Comparative analysis of trophic structure and functioning of lateral lagoons in a sub-tropical river with a cascade of reservoirs

Background/Question/Methods

Understanding the mechanisms which regulate aquatic food webs dynamics have been an important focus inquiry since ecological research started to emphasize ecosystems’ structure and functioning. The aim of this study was to investigate the trophic structure and ecosystem functioning at four different lateral lagoons of a sub-tropical river. This comparative study was conducted based on ecotrophic model outputs that describe each system’s structure and functioning. The models were constructed using the Ecopath software to differentiate main biomass flows in the systems. Six surveys from September 2004 to August 2005 were carried out in a river stretch of Rosana Reservoir, the last one in a cascade of eleven reservoirs along the Paranapanema River (southeastern, Brazil). Four lateral lagoons and one sampling site in the main river channel were selected. Among the lagoons, one was originated by the flood of mining digging; two were considered natural, since they are located inside a preservation area (Morro do Diabo State Park); and the last one is under the influence of agriculture and cattle breeding. The present study was developed under the hypothesis that the trophic structure and functioning of lateral lagoons and the main river are distinct because their unique biotic and abiotic characteristics.

Results/Conclusions

Results indicate that biomass and production in different ecosystems are concentrated in the trophic levels (TL) 2 and 3. However, lagoons reached TL 5 and 6, while the river reached TL 4. High TL biomass and production in the river may be explained by the presence of other reservoir and two tributaries close to the sampling site. High production per biomass (P/B) and food consumption per biomass (Q/B) values in lagoons indicate the high productivity of these systems and the abundance of juveniles in most groups, which utilize these systems as refuge zones and nurseries. The difference among sampling sites, however, lies between the main energy source and how it is incorporated in the food web of each ecosystem. The lagoons are detritus-driven ecosystems while the river is dominated by the plankton pathway. The lagoons showed higher detritus and nutrients cycling and higher number of ways through which energy can flow in ecosystem. Finally, lagoons and river showed similar values of maturity, defined by the ratio of primary productivity and total respiration (P/R), stability, and overhead, indicating these ecosystems are resilient and homeostatic.