12.15: Chapter Summary

Production, Consumption, Decomposition

Almost all organic matter on which marine life depends for food is synthesized from carbon dioxide and nutrients by phototrophy or chemosynthesis—a process called “primary production.” Organisms are autotrophs if they synthesize their own food, and heterotrophs if they do not. Autotrophs and heterotrophs convert some organic matter to carbon dioxide and water in aerobic respiration. Heterotrophs can be herbivores, carnivores, omnivores, detritivores, or decomposers.

Microbes in Charge.

The majority of the biomass in the oceans is now known to be microbial—consisting primarily of bacteria and archaea. Microbial species are responsible for much of the primary production and most of the decomposition of organic matter in the oceans and dominate the movement of energy and biologically important elements in the oceans. It is important to understand how these microbes interact with one another and with the environment around them. Metagenomic studies provide information on what microbes are there and what their metabolic capabilities are. 

Primary Production and Light.

Light is needed to provide energy for phototrophy (which includes photosynthesis). Light is absorbed by seawater and scattered by suspended particles. It penetrates to a depth of a few hundred meters in clear waters and much less in high-turbidity waters. The photic zone is the upper layer of the oceans where light is sufficient to support phototrophy. Most primary production is carried out by phytoplankton, which are floating microscopic algae and cyanobacteria. Benthic phototrophs are present only where the seafloor is within the photic zone. As light decreases with depth, phototrophy is slowed but respiration is not. The depth at which photosynthesis equals respiration is the compensation depth. This depth is shallower in winter and in high-turbidity water.

Primary Production and Nutrients.

Phytoplankton require nitrogen, phosphorus, iron, and other nutrients. If the concentration of a particular nutrient is too low to support growth, it is a limiting nutrient. Nutrients are taken up through the cell membrane. The small size of phytoplankton provides a large surface area (per volume) for this uptake and to slow sinking. Phosphorus is recycled rapidly to solution in liquid excretions of animals and during the decay of dead organisms. Nitrogen is recycled more slowly through several chemical forms and is more often the limiting nutrient in the ocean. In some areas where river runoff and atmospheric dustfall are both low, iron can become the limiting nutrient. Silica is an important nutrient for diatoms and other plankton that have silica-based hard parts and is recycled very slowly. Nutrients are carried below the mixed layer by the sinking of dead organisms and fecal pellets. Nutrients are provided to the photic zone primarily by rivers, recycling, and upwelling. They are relatively abundant in waters below the permanent thermocline.

Food Webs.

Phytoplankton are eaten by herbivores or omnivores, which are eaten by small carnivores, which in turn are eaten by larger carnivores. Each organism in this food chain is at a higher trophic level. The conversion efficiency of food to biomass is about 10% between each trophic level. Many carnivores and omnivores feed on organisms from more than one trophic level, forming a more complex food web.

Geographic Variation in Primary Production.

The highest primary productivity is in upwelling regions, most of which are in coastal waters along the western margins of continents. Upwelling also occurs around Antarctica and across parts of the equatorial regions. Primary productivity is lowest in the centers of the subtropical gyres.

Dissolved Oxygen and Carbon Dioxide.

Dissolved oxygen is released during photosynthesis and consumed during respiration. Gases are exchanged between ocean and atmosphere across the sea surface. Oxygen and carbon dioxide are saturated in surface waters. Oxygen can be supersaturated in shallow parts of the photic zone where photosynthesis exceeds respiration. Below the thermocline, oxygen is consumed in respiration and decomposition, and its concentration is below saturation. In some areas where the photic zone has high primary productivity and bottom waters have long residence time, bottom water is anoxic and may contain toxic hydrogen sulfide.

Organic Carbon.

Organic carbon is present in dissolved and suspended particulate forms and in phytoplankton and animal tissues. More than 95% is nonliving dissolved organic matter, and most of the rest is particulate detritus. Most of the dissolved and particulate organic matter is difficult to decompose and has limited nutritional value. Organic particles are more abundant in the photic zone and in areas of high primary productivity.

Biological Provinces and Zones.

The benthic and pelagic environments support fundamentally different biological communities. The benthic environment is divided into the hadal zone, which is the deep seafloor below 6000 m, the abyssal zone between 6000 and 2000 m, the bathyal zone between 2000 m and the continental shelf break, the sublittoral zone (water-covered continental shelf), and the intertidal zone. Benthic organisms in the hadal, abyssal, and most of the bathyal and sublittoral zones depend on detritus for food, with the exception of isolated chemosynthetic communities. The pelagic environment is separated into the neritic province in water less than 200 m deep, and the oceanic province in water deeper than 200 m. The oceanic province consists of the epipelagic zone, in which phototrophy (including photosynthesis) can occur; and the mesopelagic, bathypelagic, abyssopelagic, and hadopelagic zones, which are all below the photic zone. Communities of each of the benthic and pelagic zones are different, but there is some overlap.

In the sublittoral zone, the neritic province, and the epipelagic zone, latitudinal variations of temperature and salinity cause latitudinal zonation in biological communities. For example, tropical and polar regions support different species.

Plankton.

Plankton are organisms and viruses that drift freely with currents and include phytoplankton (which are microscopic photosynthetic algae and cyanobacteria) and zooplankton (which are herbivorous, carnivorous, or omnivorous animals). Phytoplankton are abundant in productive photic-zone waters, exceeding 1 billion individuals per liter, and have patchy distributions. Diatoms are relatively large phytoplankton with silica frustules. They are important in food chains that lead to commercially valuable species. Dinoflagellates are smaller than diatoms, and most lack hard parts. They dominate where silica is depleted, particularly in the open oceans, are not always autotrophic, and often bloom explosively. Generally less abundant types of phytoplankton include coccolithophores that are covered in tiny calcareous plates, and silicoflagellates that have a silica shell. Microscopically small ultraplankton, which include bacteria, archaea, and viruses, are extremely abundant but are not yet well characterized.

Zooplankton are either holoplankton that live their entire life cycles as plankton, or meroplankton that spend only their larval stages as zooplankton. Many zooplankton migrate to the surface layer at night to feed and return below the photic zone by day. Holoplankton include copepods, euphausiids, and other crustaceans, foraminifera, radiolaria, and pteropods, as well as gelatinous forms such as jellies, ctenophores, and salps. Meroplankton include eggs, larvae, and juveniles of many invertebrates and fishes.

Nekton.

Nekton are organisms that swim actively. They include many fish species. Sharks and rays are fishes that have cartilaginous skeletons. Many fishes have gas- or oil-filled swim bladders to maintain buoyancy. The nekton also include squid, which are extremely abundant, feed mostly near the surface at night, and migrate below the photic zone by day. Marine mammals, including dolphins, porpoises, other whales, seals, and sea lions, are also nekton. Sea turtles and sea snakes are nektonic reptiles.

Benthos.

Benthic organisms comprise a profusion of species adapted to live on or in the sediment or on hard substrates such as rocks and coral reefs.