Critical estuarine habitats for food webs supporting fisheries in Port Curtis, central Queensland, Australia

Port Curtis in central Queensland, Australia, is a large subtropical embayment with very extensive intertidal and shallow subtidal mudflats. Many economically important fish and crustacean species occur over mudflats lacking conspicuous vegetation. The autotrophic source(s) supporting food webs leading to animal production on the mudflats might be either in situ microalgae or material transported from adjacent habitats dominated by macrophytes. We measured stable isotopes of C and N values of 9 fish and 4 crustacean species, and 8 autotroph taxa (Zostera seagrass, Halophila seagrass, mangroves, saltmarsh succulents, saltmarsh grass and algal mats in adjacent habitats, in situ microalgae on mudflats, and particulate organic matter in the water column – including phytoplankton) at three locations in Port Curtis. The contribution of each autotroph to fish species was modeled using a Euclidean mixing model. Fish C isotope values lay exclusively in the enriched half of the range for autotrophs, indicating very minor contributions from depleted autotrophs (mangroves, saltmarsh succulents). Seagrass (mainly Zostera) was in the top three potential contributors for all fish species. For crustaceans such as mud crabs (Scylla serrata) and banana prawns (Fenneropenaeus merguiensis), seagrasses (including Halophila) had the highest potential contributions. Organic matter from seagrass beds is an important source for animals on adjacent unvegetated mudflats, either through outwelling of particular organic matter or via a series of predator-prey interactions (trophic relay). Saltmarsh grass (Sporobolus) also had high putative contributions for many animal species but from work elsewhere we suspect this is a spurious result, reflecting the similarity in isotope values of this autotroph to seagrass. Although macrophyte production in adjacent habitats was the dominant source of nutrition for the suite of animals over unvegetated mudflats, in situ microalgae had a high potential contribution to half of the fish species and one of the crustacean species (Scylla serrata), and particulate organic matter, including phytoplankton, was a likely contributor to several other species.