Applications of stable isotope techniques in aquatic ecological studies

Stable isotopes have been used in many research areas as natural labels and are becoming an important tool in aquatic ecological research.The theoretical basis and application of stable isotope analysis in aquatic ecology as well as its principle and method are reviewed in this paper.An organism's stable isotope ratios(δ()~(15)N and δ()~(13)C etc.) are an integration of the isotopic signatures of prey items that have been assimilated through time,the organism will come into isotopic equilibrium with its diet with the time to equilibrium depending on growth and tissue turnover rates.Stable nitrogen isotope ratios increase with trophic level by approximately 3‰ to 4‰;this provides a powerful analytical tool to quantify relative trophic position.In contrast,stable carbon isotope ratio changes very little with trophic level(0 to 1‰ enrichment per trophic level).Instead,stable carbon isotope values of organisms reflect the average δ()~(13)C of their diets.When an organism moves to a different habitat,its stable isotope ratios will change.From the stable ratios of the different habitats and the organism,we can judge the migration routine of the organism.Stable isotopes are often used to quantify the contributions of multiple sources to an organism.In general,the proportional contributions of n+1 different sources can be uniquely determined by the use of n isotope groups with linear mixing models based on mass balance equations.When there are n isotope groups and n+1 sources,all possible combinations of each sources contribution can be obtained by a computer program(IsoSource),but the proportional contributions aren't unique.The above two cases presume that the proportional contribution of a source to an organism is the same for all isotope elements,while not all cases are so.An isotope element concentration-weighted linear mixing model was developed,whose quantified results are more reasonable,but only determined the proportional contributions of n+1 sources with n isotope groups.Prey sources,trophic level and prey proportional contributions etc.are basic information needed to construct aquatic ecosystem food web patterns and functions and material and energy flows,which are fundamental to understand the dynamic of ecosystems and effects of environment to the ecosystems.The prospect of such studies in China and some existing problems are also discussed