INFLUÊNCIA DO PULSO DE INUNDAÇÃO NA COMPOSIÇÃO ISOTÓPICA (d13C E d15N) DAS FONTES PRIMÁRIAS DE ENERGIA NA PLANÍCIE DE INUNDAÇÃO DO RIO PARAGUAI (PANTANAL  MS) / SEASONAL PATTERN DETERMINING PRIMARY ENERGY SOURCES d13C AND d15N IN THE PARAGUAY RIVER FLOODPLAIN TROPHIC WEB, PANTANAL WETLAND - BRAZIL

Débora Fernandes Calheiros

The food web of the Pantanal wetland in Brazil was investigated during a complete hydrologic cycle (1998-99) using a multiple stable isotope approach (d13C and d15N) with the goal of identifying the primary carbon sources, which drive the system energy flux. The sampling was done in a representative marginal lake (Baía do Castelo) of Paraguay River floodplain and the sampled primary producers and consumers were algae (sestonic and epiphytic), vascular plants (emergent, submersed and terrestrial), invertebrates (epiphytic, planktonic and benthic), and mature detritivorous fish. The majority of vascular plants belonged to the physiological group of C3 plants (d13Cmean= -29.5), besides C4 species (d13Cmean= -12.3) with lower abundance. Algae and detritus from epiphyton and seston samples were separated for isotopic analysis by density gradient centrifugation in a colloidal silica solution. C:Chlor <100 ratio values (from both epiphytic and sestonic samples, light and heavy fractions), indicated relatively pure algae (POMa = algae) with signatures of -34.5 to -26.9 (mean ± SD = -29.3 ± 1.7) for d13C and -2.1 to 6.0 (mean= 2.1 ± 2.0) for d15N. There was no significant difference between d13C from algae and C3 plants. The d13C of algae was more enriched (D13C= only -8.3) than other tropical South American floodplains and could be a result from limiting of CO2 availability or uptaking both DIC sources (CO2 and/or HCO3-). Low and negative values of d15N denoted N2-fixing by Cyanobacteria and higher values were probably related to more enriched DIN sources, as NH4+. Clear seasonal pattern, related to the hydrologic cycle and variations of inorganic sources availability, was observed for algae (for both isotopes), showing more depleted values during rising-flood period and the opposite in the falling-dry, that reflected in the mixing zooplankton signals as well in the other primary and secondary consumers. Surprisingly the d13C values of the primary consumers were much more depleted than the values of the algae, ranging from -43 to -20 for d13C and from -2 to 12 for d15N. These results would only be possible if a more depleted source of carbon, possibly methanotrophic bacteria, were also involved, which is strengthened by the values of benthic chironomidae (d13C range= -62.4 to -49.4; d15N= -5.3 to -0.8). Thus the balance of evidence suggests a combination of both autotrophic sources (photo and chemoautotrophic) in sustaining the food webs of Pantanal wetland (mainly Pantanal of Paraguay river), with the methanotrophic branch of microbial loop being probably an alternative link between CH4 and organic carbon cycle directly through metazoan levels.

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