Recent research of the plankton and the surface sediments from Ribeirão do Campo reservoir (São Paulo, southeast Brazil) led to identification of two new cymbelloid diatom species: Encyonema acquasedis sp. nov. and Kurtkrammeria salesopolensis sp. nov. Their morphology was studied under both light and scanning electron microscopes aiming at describing the two species in detail. The morphology of each species is compared and discussed with morphologically similar taxa. The new species are different from all others in the Cymbellales due to a combination of characteristics including valve outline, striae, areolae structure, and raphe system. Both species were collected from an oligotrophic environment with both low pH and conductivity.
The type material of Cymbella schubartii was studied in order to clarify the differences and similarities within the complex of Encyonopsis species observed in southeastern Brazilian water bodies. Detailed examination using light and scanning electron microscopy of modern populations led us to propose two new Encyonopsis species, which are here described in details. Morphometric analysis of the valve shape supported the separation of four species: Encyonopsis schubartii, E. difficilis, E. sanctipaulensis, sp. nov. and E. linensis, sp. nov. Additionally, Cymbella perpusilla var. moreirae is formally transferred to the genus Encyonopsis. Regarding their ecological preferences, these species were found in rivers and reservoirs, especially in oligotrophic and oligo-mesotrophic environments.
In the present study, distance decay was used as a descriptor of the compositional variation in diatom communities’ similarity between different habitats (phytoplankton and surface sediment) and seasons (summer and winter; only for phytoplankton) along a geographical gradient of ~64km over six reservoirs located in south-east Brazil. Whether rates of decay in similarity changed with distance in tropical biological communities was tested, and the degree to which dispersal (geographic distance) and niche (habitat association) processes explain variation in the diatom communities was estimated. In addition, whether the diatom assemblage captured in the surface sediment reflected the water column assemblage events was evaluated. Distance–decay curves were estimated using linear regressions. Partial Mantel tests were performed to examine the significance of relationships among the biological, environmental and spatial datasets. Similarity decreased significantly with distance between habitats and seasons, but the results were not statistically significant for surface sediment assemblages. All diatom communities were controlled more by limited dispersal than by environmental factors, probably as a result of the water quality and scale of the study area.
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