Changing water, phosphorus and nitrogen budgets for Valle de Bravo reservoir, water supply for Mexico City Metropolitan Area

Valle de Bravo reservoir (VB) provides water supply to the Mexico City Metropolitan Area and other surrounding cities. Nutrient loading to this reservoir increased 276% for phosphorus (P) and 203% for nitrogen (N) in a single decade. During 2002-2005, P and N mean loadings to VB were 120.8 x 10(3) kg P/y and 591.8 x 10(3) kg N/y. These loadings were quite variable because of source variations from uncontrolled domestic and agricultural inputs. More than half (56%) of the maximal water storage of the reservoir was withdrawn annually. Water withdrawal removed 22% of the P input. Comparative examination of P and N mass balances showed that most (85%) of the P input to VB accumulates in the sediments. Despite the hypolimnetic anoxia that VB exhibits from March to October, net P accumulation in the sediments is normally observed. Our results confirm that although VB behaves as a warm monomictic water body, its mean hypolimnetic temperature increases throughout the stratification period. Nitrogen limitation was indicated by the dissolved inorganic nitrogen (DIN) to total dissolved P (DIN: TDP = 8.3, molar) ratio during stratification periods. We estimated that N-2 fixation exceeded denitrification. This net fixation could double the N loading from rivers and sewage. Management recommendations include (a) gauging of river and sewage inputs to VB, (b) reduction of P input through treatment of sewage from VB town and (c) monitoring and regulating fertilizers and other nonpoint source inputs in the Amanalco watershed. Valle de Bravo reservoir is similar to other eutrophic tropical lakes and reservoirs that could also improve their water quality through these management practices.