Ecological niche is fundamental to study ecology which characterizes the relational position of a species within an ecosystem and comprises species habitat requirements as well as its functional role. It was first proposed by naturalist Joseph Grinnell in 1917 who emphasized the response of species to multitude of environmental requirements. Elton described the niche as "the status of an animal in its community" who stressed the impact of species on the environment. GE Hutchinson defined the ecological niche as a multidimensional space with environmental variables as axes that species could survive and reproduce. The niche concept has undergone many changes after Hutchinson's original formalization, but all alternative contemporary definitions retain the formalization of the niche as a multidimensional space. The trophic niches describing about the spectrum of consumers within the food webs and the trophic position has proven more tractable than other niche dimensions and often use dietary diversity to assess niche width. Stable isotopes especially carbon and nitrogen which are tightly linked to their diet are widely used to quantify the trophic niche. Resource richness, availability and interactions between organisms are among the factors that influence animals' trophic niches, but recently, the investigation concerning the impacts of habitat environment on trophic niches is very limited.
In the present study, we used the convex hull occupied by all species in δ¹³C versus δ¹⁵N bi-plot space to represent the total extent of trophic diversity within a food web. In order to make the metrics of the isotopic niche comparable, we converted the stable isotope ratios to diet proportions using mixing models. Using stable isotope analysis and correlation analysis, we calculated the total trophic niche widths and correlation coefficients between the habitat environmental factors such as ammonia nitrogen, nitrite nitrogen, nitrate nitrogen, total nitrogen, dissolved total nitrogen, soluble reactive phosphatesalt, total phosphorus, dissolved total phosphorus, chlorophyll a and the trophic niches of high trophic position fish yellow catfish respectively. The results show that the total trophic niches of yellow catfish reach maximum value in June and down to minimum value in November. Meanwhile, the percentage of planktonivory and benthivory is relatively high in spring but changes to a high percentage of piscivory later in the season. The correlation analysis indicates that there are no direct correlations between environmental factors and trophic niches, but some of environment factors can influence the subentry trophic niche (planktonivory, benthivory and piscivory). The ammonia nitrogen, nitrate nitrogen, total nitrogen, dissolved total nitrogen and dissolved total phosphorus perform a significant positive correlation to planktonivory while ammonia nitrogen, nitrate nitrogen, total nitrogen, dissolved total nitrogen and dissolved total phosphorus show a significant negative correlation to piscivory. However, benthivory is mainly affected by dissolved total phosphorus and is significantly and positively correlated with it.
In conclusion, the total trophic niches of yellow catfish are mainly affected by food resources abundance, availability and interspecific or intraspecific interactions but the impact of habitat environment on three subentry trophic niches is significant. Seasonal changes of water environmental factors influence the distribution of fish bait in water body and then affect the dietary composition of fish.