Fatores que influenciam a variação espacial e temporal nas fontes autotróficas de energia e nível trófico do Paracheirodon axelrodi (Osteichthyes, Characidae) num sistema interfluvial do médio rio Negro

Abstract

Factors that influence the spatial and seasonal variations in the autotrophic energy sources and trophic level of the cardinal tetra (Paracheirodon axelrodi) were investigated in an interfluvial region of the middle Negro River. A secondary objective involved investigating mercury biomagnification in the cardinal food chain and the potential mechanisms controlling this process. Stomach content analyses were performed to determine the cardinal diet in these habitats. Carbon stable isotopes (δ 13 C) were utilized to trace organic carbon between the plants (tree/shrub leaves, aquatic emergent and submersed herbaceous plants and epiphytic and floating metaphytic algae), aquatic invertebrates and the cardinals. Nitrogen stable isotopes (δ 15 N) were used to indicate the trophic level of the cardinal in relation to the aquatic invertebrates and plants. A mass balance mixing model called IsoSource was used to determine the relative contributions of autotrophic energy sources for the aquatic invertebrates and fish. Total mercury concentrations and δ 15 N values of the plants, invertebrates and fish were used to examine trophic levels and mercury biomagnification along the food chain. Cardinals were collected in three streams and three interfluvial swamps during a period of 12 months. Water samples were collected at all sampling points to measure total suspended sediments (TSS), chlorophyll concentrations and δ 13 C and δ 15 N values of fine particulate organic matter (FPOM). Water physical-chemical parameters including dissolved oxygen, pH and water temperature were measured at all sampling points. Daily changes in water level and water temperature were recorded with Leveloggers (hydrostatic sensors) installed in two interfluvial swamps. Local precipitation was measured with rain gauges installed in the same areas as the Leveloggers. Daily primary productivity rates of epiphytic and floating metaphytic algae, tree and shrub leaf-fall, emergent herbaceous plants and phytoplankton were estimated and extrapolated over the area of influence in the interfluvial region utilizing a dual-season classification map for wetland habitats developed by Hess et al. (2003). Plant selectivity indexes were devised using the relative contributions of plant groups to fish and invertebrate carbon and nitrogen and the productivity of each plant group in relation to total primary production for the study area. Stomach content analyses demonstrated that the cardinal in the interfluvial region consumes predominantly a mix of microcrustaceans and insect larva; however no seasonal variations were observed. The relative contributions of autotrophic energy sources varied seasonally and spatially, with tree and shrub leaves and emergent herbaceous plants as the principal energy sources for the cardinals from the campos and streams, respectively, during the high water period. In the dry season, the principal energy sources were epiphytic algae for the cardinals from the campos and submerged herbaceous plants for the cardinals from the streams. The selectivity index results showed that the cardinal food chain was preferentially selecting energy from epiphytic algae in all seasons, although the production from this plant group was small in comparison to the flooded forest and herbaceous plants. δ 15 N results indicated that the cardinals were one trophic level above the aquatic invertebrates, who, in turn, were two levels above the plants. Total mercury concentrations in relation to δ 15 N values of shrimp and different fish species showed strong biomagnifications through the food chain. Higher mercury levels were encountered in cardinals from the low water period when production of epiphytic algae and herbaceous plants was highest. The selective incorporation of epiphytic algae and/or submerged herbaceous plants into the tissues of aquatic invertebrates and cardinals could be an important link in the biomagnification of mercury in this food chain. The high mercury levels encountered in cardinals and predators of the cardinal like Cichla sp. in a remote area far removed from anthopogenic mercury inputs shows additional supporting evidence for large, natural-occurring sources of mercury in the Negro River.