Editing Isotope analysis (section)

=== Ecology ===
All biologically active elements exist in a number of different isotopic forms, of which two or more are stable. For example, most carbon is present as <sup>12</sup>C, with approximately 1% being <sup>13</sup>C. The ratio of the two isotopes may be altered by biological and geophysical processes, and these differences can be utilized in a number of ways by ecologists. 
The main elements used in isotope [[ecology]] are carbon, nitrogen, oxygen, hydrogen and sulfur, but also include silicon, iron, and strontium.<ref>{{cite book |editor1-last=Michener |editor1-first=Robert |editor2-last=Lajtha |editor2-first=Kate |editor-link2=Kate Lajtha|title=Stable isotopes in ecology and environmental science |url=https://archive.org/details/stableisotopesec00mich |url-access=limited |publisher=Blackwell Pub |isbn=978-1-4051-2680-9 |pages=[https://archive.org/details/stableisotopesec00mich/page/n31 4]–5 |edition=2nd|date=2007-10-08 }}</ref>

====Stable isotope analysis in aquatic ecosystems====

[[Stable isotopes]] have become a popular method for understanding [[aquatic ecosystem]]s because they can help scientists in understanding source links and process information in marine food webs. These analyses can also be used to a certain degree in terrestrial systems. Certain isotopes can signify distinct primary producers forming the bases of [[food web]]s and [[trophic level]] positioning. The stable isotope compositions are expressed in terms of delta values (δ) in [[permil]] (‰), i.e. parts per thousand differences from a [[Reference Materials for Stable Isotope Analysis|standard]]. They express the proportion of an isotope that is in a sample. The values are expressed as:

: ''δX'' = [(''R''<sub>sample</sub> / ''R''<sub>standard</sub>) – 1] × 10<sup>3</sup>

where X represents the isotope of interest (e.g., [[carbon-13|<sup>13</sup>C]]) and R represents the ratio of the isotope of interest and its natural form (e.g., <sup>13</sup>C/<sup>12</sup>C).<ref name="Fry1987">{{cite journal |doi=10.1146/annurev.es.18.110187.001453 |title=Stable Isotopes in Ecosystem Studies |journal=Annual Review of Ecology and Systematics |volume=18 |pages=293–320 |year=1987 |last1=Peterson |first1=B J |last2=Fry |first2=B |issue=1 |bibcode=1987AnRES..18..293P |s2cid=21559668 }}</ref> Higher (or less negative) delta values indicate increases in a sample's isotope of interest, relative to the [[Reference Materials for Stable Isotope Analysis|standard]], and lower (or more negative) values indicate decreases. The standard reference materials for carbon, nitrogen, and sulfur are [[Δ13C|Pee Dee Belamnite]] limestone, nitrogen gas in the atmosphere, and Cañon Diablo meteorite respectively. Analysis is usually done using a mass spectrometer, detecting small differences between gaseous elements. Analysis of a sample can cost anywhere from $30 to $100.<ref name="Fry1987"/> Stable isotopes assist scientists in analyzing animal diets and food webs by examining the animal [[Tissue (biology)|tissues]] that bear a fixed isotopic enrichment or depletion vs. the diet. Muscle or protein fractions have become the most common animal tissue used to examine the isotopes because they represent the assimilated nutrients in their diet. The main advantage to using stable isotope analysis as opposed to stomach content observations is that no matter what the status is of the animal's stomach (empty or not), the isotope tracers in the tissues will give us an understanding of its trophic position and food source.<ref name="Les">{{cite book |doi=10.1002/9780470691854.ch9 |chapter=Stable Isotope Ratios as Tracers in Marine Food Webs: An Update |title=Stable Isotopes in Ecology and Environmental Science |pages=238–82 |year=2007 |last1=Michener |first1=Robert H |last2=Kaufman |first2=Les |isbn=978-0-470-69185-4 |chapter-url={{Google books|vWs_hN1oSwYC|page=238|plainurl=yes}} }}</ref> The three major isotopes used in aquatic ecosystem food web analysis are <sup>13</sup>C, [[nitrogen-15|<sup>15</sup>N]] and [[sulfur-34|<sup>34</sup>S]]. While all three indicate information on [[trophic dynamics]], it is common to perform analysis on at least two of the previously mentioned three isotopes for better understanding of marine trophic interactions and for stronger results.

=====Hydrogen-2=====

The ratio of <sup>2</sup>H, also known as [[deuterium]], to <sup>1</sup>H has been studied in both plant and animal tissue. Hydrogen isotopes in plant tissue are correlated with local water values but vary based on fractionation during [[photosynthesis]], transpiration, and other processes in the formation of cellulose. A study on the isotope ratios of tissues from plants growing within a small area in Texas found tissues from [[CAM photosynthesis|CAM]] plants were enriched in deuterium relative to [[C4 carbon fixation|C4]] plants.<ref>{{cite journal|last1=Sternberg|first1=Leonel|last2=DeNiro|first2=Michael|last3=Johnson|first3=Hyrum|date=1984|title=Isotope ratios of cellulose from plants having different photosynthetic pathways|url=http://www.plantphysiol.org/content/plantphysiol/74/3/557.full.pdf|journal=Plant Physiology|volume=74|issue=3|pages=557–561|doi=10.1104/pp.74.3.557|pmc=1066725|pmid=16663460|access-date=15 March 2019|doi-access=free}}</ref> Hydrogen isotope ratios in animal tissue reflect diet, including drinking water, and have been used to study bird migration<ref>{{cite journal|last1=Kelly|first1=Jeffrey F.|last2=Atudorei|first2=Viorel|last3=Sharp|first3=Zachary D.|last4=Finch|first4=Deborah M.|date=1 January 2002|title=Insights into Wilson's Warbler migration from analyses of hydrogen stable-isotope ratios|journal=Oecologia|volume=130|issue=2|pages=216–221|bibcode=2002Oecol.130..216K|doi=10.1007/s004420100789|pmid=28547144|s2cid=23355570}}</ref> and aquatic food webs.<ref>{{cite journal|last1=Doucett|first1=Richard R.|last2=Marks|first2=Jane C.|last3=Blinn|first3=Dean W.|last4=Caron|first4=Melanie|last5=Hungate|first5=Bruce A.|date=June 2007|title=Measuring Terrestrial Subsidies to Aquatic Food Webs Using Stable Isotopes of Hydrogen|journal=Ecology|volume=88|issue=6|pages=1587–1592|doi=10.1890/06-1184|pmid=17601150|bibcode=2007Ecol...88.1587D }}</ref><ref>{{cite journal|last1=Cole|first1=Jonathan J.|last2=Carpenter|first2=Stephen R.|last3=Kitchell|first3=Jim|last4=Pace|first4=Michael L.|last5=Solomon|first5=Christopher T.|last6=Weidel|first6=Brian|date=1 February 2011|title=Strong evidence for terrestrial support of zooplankton in small lakes based on stable isotopes of carbon, nitrogen, and hydrogen|journal=Proceedings of the National Academy of Sciences|volume=108|issue=5|pages=1975–1980|bibcode=2011PNAS..108.1975C|doi=10.1073/pnas.1012807108|pmc=3033307|pmid=21245299|doi-access=free}}</ref>

=====Carbon-13=====

[[Carbon isotope]]s aid us in determining the [[primary production]] source responsible for the energy flow in an ecosystem. The transfer of <sup>13</sup>C through trophic levels remains relatively the same, except for a small increase (an enrichment < 1 ‰). Large differences of δ<sup>13</sup>C between animals indicate that they have different food sources or that their food webs are based on different primary producers (i.e. different species of phytoplankton, marsh grasses.) Because δ<sup>13</sup>C indicates the original source of primary producers, the isotopes can also help us determine shifts in diets, both short term, long term or permanent. These shifts may even correlate to seasonal changes, reflecting phytoplankton abundance.<ref name="Les"/> Scientists have found that there can be wide ranges of δ<sup>13</sup>C values in phytoplankton populations over a geographic region. While it is not quite certain as to why this may be, there are several hypotheses for this occurrence. These include isotopes within dissolved inorganic carbon pools (DIC) may vary with temperature and location and that growth rates of phytoplankton may affect their uptake of the isotopes. δ<sup>13</sup>C has been used in determining migration of juvenile animals from sheltered inshore areas to offshore locations by examining the changes in their diets. A study by Fry (1983) studied the isotopic compositions in juvenile shrimp of south Texas grass flats. Fry found that at the beginning of the study the shrimp had isotopic values of δ<sup>13</sup>C = -11 to -14‰ and 6-8‰ for δ<sup>15</sup>N and δ<sup>34</sup>S. As the shrimp matured and migrated offshore, the isotopic values changed to those resembling offshore organisms (δ<sup>13</sup>C=  -15‰ and δ<sup>15</sup>N = 11.5‰ and δ<sup>34</sup>S = 16‰).<ref name="Fry1983">{{cite journal |hdl=1969.3/19268 |last1=Fry |first1=B. |year=1983 |title=Fish and shrimp migrations in the northern Gulf of Mexico analyzed using stable C, N, and S isotope ratios |journal=Fishery Bulletin |volume=81 |pages=789–801 }}</ref>

=====Sulfur-34=====

While there is no enrichment of <sup>34</sup>S between trophic levels, the stable isotope can be useful in distinguishing [[Benthos|benthic]] vs. [[pelagic]] producers and [[marsh]] vs. [[phytoplankton]] producers.<ref name="Les"/> Similar to <sup>13</sup>C, it can also help distinguish between different phytoplankton as the key primary producers in food webs. The differences between seawater sulfates and sulfides (c. 21‰ vs -10‰) aid scientists in the discriminations. Sulfur tends to be more plentiful in less aerobic areas, such as benthic systems and marsh plants, than the pelagic and more aerobic systems. Thus, in the benthic systems, there are smaller [[Δ34S|δ<sup>34</sup>S]] values.<ref name="Les"/>

=====Nitrogen-15=====

[[Nitrogen isotope]]s indicate the trophic level position of organisms (reflective of the time the tissue samples were taken). There is a larger enrichment component with δ<sup>15</sup>N because its retention is higher than that of <sup>14</sup>N. This can be seen by analyzing the waste of organisms.<ref name="Les"/> Cattle urine has shown that there is a depletion of <sup>15</sup>N relative to the diet.<ref>{{cite journal |doi=10.1017/S002185960004853X |title=Fractionation of nitrogen isotopes by animals: A further complication to the use of variations in the natural abundance of 15N for tracer studies |journal=The Journal of Agricultural Science |volume=90 |pages=7–9 |year=2009 |last1=Steele |first1=K. W |last2=Daniel |first2=R. M |hdl=10289/4600 |s2cid=96956741 |url=https://researchcommons.waikato.ac.nz/bitstream/10289/4600/1/Fractionation%20of%20nitrogen%20isotopes.pdf |hdl-access=free }}</ref> As organisms eat each other, the <sup>15</sup>N isotopes are transferred to the predators. Thus, organisms higher in the [[trophic pyramid]] have accumulated higher levels of <sup>15</sup>N ( and higher δ<sup>15</sup>N values) relative to their prey and others before them in the food web. Numerous studies on marine ecosystems have shown that on average there is a 3.2‰ enrichment of <sup>15</sup>N vs. diet between different trophic level species in ecosystems.<ref name="Les"/> In the Baltic sea, Hansson et al. (1997) found that when analyzing a variety of creatures (such as [[Particle (ecology)|particulate]] organic matter (phytoplankton), [[zooplankton]], [[mysid]]s, sprat, smelt and herring,) there was an apparent fractionation of 2.4‰ between consumers and their apparent prey.<ref name="Strue">{{cite journal |doi=10.1890/0012-9658(1997)078[2249:TSNIRA]2.0.CO;2 |year=1997 |volume=78 |issue=7 |pages=2249 |title=The Stable Nitrogen Isotope Ratio As a Marker of Food-Web Interactions and Fish Migration |journal=Ecology |last1=Hansson |first1=Sture |last2=Hobbie |first2=John E |last3=Elmgren |first3=Ragnar |last4=Larsson |first4=Ulf |last5=Fry |first5=Brian |last6=Johansson |first6=Sif }}</ref>

In addition to trophic positioning of organisms, δ<sup>15</sup>N values have become commonly used in distinguishing between land derived and natural sources of nutrients. As water travels from septic tanks to aquifers, the nitrogen rich water is delivered into coastal areas. Waste-water nitrate has higher concentrations of <sup>15</sup>N than the nitrate that is found in natural soils in near shore zones.<ref name="Kreitler">{{cite journal |doi=10.1111/j.1745-6584.1978.tb03254.x |title=N15/N14 Ratios of Ground-Water Nitrate, Long Island, New Yorka |journal=Ground Water |volume=16 |issue=6 |pages=404 |year=1978 |last1=Kreitler |first1=Charles W |last2=Ragone |first2=Stephen E |last3=Katz |first3=Brian G |bibcode=1978GrWat..16..404K }}</ref> For bacteria, it is more convenient for them to uptake <sup>14</sup>N as opposed to <sup>15</sup>N because it is a lighter element and easier to metabolize. Thus, due to bacteria's preference when performing [[Biogeochemical cycle|biogeochemical processes]] such as [[denitrification]] and [[volatilization]] of ammonia, <sup>14</sup>N is removed from the water at a faster rate than <sup>15</sup>N, resulting in more <sup>15</sup>N entering the aquifer. <sup>15</sup>N is roughly 10-20‰ as opposed to the natural <sup>15</sup>N values of 2-8‰.<ref name="Kreitler"/> The inorganic nitrogen that is emitted from septic tanks and other human-derived sewage is usually in the form of <chem>NH4+</chem>. Once the nitrogen enters the estuaries via groundwater, it is thought that because there is more <sup>15</sup>N entering, that there will also be more <sup>15</sup>N in the inorganic nitrogen pool delivered and that it is picked up more by producers taking up N. Even though <sup>14</sup>N is easier to take up, because there is much more <sup>15</sup>N, there will still be higher amounts assimilated than normal. These levels of δ<sup>15</sup>N can be examined in creatures that live in the area and are non migratory (such as [[macrophyte]]s, clams and even some fish).<ref name="Strue"/><ref>{{cite journal |doi=10.4319/lo.1998.43.4.0577 |title=Linking nitrogen in estuarine producers to land-derived sources |journal=Limnology and Oceanography |volume=43 |issue=4 |pages=577 |year=1998 |last1=McClelland |first1=James W |last2=Valiela |first2=Ivan |bibcode=1998LimOc..43..577M |doi-access=free }}</ref><ref>{{cite journal |doi=10.3354/ab00106 |title=Nitrogen stable isotopes in the shell of Mercenaria mercenaria trace wastewater inputs from watersheds to estuarine ecosystems |journal=Aquatic Biology |volume=4 |pages=99–111 |year=2008 |last1=Carmichael |first1=RH |last2=Hattenrath |first2=T |last3=Valiela |first3=I |last4=Michener |first4=RH |url=http://darchive.mblwhoilibrary.org/bitstream/1912/4525/1/b004p099.pdf |doi-access=free }}</ref> This method of identifying high levels of nitrogen input is becoming a more and more popular method in attempting to monitor nutrient input into estuaries and coastal ecosystems. Environmental managers have become more and more concerned about measuring anthropogenic nutrient inputs into estuaries because excess in nutrients can lead to [[eutrophication]] and [[Hypoxia (environmental)|hypoxic events]], eliminating organisms from an area entirely.<ref>{{cite journal |doi=10.4319/lo.1997.42.5.0930 |title=Nitrogen-stable isotope signatures in estuarine food webs: A record of increasing urbanization in coastal watersheds |journal=Limnology and Oceanography |volume=42 |issue=5 |pages=930 |year=1997 |last1=McClelland |first1=James W |last2=Valiela |first2=Ivan |last3=Michener |first3=Robert H |bibcode=1997LimOc..42..930M |doi-access=free }}</ref>

=====Oxygen-18=====

Analysis of the ratio of <sup>18</sup>O to <sup>16</sup>O in the [[seashell|shells]] of the [[Colorado Delta clam]] was used to assess the historical extent of the [[estuary]] in the [[Colorado River Delta]] prior to construction of upstream dams.<ref>{{cite journal |doi=10.1006/jare.2001.0845 |title=Macrofaunal and isotopic estimates of the former extent of the Colorado River estuary, upper Gulf of California, México |journal=Journal of Arid Environments |volume=49 |issue=1 |pages=183–93 |year=2001 |last1=Rodriguez |first1=Carlie A |last2=Flessa |first2=Karl W |last3=Téllez-Duarte |first3=Miguel A |last4=Dettman |first4=David L |last5=Ávila-Serrano |first5=Guillermo A |bibcode=2001JArEn..49..183R }}</ref>