The importance of organic matter produced in seagrass meadows (seagrass and epiphytic algae) to the nutrition of a valuable fisheries species (yellowfin whiting Sillago schomburgkii Peters) occurring over unvegetated mudflats was measured using the isotopic composition (d13C, d15N) of fish, their polychaete prey, and available autotrophic sources at several locations in southern Australia during 2 periods (summer, winter). Values for d13C and d15N for autotrophs and fishes varied little between seasons. Sources could be separated into 3 groups based on d13C: seagrass and epiphytes (mean d13C = -10.5马 benthic microalgae and macroalgae (-19.5马 and saltmarsh and mangroves (-26.5驮 Values of d15N for the sources were 2 to 5鮠Values of d13C for fish (-13.3驠corresponded with those of their polychaete prey (-12.5驠and ultimately with those of seagrass and epiphytes. Values of d15N were 5 to 6頭ore enriched than sources. Modelling of feasible source mixtures showed that seagrass and epiphytes were the most important contributors to the nutrition of fish, but their relative importance varied between seasons. The median contribution by other sources was <10%. Spatial analyses showed that saltmarsh plants contributed significantly to the variability in S. schomburgkii nutrition among locations, while macroalgae contributed in summer. The similarity in d13C values of polychaetes and S. schomburgkii is consistent with source material from a subtidal habitat being incorporated into food webs supporting a fisheries species in adjacent intertidal habitats via a largely sedentary intermediary.