Artificial eutrophication in a reservoir makes the use of water unviable by the human
population, destabilizes all ecosystemic attributes and can promote irreducible changes in
biodiversity in the aquatic environment. Thus, to understand how fish play their trophic
functions and their ecological role in the face of limnological changes, the present study aimed
to evaluate the relationship between the water quality of the reservoir of the Curuá-Una
hydroelectric power plant, the energy flow and the trophic structure of the food chain of the
main species of fish belonging to different guilds. The samples were taken at the Curuá-Una
reservoir, the first in Central Amazonia, in two periods of a hydrological cycle, upstream and
downstream of the dam. In the first part of the study, the diet of 22 fish species was evaluated
through the calculation of the Food Importance Index. The results obtained allowed the
identification of trophic interactions that suggest the development of trophic stabilization after
four decades of reservoir formation. In the second part of the study, the correlations between
energy flow and food chain structure were investigated with changes in limnological parameters
as a function of a possible artificial eutrophication. Carbon (δ13C) and nitrogen (δ15N) isotopic
compositions of fish and autotrophic sources were analyzed and nine limnological parameters
were measured. According to the results obtained, the reservoir does not present an advanced
artificial eutrophication, but the isotopic enrichment of nitrogen in the fish and in the
autotrophic sources indicates that there is an increase in the concentration of nutrients in the
aquatic environment and this in turn is incorporated in the base of the chain and transferred to
the other trophic levels. Although conventional water analysis methods do not yet detect
changes in nutrient content, the nitrogen isotope has proved to be a sensitive ecological
indicator for such a diagnosis. On the other hand, there were no significant changes in the
energy flow of the food chain. Thus, it was concluded that after forty years of formation of the
Curuá-Una reservoir the ichthyofauna reached trophic stabilization, but this stage has signs of
anthropogenic disturbance that are likely to compromise the ecological homeostasis of the
reservoir.
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