Browsing by Autor "Xavier Lazzaro"

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Algal Bloom Exacerbates Hydrogen Sulfide and Methylmercury Contamination in the Emblematic High-Altitude Lake Titicaca
(Multidisciplinary Digital Publishing Institute, 2018) Darío Achá; Stéphane Guédron; David Amouroux; David Point; Xavier Lazzaro; Pablo Fernandez; Géraldine Sarret
Algal blooms occurrence is increasing around the globe. However, algal blooms are uncommon in dominantly oligotrophic high-altitude lakes. Lake Titicaca, the largest freshwater lake in South America, located at 3809 m above the sea level, experienced its first recorded algal bloom covering a large fraction of its southern shallow basin in March–April 2015. The dominant algae involved in the bloom was Carteria sp. Water geochemistry changed during the bloom with a simultaneous alkalinization in heterotrophic parts of the lake and acidification in eutrophic shallow areas. A decrease in oxygen saturation (from 105 to 51%), and a dramatic increase in hydrogen sulfide (H2S) concentrations (from <0.02 to up to 155 µg∙L−1) resulted in the massive death of pelagic organisms. Such changes were brought by the exacerbated activity of sulfate-reducing bacteria (SRB) in this sulfate-rich lake. Although levels in total mercury remained stable during the event, MMHg % rose, highlighting higher conservation of produced MMHg in the water. Such an increase on MMHg % has the potential to produce exponential changes on MMHg concentrations at the end food web due to the biomagnification process. Our physicochemical and climatological data suggest that unusually intense rain events released large amounts of nutrients from the watershed and triggered the bloom. The observed bloom offers a hint for possible scenarios for the lake if pollution and climate change continue to follow the same trend. Such a scenario may have significant impacts on the most valuable fish source in the Andean region and the largest freshwater Lake in South America. Furthermore, the event illustrates a possible fate of high altitude environments subjected to eutrophication.
Aquatic biota responses to temperature in a high Andean geothermal stream
(Wiley, 2021) Estefania Quenta; Antonio Daza; Xavier Lazzaro; Dean Jacobsen; Olivier Dangles; Sophie Cauvy‐Fraunié
Abstract The impact of global warming on mountain ecosystems is predicted to be high, and particularly in the tropical region. Geothermal streams have provided comprehensive evidence about how aquatic biodiversity changes across natural thermal gradients, but current knowledge is restricted to arctic and temperate zones. Thermal tolerances are different in tropical biological communities, resulting in high thermal sensitivity and low capacity to endure change in their thermal environments. This feature can change the response of aquatic organisms to warming, yet there is little empirical evidence to support this assumption. In this study, we address this issue by evaluating how water temperature affects biodiversity, and the structure of primary and secondary producers of a high‐elevation geothermal stream system (4,500 m above sea level) in the Bolivian Andes. We analysed multi‐taxa responses to increased water temperature using benthic macroinvertebrate families, benthic algae and cyanobacteria, fishes, and macrophytes as study organisms. Different models were run to assess the response of aquatic biota to temperature. In addition, threshold indicator taxa analysis (TITAN) was used to identify changes in macroinvertebrate taxa distributions along the thermal gradient. We found that macroinvertebrate richness decreased at 24–25°C due to the different taxon‐specific responses to temperature. Threshold indicator taxa analysis identified 17 temperature thresholds for each family of macroinvertebrates. Changes in macroinvertebrate community composition were significantly associated with changes in water temperature. Similarly, changes in macrophytes were associated with temperature differences, and high macrophyte richness was found at 19–20°C. Chlorophyll ‐a concentration of green algae and diatoms was higher at intermediate temperatures 20–22°C, macroinvertebrates density peaked at 27°C, and fish body size reduced linearly with temperature. Temperature increase in the geothermal stream resulted in a reduction of aquatic diversity and primary and secondary producers by simplifying the community structure to a few warm‐adapted taxa and reduced body size. These patterns differed from those obtained in temperate/arctic geothermal streams, but are similar to other studies at high‐elevation. In a context of increasing warming, the ecological structure of high‐elevation streams might lose cold‐adapted taxa, and change to smaller populations. Additional studies based on ecosystem functioning of geothermal streams could lead to a better understanding on how warming affects high‐elevation streams.
Association of a Specific Algal Group with Methylmercury Accumulation in Periphyton of a Tropical High-Altitude Andean Lake
(Springer Science+Business Media, 2016) William G. Lanza; Darío Achá; David Point; Jérémy Masbou; L. Alanoca; David Amouroux; Xavier Lazzaro
Contaminación de la Bahía de Cohana, Lago Titicaca (Bolivia): Desafíos y oportunidades para promover su recuperación
(2017) Carlos I. Molina; Xavier Lazzaro; Stéphane Guédron; Darío Achá
HydroMet: La primera boya automática perfiladora con alta-frecuencia en el Lago Titicaca, el más alto de los Grandes Lagos del mundo
(2021) Xavier Lazzaro; Darío Achá Cordero; Viviana Cruz Hernández; Jhasmin Duarte Tejerina; Mishel Justiniano Ayllón; Gustavo Lanza Aguilar; Javier Maldonado Alfaro; Carlos Molina Arzabe; Javier Nuñez Villalba; Marcela Ormachea Rojas
In Situ Photochemical Transformation of Hg Species and Associated Isotopic Fractionation in the Water Column of High-Altitude Lakes from the Bolivian Altiplano
(American Chemical Society, 2022) Sylvain Bouchet; Emmanuel Tessier; Jérémy Masbou; David Point; Xavier Lazzaro; Mathilde Monperrus; Stéphane Guédron; Darío Achá; David Amouroux
Photochemical reactions are major pathways for the removal of Hg species from aquatic ecosystems, lowering the concentration of monomethylmercury (MMHg) and its bioaccumulation in foodwebs. Here, we investigated the rates and environmental drivers of MMHg photodegradation and inorganic Hg (IHg) photoreduction in waters of two high-altitude lakes from the Bolivian Altiplano representing meso- to eutrophic conditions. We incubated three contrasting waters in situ at two depths after adding Hg-enriched isotopic species to derive rate constants. We found that transformations mostly occurred in subsurface waters exposed to UV radiation and were mainly modulated by the dissolved organic matter (DOM) level. In parallel, we incubated the same waters after the addition of low concentrations of natural MMHg and followed the stable isotope composition of the remaining Hg species by compound-specific isotope analysis allowing the determination of enrichment factors and mass-independent fractionation (MIF) slopes (Δ199Hg/Δ201Hg) during in situ MMHg photodegradation in natural waters. We found that MIF enrichment factors potentially range from -11 to -19‰ and average -14.3 ± 0.6‰ (1 SE). The MIF slope diverged depending on the DOM level, ranging from 1.24 ± 0.03 to 1.34 ± 0.02 for the low and high DOM waters, respectively, and matched the MMHg MIF slope recorded in fish from the same lake. Our in situ results thus reveal (i) a relatively similar extent of Hg isotopic fractionation during MMHg photodegradation among contrasted natural waters and compared to previous laboratory experiments and (ii) that the MMHg MIF recorded in fish is characteristic for the MMHg bonding environment. They will enable a better assessment of the extent and conditions conducive to MMHg photodegradation in aquatic ecosystems.
Intermediate predation pressure leads to maximal complexity in food webs
(Wiley, 2015) Benoît Gauzens; Stéphane Legendre; Xavier Lazzaro; Gérard Lacroix
Responses of ecosystems to modifications of their environmental conditions are usually considered in terms of biodiversity or function. Maybe because they represent a hidden part of ecosystems, responses of ecological interactions are rarely studied. A more comprehensive view of the processes underlying the restructuring of food webs under environmental gradients appears crucial to understand how ecosystems functionalities are altered. We address this general issue in an experiment where trophic interactions are reorganized under a gradient of top–down effects (predation pressure) and bottom–up effects (nutrient availability). Unimodal relationships of species diversity are pervasive in ecology; we extend this principle to food‐web topology: in our study, most topological descriptor values peak at intermediate predation intensity. The same unimodal pattern holds for network complexity, measured by entropy and scaled entropy (a measure independent of species diversity). Moreover, food web complexity is maximized at higher fish abundance when nutrient availability is increased. We infer that whereas in absence of top predators, a mechanism of competitive exclusion takes place, and indirect facilitation process underlies systems with a moderate predation pressure. These results highlight the importance of the dynamic reorganization of trophic links in response to bottom–up and top–down effects. Interaction between bottom–up and top–down forces argue for multifactorial studies of ecological effects.
Mutualism between euryhaline tilapia Sarotherodon melanotheron heudelotii and Chlorella sp.—Implications for nano-algal production in warmwater phytoplankton-based recirculating systems
(Elsevier BV, 2008) Sylvain Gilles; Gérard Lacroix; Daniel Corbin; Ngansoumana Bâ; Carla Ibañez Luna; Jacob Nandjui; Allassane Ouattara; Ousséni Ouédraogo; Xavier Lazzaro
Responses of phytoplankton and periphyton community structure to an anthropic eutrophication gradient in tropical high-altitude Lake Titicaca
(Elsevier BV, 2024) William G. Lanza; Viviana Cruz Hernández; Darío Achá; Xavier Lazzaro