Testing the Tea Bag Index as a potential indicator for assessing litter decomposition in aquatic ecosystems

Abstract

The Tea Bag Index (TBI) approach is a standardized method for assessing litter decomposition in terrestrial ecosystems. This method allows determination of the stabilized portion of the hydrolysable fraction during the decomposition process, and derivation of a decomposition constant (k) using single measurements of the mass-loss ratios of green and rooibos teas. Although this method is being applied to aquatic systems, it has not been tested if the TBI-based decomposition curves accurately represent the changes in remaining mass of the teas in these environments, where initial leaching tends to be higher than in terrestrial ecosystems. Here, we first tested a critical assumption of the TBI method that green tea decomposition plateaus during the standard incubation period of 90 days, and then tested the accuracy of a TBI-based asymptote model, which was determined using another essential assumption of the TBI approach: the ratio of the decomposable hydrolysable fraction to the total hydrolysable fraction of rooibos tea is the same as that of green tea. Validation data were obtained by incubating tea bags in water samples taken from a stream, a pond, and the ocean in Kumamoto, Japan. We found that green tea decomposition did not plateau during the 90-day period, contradicting a key assumption of the TBI method. The other key assumption was also violated, because the TBI-based asymptote models disagreed with actual decomposition data. Subtracting the leachable fraction from the initial tea mass improved the TBI-based model, but discrepancies with the actual decomposition data remained. Thus, we conclude that the TBI approach, which was developed for a terrestrial environment, is not appropriate for aquatic ecosystems.