Long-term and seasonal changes in production and respiration were surveyed in the Valle de Bravo reservoir, Mexico, in a period during which high water-level fluctuations occurred (2006-2015). We assessed the community metabolism through oxygen dynamics in this monomictic water-body affected by strong diurnal winds. The multiple-year data series allowed relationships with some environmental drivers to be identified, revealing that water level-fluctuations strongly influenced gross primary production and respiratory rates. Production and respiration changed mainly vertically, clearly in relation to light availability. Gross primary production ranged from 0.15 to 1.26 gO₂ m^-2 h^-1, respiration rate from -0.13 to -0.83 gO₂  m^-2 h^-1 and net primary production from -0.36 to 0.66 gO₂  m^-2 h ^-1 within the production layer, which had a mean depth of 5.9 m during the stratification periods and of 6.8 m during the circulations. The greater depth of the mixing layer allowed the consumption of oxygen below the production layer even during the stratifications, when it averaged 10.1 m. Respiration below the production layer ranged from -0.23 to -1.38 gO₂ m^-2 h^-1. Vertically integrated metabolic rates (per unit area) showed their greatest variations at the intra-annual scale (stratification-circulation). Gross primary production and Secchi depth decreased as the mean water level decreased between stratification periods. VB is a highly productive ecosystem; its gross primary production averaged 3.60 gC m^-2 d^-1 during the 10 years sampled, a rate similar to that of hypertrophic systems. About 45% of this production, an annual average net carbon production of 599 g C m^-2 year^-1, was exported to the hypolimnion, but on the average 58% of this net production was recycled through respiration below the production layer. Overall, only 19% of the carbon fixed in VB is buried in the sediments. Total ecosystem respiration rates averaged -6.89 gC  m^-2 d^-1 during 2006-2015, doubling the gross production rates. The reservoir as a whole exhibited a net heterotrophic balance continuously during the decade sampled, which means it has likely been a net carbon source, potentially releasing an average of 3.29 gC m^-2 d^-1 to the atmosphere. These results are in accordance with recent findings that tropical eutrophic aquatic ecosystems can be stronger carbon sources than would be extrapolated from temperate systems, and can help guide future reassessments on the contribution of tropical lakes and reservoirs to carbon cycles at the global scale. Respiration was positively correlated with temperature both for the stratification periods and among the circulations, suggesting that the contribution of C to the atmosphere may increase as the reservoirs and lakes warm up owing to climate change and as their water level is reduced through intensification of their use as water sources.

Physical processes play important roles in controlling eutrophication and oligotrophication. In stratified lakes, internal waves can cause vertical transport of heat and nutrients without breaking the stratification, through boundary mixing events. Such is the case in tropical Valle de Bravo (VB) reservoir lake, where strong diurnal winds drive internal waves, boundary mixing, and hypolimnetic warming during stratification periods. We monitored VB during 21 years (2001–2021) when important water-level fluctuations occurred, affecting mixing and nutrient flux. Stability also varied as a function of water level. Hypolimnetic warming (0.009–0.028◦ C day−1) occurred in all the stratifications monitored. We analyzed temperature distributions and modeled the hypolimnion heat budget to assess vertical mixing between layers (0.639–3.515 × 10−6 m3 day−1), vertical diffusivity coefficient KZ (2.5 × 10−6 –13.6 × 10−6 m2 s−1), and vertical nutrient transport to the epilimnion. Nutrient flux from the metalimnion to the epilimnion ranged 0.42–5.99 mg P m−2 day−1 for soluble reactive phosphorus (SRP) and 5.8–101.7 mg N m−2 day−1 for dissolved inorganic nitrogen (DIN). Vertical mixing and the associated nutrient fluxes increase evidently as the water level decreases 8 m below capacity, and they can increase up to fivefold if the water level drops over 12 m. The observed changes related to water level affect nutrient recycling, ecosystemic metabolic balance, and planktonic composition of VB.