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Ecohydrology
Deuterium acts as a conservative tracer and as such works well for tracking water movement through plants and ecosystems. The field of ecohydrology is concerned with the impact ecosystems have on water cycling, from measuring the small scale drainage of water into soil to tracking the broad movements of water evaporating off from trees. Tracking water movement through, for example, cloud forests, can be complicated by the multiple precipitation sources and is more difficult than tracking evaporation or other single-process phenomena. Isotope spiking can also be done to determine water transport through soil and into plants via injection of deuterated water directly into the ground.

Stable isotope analysis of xylem water can be used to follow the movement of water from soil into the plants and therefore provide a record of the depth of water acquisition. An advantage to using xylem water is that in theory, the deuterium content should directly reflect the input water without being affected by leaf transpiration. For example, Dawson and Ehleringer used this approach to determine whether trees that grow next to streams are using the surface waters from that stream. Water from the surface would have the same isotopic composition as the stream, while water from farther below in the ground would be from past precipitation inputs. In this case, younger trees had a xylem water isotopic composition very close to the adjacent stream and likely used surface waters to get established. Older trees had depleted xylem water relative to the stream, reflecting that they source their water from deeper underground. These stable isotope studies have also determined that plants in redwood forests do not just take up water from their roots but acquire a significant proportion of water via stomatal uptake on leaves.

Plant water can be used to characterize other plant physiological processes that affect the hydrologic cycle; for example, leaf waters are widely used for modeling transpiration and water-use efficiency. In transpiration, the Craig-Gordon model for lake water enrichment through evaporation has been found experimentally to fit well for modelling leaf water enrichment. Transpiration can be measured by direct injection of deuterated water into the base of the tree, trapping all water vapor transpired from the leaves and measuring the subsequent condensate. Water use can also be measured and is calculated from a heavy water injection as follows:

$$\frac{M}{\sum_{1}^{T}C_{i}\Delta t_{i}}$$ ,

Where M is the mass of deuterated water injected, T is the final day of the experiment, Ci is the concentration of deuterium at time interval i, and Δti is the length of time interval i. However, though the calculated water use via thermal dissipation probing of some tropical plants such as bamboos correlates strongly with measured water use found by tracking D2O movement, the exact values are not the same. In fact, with tree species Gliricidia sepium, which produces a heartwood, transpired water did not even correlate strongly with injected D2O concentrations, which would further complicate water use measurements from direct injections. This possibly occurred because heartwoods could accumulate heavy water rather than the water move directly through xylem and to leaves.

Water use efficiency (WUE), which is the ratio of carbon fixation to transpiration, has previously been associated with 13C ratios using the following equation:

WUE = $$\frac{(1-\phi)p_{CO_{2}}}{1.6(\epsilon_{carb}-\epsilon_{diff})}(\epsilon_{carb}-\Delta^{13}C)$$ ,

In this equation,

$$\Delta^{13}C = \frac{\delta^{13}C_{atm}-\delta^{13}C_{plant}}{1+\delta^{13}C_{atm}}$$, φ is the fraction of fixed carbon that is respired, pCO2 is the partial pressure of CO2 in the atmosphere, εcarb is the fractionation of carboxylation, and εdiff is the fractionation of diffusion in air. The relation of δD in plant leaf waxes to δ13C has been empirically measured and results in a negative correlation of δD to water use efficiency. This can be explained in part by lower water use efficiency resulting in higher transpiration rates. Transpiration exhibits a normal isotope effect, causing enrichment of deuterium in plant leaf water and therefore enrichment of leaf waxes.

Tracking Animal Migration
Deuterium abundance can be useful in tracking migration of various animals. Animals with metabolically inert tissue (e.g. feathers or hair) will have synthesized that tissue using hydrogen from source water and food but ideally not incorporate subsequent water over the course of the migration. Because δD tends to vary geographically, the difference between animal tissue δD and post-migration water δD, after accounting for the biological fractionation of assimilation, can provide information regarding animal movement. In monarch butterflies, for example, wing chitin is metabolically inert after it has been built, so it can reflect the isotopic composition of the environmental water at the time and location of wing growth. This then creates a record of butterfly origin and can be used to determine migration distance. This approach can also be used in bats and birds, using hair and feathers, respectively. Since rainwater becomes linearly depleted as elevation is increased, this method can also track altitudinal migration. However, this is technically difficult to do, and the resolution appears to be too poor to track small altitudinal changes. Deuterium is most useful in tracking movement of species between areas with large continental water variation, beause species movement can be complicated by the similarity of local water δD values between different geographic regions. For example, source water from Baja California may have the same δD as water from Maine. Further, a proportion of the hydrogen isotope composition within the tissue can exchange with water and complicate measurements. In order to determine this percentage of isotopic exchange, which varies according to local humidity levels, standards of metabolically inert tissue from the species of interest can be constructed and equilibrated to local conditions. This allows measured δD from different geographic regions to be compared against each other.

Trophic Interactions
Assimilation of diet into tissue has a tissue-specific fractionation known as the trophic discrimination factor. Diet sources can be tracked through a food web via deuterium isotope profiles, although this is complicated by deuterium having two potential sources - water and food. Food more strongly impacts δD than does exchange with surrounding water, and that signal is seen across trophic levels. However, different organisms derive organic hydrogen in varying ratios of water to food: for example, in quail, 20-30% of organic hydrogen was from water and the remainder from food. The precise percentage of hydrogen from water was dependent on tissue source and metabolic activity. In chironomids, 31-47% of biomass hydrogen derived from water, and in microbes as much as 100% of fatty acid hydrogen can be derived from water depending on substrate. In caterpillars, diet δD from organic matter correlates linearly with tissue δD. The same relationship does not appear to hold consistently for diet δD from water, however - water derived from either the caterpillar or its prey plant is more deuterium enriched than their organic material. Going up trophic levels from prey (plant) to predator (caterpillar) results in an isotopic enrichment. This same trend of enrichment is seen in many other animals - carnivores, omnivores, and herbivores - and appears to follow 15N relative abundances. Carnivores at the same trophic level tend to exhibit the same level of 2H enrichment. Because, as mentioned earlier, the amount of organic hydrogen produced from water varies between species, a model of trophic level related to absolute fractionation is difficult to make if the participating species are not known. Consistency in measuring the same tissues is also important, as different tissues fractionate deuterium differently. In aquatic systems, tracking trophic interactions is valuable for not only understanding the ecology of the system, but also for determining the degree of terrestrial input. The patterns of deuterium enrichment consistent within trophic levels is a useful tool for assessing the nature of these interactions in the environment.

Microbial Metabolism
Biological deuterium fractionation through metabolism is very organism and pathway dependent, resulting in a wide variability in fractionations. Despite this, some trends still hold. Hydrogen isotopes tend to fractionate very strongly in autotrophs relative to heterotrophs during lipid biosynthesis - chemoautotrophs produce extremely depleted lipids, with the fractionation ranging from roughly -200 to -400‰. This has been observed both in laboratory-grown cultures fed a known quantity of deuterated water and in the environment. Proteins, however, do not follow as significant a trend, with both heterotrophs and autotrophs capable of generating large and varied fractionations. In part, kinetic fractionation of the lighter isotope during formation of reducing equivalents NADH and NADPH result in lipids and proteins that are isotopically lighter.

Salinity appears to play a role in the degree of deuterium fractionation as well; in the environment, more saline waters are associated with higher evaporation rates and are isotopically lighter. This isotopically lighter water should then in theory affect lipid δD after being incorporated into biomass. In coccolithophores Emiliania huxleyi and Gephyrocapsa oceanica, alkenone δD has been found to correlate strongly to organism growth rate divided by salinity. The relationship between deuterium fractionation and salinity could potentially be used in paleoreconstruction with preserved lipids in the rock record to determine, for example, ocean salinity at the time of organismal growth. However, the degree of fractionation is not necessarily consistent between organisms, complicating the determination of paleosalinity with this method. There also appears to be a negative correlation between growth rate and fractionation in these coccolithophores. Further experiments on unicellular algae Eudorina unicocca and Volvox aureus show no effect of growth rate (controlled by nitrogen limitation) on fatty acid δD. However, sterols become more D-depleted as growth rate increases, in agreement with alkenone isotopic composition in coccolithophores.

Methanogenesis, the production of methane from more oxidized carbon compounds, also exhibits a strong isotope effect. Hydrogenotrophic methanogenesis produces less deuterium-depleted methane relative to acetoclastic methanogenesis. The location the organism is present in also affects isotopic composition: rumen methanogenesis, which is more of a closed system, is more depleted than wetland methanogenesis.