A comunidade de macrófitas aquáticas em reservatórios do médio e baixo Rio Tietê (SP) e em lagos da bacia do médio Rio Doce (MG)
Abstract
Macrophytes have a fundamental role in the functioning of aquatic ecosystems
due to their high primary productivity, participation in the nutrient cycling, and
interactions with many other components of the system. Many species of macrophytes
can be bioindicator of the trophic state or succession stage of the system. In the present
work the macrophyte communities of twelve lakes in the Middle Rio Doce (MG) and a
cascade of six reservoirs (SP) were analysed, comparing systems naturally and
artificially fragmented. Four sampling expeditions to the Rio Doce system and three to
the Tietê were performed during the 2001 2002 period. The macrophyte species
present were recorded, photographed and collected for herborization. The community
was analysed in relation to the species richness, dominant species, the frequence of
occurrence and the species composition similarity, intra and inter lake and reservoir
systems. The relationship between species richness and lake morphometry was also
analised. Macrophyte biomass was determined for selected stands, in order to verify the
existence of temporal variations. In the lakes of Middle Rio Doce system a total of 53
species were registered, belonging to 25 genera and 23 families. The occurrence of
Habenaria fastor Hoene (Orchidaceae) in this system of lakes was recorded for the first
time. The highest richness occurred in the lakes Barra and Verde. Species composition
varied considerably among lakes and between periods. No statistically significant
relationships were found between species richness and variables related to lake
morphometry, although a tendency of increase in the species richness as a function of
the increase in the area and perimeter of the lakes was observed. In Tietê reservoirs a
total of 48 species were found, belonging to 26 genus and 22 families. The species
composition in the first three reservoirs (Middle Tietê) differed from that on the last
three reservoirs (Low Tietê). The relationship among the species richness and reservoir
morphometry was not statistically significant, however there was a tendency of
decreasing richness as the reservoir area or perimeter increased. Twenty three species
were shared among both systems; 23 exclusively on lakes of Middle Rio Doce and 18
exclusively in on Tietê reservoirs. The families Cyperaceae and Onagraceae were the
ones represented by the highest number of species in both systems. Marked differences
were found between the studied systems. The macrophyte species richness was higher
in the Middle Rio Doce lakes than in the Tietê reservoirs. In the lakes the increase in the
area and perimeter were factors directly related to the increase in species richness
whereas in the reservoirs this relationship was inverse. Biomass of macrophytes in
selected stands varied greatly between periods, however total biomass per areal unity
was similar, although the highest values were recorded in Nova Avanhandava and
Ibitinga reservoirs. The results of this study have indicated that the lakes of Middle Rio
Doce being a naturally fragmented system, ancient and stable, have a higher macrophyte
diversity, whereas the Tietê reservoirs, recently and artificially fragmented and also
with greater instability have a lower diversity. Also, the species richness in the
reservoirs appeared to be not directly related to the area and perimeter, variables which
are representative of their most propitious habitats in natural systems.