5. Studying Soft Shore Trophic Ecology in Hong Kong
Stable Isotope Techniques
Stable isotope analysis is a powerful tool for tracking the movement and transfer of nutrients in ecosystems. Unlike radioisotopes, stable isotopes occur naturally in the environment in abundance and are completely safe for use as a tracer.
The use of stable isotopes as trophic tracers is underpinned by the assumption that consumers inherit the stable isotope “signatures” of their food with little or known change (the trophic discrimination factor TDF, which represents the difference between the isotopic signatures of a consumer’s tissue and its diet sources), so that the principle of “you are what you eat” may apply to help track the movement of nutrients.
The stable isotopic ratio of essential nutrients such as carbon (C), nitrogen (N) and Sulphur (S) is measured as the ratio of the abundance of the heavier, e.g. 13C, isotope to the lighter, usually more common, isotope, e.g. 12C. This ratio is compared with a standard for the respective element and expressed as δ13C, δ15N and δ34S. Since every primary producer (any material containing these elements, actually) has its characteristic stable isotopic ratio, we can trace the flow of nutrients from the producers to the consumers using their stable isotope values as a guide.
The ECF project funds have helped us purchased a combustion module that enables measurement of the carbon isotopic ratio when interfaced with our Picarro G2201-i analyser.
The setup, called the Picarro Combustion Module Cavity Ring-Down Spectroscopy (CM-CRDS) system, is able to measure the carbon isotopic composition (δ13C) of solid and liquid samples with carbon content as low as 250mg.
Picarro G2201-i analyser itself can measure the δ13C, CO2 and CH4 concentrations of gaseous samples. We are particularly interested in those of the sediment C flux. To do this, we collect 50ml of gas from a static chamber covered with aluminium foil to inhibit photosynthetic activity, and inject the gas into the Picarro G2201-i analyser. The following photo shows the field setup.
A syringe is inserted into the three-way valve opening at the top of the chamber to draw out 50ml of gas.