Estudo limnológico da Lagoa dos Tropeiros, Capitólio, MG, com ênfase na comunidade de macroinvertebrados bentônicos

Abstract

The studies on the diversity and functional role of benthic macroinvertebrates in lake ecosystems can provide relevant informations regarding the functioning and the health of these freshwater ecosystems. The aim of this study was to analyze the Tropeiros Lake, located in the municipality of Capitólio (MG), performing a limnological evaluation of the ecosystem and ecological investigation on the diversity of the benthic macroinvertebrate community. Thus, changes in the taxonomical composition, abundance, diversity, biomass and the taxa participation in the trophic functional groups were analyzed. Water and sediment were sampled in October 2006 and 2007 (dry periods) and in March 2007 and 2008 (rainy periods). The results evidenced that Tropeiros Lake is a shallow environment with maximum depth of 3 m, densely colonized by macrophytes and with high water transparency. It has low concentration of nitrogen with predominance of the ammonium ion, low concentrations of phosphorus being organic phosphate the predominant compound, low pH values with water varying from neutral to slightly acid and moderate water electrical conductivity. The sediment has high clay and organic matter contents. The Discriminant Analysis separated the sampling periods, grouping the similar rainy periods but separating the dry ones. According to the trophic state index the lake was classified as oligotrophic during the rainy periods and mesotrophic during the dry periods. Benthic community was composed by 81 taxa, mainly represented by the members of the Order Diptera, especially those belonging to the families Chironomidae and Chaoboridae. The trophic guild of collectors was the most representative, with minimum abundance of 57.2%, with the taxa Chironomus sp, Goeldichironomus sp and Tanytarsus sp, followed by that of the predators, having Chaoborus and Ablabesmyia sp as the main taxa. The mean biomass determined for the macroinvertebrate community of Tropeiros Lake was 0.94 g. m-2. Except Chironomus sp, that was abundant, regarding both, numbers and biomass, a different condition was observed for the taxa as Pisidium pulchellum, Melanoides tuberculata and a Libellulidae nymph that were numerically less abundant but contributed significantly for the total biomass. Regarding the trophic guilds, predators was more representative in the periods of October 2006 and March 2007 than the collector guild, in terms of biomass. The same was observed regarding the macroinvertebrates contribution to the biomass in each trophic guild, where predators were more abundant than collectors in October 2006 and March 2007, evidencing the importance of this approach in ecological studies. The method based on K-dominance curves (ABC curves) indicated that Tropeiros Lake is under moderate perturbation according to the cumulative values of abundance and biomass. This results corroborates those obtained for lake trophic state index, based on the mean biomass when compared to environments of different trophic states and with the species composition, formed by both, group of tolerant species as well as those more sensible to adverse environmental conditions.