Vertical Boundary Mixing Events during Stratification Govern Heat and Nutrient Dynamics in Windy Tropical Lakes with High Water-Level Fluctuations: A Long-Term (2001-2018) Study

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) lake, where strong diurnal winds drive internal waves, boundary mixing and hypolimnetic warming during stratification periods. We monitored VB during 18 years (2001-2018) when important water-level fluctuations (WLF) occurred, affecting mixing and nutrient flux. Mean hypolimnetic temperature increase (0.06–1.04°C month-1) occurred in all the stratifications monitored. We analyzed temperature distributions and modeled the hypolimnion heat budget to assess vertical mixing between layers (26,618–140,526 m-3h-1), vertical diffusivity coefficient KZ (6.2x10-7–3.3x10-6 m2s-1) and vertical nutrient entrainment to epilimnion on monthly scale. Stability also varied as a function of WLF. Nutrient flux to the epilimnion ranged 0.36–5.99 mg m-2d-1 for soluble reactive phosphorus (SRP) and 5.8–97.1 mg m-2d-1 for dissolved inorganic nitrogen (DIN). During low water-level years, vertical nutrient fluxes increase and can account for up to >40% of the total external nutrients load to the lake. Vertical mixing changes related to WLF affect nutrient recycling, their flux to sediments, ecosystemic metabolic balance and planktonic composition of VB.

Nitrogen and Phosphorous Retention in Tropical Eutrophic Reservoirs with Water Level Fluctuations: A Case Study Using Mass Balances on a Long-Term Series

“…All of the above have been identified as causes of the eutrophication of VB [26,29]. VB is a warm monomythic water body with strong WLF linked to water management for supply purposes [33]. VB is located in a mountainous region where strong diurnal wind flows [26].…”

“…VB is located in a mountainous region where strong diurnal wind flows [26]. Both WLF and strong wind flows impact diverse limnological and ecological aspects such as stability of stratification, nutrient exchange between the epilimnion and the hypolimnion [33], composition of the phytoplankton and zooplankton community and ecosystem metabolism [31,32,40].…”

“…However, in the following years, important WLF occurred in VB. Fortunately, the continuous monitoring of the reservoir maintained since then makes possible the VB is a warm monomythic water body with strong WLF linked to water management for supply purposes [33]. VB is located in a mountainous region where strong diurnal wind flows [26].…”

“…However, in the following years, important WLF occurred in VB. Fortunately, the continuous monitoring of the reservoir maintained since then makes possible the assessment of the effects of WLF on N and P fluxes, as was previously done for vertical water fluxes [33].…”

“…In an initial (2002)(2003)(2004)(2005) time-series mass balance assessment in VB during which high WLF did not occur, Ramírez-Zierold et al [29] estimated that N 2 fixation exceeded denitrification and doubled the N loading that the reservoir received from rivers and sewage at the time. Nevertheless, important WLF have occurred since then in VB and their significant effect on vertical mixing has been outlined [33].…”