Spatial variation in aquatic food web structure across tropical riverine landscapes, International Society of River Science Congress, Florida USA, July 2009

Rivers across Australia’s tropical north have largely unmodified flow regimes and are characterised by their strongly seasonal flows and hydrological connectivity (or lack of it). They are considered to be amongst the most biologically diverse and healthy ecosystems in Australia, and are highly valued for their ecological, cultural, social and economic significance. However, there is growing interest in developing these water resources, and, combined with the degradation that has already been caused by feral species and changes to land-use, there is some urgency to build on our basic understanding of how these systems function to better predict the consequences of development. A survey of aquatic food web structure was conducted to identify the dominant food sources supporting tropical food webs, and the key consumers responsible for transferring energy within these webs. The survey was conducted 1) to test the hypothesis that river and floodplain food webs are strongly dependent on algal production, and 2) to identify macroconsumer species which may have a strong influence on benthic food webs. Potential aquatic and terrestrial food sources and a range of aquatic consumers were sampled from almost 70 sites across three catchments, the Daly River (NT), Fitzroy River (WA) and the Mitchell River (QLD), for analysis of their carbon and nitrogen stable isotopes signatures. Sites were specifically selected to encompass the spatial variation inherent in these tropical riverine landscapes and included tributary streams, rivers and floodplain waterholes, as well as the range of hydrological permanence characteristic of the region, from extremely intermittent to groundwater-dependent baseflows. Carbon sources underpinning food webs were found to vary between catchments, but consistently within catchments, as was the strength of coupling between consumers and their resources. The results suggest the trophic structure of these food webs may be a function of hydrological connectivity, and thus provide a basis for assessing the spatial variability of aquatic food web structure in Australia’s wet-dry tropics.