What Are The Two Primary Factors That Control Biologic Productivity In The Surface Oceans?

What is Bounding main Productivity?

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

The many nested cycles of carbon associated with ocean productivity are revealed by the following definitions (Bender et al. 1987) (Figure i). "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 charge per unit of heterotrophic biomass. Only a small fraction of the organic matter ingested by heterotrophic organisms is used to abound, the majority existence respired back to dissolved inorganic carbon and nutrients that tin exist reused by autotrophs. Therefore, SP in the body of water is small in comparison to NPP. Fisheries rely on SP; thus they depend on both NPP and the efficiency with which organic thing 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 divers for the ecosystem. If i considers the sunlit surface sea down to the 1% light level (the "euphotic zone") over the form of an unabridged year, then NEP is equivalent to the particulate organic carbon sinking into the nighttime bounding main interior plus the dissolved organic carbon existence circulated out of the euphotic zone. In this case, NEP is besides oft referred to as "export product" (or "new production" (Dugdale & Goering 1967), equally discussed below). In dissimilarity, 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 affair entering from the continents.

Productivity in the surface ocean, the definitions used to describe it, and its connections to nutrient cycling.

Figure one

Productivity in the surface body of water, the definitions used to depict it, and its connections to nutrient cycling. The blue cycle for "net ecosystem production" (NEP) (i.e. "new" or "consign" production) encompasses the "new" nutrient supply from the ocean interior, its uptake past autotrophic phytoplankton growth, packaging into large particles by heterotrophic grazing organisms, and sinking of organic matter out of the surface sea. 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 h2o where they can be taken upward past phytoplankton again to fuel "regenerated product." The green wheel represents the internal respiration of phytoplankton themselves, that is, their ain use of the products of photosynthesis for purposes other than growth. These nested cycles combine to yield (one) "gross primary production" (GPP) representing the gross photosynthesis and (2) "net master 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 past mass balance, in that location may exist imbalances on small scales of space and fourth dimension, 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 biological science is responsible for the storage of more than carbon away from the temper than is the terrestrial biosphere (Broecker 1982). This is achieved past the sinking of organic matter out of the surface ocean and into the ocean interior before information technology is returned to dissolved inorganic carbon and dissolved nutrients past bacterial decomposition. Oceanographers ofttimes refer to this process as the "biological pump," as it pumps carbon dioxide (CO₂) out of the surface bounding main and temper and into the voluminous deep ocean (Volk & Hoffert 1985).

Only a fraction of the organic matter produced in the surface body of water has the fate of existence exported to the deep ocean. Of the organic affair produced by phytoplankton (NPP), most is respired back to dissolved inorganic forms within the surface ocean and thus recycled for employ by phytoplankton (Eppley & Peterson 1979) (Figure 1). Virtually phytoplankton cells are as well small-scale to sink individually, so sinking occurs only one time 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 whatsoever given particle in the surface body of water, biomass and organic matter do non accumulate in the surface sea, and then consign of organic affair 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 exist 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 non address further. As organic matter settles through the body of water interior and onto the seafloor, information technology is near 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 information technology as their sole source of chemical energy; in nearly of the open up 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 littoral ecosystems is often distinct from that of the open ocean. Along the coasts, the seafloor is shallow, and sunlight can sometimes penetrate all the mode through the water cavalcade to the bottom, thus enabling bottom-dwelling house ("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 and so immediately bachelor for main production. The proximity to country and its nutrient sources, the interception of sinking organic matter by the shallow seafloor, and the propensity for littoral upwelling all outcome in highly productive ecosystems. Here, we mainly accost the productivity of the vast open ocean; still, many of the same concepts, albeit in modified form, employ to littoral systems.

What Does Sea Productivity Need?

Phytoplankton require a suite of chemicals, and those with the potential to exist scarce in surface waters are typically identified as "nutrients." Calcium is an example of an element that is chop-chop assimilated past some plankton (for production of calcium carbonate "difficult parts") but is non typically considered a nutrient because of its uniformly high concentration in seawater. Dissolved inorganic carbon, which is the feedstock for organic carbon product by photosynthesis, is also abundant and so is not typically listed among the nutrients. All the same, its acidic course dissolved CO₂ is ofttimes at fairly low concentrations to affect the growth of at least some phytoplankton.

Broadly important nutrients include nitrogen (North), phosphorus (P), fe (Atomic number 26), and silicon (Si). There appear to be relatively uniform requirements for North and P among phytoplankton. In the early on 1900s, oceanographer Alfred Redfield constitute that plankton build their biomass with C:N:P stoichiometric ratios of ~106:xvi:1, to which we now refer every bit the Redfield ratios (Redfield 1958). As Redfield noted, the dissolved Northward:P in the deep ocean is close to the 16:1 ratio of plankton biomass, and we will argue beneath that plankton impose this ratio on the deep, not vice versa. Iron is plant in biomass only in trace amounts, but it is used for various essential purposes in organisms, and it has get clear over the terminal 25 years that iron's scarcity often limits or affects productivity in the open sea, especially those regions where loftier-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 brand opal hard parts. However, the typical dominance of diatoms in Si-begetting waters, and the tendency of diatom-associated organic matter to sink out of the surface ocean, make Si availability a major gene in the broader environmental and biogeochemistry of surface waters.

Sunlight is the ultimate energy source — direct or indirectly — for about all life on Earth, including in the deep ocean. However, light is absorbed and scattered such that very niggling of information technology penetrates below a depth of ~80 m (as deep as 150 m in the least productive subtropical regions, but equally shallow as ten k in highly productive and coastal regions) (Figure ii). Thus, photosynthesis is largely restricted to the upper lite-penetrated skin of the ocean. Moreover, beyond nearly of the ocean's expanse, 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 2 known equally the "pycnocline" (for "density slope") 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 beyond 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 low-cal on photosynthesis and seawater buoyancy is disquisitional for the success of ocean phytoplankton. If the ocean did not accept a sparse buoyant surface layer, mixing would deport algae out of the light and thus away from their energy source for most of the fourth dimension. Instead of nearly neutrally buoyant single celled algae, larger, positively buoyant photosynthetic organisms (due east.1000., pelagic seaweeds) might dominate the open bounding main. This hypothetical case aside, although viable phytoplankton cells are constitute (albeit at low concentrations) in deeper waters, photosynthesis limits agile phytoplankton growth to the upper skin of the body of water, while upper ocean density stratification prevents them from being mixed down into the dark abyss. Thus, most open sea biomass, including phytoplankton, zooplankton, and nekton, is plant within ~200 g of the ocean surface.

Typical conditions in the subtropical ocean, as indicated by data collected at the Bermuda Atlantic Time-series Station in July, 2008.

Effigy two

Typical conditions in the subtropical ocean, as indicated by data collected at the Bermuda Atlantic Time-serial Station in July, 2008. The thermocline (vertical temperature slope) stratifies the upper water cavalcade. 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 style to the surface. Very piffling sunlight penetrates deeper than ~100 thousand. New supply of the major nutrients N and P is express by the slow mixing across the upper thermocline (showing here only the North nutrient nitrate, NO₃ ^-). Inside the upper euphotic zone, the boring food supply is completely consumed past 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 low-cal for photosynthesis and however 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 information shown here is fabricated available the Bermuda Plant of Sea Sciences (http://bats.bios.edu) and the Bermuda Bio Optics Project (http://www.icess.ucsb.edu/bbop/).

At the aforementioned time, the existence of a thin buoyant surface layer conspires with other processes to impose food limitation on body of water productivity. The consign 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 (Effigy 2). Considering of the density difference betwixt surface water and the deep body of water across about of the bounding main, bounding main 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'south Law of the Minimum, which states that constitute growth will be as neat as allowed past the to the lowest degree available resources, the "limiting nutrient" that sets the productivity of the organisation (de Baar 1994). While this view is powerful, interactions amid nutrients and between nutrients and light tin as well control productivity. A uncomplicated only of import case of this potential for "co-limitation" comes from polar regions, where oblique solar insolation combines with deep mixing of surface waters to yield depression light availability. In such environments, college iron supply can increase the efficiency with which phytoplankton capture light free 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 metallic nutrients (Morel 2008).

Who Are the Major Players in Bounding main Productivity?