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 (delta(13)C, delta(15)N) of fish, their polychaete prey, and available autotrophic sources at several locations in southern Australia during 2 periods (summer, winter). Values for delta(13)C and delta(15)N for autotrophs and fishes varied little between seasons. Sources could be separated into 3 groups based on delta(13)C: seagrass and epiphytes (mean delta(13)C = -10.5parts per thousand), benthic microalgae and macroalgae (-19.5parts per thousand), and saltmarsh and mangroves (-26.5parts per thousand). Values of delta(15)N or the sources were 2 to 5parts per thousand. Values of delta(13)C for fish (-13.3parts per thousand) corresponded with those of their polychaete prey (-12.5parts per thousand) and ultimately with those of seagrass and epiphytes. Values of delta(15)N were 5 to 6parts per thousand. 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 delta(13)C 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.