Seagrass and epiphytic algae support nutrition of a fisheries species, Sillago schomburgkii, in adjacent intertidal habitats

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 (δ13C, δ15N) of fish, their polychaete prey, and available autotrophic sources at several locations in southern Australia during 2 periods (summer, winter). Values for δ13C and δ15N for autotrophs and fishes varied little between seasons. Sources could be separated into 3 groups based on δ13C: seagrass and epiphytes (mean δ13C = –10.5‰), benthic microalgae and macroalgae (–19.5‰), and saltmarsh and mangroves (–26.5‰). Values of δ15N for the sources were 2 to 5‰. Values of δ13C for fish (–13.3‰) corresponded with those of their polychaete prey (–12.5‰) and ultimately with those of seagrass and epiphytes. Values of δ15N were 5 to 6‰ more 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 δ13C 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.