Aim: This study aimed to evaluate the influence of river-dam axis and abiotic factors on the composition of Closteriaceae, Gonatozygaceae, Mesotaeniaceae and Peniaceae in a tropical reservoir. Methods: Water samples for physical, chemical and periphyton analysis were collected in April and August 2002 in different regions along the axis of the river-dam of Rosana Reservoir, River Basin Paranapanema. The substrates collected, always in the litoranea region, were petioles of Eichhornia azurea (Swartz) Kunth. To examine the relationship of abiotic variables with reservoir zones and between the floristic composition of desmids, we used principal component analysis (PCA) and canonical correspondence analysis (CCA). Results: The results of the PCA explained 81.3% of the total variability in the first two axes. In the first axis, the variables of conductivity, water temperature and the pH were related to the sampling regions of April with higher values, while for the month of August, nitrate, total phosphorus and dissolved oxygen showed higher values. We identified 20 taxa, distributed in the genera Closterium (14), Gonatozygon (4), Netrium (1) and Penium (1). Spatially, the higher taxa were recorded in the lacustrine region for both collection periods. The canonical correspondence analysis (CCA) summarized 62.2% of total data variability of taxa in the first two axes, and in August, Closterium incurvum Brébisson, C. cornu Ehrenberg ex Ralfs and Gonatozygon monotaenium De Bary, were related to higher values of turbidity and nitrate to the lacustrine and intermediate regions. Conclusion: Thus, the formation of groups was due to the regions along the longitudinal axis, then the seasonal period, which must be related to the low current velocity, the higher values of temperature and the water transparency, especially in late summer.
Aim Light is an essential component in the process of synthesis of organic compounds by photosynthetic organisms. Assuming that a higher level of luminosity would positively influence the production of biomass, we evaluated the influence of different levels of shading on the biomass of periphytic algae, through an experimental study. Methods Glass slides were used for this as an artificial substrate for the colonization and succession of algae in Garças Lake, on the upper Paraná River floodplain, and later transferred to nine aquaria. These were divided into three treatments: three without cover (AC), three with 50% shading screens (A50) and three with 80% shading screens (A80). Abiotic variables (pH, conductivity, turbidity, temperature and dissolved oxygen) and biomass were measured every five days for 15 days. The biomass was evaluated using the chlorophyll-a method. Concentrations of total phosphorus, phosphate, total nitrogen, nitrate and ammoniacal nitrogen were evaluated every five days. Results There was no significant variation of the biomass over time, however, there is a significant difference between the treatments. The highest biomass was found in the control treatment and the lowest was found in the treatment with 80% shading. The other limnological variables evaluated did not show significant changes over time. Conclusions The abiotic variables did not influence the biomass of the phycoperiphyton community, which was influenced only by luminosity. Thus, we conclude that light is a variable with direct influence on the production of periphytic biomass. At low intensity it is a variable that can limit the production of biomass. In high intensity however, it influences by increasing its production.
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