No abstract

Bottom-up and top-down effects have been widely studied in the context of food webs. However, there is still inadequate knowledge about how different prey assemblages respond to simultaneous manipulations of consumers and nutrients in benthic meiofaunal and macrofaunal systems, and how these responses affect other trophic levels. Using a factorial experimental design in a salt marsh ecosystem, we investigated the direct influence of nutrients (natural versus enriched) and snail grazing pressure (with versus without Assiminea latericera) on 2 different prey communities (microalgae and bacteria). In addition, the indirect influences on nematode communities at other trophic levels were investigated. The abundance of benthic microalgae was regulated mainly by bottom-up forces. Both nutrient supply and grazing pressure significantly affected the diversity of diatoms and bacteria in terms of the Shannon-Wiener diversity index measurement. The amount of nutrient supply significantly reduced diatom and bacteria diversity. The presence of grazers reduced diatom diversity under both ambient and enriched nutrient levels; thus, the algal diversity responses did not support the grazer-reversal hypothesis (species richness of prey decreases with high grazing in nutrient-poor ecosystems, while it increases with high grazing in nutrient-rich ecosystems). The presence of grazers enhanced bacterial diversity mainly under the ambient nutrient level. The feeding selectivity of snail consumers was the likely mechanism influencing the different responses of microalgae and bacteria. Nematodes did not respond to experimental manipulations, indicating the weak indirect effects of bottom-up and top-down forces. We suggest that different prey components within a system may respond to bottom-up and top-down effects in different ways, and other trophic levels in below-ground food webs may not be sensitive to cascade effects.