Jörg Gelbrecht

Agriculture has changed the amount and composition of dissolved organic matter in Central European headwater streams

Dissolved organic matter (DOM) is an important part of the global carbon cycle and significantly influences aquatic ecosystem functions. Recent studies suggest that its amount and composition in freshwaters may be altered by agricultural land use, whereby the influence of preceding in-stream production and processing is not clear. To assess the land use effect on DOM amount and composition for the export from terrestrial to freshwater systems at the land-water interface, we sampled headwater streams draining agricultural and near-pristine catchments (forested and wetland) in the North German plains. To account for spatial and seasonal variation, we conducted a screening of DOM amount (53 sites) and composition (42 sites), and conducted bi-weekly samplings to investigate seasonal variation at eight sites over one year. Concentrations of dissolved organic carbon (DOC) were significantly higher for agricultural and wetland catchments than for forested catchments. Moreover, DOC loads exhibited higher seasonal variation for agricultural and wetland catchments than for forested catchments, which was due to higher variation in discharge. Parallel Factor Analysis revealed that the composition of DOM in agricultural catchments was significantly different from the other studied catchment types, and was characterized by low redox state and high structural complexity. Moreover, a gradient from protein- to humic-like fluorescence significantly separated forested from agricultural and wetland catchments. The contribution of humic-like DOM was strongly and positively related to DOC concentration, suggesting a mechanistic coupling of both. The effects of land use on patterns of DOC concentration and DOM composition were consistent across seasons, implying that land use strongly regulates DOM export. Overall, this study clearly shows the seasonally independent importance of agricultural land use for the amount and composition of DOM fluxes from the terrestrial zone to surface waters. These altered fluxes may affect ecosystem metabolism and health of agricultural headwaters and downstream situated aquatic ecosystems.

RELATIONSHIPS BETWEEN LITTORAL DIATOMS AND THEIR CHEMICAL ENVIRONMENT IN NORTHEASTERN GERMAN LAKES AND RIVERS1

We explored statistical relationships between the composition of littoral diatom assemblages and 21 chemical and physical environmental variables in 69 lakes and 15 river sites in the lowland of northeastern Germany. Canonical correspondence analysis with single treatment and with forward selection of environmental variables was used to detect 11 important ecological variables (dissolved inorganic carbon [DIC], Na+, total phosphorus [TP], dissolved organic carbon [DOC], total nitrogen [TN], pH, oxygen saturation, dissolved iron, SO42−, NH4+, soluble reactive silicium) and maximum water depth or Ca2+ or soluble reactive phosphorus that most independently explain major proportions of the total diatom variance among the habitats. Monte Carlo permutation tests showed that each contributed a significant additional proportion (P < 0.05) of the variance in species composition. Together, these 11 most important environmental variables explained 34% of the total variance in species composition among the sites and captured 73% of the explained variance from the full 21 parameters model. Weighted‐averaging regression and calibration of 304 indicator taxa with tolerance down‐weighting and classic deshrinking was used to develop transfer functions between littoral diatoms and DIC, pH, TP, TN, and Cl−. The DOC:TP ratio was introduced and a weighted‐averaging model was developed to infer allochthonous DOC effects in freshwater ecosystems. This diatom‐DOC/TP model was significant (P < 0.001) and explained 7.6% of the total diatom variance among the sites, surpassing the inferential power of the diatom‐TP‐transfer function (7.3% explained variance). The root‐mean‐square errors of prediction of the models were estimated by jack‐knifing and were comparable with published data sets from surface sediment diatom samples. The data set of littoral diatoms and environmental variables allows use of the diatom‐environmental transfer functions in biomonitoring and paleolimnological approaches across a broad array of natural water resources (such as floodplains, flushed lakes, estuaries, shallow lakes) in the central European lowland ecoregion.

Characterization of dissolved organic carbon (DOC) in a dystrophic lake and an adjacent fen

Dissolved organic carbon (DOC), humicsubstances (HS), polysaccharides (PS) and lowmolecular weight acids (LMWA) werecharacterized in water from the dystrophicLake Große Fuchskuhle over a period ofseven months. In addition, porewater from anadjacent fen was investigated in order toobtain information about the DOC in thecatchment area. Size-exclusion-chromatographycombined with UV- and organic carbon(IR)-detection was used to quantify DOC andits fractions. The lake had previously beendivided into four separate sections by largesheets of plastic, and the DOC compositiondiffered markedly between the fourcompartments. Spatial variations in HS and PSconcentrations were greater than seasonalvariations. The high amounts of HS (up to58%) in the western sections of the lake,indicated influence by subsurface water fromthe fen, whereas the eastern sections weredominated by PS (up to 35%) of algal origin.These differences could be explained byhydrological conditions, indicating thatcompletely different catchment areasinfluenced the water chemistry in the separatecompartments. By characterizing the HS bytheir average molecular weight and theiraromaticity, three different groups of HScould be distinguished depending on theirorigin and fate. Microbial degradation of DOCand its fractions differed between two of thecompartments during incubation studies over aperiod of six weeks.

Phosphorus mobilization in rewetted fens: the effect of altered peat properties and implications for their restoration

Rewetting of drained fens is necessary to stop further soil degradation and to reestablish important ecological functions. However, substantial changes of peat characteristics in the upper soil layers, due to drainage and land use, could counteract their recovery as nutrient-poor systems for an unknown period. We assessed the importance of altered peat properties, such as the degree of peat decomposition and the amount of redox-sensitive phosphorus (P) compounds, for P mobilization in different degraded fens. An experimental design involving 63 intact peat cores from fens with varying drainage and land-use histories was developed to quantify the mobilization of P, as well as that of iron (Fe), ammonium, carbon dioxide, and methane, all indicators of organic-matter decomposition and/or P-releasing processes. We found that net P release rates in peat cores with highly decomposed peat (range: 0.1-52.3 mg P x m(-2) x d(-1)) were significantly correlated to the amount of P bound to redox-sensitive compounds and the molar Fe:P as well as Al:P ratios of peat. We conclude that the following general rules apply for P mobilization in rewetted fens: (1) elevated levels of P release rates and P concentrations in pore water up to three orders of magnitude larger than under natural reference conditions can only be expected for rewetted fens whose surface soil layers consist of highly decomposed peat; (2) peat characteristics, such as the amount of P bound to redox-sensitive Fe(III) compounds (positive correlation) and molar ratios of Fe:P or Al:P (negative correlations), explain the high range of P release rates; and (3) a critical P export to adjacent lakes or rivers can only be expected if molar Fe:P ratios of highly decomposed peat are less than 10.

Global effects of agriculture on fluvial dissolved organic matter

Agricultural land covers approximately 40% of Earth’s land surface and affects hydromorphological, biogeochemical and ecological characteristics of fluvial networks. In the northern temperate region, agriculture also strongly affects the amount and molecular composition of dissolved organic matter (DOM), which constitutes the main vector of carbon transport from soils to fluvial networks and to the sea, and is involved in a large variety of biogeochemical processes. Here, we provide first evidence about the wider occurrence of agricultural impacts on the concentration and composition of fluvial DOM across climate zones of the northern and southern hemispheres. Both extensive and intensive farming altered fluvial DOM towards a more microbial and less plant-derived composition. Moreover, intensive farming significantly increased dissolved organic nitrogen (DON) concentrations. The DOM composition change and DON concentration increase differed among climate zones and could be related to the intensity of current and historical nitrogen fertilizer use. As a result of agriculture intensification, increased DON concentrations and a more microbial-like DOM composition likely will enhance the reactivity of catchment DOM emissions, thereby fuelling the biogeochemical processing in fluvial networks, and resulting in higher ecosystem productivity and CO2 outgassing.

Preface: Restoration, biogeochemistry and ecological services of wetlands

Natural wetlands, including fens, shallow lakes and floodplains, provide many benefits to society (Costanza et al., 1997). In particular, these wetlands deliver important functions as long-term sinks for nutrients and carbon, as hydrological buffers, and as habitats for many endangered plant and animal species (Millennium Ecosystem Assessment, 2005). In the face of eutrophication of watercourses, decreasing biodiversity, and expected climate changes, the importance of these multi-functional wetlands is being recognized more and more (McInnes, 2011). Currently, there are international efforts to protect wetlands or to repair their functions in the landscape through restoration measures (Erwin, 2009). However, their ecological rehabilitation has long been based on trial and error. Increasingly knowledge of the biogeochemical drivers of natural and biodiverse wetlands, and how these pertain to successful restoration, is vital in order to prevent costly operations with an unpredictable outcome (Klotzli & Grootjans, 2001). With relevant knowledge the restoration outcomes can be forecasted and choices should, therefore, be based on the actual restoration potential of an area rather than on the reconstruction of the prior ecosystem (van Diggelen et al., 2001). This approach requires information on causal relations and therefore experiments and investigations at different scales are of pivotal importance. Successful restoration of wetlands calls for interdisciplinary experimental research in which ecology, hydrology, microbiology, and geochemistry merge into a system-ecological approach. Furthermore, different scientific disciplines must be integrated with wetland managers, stakeholders, and crosssectoral government agencies throughout all stages of wetland restoration (from objective setting to postrestoration monitoring), in order to optimize restoration measures (Trepel, 2007). Embracing this approach, the 4th Annual Meeting of the European Chapter of the Society of Wetland Scientists (SWS) was held between 20th and 24th of May 2009 in Erkner, near Berlin, both scientists and practitioners came together to share their knowledge. The SWS is a 3500? member organisation that promotes scientific understanding, scientificallybased management and sustainable use of wetlands. Under the theme ‘‘Progress and problems in wetland science—with a particular focus upon wetland restoration in Europe’’, the 4th Annual Meeting attracted Guest editors: Dominik Zak, Robert McInnes, Jorg Gelbrecht / Restoration, biogeochemistry and ecological services of wetlands

Mitigation of Sulfate Pollution by Rewetting of Fens — A Conflict with Restoring Their Phosphorus Sink Function?

Sulfate pollution of lakes and rivers is recognized as a serious problem in many regions of Central Europe, thus we evaluated the role of rewetted fens in mitigating sulfate pollution and tested if high sulfate concentrations in fen-feeding water counteract the re-establishment of their function as sinks for phosphorus (P). A long-term incubation experiment was conducted with highly decomposed peat from upper soil layers of fens that have been rewetted for 1 to 15 years. Periodic sulfate pulses to inundated peat mesocosms, equating to an annual loading of 50 g S m−2, induced significant changes of sulfate consumption and phosphorus mobilization. Sulfate consumption of highly decomposed peat from all sampling sites was related to sulfate concentrations in overlying water (linear regression, p<0.01). Sulfate additions also led to significant increases of P concentrations or P mobilization in peat porewater (t test, p<0.05) and P concentrations in the overlying water were 2–3 times higher than in non-treated controls. In conclusion, rewetting of fens is an important tool to mitigate sulfate pollution of adjacent lakes and rivers. However, an elevated sulfate concentration in waters feeding the fen impairs P retention and increases P losses to adjacent surface waters.

Dialysis is superior to anion exchange for removal of dissolved inorganic nitrogen from freshwater samples prior to dissolved organic nitrogen determination

Environmental context Aquatic ecosystem health may be adversely affected by dissolved organic nitrogen pollution, and accurate analytical techniques are needed to assess these effects. Our study shows that dialysis is the best sample pre-treatment technique to increase the accuracy of dissolved organic nitrogen determination. It will improve analysis and understanding of the role of dissolved organic nitrogen in the nitrogen cycle of affected aquatic ecosystems. Abstract Dissolved organic nitrogen (DON) is usually determined as the difference between total dissolved nitrogen (TDN) and dissolved inorganic nitrogen (DIN). When applying this approach to samples with high DIN concentrations, there is a risk that small relative errors in TDN and DIN measurements may propagate into high absolute errors of the determined DON concentration. To reduce such errors, two pre-treatment methods have been suggested for the removal of DIN before the determination of DON: anion-exchange pre-treatment (AEP) and dialysis pre-treatment (DP). In this study, we tested the suitability of AEP and DP for DIN removal in order to increase the accuracy of DON determination of freshwater samples. The AEP performed well for standard compounds, yielding high dissolved organic carbon (DOC) recovery rates and >99 % removal of nitrate, whereas DON recovery rates varied and no removal occurred for ammonium and nitrite. However, AEP proved not to be suitable for natural samples, as it removed 36–74 % DOC and up to 83 % DON. In contrast, after 72 h of DP, 17–32 % DOC and DON had been removed from the natural samples, whereas >98 % nitrate was removed in all but one case, and >87 % of the ammonium and nitrite were removed. Moreover, we found that DP results in a significant increase in DON determination accuracy. In conclusion, DP is a useful measure to increase DON determination accuracy in natural freshwater samples with high DIN-to-TDN ratios, whereas AEP is not recommended for DON determination of natural freshwater samples.

Living on the pH Edge: Diatom Assemblages of Low-pH Lakes in Western Pomerania (NW Poland)

Diatom communities were studied in 2003 in three low-pH peatbog lakes in Western Pomerania (NW Poland); two of the lakes (LK and ZB) represented natural acid water bodies (NAWB), the third lake (LP) being least acid stressed. The three lakes differed markedly in their abiotic regimes, mainly on account of water pH in the two NAWBs being consistently below 5.0, while the pH range in the non-NAWB extended well into the alkaline part of the pH scale. In terms of all the important environmental variables studied (pH, conductivity, dissolved organic C, dissolved Si, dissolved P, nitrate and ammonia N), the three lakes followed an environmental gradient with LP on one end and ZB, a fully natural peatbog reservoir with high acidity, lower conductivity, higher upper limit of dissolved organic C and higher dissolved P contents and much lower dissolved Si contents, on the other. The study confirmed the general pattern of a low taxonomic richness in waterborne and bryophyte-associated diatom assemblages inhabiting low-pH habitats. The diatom assemblages differed between the three lakes and conformed to the environmental gradient. The LP assemblage (216 taxa), dominated by the common neutrophile Achnanthidium minutissimum, showed a preponderance of circumneutral and alkaliphilous taxa; the LK diatoms (15 taxa) represented mostly acidobiontic and acidophilous taxa, while the ZB assemblage (11 taxa) consisted predominantly of acidobionts. The study supplied information on the distribution of taxa having a high conservation value on the European scale. Important in the NAWBs were rare and endangered diatoms, primarily members of Eunotia, as well as Frustulia crassinervia. Their occurrence, coupled with other environmental and biotic characteristics of the lakes studied, makes the latter ideal candidates for conservation and protection.

Spatiotemporal redox dynamics in a freshwater lake sediment under alternating oxygen availabilities: combined analyses of dissolved and particulate electron acceptors

Environmental context At sediment surfaces, the availability of oxygen is controlled by its downward transport from the water surface and its consumption in microbial metabolism. Microorganisms can also consume substances other than oxygen to dispose of the surplus charge that is generated during microbial metabolism. We investigate the complex dynamics of these other substances when the oxygen availability fluctuates, and thereby contribute to the mechanistic understanding of oxygen-consuming processes in aquatic environments. Abstract Benthic mineralisation in lakes largely controls the availability of oxygen in the water column above the sediment. In stratified lakes with anoxic hypolimnetic waters, mineralisation proceeds by anaerobic respiration using terminal electron acceptors (TEAs) other than O2. In past work, hypolimnetic oxygen consumption has been estimated from vertical concentration profiles of redox-active dissolved species in the water column and the underlying sediment. Electron transfer to and from particulate mineral and organic phases in the sediments was, however, not accounted for, mainly because of methodological constraints. In this work we use an electrochemical approach, mediated electrochemical analysis, to directly quantify changes in the redox states of particulate geochemical phases in a lake sediment. In mesocosm incubations, sediments were subjected to shifting oxygen availability similar to conditions during and after lake overturn events. The temporal redox dynamics of both dissolved and particulate phases in sediments were monitored at a high spatial resolution. We used a combination of experimental and modelling approaches to couple the observed changes in the redox state of dissolved and particulate species in the sediment to the oxygen turnover in the overlying water column. For the studied freshwater sediment, the amount of O2 consumed during the re-oxidation of these phases in the top 21mm of the sediment after switching from hypoxic to oxic conditions corresponded to ~50% of the total sediment oxygen consumption that was estimated from in-lake measurements after the onset of summer stratification. We found that solid phases in the sediments play a more profound role in electron accepting processes than previously considered. Based on these results, we propose that the herein presented analytical method offers the possibility to constrain parameters in theoretical models that simulate benthic redox dynamics including the electron transfer to and from geochemical phases in the sediments.