A deep investigation on the trophic structure of a fish assemblage in a reference condition river is important since allow the comparison and determination of the extent in which other sites are degraded. It is especially relevant when we consider a reference condition river located in a basin under influence of several anthropogenic activities, as is the case of the Rio Cipo, an important tributary of the polluted Rio das Velhas Basin, Brazil. We aimed to characterize the trophic structure of the fish assemblage of the Rio Cipo using the carbon (δ13C) and nitrogen (δ15N) stable isotopes of fish and several available food resources sampled at two sites along the Rio Cipo in the dry and wet seasons. We found that the δ15N of fishes change among trophic guilds, but not change between sites and seasons. The δ13C values in turn, varies among trophic guilds and between seasons, and the changes between sites are not very expressive. Detritivorous fish presented the greatest variation in δ13C values. Although all detritivorous species appeared to consume the same basal resources (algae, periphyton, POM and CPOM), the proportion of assimilation of each resource varied among them. Two individuals of Myleus micans (herbivorous) and two of Piabarchus stramineus (omnivorous), were found to differ substantially within the food web by presenting the most enriched values of δ15N and δ13C in the wet season at the site close to the confluence with other rivers. These outliers had influence in the broad isotopic niches of herbivorous and omnivorous fish. This result suggest that these species have a large spatial range and may be feeding in places with resources enriched in δ15N and δ13C, outside of the Rio Cipo, as well as highlights the importance of considering the influence of downstream polluted areas on the trophic structure of this reference condition river.

中文翻译：

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位于受污染流域的参考条件河流中鱼类群落的营养结构

对参考条件河流中鱼类组合的营养结构进行深入调查很重要，因为可以比较和确定其他地点退化的程度。当我们考虑位于受多种人为活动影响的盆地中的参考条件河流时，这一点尤其重要，例如巴西里约热内卢盆地的重要支流 Rio Cipo。我们旨在利用鱼类的碳 (δ13C) 和氮 (δ15N) 稳定同位素以及在 Rio Cipo 沿线两个地点在旱季和雨季采样的几种可用食物资源来表征 Rio Cipo 鱼类群落的营养结构。我们发现鱼类的δ15N在营养级之间变化，但在地点和季节之间没有变化。δ13​​C 值依次为 不同的营养公会和不同的季节，站点之间的变化不是很明显。食腐鱼类的δ13C 值变化最大。尽管所有食腐物种似乎都消耗相同的基础资源（藻类、附生生物、POM 和 CPOM），但每种资源的同化比例各不相同。通过在靠近与其他河流汇合处的湿季呈现最丰富的 δ15N 和 δ13C 值，发现两个 Myleus micans（草食性）和两个 Piabarchus stramineus（杂食性）在食物网中存在显着差异. 这些异常值对草食性和杂食性鱼类的广泛同位素生态位有影响。