Abstract: Size spectrum slope has been considered as an indicator for system productivity and size-dependent trophic interactions in plankton communities. We investigated the effect of new nutrient flux (Nin) and feeding breadth of zooplankton (σ2) on spectral slopes of the plankton community using a size-structured nutrient-phytoplankton-zooplankton model. We studied two types of plankton systems (1) at a steady state and (2) with fluctuating plankton populations. For the steady state systems, the spectral slopes increased with Nin and σ2. In contrast, for the systems in fluctuation, spectral slopes constructed from long-term average plankton populations decreased with increasing Nin and σ2. In addition to spectral slopes constructed from long-term average population densities, we investigated slope variability in systems with fluctuating plankton populations. The variability of spectral slopes was found to increase with Nin and σ2, indicating that spectral slope variation is large in a eutrophic system or when zooplankton feed on a wide size range of prey. Empirically, because Nin is difficult to measure, nutrient concentration is investigated instead. Our model predicts that the spectral slope is positively correlated with nutrient concentration due to the accumulated mesozooplankton biomass, which was confirmed by our study in plankton community in a subtropical reservoir in Taiwan.