Stable isotopes reveal the importance of saltmarsh-derived nutrition for two exploited penaeid prawn species in a seagrass dominated system

Estuaries represent highly important nursery habitats for a range of species, with refuge and nutrition being two key benefits derived from estuaries. Quantifying these benefits provides us with a means for enhancing fisheries productivity. Metapenaeus macleayi (School Prawn) and Penaeus plebejus (Eastern King Prawn) are two commercially and recreationally important species in New South Wales that utilise estuarine nurseries throughout their life history. In this study, stable isotopes of carbon, nitrogen and sulfur were used to determine the proportional contribution of primary producers to prawn nutrition in Brisbane Water (NSW). Both the saltmarsh grass Sporobolus virginicus and seagrass Zostera muelleri were found to support a high trophic contribution to prawns (up to 53% and 40%, respectively). The contributions of other primary producers such as mangroves, fine benthic organic matter (FBOM) and C₃ saltmarsh plants were generally found to be much lower (0.7-15%). Such findings are generally consistent with patterns observed in other south-east Australian estuaries, however such a dominant role of saltmarsh in the presence of seagrass is a novel finding. These results highlight linkages between habitats of conservation concern and highly valuable fisheries species, and the benefit of using sulfur as an additional marker in Bayesian mixing models examining estuarine food webs.