Editing Isotope analysis (section)

=====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>