text: Nitrogen isotopes indicate the trophic level position of organisms (reflective of the time the tissue samples were taken). There is a larger enrichment component with δN because its retention is higher than that of N. This can be seen by analyzing the waste of organisms. Cattle urine has shown that there is a depletion of N relative to the diet. As organisms eat each other, the N isotopes are transferred to the predators. Thus, organisms higher in the trophic pyramid have accumulated higher levels of N ( and higher δN 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 N vs. diet between different trophic level species in ecosystems. In the Baltic sea, Hansson et al. (1997) found that when analyzing a variety of creatures (such as particulate organic matter (phytoplankton), zooplankton, mysids, sprat, smelt and herring,) there was an apparent fractionation of 2.4‰ between consumers and their apparent prey.
  In addition to trophic positioning of organisms, δN 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 N than the nitrate that is found in natural soils in near shore zones. For bacteria, it is more convenient for them to uptake N as opposed to N because it is a lighter element and easier to metabolize. Thus, due to bacteria's preference when performing biogeochemical processes such as denitrification and volatilization of ammonia, N is removed from the water at a faster rate than N, resulting in more N entering the aquifer. N is roughly 10-20‰ as opposed to the natural N values of 2-8‰. The inorganic nitrogen that is emitted from septic tanks and other human-derived sewage is usually in the form of . Once the nitrogen enters the estuaries via groundwater, it is thought that because there is more N entering, that there will also be more N in the inorganic nitrogen pool delivered and that it is picked up more by producers taking up N. Even though N is easier to take up, because there is much more N, there will still be higher amounts assimilated than normal. These levels of δN can be examined in creatures that live in the area and are non migratory (such as macrophytes, clams and even some fish). 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 hypoxic events, eliminating organisms from an area entirely.