The dynamics of total phosphorus (TP) in 18 strategic reservoirs of the high-density reservoir network of the Brazilian semiarid was evaluated during the wet and dry periods for the past 12 years. Seasonal overlying concentrations presented no significant differences for about 90% of the reservoirs (p>0.05). This was attributed to a trade-off between the hydrological/limnological processes occurring in the two seasons. Then, a transient complete-mix mass balance model was applied with particular adaptations for the tropical semiarid reservoirs to estimate the TP load for each season. Because of the relatively well-mixed conditions and high hypolimnetic dissolved oxygen concentrations during the wet season, the wet load was assumed to represent the external TP load. On the other hand, because of the absence of reservoir inflow during the dry season, phosphorus release under anoxic sediment conditions and wind-induced resuspension under shallow water depths, the dry load was assumed to reflect the internal TP load. The maximum external loads were related to peak inflows, notably after drought periods. Consistently, the largest internal loads were obtained during the drought periods, when the reservoirs were shallower and more prone to phosphorus release and resuspension. By comparing the impact of the two input load types, the wet period load was predominant in 72% of the reservoirs. The areal phosphorus loads ranged from 0.66 to 7.29 gP m 2 year −1 , which were consistent with the literature, despite the very high density of reservoirs. Finally, power-law curves including data for all studied reservoirs were adjusted between the dry period load and volume, dry and wet period loads, wet period load and inflow, and total load and catchment area, resulting in satisfactory R 2 (0.84-0.98).
The nitrogen (N) cycle from the catchment to the downstream reservoir is complex, particularly the quanti cation of N losses. However, in order to assess the nitrogen impact in a reservoir ecosystem, simpli ed models may be applicable regarding the TN load production and the magnitude of lake TN removal. This study presented a methodology to perform and validate a TN mass balance to further calibrate a simpli ed coe cient for TN losses (v f .) in 29 tropical semiarid reservoirs. The study reservoirs were highly productive ecosystems with an average total nitrogen (TN) concentration, accounting for all measurements in all reservoirs, ranging from 0.59 mg L − 1 to 3.84 mg L − 1 . Regarding the production of TN load, the median values ranged from 4.35 to 2,499.43 t yr − 1 with median of 80.34 t yr − 1 . The TN loads were estimated through an annual mass balance over a twenty-four-year period. The median of the estimates was compared with reference values obtained by using the export modelling coe cient. The correlation between the median estimated and reference loads resulted in satisfactory agreement (r² 0.88) and reinforced the reliability of the mass balance alternative. From the validated TN loads, the TN uptake velocity (v f ) was estimated for all reservoirs (44.9 ± 20.1 m yr − 1 ) and could be described as a general function of the water residence time. The reservoirs of the study region have demonstrated higher v f than temperate lakes and reservoirs and similar v f with Latin America/Caribbean ones. As expected, reservoirs of warmer climates tend to present intensi ed N loss processes as compared to lakes and reservoirs of temperate regions. The methodology proposed in the present study can be used to potentially improve water quality management in tropical semiarid reservoirs.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.