Ecological preferences and distribution of Aulacoseira species in southeastern Brazilian reservoirs with varying trophic states were studied. One hundred and fourteen plankton samples (winter and summer) from 57 sites located in 16 reservoirs were analysed. Ten water quality parameters were measured. Ten Aulacoseira species were identified using light, scanning and transmission electron microscopy, and new information on their ecological preferences is provided here. Our results indicate that trophic gradient is the main driver of species distribution. Principal components analysis and calculation of weighted average nutrient optima revealed three indicator taxa, Aulacoseira tenella characteristic for oligotrophic waters and two varieties of Aulacoseira granulata (nominate and var. angustissima) typical for eutrophic reservoirs. This is the first ecological study of Aulacoseira in Brazil, adding information on the distribution of this genus in the tropics, and highlighting the need for species-level identification and regional studies to improve the use of diatoms in water quality assessment.

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A metacommunity is defined as a set of local communities that are connected by dispersal and are controlled by environmental drivers operating at different spatial and temporal scales. Although diatom ecology has been studied extensively in temperate and lotic ecosystems, knowledge of tropical lentic systems is more limited. We evaluated how planktonic and surface‐sediment diatom communities, including trait‐based communities, were structured by environmental, spatial and land‐use variables in tropical reservoirs. Diatoms were sampled in seven reservoirs (31 sites in total) in south‐east Brazil, from the surface sediment and the plankton in summer and winter. Diatom communities responded primarily to a trophic gradient and secondarily to a light‐availability gradient. Connectivity was also a strong predictor, whereas land use had a weaker influence on community variation. Our results further suggested that surface‐sediment and planktonic diatom groups were structured by similar environmental variables. Trait‐based analysis also indicated that these communities were related to environmental and spatial factors. We conclude that (1) trophic state, light availability and connectivity were the main drivers of diatoms in this set of tropical reservoirs, resulting in spatially structured communities; (2) patterns for surface‐sediment diatoms resembled both summer and winter diatom communities in the plankton; and (3) the effects of the explanatory variables (local environment, connectivity and land use) varied among the different biological datasets (i.e. when organised taxonomically, or according to growth morphology, cell size or trophic preference). We emphasize that studies in tropical reservoirs should take spatial variables into account, due to the connectivity of the sites.

This, study aimed to inventory the diatom flora of tropical reservoirs from southeastern region of Brazil under oligo- to hypereutrophic conditions. We collected diatom samples from the surface sediment and phytoplankton (summer and winter) in 41 sites (ten reservoirs). Seventy-eight taxa, distributed in 28 genera, were identified in the surface sediment and phytoplankton samples. Nizschia was the most representative genus, with nine species. Six taxa represent new records for Brazil and six for São Paulo State; ten taxa were identified at the genus level and probably represent new species. Additionally, this study contributed with data on the ecology of the species and evidenced the necessity of floristic surveys to improve the knowledge about tropical diatom biodiversity.

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