TY - JOUR A1 - Berger, S.A. A1 - Diehl, S. A1 - Stibor, H. A1 - Trommer, G. A1 - Ruthestroth, M. A1 - Wild, A. A1 - Weigert, A. A1 - Jäger, Christoph G. A1 - Striebel, M. T1 - Water temperature and mixing depth affect timing and magnitude of events during spring succession of the plankton JF - Oecologia N2 - In many lakes, the most conspicuous seasonal events are the phytoplankton spring bloom and the subsequent clear-water phase, a period of low-phytoplankton biomass that is frequently caused by mesozooplankton (Daphnia) grazing. In Central European lakes, the timing of the clear-water phase is linked to large-scale climatic forcing, with warmer winters being followed by an earlier onset of the clear-water phase. Mild winters may favour an early build-up of Daphnia populations, both directly through increased surface temperatures and indirectly by reducing light limitation and enhancing algal production, all being a consequence of earlier thermal stratification. We conducted a field experiment to disentangle the separate impacts of stratification depth (affecting light supply) and temperature on the magnitude and timing of successional events in the plankton. We followed the dynamics of the phytoplankton spring bloom, the clear-water phase and the spring peak in Daphnia abundance in response to our experimental manipulations. Deeper mixing delayed the timing of all spring seasonal events and reduced the magnitudes of the phytoplankton bloom and the subsequent Daphnia peak. Colder temperatures retarded the timing of the clear-water phase and the subsequent Daphnia peak, whereas the timing of the phytoplankton peak was unrelated to temperature. Most effects of mixing depth (light) and temperature manipulations were independent, effects of mixing depth being more prevalent than effects of temperature. Because mixing depth governs both the light climate and the temperature regime in the mixed surface layer, we propose that climate-driven changes in the timing and depth of water column stratification may have far-reaching consequences for plankton dynamics and should receive increased attention. KW - Algal spring bloom KW - Clear-water phase KW - Daphnia hyalina KW - Enclosure experiment KW - Phytoplankton KW - Zooplankton Y1 - 2007 UR - https://doi.org/10.1007/s00442-006-0550-9 VL - 150 IS - 4 SP - 643 EP - 654 ER - TY - JOUR A1 - Jäger, Christoph G. A1 - Diehl, S. A1 - Matauschek, C. A1 - Klausmeier, C. A. A1 - Stibor, H. T1 - Transient dynamics of pelagic producer-grazer systems in a gradient of nutrients and mixing depths JF - Ecology N2 - Phytoplankton–grazer dynamics are often characterized by long transients relative to the length of the growing season. Using a phytoplankton–grazer model parameterized for Daphnia pulex with either flexible or fixed algal carbon : nutrient stoichiometry, we explored how nutrient and light supply (the latter by varying depth of the mixed water column) affect the transient dynamics of the system starting from low densities. The system goes through an initial oscillation across nearly the entire light–nutrient supply space. With flexible (but not with fixed) algal stoichiometry, duration of the initial algal peak, timing and duration of the subsequent grazer peak, and timing of the algal minimum are consistently accelerated by nutrient enrichment but decelerated by light enrichment (decreasing mixing depth) over the range of intermediate to shallow mixing depths. These contrasting effects of nutrient vs. light enrichment are consequences of their opposing influences on food quality (algal nutrient content): algal productivity and food quality are positively related along a nutrient gradient but inversely related along a light gradient. Light enrichment therefore slows down grazer growth relative to algal growth, decelerating oscillatory dynamics; nutrient enrichment has opposite effects. We manipulated nutrient supply and mixing depth in a field enclosure experiment. The experimental results were qualitatively much more consistent with the flexible than with the fixed stoichiometry model. Nutrient enrichment increased Daphnia peak biomass, decreased algal minimum biomass, decreased the seston C:P ratio, and accelerated transient oscillatory dynamics. Light enrichment (decreasing mixing depth) produced the opposite patterns, except that Daphnia peak biomass increased monotonously with light enrichment, too. Thus, while the model predicts the possibility of the “paradox of energy enrichment” (a decrease in grazer biomass with light enrichment) at high light and low nutrient supply, this phenomenon did not occur in our experiment. KW - algae-grazer interaction KW - C:P ratio KW - Daphnia KW - ecological stoichiometry KW - food quality KW - light enrichment KW - nutrient enrichment KW - paradox of enrichment KW - population model KW - population oscillations KW - seasonal succession Y1 - 2008 UR - https://doi.org/10.1890/07-0347.1 VL - 89 IS - 5 SP - 1272 EP - 1286 ER - TY - JOUR A1 - Yang, Soohyun A1 - Büttner, Olaf A1 - Kumar, Rohini A1 - Jäger, Christoph G. A1 - Jawitz, James W. A1 - Rao, P.S.C. A1 - Borchardt, Dietrich T1 - Spatial patterns of water quality impairments from point source nutrient loads in Germany's largest national River Basin (Weser River) JF - Science of The Total Environment N2 - 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. KW - Point source nutrient loads KW - Dilution KW - In-stream nutrient uptake KW - Eutrophication KW - Hydrological alteration Y1 - 2019 UR - https://doi.org/10.1016/j.scitotenv.2019.134145 VL - 697 ER - TY - JOUR A1 - Wentzky, Valerie C. A1 - Tittel, Jörg A1 - Jäger, Christoph G. A1 - Bruggeman, Jorn A1 - Rinke, Karsten T1 - Seasonal succession of functional traits in phytoplankton communities and their interaction with trophic state JF - Journal of Ecology N2 - 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. KW - freshwater ecology KW - functional groups KW - oligotrophication KW - plankton ecology group model KW - Rappbode Reservior KW - seasonal dynamics KW - trait-based approaches Y1 - 2020 UR - https://doi.org/10.1111/1365-2745.13395 VL - 108 IS - 4 SP - 1649 EP - 1663 ER - TY - JOUR A1 - Jäger, Christoph G. A1 - Diehl, S. T1 - Resource competition across habitat boundaries: asymmetric interactions between benthic and pelagic producers JF - Ecological Monographs N2 - 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. KW - Benthic algae KW - Consumption vector KW - Light KW - Meta-ecosystem KW - Nutrients KW - Pelagic algae KW - Resource competition KW - Spatial subsidies KW - Supply point KW - trait values Y1 - 2014 UR - https://doi.org/10.1890/13-0613.1 VL - 84 IS - 2 SP - 287 EP - 302 ER - TY - JOUR A1 - Jäger, Christoph G. A1 - Diehl, S. A1 - Emans, M. T1 - Physical determinants of phytoplankton production, algal stoichiometry, and vertical nutrient fluxes JF - The American Naturalist N2 - Most phytoplankters face opposing vertical gradients in light versus nutrient supplies but have limited capacities for vertical habitat choice. We therefore explored a dynamical model of negatively buoyant algae inhabiting a one-dimensional water column to ask how water column depth and turbulence constrain total (areal) phytoplankton biomass. We show that the population persistence boundaries in water column depth-turbulence space are set by sinking losses and light limitation but that nutrients are most limiting to total biomass in water columns that are neither too shallow or too weakly mixed (where sinking losses prevail) nor too deep and turbulent (where light limitation prevails). In shallow waters, the most strongly limiting process is nutrient influx to the bottom of the water column (e.g., from sediments). In deep waters, the most strongly limiting process is turbulent upward transport of nutrients to the photic zone. Consequently, the highest total biomasses are attained in turbulent waters at intermediate water column depths and in deep waters at intermediate turbulences. These patterns are insensitive to the assumption of fixed versus flexible algal carbon-to-nutrient stoichiometry, and they arise irrespective of whether the water column is a surface layer above a deep water compartment or has direct contact with sediments. KW - Water depth KW - Nutrient solutions KW - Turbulence KW - Nutrient transport KW - Sediments KW - Phytoplankton KW - Algae KW - Biomass production KW - Nutrient uptake Y1 - 2010 UR - https://doi.org/10.1086/650728 VL - 175 IS - 4 SP - E91 EP - E104 ER - TY - JOUR A1 - Wentzky, V. C. A1 - Tittel, J. A1 - Jäger, Christoph G. A1 - Rinke, K. ED - Wiley, T1 - Mechanisms preventing a decrease in phytoplankton biomass after phosphorus reductions in a German drinking water reservoir—results from more than 50 years of observation JF - Freshwater Biology N2 - 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. KW - community composition KW - long-term monitoring, KW - mixotrophy KW - oligotrophication KW - sedimentation Y1 - 2018 UR - https://doi.org/10.1111/fwb.13116 VL - 63 IS - 9 SP - 1063 EP - 1076 ER - TY - JOUR A1 - Jäger, Christoph G. A1 - Borchardt, D. T1 - Longitudinal patterns and response lengths of algae in riverine ecosystems: A model analysis emphasising benthic-pelagic interactions JF - Journal of Theoretical Biology N2 - 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. KW - Nutrients KW - Light KW - Longitudinal gradient KW - Resilience Y1 - 2018 UR - https://doi.org/10.1016/j.jtbi.2018.01.009 VL - 442 SP - 66 EP - 78 ER - TY - JOUR A1 - Jäger, Christoph G. A1 - Vrede, T. A1 - Persson, L. A1 - Jansson, M. T1 - Interactions between metazoans, autotrophs, mixotrophs and bacterioplankton in nutrient‐depleted high DOC environments: a long‐term experiment JF - Freshwater Biology N2 - 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. KW - Daphnia KW - Dinobryon KW - Food quality KW - Indirect effects KW - Scenedesmus Y1 - 2014 UR - https://doi.org/10.1111/fwb.12366 VL - 59 IS - 8 SP - 1596 EP - 1607 ER - TY - JOUR A1 - Jäger, Christoph G. A1 - Diehl, S. A1 - Schmidt, G. M. T1 - Influence of water-column depth and mixing intensity on phytoplankton biomass and functional community composition JF - Limnology and Oceanography N2 - We independently manipulated mixing intensity (strong artificial mixing vs. background turbulence) and water‐column depth (2 m, 4 m, 8 m, and 12 m) in order to explore their separate and combined effects in a field enclosure experiment. To accentuate the vertical light gradient, enclosures had black walls, resulting in a euphotic depth of only 3.7 m. All enclosures were placed in a well‐mixed water bath to equalize temperature across treatments. Phytoplankton responded to an initial phosphorus pulse with a transient increase in biomass, which was highest in the shallowest, least light‐limited water columns where dissolved mineral phosphorus subsequently became strongly limiting. As a consequence, the depth‐averaged mineral phosphorus concentration increased and the seston carbon (C) : phosphorous (P) ratio decreased with increasing water‐column depth. Low turbulence enclosures became quickly dominated by motile taxa (flagellates) in the upper water column, whereas mixed enclosures became gradually dominated by pennate diatoms, which resulted in higher average sedimentation rates in the mixed enclosures over the 35‐d experimental period. Low turbulence enclosures showed pronounced vertical structure in water columns >4 m, where diversity was higher than in mixed enclosures, suggesting vertical niche partitioning. This interpretation is supported by a primary production assay, where phytoplankton originating from different water depths in low‐turbulence treatments had the relatively highest primary productivity when incubated at their respective depths of origin. KW - aquatic ecology Y1 - 2008 UR - https://doi.org/10.4319/lo.2008.53.6.2361 VL - 53 IS - 6 SP - 2361 EP - 2373 ER - TY - JOUR A1 - Weitere, Markus A1 - Altenburger, Rolf A1 - Anlanger, Christine A1 - Baborowski, Martina A1 - Bärlund, Ilona A1 - Beckers, Liza-Marie A1 - Borchardt, Dietrich A1 - Brack, Werner A1 - Brase, Lisa A1 - Busch, Wibke A1 - Chatzinotas, Antonis A1 - Deutschmann, Björn A1 - Eligehausen, Jens A1 - Frank, Karin A1 - Graeber, Daniel A1 - Griebler, Christian A1 - Hagemann, Jeske A1 - Herzsprung, Peter A1 - Hollert, Henner A1 - Inostroza, Pedro A. A1 - Jäger, Christoph G. A1 - Kallies, René A1 - Kamjunke, Norbert A1 - Karrasch, Bernhard A1 - Kaschuba, Sigrid A1 - Kaus, Andrew A1 - Klauer, Bernd A1 - Knöller, Kay A1 - Koschorreck, Matthias A1 - Krauss, Martin A1 - Kunz, Julia V. A1 - Kurz, Marie J. A1 - Liess, Matthias A1 - Mages, Margarete A1 - Müller, Christin A1 - Muschket, Matthias A1 - Musolff, Andreas A1 - Norf, Helge A1 - Pöhlein, Florian A1 - Reiber, Lena A1 - Risse-Buhl, Ute A1 - Schramm, Karl-Werner A1 - Schmitt-Jansen, Mechthild A1 - Schmitz, Markus A1 - Strachauer, Ulrike A1 - von Tümpling, Wolf A1 - Weber, Nina A1 - Wild, Romy A1 - Wolf, Christine A1 - Brauns, Mario T1 - Disentangling multiple chemical and non-chemical stressors in a lotic ecosystem using a longitudinal approach JF - Science of the Total Environment N2 - 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. KW - Multiple stress KW - Running waters KW - Indicators KW - Ecological functions KW - Effect based analyses Y1 - 2021 UR - https://doi.org/10.1016/j.scitotenv.2020.144324 VL - 769 SP - 144324 ER - TY - JOUR A1 - Dupas, R. A1 - Musolff, A. A1 - Jawitz, J. W. A1 - Rao, P. S. C. A1 - Jäger, Christoph G. A1 - Fleckenstein, J. H. A1 - Rode, M. A1 - Borchardt, D. T1 - Carbon and nutrient export regimes from headwater catchments to downstream reaches JF - Biogeosciences N2 - 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. KW - Running water systems Y1 - 2017 UR - https://doi.org/10.5194/bg-14-4391-2017 VL - 14 IS - 18 SP - 4391 EP - 4407 ER - TY - JOUR A1 - Jäger, Christoph G. A1 - Hagemann, J. A1 - Borchardt, D. T1 - Can nutrient pathways and biotic interactions control eutrophication in riverine ecosystems? Evidence from a model driven mesocosm experiment JF - Water Research N2 - 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. KW - Light KW - Sinking KW - Detachment KW - Depth KW - Resource competition KW - Ecological management Y1 - 2017 UR - https://doi.org/10.1016/j.watres.2017.02.062 VL - 115 SP - 162 EP - 171 ER - TY - JOUR A1 - Kamjunke, N. A1 - Büttner, O. A1 - Jäger, Christoph G. A1 - Marcus, H. A1 - von Tümpling, W. A1 - Halbedel, S. A1 - Norf, H. A1 - Brauns, M. A1 - Baborowski, M. A1 - Wild, R. A1 - Borchardt, D. A1 - Weitere, M. T1 - Biogeochemical patterns in a river network along a land use gradient JF - Environmental Monitoring and Assessment N2 - 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. KW - TERENO KW - Land use KW - Nutrients KW - Heavy metals KW - DOC KW - Bode Y1 - 2013 UR - https://doi.org/10.1007/s10661-013-3247-7 VL - 185 IS - 11 SP - 9221 EP - 9236 ER -