Seasonal dynamics of food web energy pathways at the community-level

Stable isotopes analysis,especially carbon and nitrogen,are now commonly used to study material circulation and energy flow within ecosystems.Carbon isotope ratios(δ13C) are used to trace food sources of consumers,while nitrogen isotope ratios(δ15N) are mainly used to quantify trophic levels of organisms.Traditionally,δ13C vs.δ15N bi-plots have been used to describe food web structure for a single time period or ecosystem,but this method show less power in comparisons of food webs across time and space.Meanwhile,simple qualitative descriptions of food webs cannot test hypotheses regarding food web complex changes.Here we present statistical methodologies for quantitatively comparing stable isotope food web data.We demonstrate the utility of circular statistics and hypothesis tests for quantifying directional food web differences using a case study,a freshwater fish community from Meiliang Bay,Lake Taihu,over seasons.We used arrow diagrams for angle of change for fishes along a gradient of seasons,in which each arrow represents a single species and the direction in trophic niche space the species moved.The length of the arrow represents the magnitude of change for that particular species.Concentric circles correspond to magnitude of change.The straight line is the mean vector of change among all species;the curved line on the rim indicates the 95% confidence interval around the mean vector of change.Directional change(magnitude and angle) and statistics quantifying change in isotope niche space for fish communities were also calculated.Based on these results,we found that most species from the freshwater fish community showed statistically significant shift in the same direction in food web space toward more pelagic-based production except summer to autumn seasons.In support of these patterns,the Rayleigh′s test indicated significant pattern of consistent change among species in autumn to winter,winter to spring and spring to summer seasons.The Watson-William′s test for comparison of the community in different time indicated that significant difference between the directional change in the fish community between autumn to winter and winter to spring seasons.Seasonal dynamics of food web energy pathways at the community-level in the current study was attributed to seasonal dynamics of stable isotope composition of available resources and foraging shifts of fish species.Meanwhile,because of the high omnivory in fish community in the current system,temporal variations in pelagic primary productivity strongly drive the energy flows in local food web.Therefore,seasonal dynamics of food web energy pathways at the community-level in eutrophic ecosystem may imply the decrease of food web stability in this eutrophic system with the dynamics of resource availability.As an understanding of food web structure and functioning is of growing importance in ecology theory and ecosystem management,our study demonstrate the quantifying food web differences from the standard stable isotope analysis.