Seasonal contrasts in carbon budgets on a tropical river floodplain, International Society for River Science Congress, Florida USA, July 2009

Tropical floodplains are highly productive ecosystems where replenishing floodwaters stimulates nutrient turnover, which in turn substantially boosts both primary and secondary productivity. This is typical of many floodplain streams in the wet-dry tropics of northern Australia, where predictable seasonal changes in water levels are associated with dramatic increases in aquatic primary productivity and a shift in importance of types of primary producers. We constructed a “first-cut” carbon budget for the channel and floodplain for the Magela Creek within Kakadu National Park, thereby providing a focus to examine important ecological processes such as those encompassing seasonal interactions between aquatic and terrestrial systems. Macrophytes in the form of grasses and aquatic plants produce by far the greatest biomass on the floodplain annually. For macrophyte biomass which is not consumed in frequent fires on the floodplain in the late dry season, the labile dissolved organic carbon leaches out and is quickly consumed by microbes, without being directly used in the upper food web. Epiphytic algae are most likely the predominant energy source for aquatic consumers on the Magela Creek floodplain and macrophytes provide a suitable substrate for these epiphytes but are not an important energy source for consumers. A generalised food web contrasting the wet season and dry season for the Magela Creek system, as a representative seasonal floodplain stream in tropical northern Australia, is presented. This includes “best guess” estimates of the major sources, producers and consumers of carbon and an identification of key knowledge gaps.