Assessing food web dynamics and relative importance of organic matter sources for fish species in two Portuguese estuaries: A stable isotope approach

► Trophic webs of two estuaries were analysed with stable carbon and nitrogen isotopes. ► Ultimate organic matter sources supporting production of fish species were determined. ► Main prey of fish used mostly salt marsh-derived organic matter as nutritional source. ► Fish species trophic levels differed at inter-species level, seasonally and spatially. ► Organic matter is transferred to higher trophic positions through benthic pathways.