| Summary: | 碩士 === 國立臺灣大學 === 海洋研究所 === 98 === Effects of anthropogenic disturbance and global climate changes on ecosystems are pressing concerns. Developing reliable and efficient indicators to study trophodynamics is essential for ecosystem management and conservation. Aquatic food webs are strongly size-based; therefore, one potential indicator to investigating trophodynamics is variation in body size. Substantial evidence suggests a positive linear relationship between log size and trophic level; however, whether such positive relationship holds in plankton community remains elusive. Predator-prey mass ratio (PPMR) is an important parameter to determine the stability, structure and dynamic of foodweb. Nevertheless, only one mechanism (gape limitation) was proposed to explain the observed pattern in predator-prey body mass ratios (PPMR) so far. We hypothesized and tested that PPMR was sensitive to the variation of resource availability and used three factors as proxy for resource availability: (1) concentration of nutrient; (2) phytoplankton biomass; and (3) C:N ratio of particulate organic matter. The present study used stable isotope analysis to assess size-TL relationship and PPMR within plankton communities. Our result suggested that larger size plankton community generally showed a strong linear positive size-TL relationship, but the small size plankton community showed non-significant and sometimes even inverse pattern. The present result also manifested some patterns in the PPMR. First, PPMR was steady in highly changeable costal environments; by contrast, PPMR were relatively variable in stable offshore environments. Second, PPMR showed a nonlinear declining trend with an increase in total phytoplankton biomass. Third, food quality has influence on PPMR. Coupled analyses of size-TL and PPMR may provide a basis for assessing the structure of food webs. Our result indicated that longer food chains were found in the food webs where average PPMR were smaller, and mean PPMR were smaller in highly variable inshore environments.
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