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Data of Dissertation

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Author: Alessandra Delazari-Barroso

Title: Factors controlling the development of phytoplankton in a reservoir of public supply in Espirito Santo State, with an emphasis on cyanobacteria.

Year: 2005.    Full text (in Portuguese)

Abstract

Knowledge about key forcing functions that regulate tropical freshwater ecosystems is still under development. Physical regime, nutrient availability and trophic state are some relevant issues in tropical limnology. The contribution of these factors were evaluated on the dynamics of the Duas Bocas Reservoir (DBR) (Espírito Santo State, southeastern Brazil), a small (0.51 km2) and shallow (< 10 m) water supply reservoir surrounded by a protected forest. Monthly samplings were carried out from October 2002 to September 2003 to analyze phytoplankton community focused on physical (mixing regime and light) and chemical (mainly nutrients) properties. From October to April, strong thermal stratification and nutrient limitation for phytoplankton growth in the mixing layer were registered, besides anoxia and high nutrient concentrations in the hypolimnion. In May and June, mixing events took place. The average concentrations of total phosphorus and chlorophyll a were in agreement with meso-eutrophic systems. Nutrient concentrations showed co-limitation by N and P, suggesting that nutrient limitation rather than light limitation regulates the phytoplankton growth in the DBR most of the time. During the stratification period, the dominant small desmids (groups X3 and N) were positively associated with temperature, pH and N NO3 - and negatively associated with total phosphorus, N NH4 + and N/P ratio. On circulation period, desmids were replaced by nonheterocytic filamentous cyanobacteria (S1), associated positively with total phosphorus and NH4 + and negatively with temperature and pH, and by Cylindrospermopsis raciborskii (Sn), which showed significant positive correlation with pH, total phosphorus, and N NO3 -, and negative correlation with zeu and N NH4 +. A metalimnetic maximum of biomass composed by Limnothrix bicudoi and L. redekei were registered during high thermal stability. From 23 September to 27 October 2003, a nutrient enrichment experiment was carried out in the lacustrine zone of the reservoir, presenting mesotrophic characteristics. Nine enclosures (2,200 L each one), were set up. After P PO4 -, N NO3 - and N NH4 + additions at three days intervals, water samples for phytoplankton, chlorophyll a, and nutrients were collected from each enclosure and from the reservoir. At the beginning of the experiment, phytoplankton in controls, treatments, and reservoir were dominated by picoplanktonic and nanoplanktonic desmids, followed by cyanobacteria, coccal green, and criptophytes. The phytoplankton growth response to the enrichment was more rapid at long eutrophication than that at short eutrophication treatment. Biomass and diversity in enriched enclosures were higher than at controls and water reservoir, although no statistical significances were found. The dominance of desmids (functional group X3 and N) was replaced by filamentous cyanobacteria (groups Sn and S1), which showed a rapid growth response related to the increasing pH values and water x turbidity. The growth of C. raciborskii (Sn) was more pronounced in short eutrophication treatment, where it was associated to higher temperatures and low ammonium concentrations. Desmids increased significantly after 10th day of the experiment and were related to decreasing pH and high growth rates under higher temperatures. Nutrient input from rainfall was important as external sources that improved the phytoplankton development in a reservoir with N-limitation characteristics.

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