Aquatic ecosystems are susceptible to changes in nutrient supply and rainwater input. Both litter quality and precipitation amount are important components of global change and appear to regulate aquatic ecosystem functioning by altering nutrient cycling and detrital processing rates by macroinvertebrates. These effects on nutrient cycling and detrital processing rates are caused by changes in litter quality (i.e., elemental ratios) and by changes in rainfall patterns, which can change the physical conditions of natural water bodies (i.e., lakes and water-holding bromeliads) by isolating and/or homogenizing aquatic communities through changes in the volume of water of these aquatic systems or by modifying their chemistry to alter the rates of mobilization and dilution of nutrients. Although the main role of these two drivers on ecosystem structure and functioning, the interactive effects of nutrient supply and rainwater input on ecosystems are still poorly known. In this study, we manipulated litter quality by changing N:P stoichiometry (natural N:P = 49 and high N:P = 64) and rainfall (current levels and 40% increase) to understand how these orthogonally combined factors affect the structure and the functioning of bromeliad phytotelmata systems.
Results/Conclusions
Our results showed that the abundance of Naididae (Oligochaeta) decreased by 23% with natural N:P ratio and increased rainfall. But under high detritus N:P ratio, the abundance of Naididae increased by 63% under high rainfall. As a consequence, NH4+ concentration in the bromeliad tank increased as Naididae abundance increased suggesting that Naididae boost N cycling in the system. The lowest CDOM (colored dissolved organic matter) concentrations in the bromeliad tank were found at the highest rainfall level and high detritus N:P. However, under natural detritus conditions, increased rainfall caused an increase in fine detritus biomass, probably because of the detritus ingestion by Naididae and their bioturbation impacts. Increased rainfall caused a 50% decrease in N flux from detritus to bromeliad leaves, and diluted PO34- concentrations in the water tank in both N:P treatments. Further, rainfall increases also caused water runoff, which leached considerable amounts of nutrients, bacteria, and zooplankton out from bromeliads. Despite that, the community of microorganisms (bacteria, algae, and zooplankton) and other macroinvertebrates inside bromeliads did not respond to detritus quality and changes in rainfall. These findings suggest that aquatic communities in bromeliad phytotelmata are stable and resistant to changes in detritus quality and precipitation.