Abstract
This research investigated the source and fate of different chemical species of N and P on a deep tropical urban reservoir, the artificial Lake Paranoá, located in the city of Brasilia (Brazil). To determine an N and P budget, nutrient input from the external load (four main tributaries and two wastewater treatment plants), internal load (from sediment) and nutrient output (from a downstream dam) were estimated empirically. Nutrient storage was evaluated in two compartments: water column and sediment. Nutrient input from the tributaries varied by season presenting higher loads in the wet season, especially N. Nutrient budgets in our study indicated that Lake Paranoá retained dissolved inorganic nitrogen (DIN), PO43−-P, total organic phosphorus (TOP) and exported total organic nitrogen (TON), both on a seasonal and annual scale. Surface sediment is the major storage compartment for both N and P. These results show the pressing need for action to reduce the P outcome charges, mainly, from the wastewater treatment plants. The data here presented contributes to the recognition of this situation and to a better comprehension of these nutrient dynamics, as well as an understanding of the behavior of tropical deep-water reservoirs. This can help to promote more effective management, providing a reference for other similar systems.
Highlights
Water availability and demand issues are exacerbated by the lack of planning regarding usage and occupation of a hydrographic basin
The annual N budget for dissolved inorganic nitrogen (DIN), showed retention of this form of N, equivalent to 85% of the total incoming load (Figure 6)
The approach used in this study, based on a mass balance for N and P, including several inorganic and organic species of these elements in the different compartments of Lake Paranoá, is shown to be an appropriate and important tool for the implementation of hydric resources management mechanisms that focus on the reduction of nutrient input, and resulting development of eutrophication processes, mainly in deep tropical reservoirs, where studies are yet incipient
Summary
Water availability and demand issues are exacerbated by the lack of planning regarding usage and occupation of a hydrographic basin. Through ‘point sources’ or ‘diffuse sources’ in a hydrographic basin (external load), considerable amounts of nutrients can reach lakes and reservoirs, inducing eutrophication with P perceived as the limiting nutrient to primary production [1,2] and the key factor in determining the trophic state in continental water systems. The P input from sediment (internal load) has been reported to be an important source for the eutrophication of water bodies [3,4]. Eutrophication can significantly compromise the quality of lake and reservoir waters, undermining multiple uses of the water, for instance: for agricultural irrigation, industrial use, human and animal consumption, and preservation of aquatic ecosystems. One of the main concerns related to the eutrophication process is the proliferation of cyanobacteria over other aquatic species.
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