Uncertainty in estimating fish trophic positions and food web structure in highly polluted river basins

Tributaries greatly influence the supply of organic matter to large rivers. In this study, we used carbon (ẟ13C) and nitrogen (ẟ15N) stable isotopes to assess if organic matter from tributaries with different 15N compositions influenced the food webs of downstream fish assemblages. In addition, we aimed to evaluate if estimates of fish trophic position (TP) are influenced by 15N-depleted and 15N-enriched baselines. We compared the food web of a fish assemblage upstream from tributaries (control site), with food webs downstream from 15N-depleted (site 1) and 15N-enriched tributaries (site 2). Fishes and resources collected from sites 1 and 2 had significant differences in ẟ15N composition (15 N-depleted and 15N-enriched, respectively) relative to their conspecifics from the control site. At the control site, fish species’ TPs were more reliable and realistic based on their known feeding ecology, with TPs ranging between 2 and 3. In contrast, fish TPs at sites 1 and 2 ranged between 2 and 6, and no consistency was observed among trophic guilds. We conclude that tributaries exert a great influence on fish food webs at downstream sites, either by direct assimilation of organic matter by consumers, or by assimilation of the dissolved inorganic nitrogen by primary producers. In addition, fishes under influence of pollution may exhibit an inaccurate representation of their TP, drawing attention to possible source of bias for TP estimates, and consequently, in the food web structure of nitrogen-polluted areas.