Abstract
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−2day−1 for soluble reactive phosphorus (SRP) and 5.8–101.7 mg N m−2day−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.
Highlights
Understanding mechanisms that control eutrophication and oligotrophication is an issue that involves fascinating theoretical and applied possibilities
A seasonal variation on the water level of Valle de Bravo (VB) was observed during the full study period (Figure 3a), which followed the seasonal rain variations for the region, where the rainy season is from June to October
The approach used here was derived from long-term observation throughout two decades of monthly monitoring at VB, where strong winds, eutrophication, and water extraction for Mexico City interact in a system where explosive phytoplankton blooms are frequent [11,17]
Summary
Understanding mechanisms that control eutrophication and oligotrophication is an issue that involves fascinating theoretical and applied possibilities. The vertical exchange between the warm surface layer (epilimnion) and the colder hypolimnion below the thermocline boundary in stratified systems is an understudied critical process that may affect the nutrient dynamics and, their trophic condition [1]. As crossing the metalimnetic density barrier requires energy, entrainment—the transport of nutrients from the deep water—has been considered low or negligible during strong stratification [4]. Suggest that the vertical flux of nutrients across the thermocline may be an important driver of epilimnetic metabolism in some stratified lakes [5]. In this case, the energy required has often been shown to come from processes such as wind mixing and shear production within boundary layers of the lake [6]. Processes driving the enhancement of boundary mixing events and hypolimnetic entrainment are still understudied, and even more seldom approached in tropical lakes [9,10]
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