@article{WeitereAltenburgerAnlangeretal.2021, author = {Weitere, Markus and Altenburger, Rolf and Anlanger, Christine and Baborowski, Martina and B{\"a}rlund, Ilona and Beckers, Liza-Marie and Borchardt, Dietrich and Brack, Werner and Brase, Lisa and Busch, Wibke and Chatzinotas, Antonis and Deutschmann, Bj{\"o}rn and Eligehausen, Jens and Frank, Karin and Graeber, Daniel and Griebler, Christian and Hagemann, Jeske and Herzsprung, Peter and Hollert, Henner and Inostroza, Pedro A. and J{\"a}ger, Christoph G. and Kallies, Ren{\´e} and Kamjunke, Norbert and Karrasch, Bernhard and Kaschuba, Sigrid and Kaus, Andrew and Klauer, Bernd and Kn{\"o}ller, Kay and Koschorreck, Matthias and Krauss, Martin and Kunz, Julia V. and Kurz, Marie J. and Liess, Matthias and Mages, Margarete and M{\"u}ller, Christin and Muschket, Matthias and Musolff, Andreas and Norf, Helge and P{\"o}hlein, Florian and Reiber, Lena and Risse-Buhl, Ute and Schramm, Karl-Werner and Schmitt-Jansen, Mechthild and Schmitz, Markus and Strachauer, Ulrike and von T{\"u}mpling, Wolf and Weber, Nina and Wild, Romy and Wolf, Christine and Brauns, Mario}, title = {Disentangling multiple chemical and non-chemical stressors in a lotic ecosystem using a longitudinal approach}, series = {Science of the Total Environment}, volume = {769}, journal = {Science of the Total Environment}, pages = {144324}, year = {2021}, abstract = {Meeting ecological and water quality standards in lotic ecosystems is often failed due to multiple stressors. However, disentangling stressor effects and identifying relevant stressor-effect-relationships in complex environmental settings remain major challenges. By combining state-of-the-art methods from ecotoxicology and aquatic ecosystem analysis, we aimed here to disentangle the effects of multiple chemical and non-chemical stressors along a longitudinal land use gradient in a third-order river in Germany. We distinguished and evaluated four dominant stressor categories along this gradient: (1) Hydromorphological alterations: Flow diversity and substrate diversity correlated with the EU-Water Framework Directive based indicators for the quality element macroinvertebrates, which deteriorated at the transition from near-natural reference sites to urban sites. (2) Elevated nutrient levels and eutrophication: Low to moderate nutrient concentrations together with complete canopy cover at the reference sites correlated with low densities of benthic algae (biofilms). We found no more systematic relation of algal density with nutrient concentrations at the downstream sites, suggesting that limiting concentrations are exceeded already at moderate nutrient concentrations and reduced shading by riparian vegetation. (3) Elevated organic matter levels: Wastewater treatment plants (WWTP) and stormwater drainage systems were the primary sources of bioavailable dissolved organic carbon. Consequently, planktonic bacterial production and especially extracellular enzyme activity increased downstream of those effluents showing local peaks. (4) Micropollutants and toxicity-related stress: WWTPs were the predominant source of toxic stress, resulting in a rapid increase of the toxicity for invertebrates and algae with only one order of magnitude below the acute toxic levels. This toxicity correlates negatively with the contribution of invertebrate species being sensitive towards pesticides (SPEARpesticides index), probably contributing to the loss of biodiversity recorded in response to WWTP effluents. Our longitudinal approach highlights the potential of coordinated community efforts in supplementing established monitoring methods to tackle the complex phenomenon of multiple stress.}, language = {en} } @article{WentzkyTittelJaegeretal.2020, author = {Wentzky, Valerie C. and Tittel, J{\"o}rg and J{\"a}ger, Christoph G. and Bruggeman, Jorn and Rinke, Karsten}, title = {Seasonal succession of functional traits in phytoplankton communities and their interaction with trophic state}, series = {Journal of Ecology}, volume = {108}, journal = {Journal of Ecology}, number = {4}, pages = {1649 -- 1663}, year = {2020}, abstract = {1. Understanding and explaining the structure of communities in response to environmental gradients is a central goal in ecology. Trait-based approaches are promising but yet rarely applied to understand community dynamics in response to changing environmental conditions. 2. Here, we investigate seasonal succession patterns of functional traits in phytoplankton communities and how nutrient reductions (oligotrophication) alter these patterns. We used phytoplankton data from 40 years of observation from the Rappbode Reservoir (Germany), which underwent a strong shift in trophic conditions, and translated taxonomic composition into functional traits by assigning trait values compiled from the literature. 3. All studied traits (morphological, behavioural and physiological traits) responded to changing environmental conditions and showed consistent, reoccurring seasonal developments. The seasonal succession of phytoplankton communities was shaped by a trade-off between small-celled, fast-growing species that are able to rapidly incorporate existing resources (r -strategists) and large-celled species with more complex and efficient mechanisms to exploit scarce mineral nutrients or acquire previously unexploited nutrient pools (k -strategists). In summer, when nutrients were scarce, the k -strategy was prevailing (important traits: phosphate affinity, nitrogen fixation, motility and mixotrophy). During the rest of the year, nutrients and turbulence were high and r -strategists dominated (important traits: maximum growth rate and light affinity). 4. A comparison between eutrophic and oligotrophic years revealed that the main features of functional trait succession were largely preserved, but intra-annual fluctuations from spring to summer were stronger during eutrophic years. Nutrient reductions mainly affected functional traits and biomass in spring, while in summer the functional community composition changed little. 5. Synthesis. This study provides for the first time a quantitatively supported functional template for trait-based succession patterns in lakes under different nutrient conditions. By translating taxonomic composition into trait information, we demonstrate that the quantification of functional characteristics enables ecological interpretation of observed community dynamics and provides not only a testable template but also a powerful tool towards a more mechanistic understanding. The quantification of functional traits further improves the predictability of community shifts in response to changing environmental conditions and thus opens new perspectives for predictive limnology using lake ecosystem models.}, language = {en} } @article{YangBuettnerKumaretal.2019, author = {Yang, Soohyun and B{\"u}ttner, Olaf and Kumar, Rohini and J{\"a}ger, Christoph G. and Jawitz, James W. and Rao, P.S.C. and Borchardt, Dietrich}, title = {Spatial patterns of water quality impairments from point source nutrient loads in Germany's largest national River Basin (Weser River)}, series = {Science of The Total Environment}, volume = {697}, journal = {Science of The Total Environment}, year = {2019}, abstract = {We employed the well-established Horton-Strahler, hierarchical, stream-order (ω) scheme to investigate scaling of nutrient loads (P and N) from ~845 wastewater treatment plants (WWTPs) distributed along the river network in urbanized Weser River, the largest national basin in Germany (~46K km2; ~8.4 million population). We estimated hydrologic and water quality impacts at the reach- and basin-scales, at two steady river discharge conditions (median flow, QR50; low-flow, QR90). Of the five WWTPs class-sizes (1 ≤ k ≤ 5), ~68\% discharge to small low-order streams (ω < 3). We found large variations in capacity to dilute WWTP nutrient loads because of variability in (1) treated wastewater discharge (QU) within and among different class-sizes, and (2) river discharge (QR) within low-order streams (ω < 3) resulting from differences in drainage areas. For QR50, reach-scale water quality impairment assessed by nutrient concentration was likely at 136 (~16\%) locations for P and 15 locations (~2\%) for N. About 90\% of these locations were lower-order streams (ω < 3). At QR50 and only with dilution, basin-scale cumulative nutrient loads from multiple upstream WWTPs increase impaired locations to 266 (~32\% of total) for P. Considering in-stream uptake decreased P-impaired streams to 225 (~27\%), suggesting the dominant role of dilution in the Weser River basin. Role of in-stream uptake diminished along the flow paths, while dilution in larger streams (4 ≤ ω ≤ 7) minimizes the impact of WWTP loads. Under QR90 conditions [(QR50/QR90) ~ 2.5], water quality impaired locations will likely double for the basin-scale analyses. Long-term water quality data suggested that diffuse sources are the primary contributors for water quality impairments in large streams. Our data-modeling synthesis approach is transferable to other urbanized river basins and extends understanding of point source impacts on water quality across spatial scales.}, language = {en} } @article{WentzkyTittelJaegeretal.2018, author = {Wentzky, V. C. and Tittel, J. and J{\"a}ger, Christoph G. and Rinke, K.}, title = {Mechanisms preventing a decrease in phytoplankton biomass after phosphorus reductions in a German drinking water reservoir—results from more than 50 years of observation}, series = {Freshwater Biology}, volume = {63}, journal = {Freshwater Biology}, number = {9}, editor = {Wiley,}, pages = {1063 -- 1076}, year = {2018}, abstract = {1. To counteract the severe consequences of eutrophication on water quality and ecosystem health, nutrient inputs have been reduced in many lakes and reservoirs during the last decades. Contrary to expectations, in some lakes phytoplankton biomass did not decrease in response to oligotrophication (nutrient reduction). The underlying mechanisms preventing a decrease in biomass in these lakes are the subject of ongoing discussion. 2. We used a hitherto unpublished long-term data set ranging from 1961 until 2016 from a German drinking water reservoir (Rappbode Reservoir) to investigate the underlying mechanisms preventing a decrease in biomass. Total phosphorus (TP) concentrations in the Rappbode Reservoir dropped abruptly in 1990 from 0.163 to 0.027 mg/L within three consecutive years, as a result of banning phosphate-containing detergents. Despite substantial reductions in TP, total annual phytoplankton biomass did not decline in the long-run, and therefore, the yield of total phytoplankton biomass per unit phosphorus largely increased. 3. Regression analysis revealed a positive association between the yield and potentially phagotrophic mixotrophs (R2 = .465, p < .001). We infer that by ingesting bacteria, mixotrophic species were capable of exploiting additional P sources that are not accessible to obligate autotrophic phytoplankton, eventually preventing a decrease in algal biomass after TP reductions. 4. Long-term epilimnetic phosphorus concentrations during the winter mixing period decreased to a greater degree than summer phosphorus concentrations. Apparently, TP losses over the season were less intense. Spring diatom biomass also markedly decreased after oligotrophication. In fact, spring diatom biomass was positively related to the TP loss over the season suggesting diatoms play an important role in P reduction. However, this intraannual P processing was not the primary factor when focusing on the average yearly yield, which remained to be fully explained by mixotrophs. 5. Our study demonstrates this ecosystem's ability to compensate for changes in resource availability through changes in phytoplankton community composition and functional strategies. We conclude that an increase in mixotrophy and the ability to make bacterial phosphorus available for phytoplankters were the main factors that allowed the phytoplankton community of the Rappbode Reservoir to adapt to lower nutrient levels without a loss in total biomass.}, language = {en} } @article{JaegerBorchardt2018, author = {J{\"a}ger, Christoph G. and Borchardt, D.}, title = {Longitudinal patterns and response lengths of algae in riverine ecosystems: A model analysis emphasising benthic-pelagic interactions}, series = {Journal of Theoretical Biology}, volume = {442}, journal = {Journal of Theoretical Biology}, pages = {66 -- 78}, year = {2018}, abstract = {In riverine ecosystems primary production is principally possible in two habitats: in the benthic layer by sessile algae and in the surface water by planktonic algae being transported downstream. The relevance of these two habitats generally changes along the rivers' continuum. However, analyses of the interaction of algae in these two habitats and their controlling factors in riverine ecosystems are, so far, very rare. We use a simplified advection-diffusion model system combined with ecological process kinetics to analyse the interaction of benthic and planktonic algae and nutrients along idealised streams and rivers at regional to large scales. Because many of the underlying processes affecting algal dynamics are influenced by depth, we focus particularly on the impact of river depth on this interaction. At constant environmental conditions all state variables approach stable spatial equilibria along the river, independent of the boundary conditions at the upstream end. Because our model is very robust against changes of turbulent diffusion and stream velocity, these spatial equilibria can be analysed by a simplified ordinary differential equation (ode) version of our model. This model variant reveals that at shallower river depths, phytoplankton can exist only when it is subsidised by detaching benthic algae, and in turn, at deeper river depths, benthic algae can exist only in low biomasses which are subsidised by sinking planktonic algae. We generalise the spatial dynamics of the model system using different conditions at the upstream end of the model, which mimic various natural or anthropogenic factors (pristine source, dam, inflow of a waste water treatment plant, and dilution from e.g. a tributary) and analyse how these scenarios influence different aspects of the longitudinal spatial dynamics of the full spatial model: the relation of spatial equilibrium to spatial maximum, the distance to the spatial maximum, and the response length. Generally, our results imply that shallow systems recover within significantly shorter distances from spatially distinct disturbances when compared to deep systems, independent of the type of disturbance.}, language = {en} } @article{DupasMusolffJawitzetal.2017, author = {Dupas, R. and Musolff, A. and Jawitz, J. W. and Rao, P. S. C. and J{\"a}ger, Christoph G. and Fleckenstein, J. H. and Rode, M. and Borchardt, D.}, title = {Carbon and nutrient export regimes from headwater catchments to downstream reaches}, series = {Biogeosciences}, volume = {14}, journal = {Biogeosciences}, number = {18}, pages = {4391 -- 4407}, year = {2017}, abstract = {Excessive amounts of nutrients and dissolved organic matter in freshwater bodies affect aquatic ecosystems. In this study, the spatial and temporal variability in nitrate (NO3-), dissolved organic carbon (DOC) and soluble reactive phosphorus (SRP) was analyzed in the Selke (Germany) river continuum from three headwaters draining 1-3 km2 catchments to two downstream reaches representing spatially integrated signals from 184-456 km2 catchments. Three headwater catchments were selected as archetypes of the main landscape units (land use  ×  lithology) present in the Selke catchment. Export regimes in headwater catchments were interpreted in terms of NO3-, DOC and SRP land-to-stream transfer processes. Headwater signals were subtracted from downstream signals, with the differences interpreted in terms of in-stream processes and contributions from point sources. The seasonal dynamics for NO3- were opposite those of DOC and SRP in all three headwater catchments, and spatial differences also showed NO3- contrasting with DOC and SRP. These dynamics were interpreted as the result of the interplay of hydrological and biogeochemical processes, for which riparian zones were hypothesized to play a determining role. In the two downstream reaches, NO3- was transported almost conservatively, whereas DOC was consumed and produced in the upper and lower river sections, respectively. The natural export regime of SRP in the three headwater catchments mimicked a point-source signal (high SRP during summer low flow), which may lead to overestimation of domestic contributions in the downstream reaches. Monitoring the river continuum from headwaters to downstream reaches proved effective to jointly investigate land-to-stream and in-stream transport, and transformation processes.}, language = {en} } @article{JaegerHagemannBorchardt2017, author = {J{\"a}ger, Christoph G. and Hagemann, J. and Borchardt, D.}, title = {Can nutrient pathways and biotic interactions control eutrophication in riverine ecosystems? Evidence from a model driven mesocosm experiment}, series = {Water Research}, volume = {115}, journal = {Water Research}, pages = {162 -- 171}, year = {2017}, abstract = {Ecological theory predicts that the relative importance of benthic to planktonic primary production usually changes along the rivers' continuum from a predomination of benthic algae in lower stream orders to a predomination of planktonic algae at higher orders. Underlying mechanisms driving the interaction between algae in these habitats, its controlling factors and consequences for riverine ecosystems are, however, only partly understood. We present a mechanistic analysis of the governing ecological processes using a simplified, numerical model and examine how abiotic factors and biotic interactions influence benthic and planktonic algae by changing resource competition. We compare the outcome of the model with the results of a factorial mesocosm experiment mimicking the parameter spaces of the model. The results show a remarkable similarity with regard to the temporal development of benthic and pelagic algal biomass and shifting dominance patterns. In particular we analyse the effects of the pathways of nutrient supply (upwelling from the hyporheic zone, direct supply to the surface water, or via both pathways) and grazing in a gradient of river depths. Our results show that detachment of benthic algae, sinking of planktonic algae and the pathway of nutrient supply are key processes determining the respective algal biomass distributions particularly in shallow and intermediate deep systems. Increasing nutrient supply increases algal biomasses, but does not change the general pattern of the interactions. Decreasing light supply decreases the dominance of planktonic algae, but increases dissolved nutrients. At intermediate to high grazing rates algal biomass can be controlled by grazers, but however, at high grazing rates, dissolved nutrients accumulate in the surface water. Our results indicate that nutrient pathways, resource competition and internal control by grazing need to be considered explicitly for the understanding and explanation of eutrophication phenomena in riverine ecosystems. As a consequence, ecologically effective eutrophication management of running water systems has to go beyond the control of nutrient emissions or the achievement of limiting threshold values in the receiving waters, but requires the consideration of the nutrient pathways (surface water versus groundwater) and the shifting biological controls from lower to higher order stream ecosystems.}, language = {en} } @article{JaegerVredePerssonetal.2014, author = {J{\"a}ger, Christoph G. and Vrede, T. and Persson, L. and Jansson, M.}, title = {Interactions between metazoans, autotrophs, mixotrophs and bacterioplankton in nutrient-depleted high DOC environments: a long-term experiment}, series = {Freshwater Biology}, volume = {59}, journal = {Freshwater Biology}, number = {8}, pages = {1596 -- 1607}, year = {2014}, abstract = {1. Humic lakes with a high external supply of DOC and low input of nutrients can often support a high biomass of metazoan zooplankton. In such lakes, autotrophic algae compete with bacteria for inorganic nutrients, but bacteria support mixotrophic growth. Consequently, planktonic communities are often dominated by mixotrophic flagellates, while obligate autotrophic phytoplankton occurs in low numbers for extended periods. 2. To test the importance of autotrophic phytoplankton and mixotrophic flagellates as food resources for metazoan grazers and, in turn, the feedback effects of grazers on basal food-web interactions, we conducted a long-term experiment where we simulated abiotic resource relationships of humic lakes (high DOC [glucose] and low P input). We examined the population dynamics of Daphnia galeata when inoculated in systems with autotrophic algae only, mixotrophic algae only and a mixture of autotrophic and mixotrophic algae, and how the systems changed after the inoculation of Daphnia. All combinations were run at high- and low-light conditions to analyse the effects of light on food quantity and quality. 3. Daphnia grew to high densities only when mixotrophs were present at high-light conditions and showed no or only weak growth at low-light conditions or with autotrophs as the only food source. 4. Autotrophic algae and bacteria showed a strong competition for nutrients. Autotrophic algae were released from competition for nutrients after Daphnia grazed on bacteria, which led to a probable change of the bacteria community to less edible but less competitive taxa. As a consequence, there was a mutualistic interaction between autotrophs and mixotrophs before Daphnia were introduced which turned into competition after Daphnia inoculation. 5. We suggest that mixotrophic flagellates can be a critical resource for cladocerans and thereby also have a cascading effect on higher trophic levels, and cladocerans, in turn, have important indirect effects on basal planktonic food webs; hence, both might affect whole lake ecosystems.}, language = {en} } @article{JaegerDiehl2014, author = {J{\"a}ger, Christoph G. and Diehl, S.}, title = {Resource competition across habitat boundaries: asymmetric interactions between benthic and pelagic producers}, series = {Ecological Monographs}, volume = {84}, journal = {Ecological Monographs}, number = {2}, pages = {287 -- 302}, year = {2014}, abstract = {In shallow aquatic systems, benthic and pelagic primary producers typically compete for light and nutrients along opposing vertical supply axes: pelagic algae shade the benthic habitat; conversely, benthic algae intercept the nutrient flux from the sediment to the pelagic habitat. We present a general framework for analyzing such spatially asymmetric resource competition across habitat boundaries using a mechanistic, dynamical model. We visualize the mechanisms determining the outcome of these cross-habitat interactions using zero-net-growth isoclines, resource supply points, and resource consumption vectors. In extensive invasion analyses, we characterize the abiotic and competitive persistence boundaries of pelagic and benthic primary producers, which are set by environmental factors determining nutrient and light supply and are modified by resource use by the competitor in the respective other habitat. We note several qualitative differences between cross-habitat and "classical" within-habitat resource competition. First, coexistence of cross-habitat competitors is facilitated by, but does not require niche differentiation with respect to, the utilization of resources. Because each species has a competitive edge for the resource that is supplied from "its" side of the system, a competitor that is inferior in utilizing both resources can sometimes coexist with, or even exclude, a superior competitor. Second, increasing the external supply of one resource (the nutrient) may initially favor both competitors, until a breakpoint is reached where the benthic producer goes abruptly extinct. Finally, whether a given pair of cross-habitat competitors coexist or shows alternative states may depend on the environment. Specifically, benthic and pelagic algae may coexist at low nutrient and light supply but produce alternative states at high nutrient and light supply. Alternative states are, in turn, promoted by any algal trait combination that increases the spatial asymmetry in resource consumption, i.e., leads to a higher nutrient consumption in the benthic habitat and/or a higher light consumption in the pelagic habitat. In a first empirical application, we show that predictions from our model give a good fit to published data on benthic and pelagic primary production in temperate and arctic lakes spanning a broad range of nutrient environments.}, language = {en} } @article{KamjunkeBuettnerJaegeretal.2013, author = {Kamjunke, N. and B{\"u}ttner, O. and J{\"a}ger, Christoph G. and Marcus, H. and von T{\"u}mpling, W. and Halbedel, S. and Norf, H. and Brauns, M. and Baborowski, M. and Wild, R. and Borchardt, D. and Weitere, M.}, title = {Biogeochemical patterns in a river network along a land use gradient}, series = {Environmental Monitoring and Assessment}, volume = {185}, journal = {Environmental Monitoring and Assessment}, number = {11}, pages = {9221 -- 9236}, year = {2013}, abstract = {The Bode catchment (Germany) shows strong land use gradients from forested parts of the National Park (23 \% of total land cover) to agricultural (70 \%) and urbanised areas (7 \%). It is part of the Terrestrial Environmental Observatories of the German Helmholtz association. We performed a biogeochemical analysis of the entire river network. Surface water was sampled at 21 headwaters and at ten downstream sites, before (in early spring) and during the growing season (in late summer). Many parameters showed lower concentrations in headwaters than in downstream reaches, among them nutrients (ammonium, nitrate and phosphorus), dissolved copper and seston dry mass. Nitrate and phosphorus concentrations were positively related to the proportion of agricultural area within the catchment. Punctual anthropogenic loads affected some parameters such as chloride and arsenic. Chlorophyll a concentration and total phosphorus in surface waters were positively related. The concentration of dissolved organic carbon (DOC) was higher in summer than in spring, whereas the molecular size of DOC was lower in summer. The specific UV absorption at 254 nm, indicating the content of humic substances, was higher in headwaters than in downstream reaches and was positively related to the proportion of forest within the catchment. CO2 oversaturation of the water was higher downstream compared with headwaters and was higher in summer than in spring. It was correlated negatively with oxygen saturation and positively with DOC concentration but negatively with DOC quality (molecular size and humic content). A principle component analysis clearly separated the effects of site (44 \%) and season (15 \%), demonstrating the strong effect of land use on biogeochemical parameters.}, language = {en} }