Critical load of phosphorus in reservoirs of a tropical semi‐arid region

Abstract

The over-enrichment of water bodies by nutrients and the increase in primary production is a natural process that happens slowly, however, this process has been occurring in recent decades rapidly, due to the increase in the external load of anthropogenic nutrients, mainly phosphorus. Lake and reservoir eutrophication models seeking to calculate the critical load of phosphorus often do not consider the temporal variation in water volume. In this study, a dynamic model was used to estimate critical loads of phosphorus from six reservoirs in Brazil's semi-arid region, all of which experience great water volume variation, to describe the variation in water balance together with the variation in phosphorus mass balance. The critical load of phosphorus was defined as the load that guaranteed total phosphorus concentrations of up to 30 mg/m3 throughout 90% of the simulation period. A total of 40 scenarios were simulated and combined two vertical flows of phosphorus, five concentrations of phosphorus in the affluent volumes, and four water demands. Results revealed that the critical load of phosphorus was strongly affected by water demand, what pose a strong dilemma toward the operation of reservoirs in the semi-arid region of Brazil between rationing stored water for use in long periods of drought or stimulating water use to increase water turnover rates. The critical load of phosphorus was also affected by phosphorus concentration in affluent volumes, and the vertical flow of phosphorus between the sediment and water columns. The findings of this study can help for better understanding of the impact of temporal variations in lake and reservoir water volumes to define critical loads of phosphorus more accurately and control eutrophication more effectively in these environments.