Basin geochemistry and isotopic ratios of fishes and basal production sources in four neotropical rivers

Abstract

We analysed stable carbon and nitrogen isotopic ratios of dissolved inorganic carbon (DIC), plants, detritus and fishes to estimate the relative importance of dominant production sources supporting food webs of four Venezuelan rivers with divergent geochemical and watershed characteristics. Based on samples taken during the dry season at each site, fishes from two nutrient-poor, blackwater rivers had significantly lower d 13 C values (mean ¼ )31.4& and )32.9&) than fishes from more productive clearwater and whitewater rivers (mean ¼ )25.2& and )25.6& respectively). Low carbon isotopic ratios of fishes from blackwaters were likely influenced by low d 13 C of DIC assimilated by aquatic primary producers. Although floodplains of three savanna rivers supported high biomass of C 4 grasses, relatively little carbon from this source appeared to be assimilated by fishes. Most fishes in each system assimilated carbon derived mostly from a combination of microalgae and C 3 macrophytes, two sources with broadly overlapping carbon isotopic signatures. Even with this broad overlap, several benthivorous grazers from blackwater and whitewater rivers had isotopic values that aligned more closely with algae. We conclude that comparative stable isotopic studies of river biota need to account for watershed geochemistry that influences the isotopic composition of basal production sources. Moreover, isotopic differences between river basins can provide a basis for discriminating spatial and temporal variation in the trophic ecology of fishes that migrate between watersheds having distinct geochemical characteristics.

Potential influence of water level changes on energy flows in a lake food web

“…These isotopic shifts in consumers were also suggested by the IsoSource model estimation results, which indicated that in the wet season consumers shift to consuming 13 C-enriched endpoints like aquatic macrophytes and terrestrial plants, not seston. These results were consistent with the findings of several authors from different geographical regions which also reported that fish assemblages collected during low and high water level periods showed shifts in their stable carbon isotope compositions in response to a change in the isotope composition of their food [8][9][10][49][50][51].…”

“…15 N values may result from a shift to different 15 N basal food sources [10,16,17,50,51]. IsoSource model results also indicated variations in the temporal shifts of potential food sources assimilated by different species of consumers.…”

Are Large Herbivores Vectors of Terrestrial Subsidies for Riverine Food Webs?

“…A study during the wet season has estimated that 3-11% of the suspended sediments in the Mara River at the NMB site are contributed by hippo feces (Dutton 2012). However, the importance of terrestrial C4 carbon relative to autochthonous production should be interpreted with caution, given that the 13 C-enriched values of periphyton could also be partially explained by a contribution of CO 2 resulting from the respiration of C4 inputs to the inorganic C pool fixed by algae during photosynthesis (Rau 1978;Jepsen and Winemiller 2007). In this study, the importance of herbivore-mediated C4 carbon for consumers has been interpreted as a direct contribution via a detrital pathway, but it may also reflect a shift in the d 13 C values of a dominant algal source.…”

“…Although uncommon, these findings show that C4 resources can be major sources of energy in some rivers. Elsewhere in the tropics, some fishes are known to incorporate C4 sources in their diets, such as Schizodon fasciatus from the Amazon River (Forsberg and others 1993) and Schizodon isognatus from a floodplain lake in Venezuela (Jepsen and Winemiller 2007).…”

Trophic ecology of Arapaima in Guyana: giant omnivores in Neotropical floodplains

“…Other studies in tropical South America also have reported low 13 C values for curimatid fishes (e.g., Jepsen & Winemiller, 2007; δ 13 C = -38.1‰). In the central Amazon, fishes with such low δ 13 C values were primarily detritivorous characiforms, especially curimatids and prochilodontids that apparently fed on phytoplankton and/or detritus derived from phytoplankton (Forsberg et al, 1993).…”