Ecologia trófica do pirarucu (arapaima sp.) em ecossistemas de lagos de ria e de várzea na bacia amazônica central

Abstract

Neotropical freshwater ecosystems are among the most productive and diverse environments on Earth. The biological productivity of tropical rivers is maintained by the inputs of nutrient, detritus and sediment and also by the seasonal variation in water level, called flood pulse, which creates riparian areas adjacent to the river that alternate between the terrestrial and aquatic stages (floodplain, várzea, flooded forest, etc.). These locations have a high diversity of features such as floodplain lakes, connecting channels, flooded forest, among others. In contrast, on the edge of the floodplain, different habitat types called ria lakes are only influenced by the main river channel during the wet season. These lakes were formed by erosion of former tributaries during glacial periods and by the subsequent impoundment exercised by the main river channel during interglacial periods, which resulted in the drowning of the lower portion of these tributaries. Ria lakes are typically dendritic in morphology and possibly more dependent on terrestrial inputs than floodplain lakes. As with most tropical food webs, floodplain and ria lakes are influenced by an abundance of organic matter and detritus, resulting in a ichthyobiomass mainly dominated by detritivorous and omnivorous fish. Omnivory is an important adaptation that allows fish to deal with the seasonal variability in prey composition. Some species in higher positions of the food chain, such as the Arapaima (Arapaima sp.), play an important ecological role on the regulation and energy transfer in the ecosystem. In this study, we aim to assess the trophic ecology of arapaima, one of the most endangered fish in the Amazon. We collected fish during the dry season of 2013 (N = 70). The stomach content analysis was combined with the stable isotopes methodology to infer about the trophic level of arapaima and the main energy sources for the species in floodplain and ria lakes. The study was conducted in the Sustainable Development Reserve Piagaçu-Purus (RDS-PP), low Purus River, 223 km away from Manaus, Amazonas state. The results confirm previous findings about the dependence of arapaima for organisms with low position in the food chain such as omnivorous and detritivorous. While in the ria lake most of the sampled stomachs were full, in floodplain lakes empty stomachs were predominant. Thus, ria lakes tend to be an important habitat for arapaima feeding during the drought, since these environments remain interconnected with an extensive stream network and main river channel throughout the year. The stable isotopes of nitrogen (δ15N) showed that the trophic position of arapaima changes during ontogeny and that the species should be considered as piscivorous. The analysis of autotrophic sources indicated that the phytoplankton and flooded forest trees are the main sources of energy for arapaima.