Abstract
The delta15N composition of bottom-feeding fish (iliophagous = Apareiodon affinis, Cyphocharax nagelii, Prochilodus lineatus, Steindachnerina brevipinna and S. insculpta; detritivorous = Loricariichthys platymetopon and Liposarcus anisitsi; benthophagous = Satanoperca pappaterra and Hoplosternum littorale) and their primary food sources were investigated in the upper Paraná River floodplain during rainy seasons in different environments (lotic and lentic). Two hypotheses were tested: i) that the trophic position and isotopic values of the investigated organisms (fish and food resources) vary spatially; and ii) that trophic position and isotopic compositions differ among iliophagous, detritivorous and benthophagous fish. C4 macrophytes, periphyton and phytoplankton were isotopically different in sites analyzed. Significant isotopic differences occurred in the species of each trophic category. Spatial differences were observed in the isotopic composition of P. lineatus and L. platymetopon, whose values were more enriched in the Paraná River and Pau Véio Lake. Significant spatial differences in trophic position were observed for L. platymetopon and H. littorale, which presented the highest values in the Paraná and Baía rivers, respectively. Trophic positions were significantly different among the species that composed each trophic category. These findings demonstrate that in energy-flow studies in detrital food chains generalizations concerning the grouping of fish into trophic categories and/or habitats should only be carried out after careful investigations of the local/specific trophic dynamics of the organisms.
Highlights
Bottom-feeding fish have an important role in the ecology of the ecosystems in which they live, as they process the pre-mineralization phase of organic material present in the mud, making it easier to decompose for microorganisms (Fugi, 1993), thereby accelerating the nutrient cycle (Gneri and Angelescu, 1951; Flecker, 1996)
In the upper Paraná River floodplain, stomach content analyses have demonstrated that the ichthyofauna exploit food resources according to their habitats, commonly showing spatial variations (Lolis and Andrian, 1996; Hahn et al, 1997). δ15N investigations carried out in the same area, have demonstrated variations in the trophic positions of P. lineatus, a bottom-feeding species, that are interrelated with the various environments (Lopes et al, 2007)
The ammonium ion (NH4+) is energetically more attractive than the nitrate ion (NO3–), due to its absorption inside the cells not requiring enzymatic reduction reactions, but as it is very scarce in the layers where phytoplankton are found, the nitrate ends up being the nitrogenous form most frequently used by vegetation (Esteves, 1998)
Summary
Bottom-feeding fish have an important role in the ecology of the ecosystems in which they live, as they process the pre-mineralization phase of organic material present in the mud, making it easier to decompose for microorganisms (Fugi, 1993), thereby accelerating the nutrient cycle (Gneri and Angelescu, 1951; Flecker, 1996) These organisms create indispensable links in the food chain, as they make the resources contained in the sediment available to pscivorous species. This species, which is regarded as a consumer, principally of mud (constituted for the most part by algae) (Fugi et al, 1996; Hahn et al, 1998; Luz et al, 2001), has reached the third trophic level in the Paraná River, which suggests that in addition to algae, it has been exploiting and assimilating food sources enriched in protein and δ15N (Lopes, 2007)
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