Assessment of the applicability of morphological and size diversity indices of bacterial
populations of reservoirs in different trophic states
RACY1, F. ; GODINHO1, M. J. L.; REGALI-SELEGHIM1, M. H. ; BOSSOLAN2, N. R. S.;
FERRARI1, A. C. & LUCCA1, J. V.
1 Departamento de Ecologia e Biologia Evolutiva, Universidade Federal de São Carlos - Via Washington Luiz, km 235, São Carlos (SP) 13565-905.
fracy@iris.ufscar.br, pmhrs@iris.ufscar.br
2 Instituto de Física de São Carlos, Universidade de São Paulo, C.P. 369, 13560-970 São Carlos, SP, Brazil.
ABSTRACT: The morphology and size of bacterial cells, in samples taken at monthly intervals over half a year from the surfaces of three Brazilian reservoirs in different trophic states (Barra Bonita – eutrophic, Broa – mesotrophic and Lagoa Dourada - oligotrophic), were analysed with the aid of a computerized system, from digitalized images obtained by epifluorescence microscopy (with DAPI fluorescent stain). For each reservoir, densities (D) were estimated, and the relative frequencies of biovolumes and of each morphotype identified (coccus, coccobacillus, bacillus, vibrio, filament and spirillum) were converted into indices of size diversity (SDI) and morphological diversity (MDI), using the Shannon Index for calculations. These values were compared with the mean trophic state index (mTSI) calculated for each environment. Among the biological variables, bacterial population density was the most significant indicator of the trophic state of the environments. The MDI proved to be significantly different between the eutrophic and oligotrophic environments, as well as between the mesotrophic and oligotrophic reservoirs, but not consistently so between the eutrophic and mesotrophic, possibly because of the similarity of the trophic states in the last two. The SDI was found to differentiate significantly between the eutrophic and oligotrophic environments, but not between either of these and the mesotrophic. In the oligotrophic environment there was an extremely significant negative linear correlation (r = - 0.9735, P = 0.001) between the monthly values of mTSI and MDI; this correlation was specially significant with chlorophyll a concentration [Chla], but not significant with the other components of mTSI.
Key-words: bacteria, morphology, size, diversity indices, trophic level.