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Anna Wressnig


Current Appointment: PhD Candidate - Marine Ecology, UTS Contact: Department of Environmental Sciences University of Technology, Sydney PO Box 123 SYDNEY NSW 2007 Australia E-mail: Anna Wressnig@uts.edu.au Project *Impact of leatherjackets Meuschenia freycineti* and M. trachylepis (Monacanthidae) on shallow estuarine Posidonia australis meadows in NSW** Seagrass meadows are important habitats for invertebrates, fishes and marine mammals. They provide shelter and protection from predation and are important nursery habitats for a variety of invertebrates and fishes, including a range of commercially important species. In addition seagrasses have also been found to serve as food for a wide range of animals, including fishes such as the leatherjackets Meuschenia freycineti (Sixspine leatherjacket) and Meuschenia trachylepis (Yellowfin leatherjacket) which inhabit estuarine seagrass meadows along the south-eastern coast of Australia. In order to develop management measures for estuarine ecosystems it is of vital importance to gain a better understanding of the ecological roles of their inhabitants. My study consists of three major components, which examine both, the direct and indirect impacts of two species of leatherjackets which live in and feed on the seagrass, Posidonia australis, and the nutritional role of the seagrass and its epiphytes and epifauna in the diet of these fishes. The fishes have a direct impact on the seagrass meadow by removing leaf and epiphyte biomass. In my study I determine sseasonal patterns of grazing pressure on the seagrass in order to calculate the amount of seagrass biomass removed by the grazing activities of the fishes. This involves estimations of the biomass of both fish species, of the amounts of seagrass they consume and determination of their bite sizes and gut throughput rates. In a feeding experiment I tested the fishes’ preference for particular parts of the seagrass shoot. In addition to their direct grazing impact the fishes also indirectly affect the seagrass meadow by contributing to its nutrient dynamics. By digesting and subsequently excreting part of the food they are thought to change the ratio of the major plant nutrients, nitrogen (N) and phosphorus (P). Several studies on nutrient recycling by zooplankton and macrofauna have demonstrated that consumers had a considerable impact on nutrient cycling within the ecosystem. In my study I examine the change in N:P between gut contents and faeces of the fish. The third part of my study is designed to shed some light on the nutritional role of P. australis and its epiphytes and epifauna in the diet of the two leatherjacket species. By analysing the fish tissue and the three possible food sources (P. australis, epiphytes and epipfauna) I am able to establish their most important food source and to see if they use the seagrass as an actual source of nutrients. My study is the first comprehensive examination of the role of grazing fishes in a seagrass meadow. It will contribute vital information to our knowledge of trophic interactions and nutrient dynamics in these important coastal ecosystems.

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