Discriminating the effects of global change drivers on headwater streams: a functional perspective – FunctionalStreams

Global change is affecting ecosystems worldwide at unprecedented rates, with strong impacts on biodiversity and ecosystem functioning and potential impairment of ecosystem services to humans. Freshwaters are particularly vulnerable to such environmental changes, due to their dependence upon land use, terrestrial subsidies, water abstraction and cumulative impact of multiple stressors. Global change drivers act simultaneously and the influence of their interactions is the major source of “ecological surprises”. Evaluating the contribution of individual drivers of global change and their interactions represents both a scientific challenge and a major issue for the society and environmental stakeholders. Warming and N enrichment through atmospheric pollution threaten to destabilize ecosystems, including headwater streams that are located in forested areas with low population density and are considered as biodiversity hotspots and essential providers of clean water for human consumption.
Leaf litter decomposition is the major source of carbon (C) and energy for woodland stream food webs. We propose to examine the alteration of litter decomposition in streams affected by global warming and nitrogen (N) deposition, and evaluate the relative contribution of these stressors and their interactions in a C budget perspective. The mechanisms underlying such effects together with the response and relative implication of decomposers will be identified. Literature on the subject remains scarce, being often limited to particular biological groups and mostly ignoring consequences at the whole ecosystem level. We hypothesize the impact of warming and N enrichment on decomposers and rate of C incorporation into stream food web to diverge depending on the nutrient status. We also expect warming to exacerbate effect of N enrichment on litter decomposition and consumers. The mechanisms underlying such interactive effects together with the response and relative implication of microbial decomposers and invertebrate detritivores will be identified. Here, we will use a wide range of approaches that are nested from ecosystem to microcosms and complementary in their specific objectives. Task 1 will consist in coordinating the workload between the participants of the two laboratories involved in the project. Budgets of C products from litter decomposition will be compared along nutrient gradients set in two regions in France (Task 2), and in the unique in natura structure dedicated to stream manipulation of temperature and nutrient in Europe (Task 3). Experiments in stream-simulating mesocosms will allow us to quantify consequences of increasing temperature under two nutrient limitation contexts (high/low N:P ratios of highly and weakly impacted streams, Task 4). Microcosm assays will aim at disentangling the combined effects of temperature and inorganic N on two interacting decomposer communities, aquatic fungi and invertebrate detritivores (Task 5). The dataset obtained within this project (Tasks 2–5) as well as a meta-analysis of an European database managed by the project leader (RivFunction) will be used to build and test a mechanistic model able to predict independent and interactive effects of nutrients and temperature on decomposition and organic C dynamics (Task 6). Finally, the project outcomes in addition to their dissemination to the scientific community will be disseminated to a broad audience through teacher training programmes, citizen science, and outreach activities to augment community awareness for global change issues (Task 7). This will be done in partnership with two (Lorraine and Midi-Pyrenees) of the four newly created schools for science (aka Maison pour la science). Moreover appropriate measures of management for headwater streams impaired by global changes will be communicated to potential end-users such as consulting companies, water agencies, and local river management actors through partnership with the pole of competitiveness Pole EAU.