Homogenization of diatom assemblages is driven by eutrophication in tropical reservoirs.

Eutrophication is one of the most widespread causes of biotic homogenization in freshwater ecosystems. Biotic homogenization can be characterized as reductions in local diversity (alpha) and occupation of available niches by more generalist species. Beta diversity is expected to decrease in more homogeneous communities, however, there is no consensus on how it responds to eutrophication. We used a space-for-time approach to analyze the process of biotic homogenization on diatom assemblages in response to eutrophication in tropical reservoirs ranging from oligotrophic to hypereutrophic conditions. Diatom assemblages were analyzed in phytoplankton and surface sediment from 12 reservoirs with different trophic levels. We calculated total beta diversity and turnover and nestedness components and used regressions to analyze their relationships with productivity differences (without distance effects). Total beta diversity had a positive influence of the trophic gradient, whereas turnover was not related to eutrophication. However, we found that eutrophication and lower species richness (alpha diversity) led to increasing rates of the nestedness component. We also observed that the homogenization process was not characterized by invasion of new species, but, on the contrary, by filtering nutrient-rich tolerant species also present in oligo-mesotrophic reservoirs and able to occupy available niches in the eutrophic reservoirs. These findings (increase in nestedness, decrease in alpha diversity, and development of tolerant species) suggest that biotic homogenization is leading to a simplification of diatom assemblages in tropical reservoirs, making assemblages from eutrophic and hypereutrophic reservoirs a subset of assemblages from oligotrophic and mesotrophic ones.

Holocene diatoms from Xingu river: new records for the Brazilian Amazon and Brazil / Diatomáceas holocênicas do rio Xingu: novos registros para a Amazônia brasileira e o Brasil

Present study is the floristic survey and the distribution of diatoms observed in two sediment cores of the Xingu river, Pará State, north Brazil, and represents the past ~6,000 cal yr BP. Thirty-eight taxa distributed in twenty genera were recorded, among which five were new records for Brazil and nineteen new for the Brazilian Amazon. The most representative genus based on species number was Gomphonema Ehrenberg with six species. In brief, present new records represent a 75% increase in the total number of records for the Brazilian Amazon, and indicates that this region has not only been poorly investigated, but also suggests that its biodiversity assessment is underestimated. These findings highlight the efficacy of the paleolimnological approach as a tool for assessing biodiversity before the human impacts begin to affect the pristine conditions of the area.

O presente estudo visou ao levantamento florístico e à distribuição de diatomáceas obtidas de dois perfis sedimentares do rio Xingu, Estado do Pará, Brasil, e representa os últimos ~6.000 anos de idade. Trinta e oito táxons distribuídos em vinte gêneros foram documentados, entre os quais cinco são novos registros de ocorrência no Brasil e dezenove na Amazônia brasileira. Gomphonema Ehrenberg foi o gênero mais representado com seis espécies. Sintetizando, as novas ocorrências representam um aumento de 75% do número total de registros para a Amazônia brasileira e indicam que essa região ainda se encontra mal investigada, mas também sugere que a avaliação da biodiversidade está subestimada. Finalmente, os resultados destacam a utilidade da abordagem paleolimnológica como uma ferramenta de peso na avaliação da biodiversidade antes que os impactos humanos comecem a afetar as condições pristinas da área.

Structure and species composition of diatom community during the wet season in three floodplain lakes of Brazilian Pantanal.

In order to access environmental conditions, the use of bioindicators that have a close relationship with environmental stressors is a largely common practice, but when evaluating environmental inferences, the individual dominant taxa need to be interpreted. Humid regions such as the marshlands are fragile ecosystems and sustain communities of microalgae, often used as bioindicators, of which diatoms are a good example. Although they provide an excellent response to chemical and physical changes in water, diatom studies in surface sediments in wetlands are scarce worldwide. To determine whether diatom species have the potential to provide unambiguous inferences in the influence of environmental factors, we have evaluated diatom abundance in surface sediment, from three Pantanal lakes, against a set of environmental gradients: pH, dissolved oxygen, turbidity, conductivity, total dissolved solids, water temperature, index of trophic water status, total phosphorus and total nitrogen. The Ferradura lake presented an oligotrophic state and both Burro and Caracará lakes presented mesotrophic state. Diatoms were more abundant in the a mesotrophic conditions, but with higher species richness in the oligotrophic conditions. Depending on the N:P ratio, the nutrients nitrogen and phosphorus can also play the role of pollutants and may have negative and unpredictable effects in the environment, such as biotic homogenization. Despite the spatial variation in species, there was a greater richness of Eunotia Ehrenberg species, with the highest relative density of Eunotia formica Ehrenberg and E. pantropica Glushchenko, Kulikovskiy & Kociolek, due to the environmental acidic conditions, a determining characteristic of this genus. It was also observed that a small increase in the level of phosphorus generated an increase in the abundance of Aulacoseira Thwaites with the highest relative density of A. pusilla (Meister) Tuji & Houki and A. veraluciae Tremarin, Torgan & T.Ludwig. However, A. italica dominated in the moderately acidic environment. The results can help with decisions in impacted areas to solve socioeconomic problems, environmental management and biodiversity.