Using isotopic techniques to assess trophic structure in northern Murray-Darling Basin wetlands.

Floodplain wetlands provide habitats for many species of terrestrial and aquatic biota (Junk et al. 1989; Junk and Wantzen 2004). Their overall productivity and biodiversity is closely linked to patterns of flooding (Kingsford and Thomas 1995; Kingsford 2000; Arthington and Pusey 2003) which mobilise resources such as carbon and nutrients from the floodplain surface and upstream watercourses. These resources are subsequently utilised by a range or organisms, from bacteria to predatory fish and waterbirds over a range of temporal and spatial scales. For example, microbial components such as bacteria and algae develop within days of inundation of the floodplain sediments (Kobayashi et al. 2009) and are then utilised by higher-level consumers such as zooplankton and fish (Lindholm et al. 2007; Burford et al. 2008). Understanding the flow of nutrients and energy, as well as the trophic linkages within food webs, is essential for developing ecological models for sustainable management of aquatic ecosystems. However, food web connections are often complex and are likely to be influenced by the dynamics of physico-chemical processes. For example, the relative contribution of energy to food webs by autochthonous (derived within the watercourse, e.g. algae) and allochthonous (derived outside the watercourse, e.g. riparian leaf litter) primary sources varies between wetlands, influenced by factors such as climate, discharge regime, canopy cover, nutrient concentrations and other abiotic characteristics(Vannote et al. 1980; Bunn 1986; Lake et al. 1986; Reid et al. 2008). Identifying linkages between sources of primary production to lower and higher trophic-order consumers and detrital pathways (e.g. bacteria and detritivores) is fundamental to our understanding of ecosystems, and ultimately their conservation. Carbon and nitrogen stable isotope ratios (δ13C and δ15N) provide an important tool with which to model trophic connectivity between species and ecosystem resources. In this chapter, we review the ecological applications of stable isotope techniques to freshwater food web research and present findings from two key wetland systems in the northern Murray-Darling basin. © 2010, CSIRO Publishin