Seasonal variation in the density of the macrophyte Scirpus cubensis poepp & kunth (Cyperaceae) in the Lagoa do Infernao, State of Sao Paulo, Brazil

This paper deals with the seasonal density dynamics of the macrophyte Scirp~is cuberzsis Poepp & Kunth (Cyperaceae) in the Lagoa do Inferngo, a lake marginal to the Mogi-Gua~u river, State of SBo Paulo, Brazil. Seasonal density dynamics of the lnacrophyte were studied in permanent plots of I m length by 25 cm width, divided into four subplots of 25 X 25 cm. An individual of S. cubensis was defined as an independent morphological unit arising from a rhizome node. Seasonal variation in density, with a range from 135.52 + 171.52 to 270.00 * 196.00 ind.m-2, is related to the drought and flood conditions to which the lake is subject. Analysis of data also suggests that I2 sample units are significant for assessment of population density. Aspects of the macrophytes sampling are also discussed. INTRODUCTION Geological, hydrological and climatological conditions in tropical regions are favorable to the occurrence of areas liable to flooding (ESTEVES, 1988). The dynamic equilibrium in these areas often coincides with fluctuations in water level, leading to a "pulse stability" (ODUM, 1971). JUNK et al. (1989) the flood pulse to be the force largely responsible for the existence of systems of rivers with related to flood-liable areas, and for their productivity and interactions. Flood-liable areas are considered to be highly productive, and their macrophytes are a very important component in the trophic level. The various macrophyte species which occur in flood-liable areas are not distributed at random; each one has its microhabitats, especially on flood gradients. The plant communities of aquatic environments are formed by a characteristic assemblage of species, in equilibrium in terms of their numbers and abundance, and occupying relatively homogeneous sites (BEST, 1988). The difficulties which explain the shortage of structural studies of macrophyte communities are principally related to analytical methods (HENRIQUES et al., 1988). It is most important that the right methods should be selected for taking samples specific to each type of community, and that sampling should be carried out with great care so as to ensure accurate and useful results (BARBOUR et al., 1987). The aim of this work is to analyze the seasonal density dynamics of the emergent macrophyte Scirpus cube~zsis Poepp & Kunth (Cj~peraceae) in the lake known as the Lagoa do Inferngo, State of SBo Paulo, Brazil. According to GREIGSMITH (1983), unit counts for plants may be regarded as a basis for indirect estimate of biomass, with the great advantage that they do not involve destruction of the vegetation. Aspects of the sampling of macrophytes are also discussed. THE STUDY AREA The work underlying this paper was carried out in the Lagoa do Infern2o. a lake marginal to the Mogi-Guay river and a component part of the Jatai Ecological Station, State of Silo Paulo, Brazil. The lake lies at 2Io 33' S. 47O 5 1 ' W (SANTOS & MOZETO, 1992). It is small and shallow, with an average depth of 2.1 m and an area of 3.05 ha. (COUTINHO, 1989), and is horseshoe-shaped (fig. I ) due to the continuous processes of'erosion and sedimentation which cut off the meanders of the river (ESTEVES, 1988). The marginal region of the Iakc is characterized by Pnnic~mz perilarnb~lcerzsis, rooted in the sediment, and then by a floating L~rnnetica, 12 ( 1 ) . 17-23 (1996) O Asociaci6n EspaRola de Limnologia, Madrid. Spain mat made up mostly of Scirp~ts c~lbensis. Further out towards the limnetic region the floating macrophyte Eichhornia azLtrea occurs, followed by the submerged Cnbombu pihauyensis (MOSCHINI-CARLOS, 1991). The sigmatist method of Braun-Blanquet (MOSCHINICARLOS, 199 1 ) and the seasonal variation in biomass (MOSCHINI-CARLOS et al., 1993) suggest that the plant association dominated by S. cubensis is homogeneous throughout the Lagoa do Inferngo. Through weather data rainfall and air temperature CAVALHEIRO et al. (1990) and TOLEDO FILHO (1984) have established two quite distinct seasons for the study region. One is rainy, with higher temperatures and rainfall between November and April; the other is dry, with lower temperatures and rainfall between May and October. The hydrology of the Lagoa do Inferno is thus characterized by fluctuations of water level in the Mogi-Gua~u river; annual patterns involve cycles of flooding and drought. In the study period occurred seasonal variation in precipitation from April to September 1989 and from April to May 1990 rainfall was low (fig. 2), varying from 21.0 to 5 1.0 mm. From October 1989 to March 1990 rainfall was much higher, varying from 141.99 to 246.9 mm.