Impacts of fish and nutrients on riverine primary production in a split stream experiment

In tropical river food webs top-down control (fish predation) has been established as a common and important structuring force. However much less is understood about the role of bottom-up control (nutrient addition) in these ecosystems. As part of the food web and biodiversity theme within the Tropical Rivers and Coastal Knowledge (TRaCK) research program, together with the Framework for the Assessment of River and Wetland Health (FARWH), we are examining the relative strength of top-down and bottom-up effects on benthic algal production within a diversity of benthic habitats. Here we ask: what are the impacts of fish and large macroinvertebrate grazers on benthic algal biomass? Do these effects vary depending on river habitat type (i.e. sandy vs. rocky)? What is the effect of nutrient addition on benthic algal biomass in grazed compared to non-grazed areas? We will address these questions experimentally using large scale exclusion cages, small scale electrical exclusion fences, and nutrient diffusing substrates. We have chosen to conduct this experiment in two reaches, which differ in their dominant substrate type (sandy vs. rocky) of a permanently flowing river in the Daly River catchment, Northern Territory. The results of this research will contribute to the understanding of tropical river food webs and provide valuable information necessary to make predictions about outcomes of future river management plans.