Stable Isotopes

Important insights into ecosystem dynamics can be revealed through the use of naturally occurring 'stable isotopes'. These alternate forms of elements can reveal both the source of material flowing through an ecosystem and its consumer's trophic position. This is because different sources of organic matter can have unique isotopic signatures which are altered in a consistent manner as materials are transferred throughout an ecosystem, from trophic level to trophic level. Consequently, stable isotopes provide powerful tools for estimating material flux and trophic positions.

The elements C, N, S, H, and O all have more than one isotope. For example, carbon has several isotopes, two of which are 13C and 12C. In nature, only 1% of carbon is 13C. Isotopic composition is typically expressed in S values, which are parts per thousand differences from a standard. For carbon,

x 103

Consequently, S values express the ratio of heavy to light isotope in a sample. Increases in these values denote increases in the amount of the heavy isotope component. The standard reference material for carbon is PeeDee limestone, while the standard for nitrogen is nitrogen gas in the atmosphere. Natural variation in stable isotopic composition can be detected with great precision with a mass spectrometer.

Stable isotopes record two kinds of information. Process information is revealed by physical and chemical reactions which alter stable isotope ratios, while source information is revealed by the isotopic signatures of source materials. When organisms take up carbon and nitrogen, chemical reactions occur which discriminate among isotopes, thereby altering the ratio of heavy to light isotope. This is known as 'fractionation'. Although carbon fractionates very little (0.4%o, 1 SD = 1%), the mean trophic fractionation of S15N is 3.4% (1 SD = 1%), meaning that S15N increases on average by 3.4% with every trophic transfer. Because the S15N of a consumer is typically enriched by 3.4% relative to its diet, nitrogen isotopes can be used to estimate trophic position. Stable isotopes can provide a continuous measure of trophic position that integrates the assimilation of energy or material flow through all the different trophic pathways leading to an organism. In contrast, S13C can be used to evaluate the ultimate sources of carbon for an organism when the isotopic signatures of the sources are different.

Stable isotopes can track the fate of different sources of carbon through an ecosystem, because a consumer's iso-topic signature reflects those of the key primary producers it consumes. For example, in both lake and coastal marine ecosystems, S13C is useful for differentiating between two major sources of available energy, benthic (nearshore) production from attached macroalgae, and pelagic (open water) production from phytoplankton. This is because macroalgae and macroalgal detritus (specifically kelp of the order Laminariales) is typically more enriched in S13C (less negative S13C) relative to phytoplankton due to boundary layer effects. Researchers have exploited this difference to answer many important ecosystem-level questions. Below are two examples.

During the late 1970s and early 1980s, in the western Aleutian Islands of Alaska, where sea otters had recovered from overexploitation and suppressed their herbivorous urchin prey, productive kelp beds dominated. There, transplanted filter feeders, barnacles and mussels, grew up to 5 times faster compared to islands devoid of kelp where sea otters were scarce and urchin densities high. Stable isotope analysis revealed that the fast-growing filter feeders were enriched in carbon suggesting that macroalgae was the carbon source responsible for this magnification of secondary production.

In four Wisconsin lakes, experimental manipulations of fish communities and nutrient loading rates were conducted to test the interactive effects of food web structure and nutrient availability on lake productivity and carbon exchange with the atmosphere. The presence of top predators determined whether the experimentally enriched lakes operated as net sinks or net sources of atmospheric carbon. Specifically, the removal of piscivorous fishes caused an increase in planktivorous fishes, a decrease in large-bodied zooplankton grazers, and enhanced primary production, thereby increasing influx rates ofatmospheric carbon into the lake. Atmospheric carbon was traced to upper trophic levels with S C. Here, naturally occurring stable isotopes and experimental manipulations conducted at the scale of whole ecosystems illustrated that top predators fundamentally alter biogeochemical processes that control a lake's ecosystem dynamics and interactions with the atmosphere.

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