Stable carbon and nitrogen isotopes (SI) of fish have been widely used to elucidate food web structure. However, the development of methodological approaches for detecting changes in food webs has lagged behind. For instance, environmentally driven changes in composition and diversity of basal resources of an aquatic food web are transferred partially into the consumers of the upper trophic level. This leads to characteristic SI patterns in fish food webs depending on e.g. consumer’s feeding behavior, time and location. We aim at detecting specific SI pattern dynamics in fish food webs e.g. in the form of rotations of SI data with geometrical methods. We present how patterns of SI signatures of fish food webs change in response to spatial and temporal gradients. We provide examples drawn from estuarine food webs across wet and dry seasons as well as low and high river inflow sites in the USA and Africa. We use novel geometrical methodologies that are based on rescaled and normalized C and N isotopes and comparisons for shifts in Euclidean distance of isotope values for individual species and pairs of species. The findings illustrate how the methodology can be used to detect asymmetric baseline shifts of SI standardized fish food webs. Rotational shifts in the normalized fish SI values are detected in all case studies. The shifts in SI fish values mainly observed for the plankton-dependent consumers rather than detritus-dependent consumers. This indicates that the allochtonous vs. autochtonous contributions in different seasons and sites can change the baseline in different ways, leading to a rotation in fish SI data.