Alpha, beta and gamma are three components of species diversity. Knowing these attributes in floodplain lake phytoplankton communities is vital when selecting conservation areas. Species diversity is commonly used with other taxonomic groups, but rarely with phytoplankton. We compared the number of phytoplankton species (alpha diversity) from 21 Middle Araguaia River floodplain lakes in the 2000 and 2001 rainy and dry seasons. From these samples we estimated complete survey species richness (gamma diversity), quantified differences in species composition between lakes (beta diversity) and assessed the influence of abiotic variables on beta diversity. We recorded a total of 577 taxa. The Sjack1 estimator indicated that 62.31% of taxa were sampled in the 2000 rainy and 67.65% dry seasons, and 68.36% in the 2001 rainy and 73.5% dry seasons. In almost all seasons, alpha diversity negatively correlated with latitude. Beta diversity (b-1) was higher in high water periods, especially in 2000. This may have been caused by isolated heavy rainfall, which would have increased environmental heterogeneity and raised beta diversity. DCA showed differences in phytoplankton composition between rainy and dry seasons in 2000 and 2001, reflecting the influence of flood pulses on phytoplankton composition. The Mantel test indicated spatial distribution patterns where geographically more distant lakes had less-similar phytoplankton communities.

-(Occurrence and autoecology of Acanthoceras zachariasii (Brun) Simonsen (Chaetocerales, Coscinodiscophyceae) in the Central-Northern Brazil). In this study we presented the structural and ultrastructural characterization of Acanthoceras zachariasii (Brun) Simonsen, as well as its geographical distribution in the Amazon River Basin and Tocantins-Araguaia River Basin, Brazil, and provided the lectotypification of the taxon. We analyzed 454 phytoplanktonic samples, but the occurrence of A. zachariasii was recorded in only 14 of them. Of the total samples collected along the two basins, A. zachariasii was recorded only in 3.08%, which demonstrates the rarity of this taxon. This species was registered in lentic environments, with mesotrophic to eutrophic conditions, and pH varying from slightly acid to alkaline. Keywords: Amazon River Basin, Tocantins-Araguaia River Basin, Attheya zachariasii, Acanthoceras magdeburgense, lectotypification RESUMO -(Ocorrência e autoecologia de Acanthoceras zachariasii (Brun) Simonsen (Chaetocerales, Coscinodiscophyceae) na região centro-norte do Brasil). Apresenta-se a caracterização estrutural e ultraestrutural de Acanthoceras zachariasii (Brun) Simonsen, bem como, a distribuição geográfica da espécie nas bacias Amazônica e do Tocantins-Araguaia, Brasil, além da lectotipificação do táxon. Foram analisadas 454 amostras fitoplactônicas, sendo registrada a ocorrência de A. zachariasii em apenas 14. Do total de amostras coletadas nos estudos das duas bacias, apenas em 3,08% foram registrados representantes de A. zachariasii, demonstrando quão rara é a frequência de ocorrência do táxon. A espécie ocorreu em ambientes lênticos, mesotróficos a eutróficos, com pH variando de levemente ácidos a alcalinos. Palavras-chave: Bacia Amazônica, Bacia do Tocantins-Araguaia, Attheya zachariasii, Acanthoceras magdeburgense, lectotipificação IntroduçãoAcanthoceras zachariasii (Brun) Simonsen é caracterizada por apresentar frústula pouco silicificada, retangulares em vista pleural, com um espinho tubular em cada ângulo da valva (Round et al. 1990). A fragilidade da frústula dificulta a preservação e observação do táxon, uma vez que no processo de limpeza por oxidação e fixação o organismo pode ser destruído (Tremarin et al. 2015). Provavelmente seja este o motivo dos poucos registros desse táxon para o Brasil e para o mundo.Apesar dos poucos registros no país, A. zachariasii possui distribuição ampla, cosmopolita, porém a espécie, em geral, apresenta populações pouco