Food webs depict complex trophic interactions among organisms in ecosystems, and food-chain length is a central index of food webs. Food-chain lengths of ecosystems could be estimated using nitrogen stable isotope ratios. Recently, using such isotope techniques, theoretical hypotheses of food-chain lengths have been tested in the field. Previous studies showed that the ecosystem size determine food-chain lengths in lakes (from 105 to 1013 lake volume m3), but productivity of the lakes were not affected on the food-chain lengths. However, in small ecosystems size, the variation of food chain lengths are little known. We hypothesized that in small system, the productivity and trophic interaction would determine the food-chain lengths in aquatic systems. In this study, we estimated food-chain lengths in the 15 small ponds (102 to 103 lake volume m3) using carbon and nitrogen stable isotope ratios. Also we estimated the planktonic community structures and biomass in the ponds. Our results showed that the food-chain length varied from 3.1 to 4.5 in the 15 ponds, and productivity and microbial trophic interactions in the ponds could determine the food chain lengths in small pond ecosystems.
