What will most likely happen if the plankton population decreases in this ocean system

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What would most likely happen if the number of shrimp increased?
Would most likely happen if the number of shrimp decreased?
If an invasive species that competes with foxes enters the community shown here what is most likely to happen?
What would most likely happen in the ecosystem if the zooplankton population significantly decreased?
What would most likely decrease the genetic variation in the human population?
What is Ocean Productivity?
What Does Ocean Productivity Need?
Who Are the Major Players in Ocean Productivity?

What would most likely happen if the number of shrimp increased?

the plankton population would decrease

Would most likely happen if the number of shrimp decreased?

The plankton population would increase

If an invasive species that competes with foxes enters the community shown here what is most likely to happen?

A decline in the size of native fox population

What would most likely happen in the ecosystem if the zooplankton population significantly decreased?

All parts in the levels above this layer will become fewer.

What would most likely decrease the genetic variation in the human population?

Which would most likely decrease the genetic variation in the human population?

The Elements of Life

In biology, the elements of life are the essential building blocks that make up living things. They are carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur. The first four of these are the most important, as they are used to construct the molecules that are necessary to make up living cells. These elements form the basic building blocks of the major macromolecules of life, including carbohydrates, lipids, nucleic acids and proteins. Carbon is an important element for all living organisms, as it is used to construct the basic building blocks of life, such as carbohydrates, lipids, and nucleic acids. Even the cell membranes are made of proteins. Carbon is also used to construct the energy-rich molecules adenosine triphosphate (ATP) and guanosine triphosphate (GTP). Hydrogen is used to construct the molecules water and organic compounds with carbon. Hydrogen is also used to construct ATP and GTP. Nitrogen is used to construct the basic building blocks of life, such as amino acids, nucleic acids, and proteins. It is also used to construct ATP and GTP. Oxygen is used to construct the basic building blocks of life, such as carbohydrates, lipids, and nucleic acids. It is also used to construct ATP and GTP. Phosphorus is used to construct the basic building blocks of life, such as carbohydrates, lipids, and nucleic acids.

The Elements of Life

What is Ocean Productivity?

Ocean productivity largely refers to the production of organic matter by "phytoplankton," plants suspended in the ocean, most of which are single-celled. Phytoplankton are "photoautotrophs," harvesting light to convert inorganic to organic carbon, and they supply this organic carbon to diverse "heterotrophs," organisms that obtain their energy solely from the respiration of organic matter. Open ocean heterotrophs include bacteria as well as more complex single- and multi-celled "zooplankton" (floating animals), "nekton" (swimming organisms, including fish and marine mammals), and the "benthos" (the seafloor community of organisms).

The many nested cycles of carbon associated with ocean productivity are revealed by the following definitions (Bender et al. 1987) (Figure 1). "Gross primary production" (GPP) refers to the total rate of organic carbon production by autotrophs, while "respiration" refers to the energy-yielding oxidation of organic carbon back to carbon dioxide. "Net primary production" (NPP) is GPP minus the autotrophs' own rate of respiration; it is thus the rate at which the full metabolism of phytoplankton produces biomass. "Secondary production" (SP) typically refers to the growth rate of heterotrophic biomass. Only a small fraction of the organic matter ingested by heterotrophic organisms is used to grow, the majority being respired back to dissolved inorganic carbon and nutrients that can be reused by autotrophs. Therefore, SP in the ocean is small in comparison to NPP. Fisheries rely on SP; thus they depend on both NPP and the efficiency with which organic matter is transferred up the foodweb (i.e., the SP/NPP ratio). "Net ecosystem production" (NEP) is GPP minus the respiration by all organisms in the ecosystem. The value of NEP depends on the boundaries defined for the ecosystem. If one considers the sunlit surface ocean down to the 1% light level (the "euphotic zone") over the course of an entire year, then NEP is equivalent to the particulate organic carbon sinking into the dark ocean interior plus the dissolved organic carbon being circulated out of the euphotic zone. In this case, NEP is also often referred to as "export production" (or "new production" (Dugdale & Goering 1967), as discussed below). In contrast, the NEP for the entire ocean, including its shallow sediments, is roughly equivalent to the slow burial of organic matter in the sediments minus the rate of organic matter entering from the continents.

Figure 1

Productivity in the surface ocean, the definitions used to describe it, and its connections to nutrient cycling. The blue cycle for “net ecosystem production” (NEP) (i.e. “new” or “export” production) encompasses the “new” nutrient supply from the ocean interior, its uptake by autotrophic phytoplankton growth, packaging into large particles by heterotrophic grazing organisms, and sinking of organic matter out of the surface ocean. The red cycle illustrates the fate of the majority of organic matter produced in the surface ocean, which is to be respired by heterotrophic organisms to meet their energy requirements, thereby releasing the nutrients back into the surface water where they can be taken up by phytoplankton once again to fuel “regenerated production.” The green cycle represents the internal respiration of phytoplankton themselves, that is, their own use of the products of photosynthesis for purposes other than growth. These nested cycles combine to yield (1) “gross primary production” (GPP) representing the gross photosynthesis and (2) “net primary production” (NPP) that represents phytoplankton biomass production that forms the basis of the food web plus a much smaller rate of organic matter export from the surface. While the new nutrient supply and export production are ultimately linked by mass balance, there may be imbalances on small scales of space and time, allowing for brief accumulations of biomass.

There are no accumulations of living biomass in the marine environment that compare with the forests and grasslands on land (Sarmiento & Bender 1994). Nevertheless, ocean biology is responsible for the storage of more carbon away from the atmosphere than is the terrestrial biosphere (Broecker 1982). This is achieved by the sinking of organic matter out of the surface ocean and into the ocean interior before it is returned to dissolved inorganic carbon and dissolved nutrients by bacterial decomposition. Oceanographers often refer to this process as the "biological pump," as it pumps carbon dioxide (CO2) out of the surface ocean and atmosphere and into the voluminous deep ocean (Volk & Hoffert 1985).

Only a fraction of the organic matter produced in the surface ocean has the fate of being exported to the deep ocean. Of the organic matter produced by phytoplankton (NPP), most is respired back to dissolved inorganic forms within the surface ocean and thus recycled for use by phytoplankton (Eppley & Peterson 1979) (Figure 1). Most phytoplankton cells are too small to sink individually, so sinking occurs only once they aggregate into larger particles or are packaged into "fecal pellets" by zooplankton. The remains of zooplankton are also adequately large to sink. While sinking is a relatively rare fate for any given particle in the surface ocean, biomass and organic matter do not accumulate in the surface ocean, so export of organic matter by sinking is the ultimate fate for all of the nutrients that enter into the surface ocean in dissolved form — with the exceptions that (1) dissolved nutrients can be returned unused to the interior by the circulation in some polar regions (see below), and (2) circulation also carries dissolved organic matter from the surface ocean into the interior, a significant process (Hansell et al. 2009) that we will not address further. As organic matter settles through the ocean interior and onto the seafloor, it is nearly entirely decomposed back to dissolved chemicals (Emerson & Hedges 2003, Martin et al. 1987). This high efficiency of decomposition is due to the fact that the organisms carrying out the decomposition rely upon it as their sole source of chemical energy; in most of the open ocean, the heterotrophs only leave behind the organic matter that is too chemically resistant for it to be worth the investment to decompose. On the whole, only a tiny fraction (typically much less than 1%) of the organic carbon from NPP in the euphotic zone survives to be buried in deep sea sediments.

Productivity in coastal ecosystems is often distinct from that of the open ocean. Along the coasts, the seafloor is shallow, and sunlight can sometimes penetrate all the way through the water column to the bottom, thus enabling bottom-dwelling ("benthic") organisms to photosynthesize. Furthermore, sinking organic matter isintercepted by the seabed, where it supports thriving benthic faunal communities, in the process being recycled back to dissolved nutrients that are then immediately available for primary production. The proximity to land and its nutrient sources, the interception of sinking organic matter by the shallow seafloor, and the propensity for coastal upwelling all result in highly productive ecosystems. Here, we mainly address the productivity of the vast open ocean; nevertheless, many of the same concepts, albeit in modified form, apply to coastal systems.

What Does Ocean Productivity Need?

Phytoplankton require a suite of chemicals, and those with the potential to be scarce in surface waters are typically identified as "nutrients." Calcium is an example of an element that is rapidly assimilated by some plankton (for production of calcium carbonate "hard parts") but is not typically considered a nutrient because of its uniformly high concentration in seawater. Dissolved inorganic carbon, which is the feedstock for organic carbon production by photosynthesis, is also abundant and so is not typically listed among the nutrients. However, its acidic form dissolved CO2 is often at adequately low concentrations to affect the growth of at least some phytoplankton.

Broadly important nutrients include nitrogen (N), phosphorus (P), iron (Fe), and silicon (Si). There appear to be relatively uniform requirements for N and P among phytoplankton. In the early 1900s, oceanographer Alfred Redfield found that plankton build their biomass with C:N:P stoichiometric ratios of ~106:16:1, to which we now refer as the Redfield ratios (Redfield 1958). As Redfield noted, the dissolved N:P in the deep ocean is close to the 16:1 ratio of plankton biomass, and we will argue below that plankton impose this ratio on the deep, not vice versa. Iron is found in biomass only in trace amounts, but it is used for diverse essential purposes in organisms, and it has become clear over the last 25 years that iron's scarcity often limits or affects productivity in the open ocean, especially those regions where high-N and -P deep water is brought rapidly to the surface (Martin & Fitzwater 1988). Research is ongoing to understand the role of other trace elements in productivity (Morel et al. 2003). Silicon is a nutrient only for specific plankton taxa-diatoms (autotrophic phytoplankton), silicoflaggellates, and radiolaria (heterotrophic zooplankton) — which use it to make opal hard parts. However, the typical dominance of diatoms in Si-bearing waters, and the tendency of diatom-associated organic matter to sink out of the surface ocean, make Si availability a major factor in the broader ecology and biogeochemistry of surface waters.

Sunlight is the ultimate energy source — directly or indirectly — for almost all life on Earth, including in the deep ocean. However, light is absorbed and scattered such that very little of it penetrates below a depth of ~80 m (as deep as 150 m in the least productive subtropical regions, but as shallow as 10 m in highly productive and coastal regions) (Figure 2). Thus, photosynthesis is largely restricted to the upper light-penetrated skin of the ocean. Moreover, across most of the ocean's area, including the tropics, subtropics, and the temperate zone, the absorption of sunlight causes surface water to be much warmer than the underlying deep ocean, the latter being filled with water that sank from the surface in the high latitudes . Warm water is more buoyant than cold, which causes the upper sunlit layer to float on the denser deep ocean, with the transition between the two known as the "pycnocline" (for "density gradient") or "thermocline" (the vertical temperature gradient that drives density stratification across most of the ocean, Figure 2). Wind or another source of energy is required to drive mixing across the pycnocline, and so the transport of water with its dissolved chemicals between the sunlit surface and the dark interior is sluggish. This dual effect of light on photosynthesis and seawater buoyancy is critical for the success of ocean phytoplankton. If the ocean did not have a thin buoyant surface layer, mixing would carry algae out of the light and thus away from their energy source for most of the time. Instead of nearly neutrally buoyant single celled algae, larger, positively buoyant photosynthetic organisms (e.g., pelagic seaweeds) might dominate the open ocean. This hypothetical case aside, although viable phytoplankton cells are found (albeit at low concentrations) in deeper waters, photosynthesis limits active phytoplankton growth to the upper skin of the ocean, while upper ocean density stratification prevents them from being mixed down into the dark abyss. Thus, most open ocean biomass, including phytoplankton, zooplankton, and nekton, is found within ~200 m of the ocean surface.

Figure 2

Typical conditions in the subtropical ocean, as indicated by data collected at the Bermuda Atlantic Time-series Station in July, 2008. The thermocline (vertical temperature gradient) stratifies the upper water column. During this particular station occupation, the shallow wind-mixed surface layer is not well defined, presumably because of strong insolation and a lack of wind that allowed continuous stratification all the way to the surface. Very little sunlight penetrates deeper than ~100 m. New supply of the major nutrients N and P is limited by the slow mixing across the upper thermocline (showing here only the N nutrient nitrate, NO3-). Within the upper euphotic zone, the slow nutrient supply is completely consumed by phytoplankton in their growth. This growth leads to the accumulation of particulate organic carbon in the surface ocean, some of which is respired by bacteria, zooplankton, and other heterotrophs, and some of which is exported as sinking material. The deep chlorophyll maximum (DCM) occurs at the contact where there is adequate light for photosynthesis and yet significant nutrient supply from below. The DCM should not be strictly interpreted as a depth maximum in phytoplankton biomass, as the phytoplankton at the DCM have a particularly high internal chlorophyll concentration. The data shown here is made available the Bermuda Institute of Ocean Sciences (http://bats.bios.edu) and the Bermuda Bio Optics Project (http://www.icess.ucsb.edu/bbop/).

At the same time, the existence of a thin buoyant surface layer conspires with other processes to impose nutrient limitation on ocean productivity. The export of organic matter to depth depletes the surface ocean of nutrients, causing the nutrients to accumulate in deep waters where there is no light available for photosynthesis (Figure 2). Because of the density difference between surface water and the deep sea across most of the ocean, ocean circulation can only very slowly reintroduce dissolved nutrients to the euphotic zone. By driving nutrients out of the sunlit, buoyant surface waters, ocean productivity effectively limits itself.

Phytoplankton growth limitation has traditionally been interpreted in the context of Liebig's Law of the Minimum, which states that plant growth will be as great as allowed by the least available resource, the "limiting nutrient" that sets the productivity of the system (de Baar 1994). While this view is powerful, interactions among nutrients and between nutrients and light can also control productivity. A simple but important example of this potential for "co-limitation" comes from polar regions, where oblique solar insolation combines with deep mixing of surface waters to yield low light availability. In such environments, higher iron supply can increase the efficiency with which phytoplankton capture light energy (Maldonado et al. 1999, Sunda & Huntsman 1997). More broadly, it has been argued that phytoplankton should generally seek a state of co-limitation by all the chemicals they require, including the many trace metal nutrients (Morel 2008).