DISTRIBUTION OF MACROALGAE IN A TROPICAL RIVER BASIN FROM SOUTHEASTERN BRAZIL

Twenty-two stream segments in the Preto River Basin (southeastern Braz il, 20-degrees-12'-20-degrees-57'S, 49-degrees-16'-49-degrees-42' W) w ere examined for macroalgal distribution. Five of these study sites we re sampled monthly during one year, four bimonthly during two years, a nd 13 twice (hot-rainy and cool-dry seasons) from May 1989 through Mar ch 1991. Twenty-five species and seven 'vegetative groups' (sterile pl ants of Oedogonium spp., Spirogyra spp. and Vaucheria sp.) were found, from which the most widespread species were Stigeoclonium belveticum (77.3 % of the sites), Lyngbya aerugineo-caerulea (59.1 %) and Batrach ospermum delicatulum (50 %). Species richness ranged from 2 to 17 (XBA R - 8 +/- 3.3) and was positively correlated to stream area and length , whereas total species cover varied from 5 to 85 % (XBAR - 52 +/- 16 %) and was negatively correlated with dissolved oxygen. Cluster analys es revealed the existence of four groups of species and three groups o f sampling sites. Among the most frequent species in the basin, S. hel veticum best fitted the pattern of frequency distribution of stream va riables measured, differing only for specific conductance and oxygen. The stream variables most closely related to the species distribution in the basin were conductance, oxygen, size and substratum type. Highe r species numbers and abundances were generally found in higher conduc tances, lower oxygen contents, larger streams and rocky substrata. The patterns of macroalgal distribution in the Preto River Basin agree wi th the predictions of the River Continuum Concept for species richness but not for abundance. The strongly developed dominance of few specie s was found in the basin.