Laguna Larga, a coastal lagoon in central Cuba, has been heavily altered by tourism infrastructure construction and sewage disposal. We hypothesize that this has decreased the circulation and caused eutrophication of the lagoon. To assess this, 12 bimonthly samplings were carried out in 2007-2008. Temperature, salinity, oxygen, nutrients and nitrogen, and phosphorous fractions (inorganic, organic, and total) were determined. Water and salt budgets, as well as biogeochemical fluxes of nitrogen and phosphorus were calculated using the LOICZ budget model for the three sections of the lagoon identified by morphological constrains and salinity patterns. Laguna Larga is a choked lagoon with restricted water circulation, low exchange, and high residence times that vary significantly along its sections. Residence time was estimated to be 0.1-0.7 years for the inner section and 1-9 days for the outer one. High levels of total nitrogen (annual means 126-137 μM, peaks up to 475 μM) and phosphorus (2.5-4.4 μM, peaks up to 14.5 μM) are evidence of eutrophication of Laguna Larga. During 2007, an average precipitation year, Laguna Larga exported water (703 m(3) d(-1)) and was a source of nitrogen (9.026 mmol m(-2) d(-1)) and phosphorus (0.112 mmol m(-2) d(-1)) to the adjacent sea. δ(15)N determinations in the seagrass Thalassia testudinum (-1.83 to +3.02 ‰) differed significantly between sites in the lagoon and offshore reference sites located W of the inlet, but were similar to those located E of the inlet. δ(15)N determinations in the seaweed Penicillus dumetosus (+1.02 to +4.2) did not show significant differences.

Microbialites are modern analogs of ancient microbial consortia that date as far back as the Archaean Eon. Microbialites have contributed to the geochemical history of our planet through their diverse metabolic capacities that mediate mineral precipitation. These mineral-forming microbial assemblages accumulate major ions, trace elements and biomass from their ambient aquatic environments; their role in the resulting chemical structure of these lithifications needs clarification. We studied the biogeochemistry and microbial structure of microbialites collected from diverse locations in Mexico and in a previously undescribed microbialite in Cuba. We examined their structure, chemistry and mineralogy at different scales using an array of nested methods including 16S rRNA gene high-throughput sequencing, elemental analysis, X-Ray fluorescence (XRF), X-Ray diffraction (XRD), Scanning Electron Microscopy-Energy Dispersive Spectroscopy (SEM-EDS), Fourier Transformed Infrared (FTIR) spectroscopy and Synchrotron Radiation-based Fourier Transformed Infrared (SR-FTIR) spectromicroscopy. The resulting data revealed high biological and chemical diversity among microbialites and specific microbe to chemical correlations. Regardless of the sampling site, Proteobacteria had the most significant correlations with biogeochemical parameters such as organic carbon (Corg), nitrogen and Corg:Ca ratio. Biogeochemically relevant bacterial groups (dominant phototrophs and heterotrophs) showed significant correlations with major ion composition, mineral type and transition element content, such as cadmium, cobalt, chromium, copper and nickel. Microbial-chemical relationships were discussed in reference to microbialite formation, microbial metabolic capacities and the role of transition elements as enzyme cofactors. This paper provides an analytical baseline to drive our understanding of the links between microbial diversity with the chemistry of their lithified precipitations.

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.

Laguna Larga (Cayo Coco, Cuba) is a eutrophic coastal lagoon due to tourism development. As part of long-term monitoring of Laguna Larga, we were able to follow the lagoon's water quality from 2015 to 2018 and could assess the impacts of Hurricane Irma (September 8–9, 2017) by intensifying our sampling frequency. Physicochemical parameters (salinity, pH, dissolved oxygen, dissolved inorganic nitrogen, dissolved reactive silicate and total nitrogen) exhibited significant variations associated with hurricane Irma. Salinity decreased due to the extraordinary rainfall of the hurricane (339.8 mm/24 h, a new record for Cayo Coco). The water level in the lagoon rose 0.85 m. Strong hurricane winds and intense runoff drove organic matter and sediment resuspension. Anoxia and an increase of nutrients occurred throughout the lagoon. The main biogeochemical impact was that it boosted these eutrophic conditions of the lagoon, to levels that lasted for several months. A significant correlation among nutrients, salinity and dissolved oxygen was found. After 6 months, water quality in the lagoon had recovered to conditions similar to those before the hurricane. The case of Laguna Larga sustains those coastal systems under anthropic pressure that can take longer to recover after extreme climatic events, and highlights the need for long-term monitoring of tropical coastal ecosystems.