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The global biodiversity loss is a consensus. The biodiversity conservations shortfalls make conservation of biological diversity even more challenging. For many taxa, the knowledge about their distributions is deficient, and this is called the Wallacean shortfall. This situation is no different within algae biodiversity, especially in Brazil. There is still an enormous inequity of sample effort, as is the case of the Parnaíba River Basin (Northeast region), which had only 10 algal species in published (the lowest number of algal species reported among the main Brazilian basins). The present work had the objective of increasing the knowledge of algal flora in Brazil by conducting a taxonomic study of the stream macroalgal species of the Parnaíba River Basin. The sampling of macroalgae was carried out in 21 segments of streams from the Middle and Lower Parnaíba Basin, in the Piauí and Maranhão states. Macroalgae were manually removed and preserved in 4% formaldehyde. Environmental characteristics of each segment were measured in order to describe the sampling sites. The taxonomic survey of the macroalgal communities resulted in the identification of 38 taxa in total, of which 32 at a specific level; three vegetative groups; two sporophytic stages of red algae and one unidentified species. Among the 38 species recorded, 37 are new records for the Parnaíba River Basin. Following the same pattern, 23 species are new records for the Brazilian Northeast region, and Microcoleus lacustris represented the first report in Brazil. Despite the fact that it remains the Brazilian basin with the lowest number of algal species documented, this study contributed to the increase of almost five times the number of species sampled in the Parnaíba River Basin (from only 10 to 47 species). These data reinforce that the differences presented in algal diversity in the Brazilian regions are more related to the sampling effort than other factors (e.g. environmental characteristics, geographic distribution, biomes, among others). In addition, this survey illustrates not only the group's lack of information in the region but also shows the importance of this type of study as a tool for expanding the knowledge about biodiversity and its conservation.

RESUMO A diversidade de macroal gas lóticas tem sido pouco estudada no Brasil. Levantamentos florísticos não têm sido publicados, gerando um déficit de informações sobre a ecologia e taxonomia desses grupos. Sendo assim, o presente trabalho teve como objetivo realizar o levantamento de espécies de macroalgas de riachos da Serra do Japi, uma área de Floresta Estacional Semidecidual (23º12’- 23º21’ S e 46º30’-46º05’ W). Foram amostrados 16 riachos e foram identificados 16 táxons (11 infragenéricos, 4 genéricos e um estágio de vida, ‘Chantransia’), distribuídos em quatro divisões. Cyanobacteria foi o grupo melhor representado, com cerca de 38% das espécies, seguido de Rhodophyta com aproximadamente 31% dos táxons amostrados. O estágio esporofí- tico ‘Chantransia’ e Phormidium aerugineocaeruleum foram os táxons os mais bem distribuídos, ocorrendo em 50% e 29% dos pontos investigados, respectivamente. O conhecimento sobre a composição e ocorrência das espécies de macroalgas pode trazer informações valiosas para a tomada de decisões sobre a conservação de recursos hídricos, particularmente em áreas protegidas como a Serra do Japi.

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Arnoldiella elbakyaniae sp. nov. is described based on epilithic material collected in a cascade of a stream in Northeastern Brazil, Piauí State. The stream belongs to the Parnaíba river basin, which encompasses one of the most poorly sampled regions in the country. We also propose here the combination of Basicladia emedii C.K. Peres & C.C.Z. Branco into Arnoldiella emedii comb. nov., following the new arrangement proposed for the group. Arnoldiella elbakyaniae can be recognized by the heterotrichous thallus with short erect, unbranched or sparsely branched filaments, and by the short and enlarged basal cells of the erect system. The new species differs from A. emedii by its shorter basal cells, the narrower and cylindrical axis cells, and the zoosporangia in terminal chains.

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