Nutrient And Grazing Effects On Different Benthic Prey Assemblages

The impacts of bottom-up and top-down forces on natural communities in food webs have been widely studied in ecology, and the strength of the two forces vary with ecosystem type, trophic state of the habitat and consumers feeding preference. However, how different prey assemblages in response to bottom-up and top-down controls in one food web system is poorly understood. Moreover, as ecologists have realized that the propagation of bottom-up and top-down effects can cause multitrophic effects, more evidence was needed to verify it. Using factorial field experiment, our study investigated the direct impact of resource (nutrient supply) and grazing (snail consumer) effects on benthic microalgae and bacteria communities in a salt marsh ecosystem, and the indirect influence on nematode community at higher trophic levels. This study aims to reveal the relative importance of bottom-up and top-down forces on different prey composition, and the indirect multitrophic effects they may induce. The main results are as follows:(1) Biomass and abundance of benthic microalgae significantly increased with nutrient supply, while the grazing effects were relatively weak. This result revealed that the biomass and abundance of microalgae were regulated mainly by bottom-up forces.(2) Both nutrient supply and grazing treatments significantly affected the diversity of diatoms and bacteria, while the grazing effects were more important. This proved that the species diversity was controlled by both bottom-up and top-down forces, and the top-down control were relative more important than the bottom-up control.(3) Nutrient supply significantly reduced diversity of both diatoms and bacteria. However, the grazing had different effects on these two prey communities, which inhibited diatom diversity but enhanced bacterial diversity. This result showed that the underlying mechanisms of the regulations of different prey assemblages by snail grazers were different.(4) Abundance of each nematode feeding group had no significant relevance with the changes of microalgal and bacterial communities, which implied that nematodes had little response to their food sources. The result revealed that multitrophic effects were not as prominent as we hypothesed in our studied salt marsh system.