesis by their symbiotic zooxanthellae.
  • Coral reefs provide an abundance of different niches for many different species, including nooks and crannies in the reef and soft sediments inside lagoons.
  • Coral reefs usually have a benthic algae–dominated reef terrace near their ocean edge where wave action limits growth of fragile corals. Seaward of this terrace the reef drops off, sometimes with a tongue and groove formation, into deeper water. The upper part of the drop off, the buttress zone, is wave-swept and supports only very robust corals. Further down the reef wall, progressively more fragile corals grow until, near the bottom of the photic zone, delicate branched forms such as black coral and sea fans are found. Thus, wave action is a primary determinant of ecological niches of corals.
  • Soft corals have no zooxanthellae and grow only where wave action is limited.

17.3 Kelp Forests

  • Kelp is a generic name for a number of species of brown macroalgae that are attached to the seafloor.
  • Extensive forests of one such kelp, Macrocystis, are present off the west coast of North America.

Kelp Community Environmental Characteristics

  • Kelp grows in cold (less than 20oC) nutrient-rich waters where the seafloor is within the photic zone so new growth can photosynthesize. These conditions are found almost exclusively in coastal upwelling zones.
  • Kelp also requires a stable, preferably rocky substrate to attach to.
  • Kelp grows upward to the surface supported by gas-filled sacs and can form a dense canopy at the surface.

Kelp Life Cycle and Communities

  • Kelp grows rapidly by vegetative (asexual) reproduction but also produces microscopic spores that germinate to form a life stage that reproduces sexually to produce a planktonic form that settles to the seafloor before growing vegetatively..
  • Kelp is eaten directly by only a few species but it is fragmented and broken down by decomposers to form detritus that supports many species.
  • The kelp forest provides shelter and a variety of habitats for many invertebrate and fish species.

Kelp, Sea Otters, and Sea Urchins

  • Sea urchins eat kelp and are themselves eaten by otters.
  • Kelp forests have disappeared off many coasts where the otter populations have been reduced drastically by hunting, leaving urchin populations free to increase due to lowered predation.
  • Where the otter populations are recovering, urchin populations are declining and kelp forests are slowly returning.

17.4 Rocky Intertidal Communities

  • The rocky intertidal community is separated into communities that live in different zones arranged in bands of different depth along the shore.
  • The primary classification of the zones is: supralittoral, high tide, middle tide, and low tide zone. Tide pools provide a separate habitat.
  • The critical environmental parameter that delineates these zones is the length of time of exposure to the atmosphere (and dehydration) during the tidal cycle.
  • The zones vary in width according to factors including the slope of the rocky shore and the tidal range.

Supralittoral Zone

  • The supralittoral zone is either permanently exposed to the atmosphere or covered by water only during occasional extreme high tides.
  • Spray reaches this zone continuously and provides nutrients and moisture for lichens and some blue-green algae to grow.
  • The width of this zone depends on the extent of ocean spray and the regional climate. It is wider in cool damp climates than in warm sunny climates.
  • Periwinkles, limpets, isopods, hermit crabs, and other grazers fed on the lichen and algae.

High-Tide Zone

  • In the high tide zone, organisms are exposed to the atmosphere for long periods and must be able to withstand large temperature and salinity changes and resist dehydration.
  • Food is abundant in this zone because waves constantly supply nutrients, which support the growth of encrusting algae and attached macroalgae, and also supply suspended particles and plankton.
  • In many areas, the upper part of the high tide zone is dominated by small barnacles called buckshot barnacles.
  • Other species in the high tide zone are similar to those in the supralittoral zone except that chitons and mussels become more abundant toward the lower end of the high-tide zone. Mussels occupy a zone that is limited at its upper end by the length of time of exposure between tides.

Middle-Tide Zone

  • Macroalgae are less abundant in the middle tide zone because of competition between mussels and barnacles.
  • In the lower part of the middle tide zone sea stars, which cannot live higher up the shore, become more abundant.
  • The zone occupied by mussels is limited at the bottom due to predation by sea stars.
  • Many species have their upper limit determined by physical environmental conditions and their lower limit by competition and predation in a similar manner.

Low-Tide Zone

  • The low-tide zone sustains a variety of macroalgae and an animal community similar to that found in kelp ecosystems, including many urchins, sea stars, nudibranchs, shrimp, crabs, sea cucumbers, sponges, anemones, and fishes.
  • Many of the species in this zone attach themselves to the substrate or are active swimmers to withstand the turbulence associated with breaking waves.

Tide Pools

  • Depressions on a rocky shore remain filled with seawater to form tide pools when the tide recedes.
  • Each tide pool is unique and its inhabitants depend primarily on factors that control changes of temperature and salinity in the tide pool water between tides.
  • Shallow tide pools, tide pools high on the shore, and tide pools in warm, dry climates tend to experience the largest range of salinity and temperature and thus only support the most tolerant species such as microscopic algae and some highly tolerant copepods.
  • Deep tide pools and pools lower on the shore can support less tolerant species including many species of algae, sea urchins, sea stars, anemones, crabs, shrimp, and small fishes.

17.5 Sargasso Sea

  • The Sargasso Sea supports a unique community of floating macroalgae called Sargassum in which many species of invertebrates and fishes find food and shelter.
  • Sargassum is very slow growing and has a very long lifetime, allowing it to survive in the very nutrient-poor surface waters of the interior of this subtropical gyre. The circulation also tends to concentrate the algae inside the gyre, allowing it to aggregate into dense mats.

17.6 Polar Regions

  • The marine ecosystems of the north and south polar regions differ physically and biologically because of the different configuration of landmasses and because they are separated by warm tropical waters.
  • In both polar regions cooling and storm winds prevent the formation of a thermocline. However, a halocline prevents vertical mixing in much of the Arctic Ocean.
  • In most parts of the Southern Ocean and most marginal seas and certain coastal regions of the Arctic Ocean, the water column is permanently well mixed and intense primary production occurs during the short summer season when light is available. However, iron may be lacking and may limit productivity in much of the south polar region.

Special Characteristics of Arctic Marine Environments

  • Primary productivity is low in most of the Arctic Ocean because ice exclusion and runoff lower the surface salinity and a permanent halocline prevents vertical mixing. However, summer productivity is high in Arctic Ocean coastal regions where runoff supplies nutrients.

Common Characteristics of Arctic and Antarctic Marine Environments

  • In both polar regions, there are substantial populations of ice algae that survive the winter in liquid brine pockets in the ice or on its underside. These grow quickly when light penetrates the ice in spring and these algae contribute a large fraction of the primary production in the near ice edge region.
  • Many animals that feed in polar regions during the short, highly productive summer migrate to lower latitudes during the winter when food is unavailable in the polar oceans.
  • Many polar animals are long-lived, late maturing, and bear only a few young per year as an adaptation to overcome the inevitable years when the variable climate causes starvation due to changes in the primary production rate or in the timing of the diatom bloom.
  • Many polar regional species have high fat contents as the stored fat acts as an energy source during the long winters when food is unavailable. This makes these animals susceptible to bioaccumulation of certain toxic substances such as PCBs.

Antarctic Communities

  • The pelagic food web of the Southern Ocean includes many species of whales, seals, penguins, and fishes that live in the region year round and feed on krill and other zooplankton.
  • Major species of animals, especially those that live year round in the Arctic and Antarctic regions, are different from each other. For example, penguins are found in the south polar region but not in the north, and polar bears are found in the north polar region but not in the south. This is because these species would have to migrate through the much warmer tropics to colonize the opposite polar region.

Arctic Communities

  • North polar regions, like the south polar regions, have a large concentration of marine mammals.
  • Many Arctic region seal species remain in the region year round and haul onto land to breed, just as other seal species do in the Antarctic region. However, most species of whales that feed in the Arctic in summer migrate to tropical or subtropical regions to breed.

Susceptibility to Climate Change    

  • Global climate models predict that climate changes will be magnified in polar regions, especially the Arctic.
  • There is strong evidence that, at least in the Arctic region, average temperatures have risen in recent decades and the sea ice extent is steadily declining year to year; it is predicted the Arctic Ocean may have no permanent ice cap by 2040.
  • Contributing to the temperature rise and loss ice in the Arctic may be the positive feedback caused when ice is melted and replaced by much less reflective open water so that more solar energy is retained by the water, accelerating the warming process.

17.7 Beyond the Sun’s Light

  • Animals that live in the aphotic zone are adapted to a low and uncertain food supply primarily supplied by detritus falling from the photic zone above.
  • They are also adapted to finding food in the dark by means other than sight.
  • In many species eyes are absent and many species of fishes have jaws that can be unhinged so they can consume an occasional meal that is sometimes larger than themselves.

17.8 Hydrothermal Vents

  • Hydrothermal vents are locations where heated water is discharged through the seafloor after percolating through marine sediments or rocks. Such vents appear to be abundant and widespread but were discovered only in 1977. Extensive biological communities surround many hydrothermal vents, supported by chemosynthetic primary production.

Hydrothermal Vent Environments

  • Hydrothermal vents have been found at many locations dispersed along the oceanic ridges and ridge flanks and on submerged volcanoes including back-arc volcanoes. The temperature and chemistry of the effluent from these vents varies widely between different types of vent.

Biological Communities Associated with Hydrothermal Vents

  • Unique communities of bacteria and animals are found at many hydrothermal vents. These communities are all supported by food supplied by chemosynthesis by archaea and bacteria. Vent animal species are typically very large, perhaps because they must produce very large numbers of larvae so that a few may find a new vent to colonize.
  • Hydrothermal vent communities are sustained because a continuous inflow of heated water from the vent provides a constant supply of reduced substances (primarily sulfides at some vents and hydrogen and methane at others) that can be used to fuel chemosynthesis. Rapidly reproducing and growing chemosynthetic bacterial species form large amounts of suspended particulate matter as they grow at vent mouths and are sloughed off by the water currents. This material provides food for species of suspension feeders in the vent communities.
  • Some species, including a species of tubeworm that is abundant and widely occurring in black smoker vent communities, feed on symbiotic chemosynthetic bacteria that grow within the animal’s body.
  • Population biomass is very high close to many vents but falls off rapidly with distance from the vent as the supply of bacterially chemosynthesized particles is reduced by sedimentation and mixing.

Unanswered Questions about Hydrothermal Vents

  • At present, there is much that is not known about hydrothermal vent communities, including how they recolonize new vents, since such vents are dispersed irregularly. At least some vents have lifetimes of only a few decades.

Other Chemosynthetic Communities

  • In addition to hydrothermal vent communities, other types of chemosynthetic communities have now been found in a number of other areas of the oceans. These include communities in the anoxic sediments of marshes, bacterial mats found at the base of some continental slopes that utilize hydrogen sulfide where the hydrogen sulfide seeps out of the sediments, and communities that use hydrogen sulfide and/or hydrocarbons in areas of the seafloor where there are seeps of oil and gas.


Critical Concept Reminders:

CC.9 The Global Greenhouse Effect (p. 515)

  • Perhaps the greatest environmental challenge faced by humans is the prospect that major climate changes may be an inevitable result of our burning fossil fuels. The burning of fossil fuels releases carbon dioxide and other gases into the atmosphere where they accumulate and act like the glass of a greenhouse trapping more of the sun’s heat. To read CC9 go to page 24CC.

CC.14 Photosynthesis, Light, and Nutrients (p. 499)

  • Photosynthesis and chemosynthesis are two processes by which simple chemical compounds are made into the organic compounds of living organisms. Photosynthesis depends on the availability of carbon dioxide, light, and certain dissolved nutrient elements including nitrogen, phosphorus, and iron. Chemosynthesis does not use light energy, but instead depends on the availability of chemical energy from reduced compounds, which occur only in limited environments where oxygen is depleted. To read CC14 go to page 46CC.

CC.16 Maximum Sustainable Yield (p. 515)

  • The maximum sustainable yield is the maximum biomass of a fish species that can be depleted annually by fishing but that can still be replaced by reproduction. This yield changes unpredictably from year to year in response to the climate and other factors. The populations of many fish species worldwide have declined drastically when they have been overfished (beyond their maximum sustainable yield) in one or more years when that yield was lower than the average annual yield on which most fisheries management is based. To read CC16 go to page 51CC.

CC.17 Species Diversity and Biodiversity (p. 500)

  • Biodiversity is an expression of the range of genetic diversity; species diversity; diversity in ecological niches and types of communities of organisms (ecosystem diversity); and diversity of feeding, reproduction, and predator avoidance strategies (physiological diversity), within the ecosystem of the specified region. Species diversity is a more precisely-defined term and is a measure of the species richness (number of species) and species evenness (extent to which the community has balanced populations with no dominant species). High diversity and biodiversity are generally associated with ecosystems that are resistant to change. To read CC17 go to page 53CC.

CC.18 Toxicity (p. 515)

  • Many dissolved constituents of seawater become toxic to marine life when the concentrations go above their natural amount. Some synthetic organic chemicals are especially significant because they are persistent and may be bioaccumulated or biomagnified. To read CC18 go to page 54CC.


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