Water Level Rise Induced Limnological Changes Indirectly Influencing the Structure of Aquatic Macrophyte Communities in a Tropical Reservoir

This study aimed to evaluate the degree of contribution of limnological variables to explaining ecological processes involving aquatic macrophytes after an event of water level rise (WLR) of an artificial lake (Cursai Reservoir - CR), located in northeastern Brazil. Initially we tested the hypotheses that limnological characteristics, and species richness and composition of aquatic macrophytes, would change post-WLR. Once these hypotheses were confirmed, we evaluated the degree of influence (direct or indirect) of the variables to structuring the community of aquatic macrophytes in CR. We carried out four pre-WLR sampling expeditions and four post-WLR sampling expeditions to a permanent plot of 5 m x 40 m, wherein we assessed species richness, composition and cover, and measured 12 limnological variables. We used partial correlation analysis to evaluate the degree of contribution of the limnological changes to post-WLR structuring of the aquatic macrophyte community. The results pointed to an indirect correlation between the limnological changes and the structuring of the aquatic macrophyte community derived from the WLR, with interespecific interactions being the determining factors in structuring. The cover and species composition in the post-WLR period confirmed this inference. In this period, we found a reduction in cover of the weeds Eichhornia crassipes, Paspalidium geminatum and Salvinia auriculata, new records of Egeria densa, Ludwigia leptocarpa and Ludwigia helminthorriza, and an increase in cover of Cyperus odoratus and Oxycaryum cubense. We consider that fast WLRs in artificial lakes, such as occurred in CR, can be considered intermediate disturbances. In such cases, the ecological processes involving the aquatic macrophytes could be explained by the intermediate disturbance hypothesis (IDH), which reinforces interespecific interactions (moreover competition) as being the restructuring factor.