Abstract: The ecosystem approach has substantially contributed to the generation of knowledge regarding biogeochemical fluxes and transformations in lake ecosystems. However, research in ecosystem ecology seldom considers the cycling of multiple elements simultaneously despite being linked via organismal stoichiometric demand. Additionally, the ‘black-box’ approach traditionally employed in ecosystem ignores processes occurring at lower levels of biological levels of organization which precludes integration across scales. This preclusion of a coupled biogeochemical and multi-scale understanding hinders efforts to identify the spatially and temporally varying importance of mechanisms underpinning observed patterns in ecosystem function. One way to incorporate a coupled, multi-scale perspective is quantitatively assessing the limitation of organismal physiology by environmental factors such as nutrients or light. Limitation is a quantitative integration of ecological processes acting across scales because it incorporates both the environmental supply and physiological demand of essential environmental factors. Supply mechanisms often act at the landscape or ecosystem-scale while demand mechanisms act at the organismal, population, or community scale. Here, we show how combining relatively simple lake ecosystem and algal physiological models ties limitation, and multi-scale drivers of limitation, to patterns in two lake ecosystem functions: primary productivity and carbon burial. First, to integrate past efforts to understand the chlorophyll–TP relationship, a common management tool that assesses patterns of lake primary productivity, we ran model simulations exploring the interactive influence of regional land use and climate, lake morphometry, and algal physiology. We found that broad shifts in algal limitation status underlie the shape of the chlorophyll–TP relationship and the occurrence of these shifts are determined by interactions between multi-scale drivers acting on supply and demand. Second, to disentangle the effects of land use and climate drivers (e.g., precipitation and temperature) on lake carbon burial we conducted a set of model simulations using a full-factorial manipulation of past/present land use and climatic forcings. We found that changes in land use and climate, such as shifts from forest to agriculture or increases in precipitation, led to changes in the limitation-status of primary producers. The degree to which primary productivity was released from, or further constrained by, limitation determined the magnitude and direction of change. Quantitatively identifying when and where shifts in limitation occur can provide a multi-scale, biogeochemically coupled understanding of ecosystem function. Incorporating this framework into management tools will enhance our ability to predict the impact of global change drivers, such as eutrophication and climate change, on ecosystem services.
