CO2 and nutrient-driven changes across multiple levels of organization inZostera noltii ecosystems

[摘要] Increasing evidence emphasizes that the effects of human impacts onecosystems must be investigated using designs that incorporate the responsesacross levels of biological organization as well as the effects of multiplestressors. Here we implemented a mesocosm experiment to investigate how theindividual and interactive effects of CO₂ enrichment andeutrophication scale-up from changes in primary producers at theindividual (biochemistry) or population level (production, reproduction,and/or abundance) to higher levels of community (macroalgae abundance,herbivory, and global metabolism), and ecosystem organization (detritusrelease and carbon sink capacity). The responses of Zostera noltii seagrass meadowsgrowing in low- and high-nutrient field conditions were compared. In bothmeadows, the expected CO₂ benefits on Z. noltii leaf production were suppressedby epiphyte overgrowth, with no direct CO₂ effect on plant biochemistryor population-level traits. Multi-level meadow response to nutrients wasfaster and stronger than to CO₂. Nutrient enrichment promoted thenutritional quality of Z. noltii (high N, low C : N and phenolics), the growth ofepiphytic pennate diatoms and purple bacteria, and shoot mortality. In thelow-nutrient meadow, individual effects of CO₂ and nutrients separatelyresulted in reduced carbon storage in the sediment, probably due to enhancedmicrobial degradation of more labile organic matter. These changes, however,had no effect on herbivory or on community metabolism. Interestingly,individual effects of CO₂ or nutrient addition on epiphytes, shootmortality, and carbon storage were attenuated when nutrients andCO₂ acted simultaneously. This suggests CO₂-induced benefits oneutrophic meadows. In the high-nutrient meadow, a striking shoot declinecaused by amphipod overgrazing masked the response to CO₂ and nutrientadditions. Our results reveal that under future scenarios of CO₂, theresponses of seagrass ecosystems will be complex and context-dependent,being mediated by epiphyte overgrowth rather than by direct effects on plantbiochemistry. Overall, we found that the responses of seagrass meadows toindividual and interactive effects of CO₂ and nutrient enrichmentvaried depending on interactions among species and connections betweenorganization levels.

[学科分类] 地球化学与岩石