Executive summary-Lakes

Water abstraction continues to increase worldwide, causing periods with extreme low-flow in many streams, which will likely intensify in the future due to climate change. Extreme low-flow may have major effects on in-stream habitats, organisms, and consequently ecosystem functions. We investigated the effects of a 2 months experimentally induced extreme low-flow scenario on the physical, biological, and functional characteristics in a macrophyte-rich lowland stream using a before-after, control-impact (BACI) approach. We quantified nutrient dynamics, including inorganic nitrogen and phosphorus concentrations, ammonium uptake, and whole-stream metabolism. We found a significant decline in the stream wetted habitat area, an increase in water temperature, and an increase in the accumulation of fine organic matter with reduced flow, but no significant changes in dissolved oxygen or benthic chlorophyll a concentrations. Furthermore, the relative demand and overall uptake of ammonium was lower in the low-flow reach relative to the control reach, whereas the relative demand and uptake of phosphate were higher at low-flow. Our results demonstrate that lowflow conditions cause resource limitation in stream biota most likely due to increased thickness of the diffusive boundary layers and an enhanced heterotrophic activity in the accumulated fine organic matter. Our results imply that the basal resources for productivity shift from autotrophic towards more heterotrophic resources causing a shift at higher trophic levels towards more detritivore based and less herbivore based food webs with implications for the invertebrate community composition and the distribution of functional feeding groups. Based on the strong links observed between low-flow and nutrient uptake, we suggest that functional metrics are suitable to assess the effects of low-flow conditions in small streams. MARS Deliverable D3.2: Manuscripts on experimental results 6 Paper 2: Multiple stress response of lowland stream benthic macroinvertebrates depends on habitat type Published in Science of the Total Environment in May 2017 with DOI: 10.1016/j.scitotenv.2017.05.102 Authors: Daniel Graeber, Tinna M. Jensen, Jes J. Rasmussen, Tenna Riis, Peter Wiberg-Larsen, Annette Baattrup-Pedersen a Department of Bioscience, Aarhus University, Vejlsøvej, Silkeborg Abstract: Worldwide, lowland stream ecosystems are exposed to multiple anthropogenic stress due to the combination of water scarcity, eutrophication, and fine sedimentation. The understanding of the effects of such multiple stress on stream benthic macroinvertebrates has been growing in recent years. However, the interdependence of multiple stress and stream habitat characteristics has received little attention, although single stressor studies indicate that habitat characteristics may be decisive in shaping the macroinvertebrate response. We conducted an experiment in large outdoor flumes to assess the effects of low flow, fine sedimentation, and nutrient enrichment on the structure of the benthic macroinvertebrate community in riffle and run habitats of lowland streams. For most taxa, we found a negative effect of low flow on macroinvertebrate abundance in the riffle habitat, an effect which was mitigated by fine sedimentation for overall community composition and the dominant shredder species (Gammarus pulex) and by nutrient enrichment for the dominant grazer species (Baetis rhodani). In contrast, fine sediment in combination with low flow rapidly affected Worldwide, lowland stream ecosystems are exposed to multiple anthropogenic stress due to the combination of water scarcity, eutrophication, and fine sedimentation. The understanding of the effects of such multiple stress on stream benthic macroinvertebrates has been growing in recent years. However, the interdependence of multiple stress and stream habitat characteristics has received little attention, although single stressor studies indicate that habitat characteristics may be decisive in shaping the macroinvertebrate response. We conducted an experiment in large outdoor flumes to assess the effects of low flow, fine sedimentation, and nutrient enrichment on the structure of the benthic macroinvertebrate community in riffle and run habitats of lowland streams. For most taxa, we found a negative effect of low flow on macroinvertebrate abundance in the riffle habitat, an effect which was mitigated by fine sedimentation for overall community composition and the dominant shredder species (Gammarus pulex) and by nutrient enrichment for the dominant grazer species (Baetis rhodani). In contrast, fine sediment in combination with low flow rapidly affected macroinvertebrate composition in the run habitat, with decreasing abundances of many species. We conclude that the effects of typical multiple stressor scenarios on lowland stream benthic macroinvertebrates are highly dependent on habitat conditions and that high habitat diversity needs to be given priority by stream managers to maximize the resilience of stream macroinvertebrate communities to multiple stress. MARS Deliverable D3.2: Manuscripts on experimental results 7 Paper 3: Responses of benthic algal communities and their traits to experimental changes in fine sediments, nutrients, and flow Accepted in Freshwater Biology in May 2017 Authors: Érika Maria Neif, Daniel Graeber, Liliana Rodrigues, Simon Rosenhøj-Leth, Tinna M. Jensen, Peter Wiberg-Larsen, Frank Landkildehus, Tenna Riis, Annette Baattrup-Pedersen a Programa de Pós-graduação em Ecologia de Ambientes Aquáticos Continentais – PEA – Nupélia, Universidade Estadual de Maringá, Maringá, PR, Brazil. b Department of Bioscience, Aarhus University, Denmark Abstract: 1Lowland stream ecosystems are subjected to multiple anthropogenic stressors, usually nutrient enrichment in combination with sedimentation of fine particles and low flow periods in summer. Here, we investigated the temporal development of the benthic algae community in response to these three stressors and linkages to the trait characteristics of the community to explore the mechanisms responsible for stress-induced community changes. 1Lowland stream ecosystems are subjected to multiple anthropogenic stressors, usually nutrient enrichment in combination with sedimentation of fine particles and low flow periods in summer. Here, we investigated the temporal development of the benthic algae community in response to these three stressors and linkages to the trait characteristics of the community to explore the mechanisms responsible for stress-induced community changes. 2We investigated the response of benthic algae species composition, traits (life forms, cell size categories), biovolume and Chl-a concentration to low flow in combination with nutrient enrichment and fine sedimentation in twelve large outdoor stream flumes (12 m long) resembling small streams in size and habitat characteristics. The experiment consisted of two phases: a normal-flow phase followed by a low-flow phase (90% current velocity reduction), each spanning four weeks. We applied a eutrophication scenario (mean increases of 1.14-5.48 mg N L-1 and 0.01-0.06 mg P L-1 in the flumes for dissolved inorganic nitrogen and phosphate, respectively) throughout the experiment. Under low flow, we supplemented this with a fine sedimentation scenario (> 90% stream bed cover). We took samples once in the normal-flow phase and every week during the low-flow phase. 3We observed strong responses in the benthic algae community to sudden changes in low flow and fine sedimentation, mediating rapid species turnover with a decreased algal biovolume and increased abundance of large, motile species. However, we did not observe any pronounced responses to nutrient enrichment. In contrast to the observations for other variables, we found a continuous increase in Chl-a concentration during low flow. This was likely due to continuous fine sedimentation during this phase, reducing light availability which probably resulted in an increase of cell-level Chl-a concentration in response to light limitation and lower rates of light-induced Chl-a degradation. 4The rapid response of the benthic algal community to the applied stressors suggests that even short periods of major stressor exposure may significantly affect benthic algae in lowland systems. We suggest that short-term stress events may have cascading effects on several important ecosystem processes given the importance of benthic algae for the productivity of these systems. MARS Deliverable D3.2: Manuscripts on experimental results 8 Paper 4: Effects of hydroand thermopeaking on benthic macroinvertebrate drift Published in Science of the Total Environment in December 2016 with DOI: 10.1016/j.scitotenv.2016.08.022 Authors: Lisa Schülting, Christian K. Feld, Wolfram Graf a Institute of Hydrobiology and Aquatic Ecosystem Management, University of Natural Resources and Life Sciences, 1180 Vienna, Austria b Aquatic Ecology, Faculty of Biology and Centre for Water and Environmental Research, University of Duisburg-Essen, 45117 Essen, Germany Abstract: The operation of storage hydropower plants is commonly linked to frequent fluctuations in discharge and water level (hydropeaking) of downstream river stretches and is often accompanied by cooling or warming of the water body downstream (cold or warm thermopeaking, respectively). The objective of this study is to assess the single and combined effects of hydropeaking and cold thermopeaking on the drift of selected aquatic macroinvertebrates in experimental flumes. The study specifically aims to (1) investigate the macroinvertebrate drift induced by hydropeaking, (2) identify taxon-specific drift patterns following combined hydropeaking and cold thermopeaking and (3) quantify diurnal drift differences under both impact types. Overall, hydropeaking induced significantly higher drift rates of most macroinvertebrate taxa. Combined hydropeaking and cold therm