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 K_Z (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.

Nitrogen and phosphorous loading drives eutrophication of aquatic systems. Lakes and reservoirs are often effective N and P sinks, but the variability of their biogeochemical dynamics is still poorly documented, particularly in tropical systems. To contribute to the extending of information on tropical reservoirs and to increase the insight on the factors affecting N and P cycling in aquatic ecosystems, we here report on a long-term N and P mass balance (2003–2018) in Valle de Bravo, Mexico, which showed that this tropical eutrophic reservoir lake acts as a net sink of N (−41.7 g N m⁻² y⁻¹) and P (−2.7 g P m⁻² y⁻¹), mainly occurring through net sedimentation, equivalent to 181% and 68% of their respective loading (23.0 g N m⁻² y⁻¹ and 4.2 g P m⁻² y⁻¹). The N mass balance also showed that the Valle de Bravo reservoir has a high net N atmospheric influx (31.6 g N m⁻² y⁻¹), which was 1.3 times the external load and likely dominated by N₂ fixation. P flux was driven mainly by external load, while in the case of N, net fixation also contributed. During a period of high water level fluctuations, the net N atmospheric flux decreased by 50% compared to high level years. Our results outlining water regulation can be used as a useful management tool of water bodies, by decreasing anoxic conditions and net atmospheric fluxes, either through decreasing nitrogen fixation and/or promoting denitrification and other microbial processes that alleviate the N load. These findings also sustain the usefulness of long-term mass balances to assess biogeochemical dynamics and its variability.

Small-sized invertebrates inhabiting hard substrates in coral reefs (a.k.a. cryptofauna) contribute substantially to reef biodiversity, but their patterns of distribution and ecological controls are poorly understood. Here, we characterized the cryptofauna community and explored “bottom-up” and “top-down” controls by benthic cover and fish abundance, respectively. We sampled the cryptofauna inhabiting the reef terrace from 13 sites along 200 km in Jardines de la Reina (Cuba), a well-preserved and protected area in the Caribbean. We counted 23,959 invertebrates of 14 higher taxa, being the most abundant Copepoda (54%), Nematoda (21%), Mollusca (7%), Ostracoda (5%), Polychaeta (5%), and Amphipoda (3%). Richness, abundance, and community structure varied across the reefs without any geographical gradient of distribution. One-third of the variance occurred at site scale (~ 10 km), and half occurred at quadrat scale (~ 1 m). Algal cover promoted cryptofauna richness and abundance likely providing substrate and food, while live coral cover negatively influenced nematode abundances, potentially due to coral defenses. Relationships between cryptofauna and reef fishes were also present, with invertivores and herbivores negatively affecting cryptofauna abundance likely due to direct or indirect predation pressures. This research highlights the important roles of bottom-up and top-down controls, by algal/coral cover and fishes, respectively, on cryptofauna and in extension to coral reef biodiversity. Current threats by climate change are expected to alter these controls on cryptofauna resulting in changes to diversity, trophodynamics and energy flows of coral reefs.

Coastal lagoons are highly sensitive ecosystems from the ecological point of view. Water samples to assess physicochemical parameters, as well as sediments to measure trace metals (Cd, Cr, Cu, Ni, Pb, Zn and Hg) were collected in seven coastal lagoons of the Sabana-Camagüey Archipelago. Los Caimanes, Tortuguilla and La Salina lagoons showed high salinity levels (>100.0 PSU). High concentrations of Total Nitrogen (TN, up to 819.3 mmol/L) and Total Phosphorus (TP, up to 7.8 mmol/L) were recorded in the lagoons with more impact from tourism development, such as Las Coloradas, Larga, Los Caimanes and La Salina lagoons. Regarding their relative abundance, metals ranked as follows: Ni > Zn > Cu > Cd > Hg > Cr >Pb. Metal concentrations ranged within the following intervals: Pb: < 50 mg/kg (all values below the quantification limit); Ni: <12.5–115.7 mg/kg; Cd: <2.0–3.4 mg/kg; Zn: 5.2–81.4 mg/kg; Cu: <5.0–45.2 mg/kg; Cr: <25.0–156.4 mg/kg and Hg: <0.1–0.30 mg/kg. The concentrations of trace metals were similar to those recorded in uncontaminated coastal systems of Cuba and other parts of the world. Nevertheless, most trace metals, except Hg, showed high concentrations at Site 1 of Laguna Las Coloradas. Laguna Tortuguilla was the lagoon with the highest ecological risk. Cd must be carefully monitored there, as concentrations detected were high (up to 3.4 mg/kg), which poses an ecological risk for all the lagoons.

Through a nested suite of methods here we contrast the coexistence of different ecosystem states in a tropical coastal lagoon, the Laguna Larga, with increasing eutrophication stress between 2007 and 2009. Water temperature averaged 27.4°C in the lagoon and showed a slight positive trend during the study period. Salinity averaged 35.0±6.2, exhibiting high spatial and temporal variability, and also a slight positive trend in time. In contrast, dissolved oxygen showed a substantial decreasing trend (–0.83 ml L^–1 y^–1; –13.3% y^–1) over the period, while nutrients increased dramatically, particularly total phosphorus (2.6 µM y^–1), in both cases sustaining the progression of eutrophication in the lagoon during the three years we sampled. The Karydis nutrient load-based trophic index showed that the lagoon has a spatial pattern of increasing eutrophication from the sea and the outer sector (oligotrophic-mesotrophic) to the central (mesotrophic) and the inner sector (mesotrophic-eutrophic). Two ecosystem states were found within the lagoon. In the outer oligotrophic sector, the dominant primary producers were macroalgae, seagrasses and benthic diatoms, while mollusc assemblages were highly diverse. In the inner and central sectors (where trophic status increased toward the inner lagoon) a phytoplankton-dominated ecosystem was found where mollusc assemblages are less diverse. In spite of the progression of eutrophication in the lagoon, these two different ecosystems coexisted and remained unchanged during the study period. Apparently, the effect of water residence time, which increases dramatically toward the inner lagoon, dominated over that of nutrient loadings, which is relatively more homogeneously distributed along the lagoon. Therefore, we consider that actions that reduce the water residence time are likely the most effective management options for this and other similarly choked lagoons.

Irma was a major hurricane that developed during the 2017 season. It was a category 5 on the Saffir–Simpson Hurricane wind scale. This hurricane caused severe damage in the Caribbean area and the Florida Keys. The social, economic, and environmental impacts, mainly related to coastal flooding, were also significant in Cuba. The maximum limits of coastal flooding caused by this hurricane were determined in this research. Field trips and the use of the GPS supported our work, which focused on both the northern and southern coasts of the Ciego de Ávila province. This work has been critical for improving coastal flooding scenarios related to a strong hurricane, as it has been the first experience according to hurricane data since 1851. Results showed that the Punta Alegre and Júcaro towns were the most affected coastal towns. The locals had never seen similar flooding in these places before. The differences between flood areas associated with Hurricane Irma and previous modeled hazard scenarios were evident (the flooded areas associated with Hurricane Irma were smaller than those modeled for categories 1, 3, and 5 hurricanes). The effects of this hurricane on the most vulnerable coastal settlements, including the impacts on the archeological site “Los Buchillones”, were also assessed.