Introduction to Ocean Sciences

Chapter 14: Foundations of Life in the Oceans

Guide to Reading and Learning

In this chapter you will discover the critical role of microscopic organisms that perform photosynthesis and become the base of the food chain on which almost all ocean life depends. You will also learn about the physical and chemical parameters that control this primary production. You may already know that plants cannot grow without light. You may also know that the microscopic ocean equivalents of plants cannot photosynthesize except in the very thin upper layer near the surface because there is no light throughout most of the depths. However, did you know that, despite the availability of sufficient light, almost all of this surface layer of the oceans is the equivalent of a desert on land? Did you know that the teeming abundance of life that we often see in underwater videos is restricted almost entirely to a few very small areas of the oceans, mostly near land masses?

Why are most of the oceans the equivalent of deserts? The answer is that there is simply not enough of vital nutrients such as nitrogen, phosphorus, and iron in most surface layer water to allow photosynthetic organisms to do their job. The irony is that ample concentrations of all these nutrients are to be found just a few tens of meters away. All ocean water that lies below the depth to which significant amounts of light penetrate contains plentiful supplies of nutrients. However, we will learn that these nutrients are locked up below a permanent thermocline that inhibits vertical mixing in most of the oceans. Only in a few limited areas where upwelling occurs, where there is no permanent thermocline, or where rivers supply nutrients can marine photosynthesizers grow and support the incredibly diverse and locally abundant animal life in the oceans.

There are two fundamental approaches to the study of life. The more traditional approach is largely descriptive and taxonomy-based. Although taxonomy is an essential underpinning of marine biology, the second, ecology-based approach, provides a better introduction to ocean life. The ecology-based approach is taken in this text. The basic relationships between the physical and chemical environment of the oceans and life in the marine environment are discussed in this chapter. Chapter 15 extends this discussion to the special characteristics of the coastal and estuarine regions, and Chapter 16 describes how marine life is adapted to address the fundamental needs of all living organisms (i.e., habitat, food, survival, and reproduction). Chapter 17 then describes the relationships between the physical environment and life in a number of special marine environments. This integrated approach to marine biology is supplemented by descriptions of various types of organisms where appropriate within this block of chapters.

In this chapter, you will learn much about ocean life but, together with the fact that most ocean life is concentrated in only a very few limited areas, probably the most important discovery is that the food this limited marine life provides to humans is consumed at a cost to the environment that far transcends the cost of food from agriculture. You will learn that the food chains and food webs in the oceans are complex and often have many levels. We will learn that the efficiency of food transfer from one level of consumer to another is low and that the seafood species prized by humans are often very high in this food chain. You will learn that a consequence of this preference is that more than one thousand times more primary production must be performed in the oceans to provide us with a certain weight of species such as tuna that must occur on land to provide us with the same weight of beef

Chapter 14 Essential to Know 

Critical Concepts used in this chapter

CC.4, CC.8, CC.10, CC.14, CC.15

14.1 How Do We Describe Life?

  • All living organisms are arranged in a hierarchical taxonomy according to their species. Classification of species within the hierarchy is determined by differences in organism physiology, anatomy, and genetics.
  • At the highest level of the taxonomic hierarchy there are three domains: Archaea, Bacteria, and Eukarya. Within each of these domains are multiple lower levels of the hierarchy into which each species is classified. Species are named by using two words: the genus name (the level immediately above species in the hierarchy) and the species name.
  • Species within the domains Archaea and Bacteria are single celled organisms with no membrane-bound nucleus or internal structure; they are known as prokaryotes. Species within the domain Eukarya are single or multi-celled, have a membrane-bound nucleus, and are called eukaryotes.
  • The Eukarya include all of the more familiar non-microscopic life forms and are separated into four kingdoms: Protista, Fungi, Plantae, and Animalia.

14.2 Production, Consumption, and Decomposition

  • Organic matter in the oceans is synthesized from inorganic compounds by photosynthesis or, in restricted special environments, by chemosynthesis. This is called primary production.
  • Organisms are either autotrophs, which can produce all of their organic matter from inorganic compounds, or heterotrophs, which need to obtain organic matter from food.


  • Most primary production in the oceans takes place through photosynthesis, which converts carbon dioxide and water into organic matter and releases oxygen.
  • Photosynthesis requires carbon dioxide and water, both of which are readily available in the oceans; light energy, which is only available in the upper layers of the oceans; and certain nutrient elements that are only available in adequate concentrations in seawater in some locations.


  • In some limited environments bacteria and archaea can produce organic matter from carbon dioxide and water by chemosynthesis, which uses energy obtained not from light but instead from the oxidation of sulfide to sulfate, of metals from reduced to oxidized form, of hydrogen to water, or of methane to carbon dioxide and water.
  • Chemosynthesis is common near hydrothermal vents and methane or hydrogen sulfide seeps on the seafloor, and also occurs in some oxygen deficient environments including some salt marshes and fjords.

Secondary Production and Decomposers

  • In the marine ecosystem, primary producer species synthesize organic matter from inorganic compounds. This organic matter is consumed by heterotrophs. Decomposers convert the waste products or dead tissues of organisms back to inorganic compounds.
  • Heterotrophs use food inefficiently. Only about 10 percent of the food consumed by any heterotroph is converted to biomass of the heterotroph. The rest is used for respiration or lost as waste.

14.3 Primary Production and Light

  • Light is the most important requirement for photosynthesis-based primary production. Light only penetrates a few meters or tens of meters into the oceans. As a result, primary production is concentrated in the upper layer of the oceans and is dominated by microscopic floating plants called phytoplankton.

Photosynthesis, Light, and Depth

  • All living organisms, including primary producers, respire, converting organic matter to carbon dioxide to fuel their life processes.
  • The rate of respiration of photosynthetic primary producers does not vary significantly with depth. However, the rate of photosynthesis is reduced with increasing depth because it depends on the amount of available light energy.
  • At shallow depths where the rate of photosynthesis exceeds the rate of respiration primary producers can produce more organic matter than they consume. This is the photic zone.
  • The depth at which respiration and photosynthesis are equal is called the compensation depth. This depth is located where the light intensity is reduced to about 1% of that at the surface.
  • Below the compensation depth, respiration exceeds photosynthesis and primary producers consume more organic matter than they can produce. This is called the aphotic zone.
  • The compensation depth varies with sun angle and intensity and the turbidity of the water column. In the open oceans this depth is often around 100 m. In coastal waters it is variable but often about 5 to 10 m.

Primary Production and the Ozone Hole

  • Photosynthesis is inhibited by high levels of ultraviolet light. Thus, photosynthesis is often lower in the upper meter of ocean water than immediately below.
  • Photosynthesis may be significantly reduced by the higher ultraviolet light intensity in areas under the polar ozone holes.

14.4 Primary Production and Nutrients

  • Primary producers require nutrients to grow, including nitrogen compounds, phosphate, and iron.

Nutrient Uptake

  • Nutrients are taken up from seawater solution by diffusion across the cell membrane. Some species of primary producers, including diatoms, have siliceous hard parts and require silicon in addition to other nutrients.
  • The small size of phytoplankton maximizes their surface area to volume. As a result, small size is advantageous for retarding sinking (so the organism can remain in the photic zone) and for providing the maximum surface area through which to take up nutrients from the surrounding seawater.

Nutrient Recycling

  • If organisms sink below the thermocline after death or if their fecal pellets pass through the thermocline, this removes nutrients from the upper mixed layer of the oceans.


  • Phosphorus is released to solution quickly by enzymes after the death of an organism. Therefore, it is partially released to solution before being transported below the thermocline and is generally available in high enough concentration to support primary production.


  • Nitrogen in organic matter is released to solution by decomposers. This process is slower than the enzyme release of phosphate so a substantial amount of the nitrogen is transported below the thermocline before being released. This nitrogen below the thermocline is no longer available to support primary production. Nitrogen is often the limiting nutrient in areas where there is a strong pycnocline.


  • Silica is released by dissolution of siliceous hard parts after death of the organism, but this is a very slow process and silica is rapidly transported below the thermocline.
  • When silica is available in the surface layer and other nutrients and light are sufficient to support photosynthesis, diatoms usually dominate the phytoplankton. When silica is depleted dinoflagellates, which do not need silica, usually become dominant. Thus, dinoflagellates are dominant where there is a strong permanent thermocline and in mid latitudes during summer when there is a strong seasonal thermocline.


  • Iron is only very slowly released during decomposition of organisms, if at all. Thus, iron is supplied primarily through terrestrial runoff and atmospheric dust deposition on the oceans and is generally present in only very low concentrations upwelled from below the pycnocline.

Nutrient Transport and Supply

  • Nutrients are transported out of the photic zone by sinking of fecal pellets and other detritus and by vertical migration of organisms. Nutrients are supplied to the photic zone primarily by rivers, and by upwelling in the limited areas where upwelling occurs.
  • Upwelling of water from below the pycnocline is a significant source of recycled phosphorus and nitrogen from below the pycnocline in those areas where upwelling occurs.

Vertical Distribution of Nutrients

  • In the open mid-latitude ocean, nutrients are generally low in concentration in the mixed layer, increase in concentration to a maximum below the thermocline, then decrease somewhat before remaining almost constant with depth in the deeper deep layer. Oxygen concentrations are almost a mirror image, except that they increase slowly with depth throughout the deep layer and may reach a maximum in the mixed layer a few meters below the surface.
  • As water masses move through the oceans out of contact with the atmosphere, nutrients are progressively released and oxygen is progressively consumed by decomposers. Thus, the nutrient concentration is highest and oxygen concentration lowest in deep waters of the North Pacific, intermediate in the South Atlantic, and lowest (nutrients) and highest (oxygen) in the North Atlantic.

14.5 Food Webs

  • Phytoplankton are consumed by grazing zooplankton, which are consumed in turn by carnivorous zooplankton, which are consumed by small fish, and so on. These are food chains and each level is a trophic level. Because many organisms feed at more than one trophic level, feeding relationships in the oceans form complex food webs.
  • Only about 10% of the food ingested by organisms at each trophic level is converted to biomass. As a consequence, human consumption of high trophic level animals such as tuna uses ocean primary production hundreds or thousands of times less efficiently than consumption of low trophic level animals such as sardines.
  • As much as half of all ocean primary production may be performed by microscopically small photosynthesizers that are consumed and recycled through a microbial food loop that may be largely disconnected from the phytoplankton based food chains leading to the fishes and other large ocean animals.

14.6 Geographic Variation in Primary Production

  • Primary productivity is high in areas of upwelling and near rivers, because the nutrient supply is ample in these areas. High productivity areas are therefore concentrated along the western boundaries of the continents and in a latitudinal band across some of the equatorial oceans.
  • In high latitudes nutrients are constantly available but productivity is high only during the summer because sufficient light is available only during the summer months.

14.7 Dissolved Oxygen and Carbon Dioxide

  • Oxygen is exchanged freely between the oceans and atmosphere at the sea surface. As a result the oxygen concentration in the surface water layer is at, or close to, the saturation solubility.
  • Oxygen is released during photosynthesis, so the oxygen concentration is higher (supersaturated) below the immediate surface layer where the rate of photosynthesis is high.
  • Below the photic zone, where the rate of photosynthesis is reduced or zero, oxygen is consumed by respiration, the oxygen concentration is progressively decreased, and carbon dioxide is released.
  • Carbon dioxide is transported below the photic zone by sinking of detritus and vertical migration of organisms. About 50% of all anthropogenic carbon dioxide has been transported to the deep ocean waters by this route and proposals have been made to inject more atmospheric carbon dioxide to these deep waters to reduce its concentration or rate of concentration increase in the atmosphere.
  • In some limited areas the dissolved oxygen in the water column below the surface layer (usually below a steep pycnocline) is depleted and the water is anoxic.
  • In anoxic environments bacteria obtain energy by reducing nitrate to ammonia and, when all the nitrate has been converted to ammonia, by converting sulfate to highly toxic sulfide.

14.8 Organic Carbon

  • Much of the organic carbon in the oceans consists of dissolved organic compounds, with progressively much smaller amounts in detritus, phytoplankton, zooplankton, and fishes.
  • Extremely small microorganisms, including many species of bacteria, archaea, and viruses utilize dissolved organic matter. These microorganisms have been found to be extremely abundant in all ocean waters but it is unclear how much of the organic matter biomass created by these organisms enters food chains leading to macroscopic higher trophic level consumers.

14.9 Biological Provinces and Zones

  • The oceans may be separated into two distinctly different habitats: the benthic and pelagic environments. Each of these environments may be separated into zones that are arranged by depth. Each zone is somewhat distinct from the others with respect to its physical characteristics, including availability of light, water temperature, or sediment type.

Benthic Environment 

  • The nature of the sediment is an important determinant of the species in the benthic environment.
  • Benthic animals are infauna if they live buried in the sediments and epifauna if they live on top of the sediments.
  • The deepest zones, the hadal and abyssal, are generally covered in fine grained muds and have little variability in the types of benthic organisms.
  • The variability of sediment characteristics is greater in the three shallower zones: the bathyal, sublittoral and intertidal zones.
  • Other than in the intertidal zone and the sublittoral zone, which are restricted to the continental shelf depths, and the upper part of the bathyal zone in the very limited areas where turbidity is very low, the benthic environment is below the photic zone. As a result, most benthos are totally dependent for their food on transport of organic matter from above.

Pelagic Environment

  • The pelagic environment is divided into the neritic province and the oceanic province. The neritic province is the entire water column where the water depth is less than 200 m, or effectively all the water overlying the continental shelf. The oceanic province is the water column in the deep oceans.
  • Salinity temperature and suspended sediment concentrations are more variable in the neritic province than in the oceanic province.
  • The oceanic province is divided into the epipelagic zone, which is the water column to 200m depth, and includes all of the photic zone, and the bathypelagic, abyssopelagic and hadopelagic zones at successively greater depths.
  • The bathypelagic, abyssopelagic, and hadopelagic zones are zones of perpetual darkness, high pressure, and low temperature and all their inhabitants feed on either detritus or each other.

Latitudinal Zones

  • Most of the pelagic and benthic environment below the photic zone has relatively invariable temperature and salinity and relatively little change of biota with latitude.
  • In the benthic and pelagic environment that is within the upper layers of ocean water, temperature and salinity vary substantially regionally and with latitude.
  • Species are generally adapted to a narrow range of temperature and salinity, so latitudinal variations in these parameters lead to substantial latitudinal variability in both benthic and pelagic species of the upper biological zones of the oceans.
  • Temperature and salinity differences can act as a barrier to prevent species from moving between latitudes.. As a result, the species that live in the Arctic region are completely different from those that live in the Antarctic region.
  • Temperature and salinity differences can also act as a barrier to prevent species from moving between oceans at the same latitude if the continents. For example, tropical species of the Atlantic Ocean are different from those of the Pacific and Indian Oceans because species cannot move between these oceans without passing through colder water at high latitudes.

14.10 Plankton

  • The plankton at any location and time consists of many species of phytoplankton and zooplankton, one or more of which may be dominant.


  • Phytoplankton are photosynthetic primary producers. They are predominantly smaller 1 mm in diameter.


  • Diatoms, which have a siliceous frustule, are among the largest phytoplankton and are desirable as food for larger zooplankton and small herbivorous fishes.


  • Dinoflagellates are smaller than diatoms and weakly motile. Many have a readily decomposed cellulose cell wall but none have hard parts. They tend to dominate the phytoplankton when silica is unavailable for diatom growth.

Coccolithophores and Other Types of Phytoplankton

  • Normally less abundant than diatoms or dinoflagellates are the coccolithophores, which have a mosaic of calcareous plates covering the cell wall. They tend to be more abundant in tropical open ocean waters.

Bacteria and Archaea

  • Microorganisms, including bacteria, archaea, and viruses are extremely abundant throughout all ocean waters. However, they are very difficult to study because of their very small size, and relatively little is known about them. However, certain bacteria and archaea are known to be primary producers and to be responsible for possibly a major proportion of ocean primary production in many parts of the oceans, especially in the tropics and subtropics.


  • Many zooplankton species are larvae or juveniles of larger invertebrates or fishes.


  • Zooplankton are holoplankton if they live all their lives as plankton. Species include euphausiids such as krill, calcium carbonate shelled foraminifera and pteropods, and silica shelled radiolarians.

Gelatinous Holoplankton

  • Holoplankton also include gelatinous forms such as jellyfish, ctenophores, and salps.


  • Zooplankton that are eggs, larvae, or juveniles of species that are benthos or nekton as adults are meroplankton. They include numerous species of crustaceans and echinoderms (such as sea urchins and sea cucumbers).

14.11 Nekton

  • Nekton are organisms that live in the water column and are able to swim actively rather than being swept along by currents.

Bony Fishes

  • More than 95% of all fishes are bony fishes, which have skeletons largely of calcium carbonate and calcium phosphate.
  • Bony fishes range in size from only a few centimeters to lengths in excess of 3 m (such as the tuna).
  • Fish species that live on or near the sea floor are demersal fishes, whereas those that live predominantly in the water column are pelagic fishes.
  • Many fish species possess a gas filled swim bladder to control their buoyancy, while others that migrate vertically or live in very deep waters possess lighter-than-water oils to maintain their buoyancy.

Sharks and Rays

  • Sharks and rays have skeletons of cartilage and are more primitive than bony fishes.
  • Sharks and rays can be predators, scavengers, or grazers. The biggest shark species are plankton eaters and can grow to 15 m in length. Manta rays also eat only plankton.

Squid and Their Relatives

  • Squid are extremely abundant in the oceans. Most species live in schools. Most species are also are fast swimmers and voracious predators of small fishes.
  • Many squids live below the photic zone by day but migrate to the surface layers at night to feed.
  • The largest squid, the giant squid, can reach 16 m in length.
  • Squid relatives include nautilus and cuttlefish. Nautilus have an external carbonate shell, cuttlefish have an internal carbonate cuttle “bone,” and squid have no shell or bone. Squid evolved from nautilus through cuttlefish, and the progressive loss of the shell or skeleton represents a change that trades off the shell’s protection for speed.

Marine Mammals

  • Marine mammals include seals, sea lions, dolphins, and other whales. They are warm blooded, breathe air, and bear live young like other mammals.
  • The largest whales are called baleen whales. They feed exclusively on plankton strained out of the water by baleen plates. The largest species, the blue whale, can reach 30 m in length.
  • The sea otter is a unique marine mammal that has a soft, thick fur coat.
  • Many marine mammals have thick blubber (fat) layers to maintain their body heat.
  • Many marine mammals were hunted for their oil, meat, bone, and fur, some almost to extinction, but are now protected and most populations are slowly recovering.


  • There are only a few species of reptiles, which are air breathing animals, that live in the oceans. These include sea snakes, turtles, and a single species of marine iguana.


  • The only species of birds that live in the oceans are penguin species. Penguins are flightless, feed on fishes, and must leave the water to lay their eggs and brood their young. Many species of birds feed exclusively on ocean fishes, including some, such as seagulls, that feed at the surface and others, such as cormorants, that dive underwater to hunt.

14.12 Benthos         

  • Organisms living in or on the sea floor are benthos. There are a vast variety of benthic species, predominantly invertebrates.
  • Benthic organisms are generally adapted to a particular type of seafloor. For example, some can burrow through mud to hide or feed, whereas others need to attach to a rocky, stable surface.

Critical Concept Reminders:

CC.4 Particle Size, Sinking, Deposition, and Resuspension (pp. 376, 381, 388)

  • Suspended particles in ocean water, including plankton, sink at rates primarily determined by particle size: large particles or plankton sink faster than small particles. Larger plankton species have various adaptations to reduce their sinking rate. Small organic particles can be aggregated into fecal pellets that are larger than the individual particles and, thus, sink faster. To read CC4 go to page 12CC.

CC.8 Residence Time (p. 391)

  • The residence time of seawater in a given segment of the oceans is the average length of time the water spends in the segment. In the deep ocean trenches water column water mass residence time can be long and this affects the character of the biota that inhabits this zone. To read CC8 go to page 19CC.

CC.10 Modeling (pp. 379, 380)

  • Complex environmental systems including the cycling of nutrients among ocean water, living matter, suspended particles and sediments can best be studied by using conceptual and mathematical models. Many oceanographic and climate models are extremely complex and require the use of the fastest supercomputers. To read CC10 go to page 26CC.

CC.14 Photosynthesis, Light, and Nutrients (pp. 373, 374, 386)

  • Photosynthesis and chemosynthesis are the 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 and 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.15 Food Chain Efficiency (pp. 374, 383, 389)

  • All organisms use some of their food as an energy source in respiration and for reproduction, and also lose some in excretions including wastes. On average, at each level in a food chain, only about 10% of food consumed is converted to growth and biomass of the consumer species. To read CC15 go to page 49CC.


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