Valle de Bravo (VB) is a tropical reservoir located (19°21 0 30@ N, 100°11 0 00@ W) in the highlands of Mexico. The reservoir is daily swept by strong (7.4 m s À1 mean speed) diurnal (12:00-19:00 h) winds that blow along its two main arms. As expected from its fetch (6.9 km) and its depth (21.1 m mean), the reservoir behaves as a warm monomictic water body. During 2001, VB was stratified from February to October, and well mixed from November to January. Its mean temperature was 19.9°C; the maximum found was 23.8°C in the epilimnion, while a minimum of 17.8°C was registered during mixing. VB exhibited a thermal regime similar to other water bodies of the Mexican tropical highlands, except for a steady increase of its hypolimnetic temperature during stratification, which is attributed to entrainment of epilimnetic water into the hypolimnion. During stratification, the hypolimnion was anoxic, while the whole water column remained under-saturated (60%) during mixing. The flushing time is 2.2 years. Mineralization and total alkalinity are low, which allows strong changes in pH. Ammonia remained low (2.4 lmol l À1 mean) in the epilimnion, but reached up to 60 lmol l À1 in the hypolimnion. Soluble reactive phosphorous had a mean of 0.28 lmol l À1 in the epilimnion and a mean of 1.25 lmol l À1 in the hypolimnion. Nitrate exhibited maxima (up to 21 lmol l À1 ) during mixing, and also in the metalimnion (2 lmol l À1 ) during stratification. Low dissolved inorganic nitrogen indicated nitrogen limitation during stratification. Eutrophication is an emerging problem in VB, where cyanobacteria dominate during stratification. At VB chlorophyll a is low during mixing (mean of 9 lg l À1 ), and high during stratification (mean 21 lg l À1 ), when blooms (up to 88 lg l À1 ) are frequent. This pattern is similar to that found in other eutrophic tropical water bodies. We propose that in VB the wind regime causes vertical displacements of the thermocline (0.58-1.10 m hr À1 ) and boundary mixing, enhancing the productivity during the stratification period in this tropical reservoir.

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The quality of groundwater is threatened in karstic regions with very high population growth, such as the eastern coast of Yucatan. As polluted groundwater flows towards the ocean, coastal ecosystems and coral reefs may also be affected. Pollution and the interaction between the coastal aquifer and the reef lagoon were assessed at a developing area (Puerto Morelos, NE Yucatan Peninsula) within the Mesoamerican Coral Reef System. Coastal environments along the land-sea gradient (wells, mangroves, beaches, submarine springs, the reef lagoon, and the open sea) were sampled. Silicate and salinity were used as tracers of groundwater and seawater, respectively. Their patterns evidence water flow and mixing among these coastal environments. High nitrate concentrations (268.6 μM) and coliform bacteria densities indicate groundwater pollution in most of the wells sampled and also in mangroves, beaches, and submarine springs. Phosphorous content peaks (14.2 μM) in mangroves, where it is likely released due to reducing conditions in the sediments. Nitrogen flux toward the lagoon reef through groundwater discharge is estimated at 2.4 ton N km −1 year −1 and phosphorous at 75 to 217 kg P km −1 year −1 . These results provide evidence of the need for more detailed groundwater studies and for the integrated management of aquifers and coastal ecosystems in karstic regions.

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.

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