Comprehensive analysis of chemical and biological problems associated with browning agents used in aquatic studies

Inland waters receive and process large amounts of colored organic matter from the terrestrial surroundings. These inputs dramatically affect the chemical, physical, and biological properties of water bodies, as well as their roles as global carbon sinks and sources. However, manipulative studies, especially at ecosystem scale, require large amounts of dissolved organic matter with optical and chemical properties resembling indigenous organic matter. Here, we compared the impacts of two leonardite products (HuminFeed and SuperHume) and a freshly derived reverse osmosis concentrate of organic matter in a set of comprehensive mesocosm- and laboratory-scale experiments and analyses. The chemical properties of the reverse osmosis concentrate and the leonardite products were very different, with leonardite products being low and the reverse osmosis concentrate being high in carboxylic functional groups. Light had a strong impact on the properties of leonardite products, including loss of color and increased particle formation. HuminFeed presented a substantial impact on microbial communities under light conditions, where bacterial production was stimulated and community composition modified, while in dark potential inhibition of bacterial processes was detected. While none of the browning agents inhibited the growth of the tested phytoplankton Gonyostomum semen, HuminFeed had detrimental effects on zooplankton abundance and Daphnia reproduction. We conclude that the effects of browning agents extracted from leonardite, particularly HuminFeed, are in sharp contrast to those originating from terrestrially derived dissolved organic matter. Hence, they should be used with great caution in experimental studies on the consequences of terrestrial carbon for aquatic systems

Wetland eutrophication : consequences for aquatic plant quality, decomposition, and C fluxes

L'eutrophisation est une des principales menaces pesant sur les écosystèmes aquatiques. Cette thèse a pour objectifs de déterminer le rôle des paramètres abiotiques liés à l'eutrophisation, notamment la concentration en phosphore total, sur 1) la composition chimique des communautés végétales aquatiques, 2) leur décomposition et 3) les flux de C dans les zones humides. 1. Trois espèces aquatiques représentatives des trois stratégies adaptatives de Grime (i.e. compétitive, rudérale et stress tolérante) sont sélectionnées dans des zones humides distribuées le long d'un gradient de phosphore. Les espèces compétitives et rudérales ont une concentration en lignine significativement plus faible que l'espèce stress tolérante. Pour une même espèce, la teneur en eau augmente avec la concentration en phosphore de l'habitat et l'allocation en composés carbonés (amidon et/ou lignine) varie également significativement. 2. La composition des plantes aquatiques a un fort effet sur leur décomposition, les espèces rudérales et compétitives se décomposant plus vite, d'autant plus si elles se sont développées dans des sites riches en nutriments. 3. Dans les milieux eutrophes, les communautés végétales contribuent à l'augmentation des émissions de CH4 et diminuent les émissions de CO2 mesurées pendant la journée, probablement directement, au travers de leur qualité, de leur vitesse de décomposition, et des quantités de matières produites, et indirectement, au travers de leur position dans la lame d'eau. Le niveau d'eutrophisation des écosystèmes doit donc être pris plus explicitement en compte dans les modèles d'estimation des flux de carbone des milieux aquatiques d'eau douceEutrophication is a current threat for wetlands. This phD thesis aims at determining the role of the abiotic parameters of eutrophication, mainly the phosphorus content, 1) on aquatic plant quality, 2) on aquatic plant decomposition, and 3) on carbon fluxes. 1. Three aquatic plant species representative of the Grime strategies, i.e. competitive, ruderal and stress tolerant, were collected in wetlands dispatched along a phosphorus gradient. For the three species, water content of populations increased with the nutrient content of the habitat. Carbon allocation (starch and/or lignin) also varied according to habitat. 2. The three species were collected and decomposed in wetlands dispatched along a nutrient gradient. Aquatic plant quality significantly affected their decomposition, in particular the ruderal and competitive species were more rapidly decomposed when they grew in nutrient rich sites. 3. Daytime CO2 and CH4 fluxes were measured in 6 floristic zones. Daytime CO2 emissions were negatively correlated with net primary productivity and CH4 emissions were positively correlated. The abundance of floating vegetation also increased CH4 emissions probably because macroalgae and to a lower extent vascular plants with floating leaves favor anoxic conditions. Eutrophication may affect aquatic plant chemical composition and increase their decomposition rate. Moreover, in eutrophic wetlands, floating vegetation may affect carbon fluxes because of their quality, their decomposition rate and the quantities produced, and indirectly because of their location in water column. Therefore the eutrophication should be taken into account in the global C budgets of softwater ecosystem

Methane formation in tropical reservoirs predicted from sediment age and nitrogen

Freshwater reservoirs, in particular tropical ones, are an important source of methane (CH4) to the atmosphere, but current estimates are uncertain. The CH4 emitted from reservoirs is microbially produced in their sediments, but at present, the rate of CH4 formation in reservoir sediments cannot be predicted from sediment characteristics, limiting our understanding of reservoir CH4 emission. Here we show through a long-term incubation experiment that the CH4 formation rate in sediments of widely different tropical reservoirs can be predicted from sediment age and total nitrogen concentration. CH4 formation occurs predominantly in sediment layers younger than 6-12 years and beyond these layers sediment organic carbon may be considered effectively buried. Hence mitigating reservoir CH4 emission via improving nutrient management and thus reducing organic matter supply to sediments is within reach. Our model of sediment CH4 formation represents a first step towards constraining reservoir CH4 emission from sediment characteristics

Carbon allocation in aquatic plants with contrasting strategies: the role of habitat nutrient content

International audienceQuestions: The link between the carbon composition of aquatic plants and (1) plant strategies and (2) habitat nutrient availability has received little attention. We tested whether three aquatic species belonging to the three adaptive strate- gies defined by Grime (ruderal, stress tolerant and competitive) had contrasting carbon allocation patterns, and if these patterns varied in the same way between populations distributed along a gradient of habitat nutrient content.Location: Wetlands in the northern Rho^ne River Basin, France.Methods: The three species were sampled in 17 wetlands along a gradient of nutrient content in the northern Rho^ne River Basin. In each population sam- pled, we measured plant water content, C/N ratio, structural compounds (lignin and structural polysaccharides) and storage compounds (free sugars and starch) in two seasons (spring and autumn 2012).Results: The stress-tolerant species had higher content of structural compounds than the competitive and ruderal species. The content of storage compounds was higher in the competitive and stress-tolerant species compared to the ruder- al species. Allocation of carbon compounds varied with habitat nutrient content in different ways for the three species, suggesting contrasting plasticities, possi- bly linked to plant strategy.Conclusion: Plant strategies and habitat nutrient content are likely key drivers in plant carbon allocation and should be taken into account when studying interactions between habitat and plant quality

Can Soil Organic Carbon Fractions Be Used as Functional Indicators of Wetlands?

International audienc

Carbon emission along a eutrophication gradient in temperate riverine wetlands: effect of primary productivity and plant community composition

International audience1. Eutrophication increases primary productivity and favours the predominance of floating vegetation in wetlands. Carbon (C) fluxes in wetlands are strongly driven by primary productivity and can differ by vegetation type. However, to the best of our knowledge, the role of eutrophication in C fluxes has rarely been assessed. 2. Consequently, we aimed to measure the seasonal variation in carbon dioxide (CO2) and methane (CH4) fluxes at six aquatic sites in four temperate wetlands, ranging along a gradient of sediment total phosphorus content, and determine whether C fluxes correlate with above-ground net primary productivity (ANPP) and plant community composition along this eutrophication gradient. 3. Daytime CO2 emissions were significantly and negatively correlated with wetland net primary productivity as a result of the greater C fixation by photosynthesis during the peak of production. Conversely, CH4 emissions were significantly and positively correlated with wetland ANPP, possibly due to higher litter production and anaerobic decomposition. 4. The highest CH4 emissions were observed above floating vegetation, which favoured hypoxic conditions in the water column. CH4 emissions including ebullition were higher above macroalgal belts than above vascular plants with floating leaves. CH4 emissions without ebullition (i.e. resulting from plant transport and diffusion) better correlated with the abundance of macroalgae than with the abundance of vascular plants with floating leaves. 5. Our results suggest that eutrophication may greatly modify CO2 and CH4 emissions from wetlands through changes in vegetation type and productivity

The interaction between wetland nutrient content and plant quality controls aquatic plant decomposition

International audienceWe conducted an in situ decomposition experiment to better understand how habitat nutrient content controls aquatic plant decomposition and, more precisely, to determine the relative importance of the wetland conditions in decomposition, and the intrinsic degradability of plant tissues. We collected the green leaves of three aquatic plant species with contrasting plant strategies from three wetlands of differing nutrient contents, and allowed them todecompose in seven wetlands along a nutrient gradient. The plant mass loss was higher for competitive and ruderal species collected in nutrient richer wetlands as well as when they were led to decompose in nutrient richer wetlands. Plant water content correlated with mass loss for the competitive and ruderal species, which may explain the increase in mass loss with increasing nutrient content in the collection wetlands. Litter decomposition rate may be enhanced by wetland eutrophication, because of both the modification of wetland decomposition conditions and by changes in plant tissue quality

Predicting Methane Formation Rates of Freshwater Sediments in Different Biogeographic Regions

Freshwater lakes and reservoirs cover a small fraction of the Earth, however their emission of the greenhouse gas methane (CH4) from the sediment to the atmosphere is disproportionately high. Currently, there is still a limited understanding of the links between sediment characteristics and CH4 formation. Earlier studies have indicated that sediment age and nitrogen content are related to sediment CH4 formation rates, but it is uncertain such relationships are valid across gradients of sediment characteristics. We therefore measured potential CH4 formation rates in multiple layers of sediment sampled from nine lakes situated in the temperate, boreal and alpine biogeographic regions of Sweden, thus differing in productivity, catchment and climate properties. Potential CH4 formation varied over 3 orders of magnitude, and was broadly related to the quantity and reactivity of organic matter, and generally decreased with sediment depth. Sediment age and total nitrogen content were found to be the key controlling factors of potential CH4 formation rates, together explaining 62% of its variability. Moreover, the model developed from the Swedish lake sediment data was able to successfully predict the potential CH4 formation rates in reservoirs situated in different biogeographic regions of Brazil (R2 = 0.62). Therefore, potential CH4 formation rates in sediments of highly contrasting lakes and reservoirs, from Amazonia to alpine tundra, could be accurately predicted using one common model (RMSE = 1.6 in ln-units). Our model provides a valuable tool to improve estimates of CH4 emission from lakes and reservoirs, and illustrates the fundamental regulation of microbial CH4 formation by organic matter characteristics.Lakes and reservoirs are important emitters of methane, a strong greenhouse gas, to the atmosphere. Methane is produced in absence of oxygen by specific microbes that degrade the organic matter in the sediment. Currently, it is still uncertain which specific sediment properties control the production of methane, and if such properties are the same across lakes and reservoirs located in different ecosystem. To test this, we collected sediment cores from several lakes across different ecosystems in Sweden, and we measured potential methane formation rates. Methane formation rates varied greatly among lakes and was related to the quality and quantity of organic matter in the sediment. From this experiment, we calculated an empirical model that can predict methane formation rates as a function of sediment age and nitrogen content. Moreover, we found that our model could well predict potential methane formation rates in tropical reservoirs. In conclusion, sediment age and nitrogen content are universal controlling factors of methane formation rates across lakes and reservoirs in different ecosystems, from tropics to arctic tundra. Our findings provide a valuable tool to improve estimates of methane emission from lakes and reservoirs and illustrates how sediment characteristics play a crucial role in regulating methane formation rates

Predicting Methane Formation Rates of Freshwater Sediments in Different Climates

<p>These datasets uploaded here contain:</p> <p>CH4 formation rates of sediment layers incubated from nine Swedish lakes (<a href="https://zenodo.org/api/files/39e74df0-e0c7-41a6-88c6-d37bcc1ffb49/CH4Formation_SwedishLakes.csv">CH4Formation_SwedishLakes.csv</a>),  their sediment characteristics measured in different layers and land use data</p> <p>CH4 formation rates retrieved from Isidorova et al. (2019,<a href="http://urn.kb.se/resolve?urn=urn%3Anbn%3Ase%3Auu%3Adiva-387547">urn:nbn:se:uu:diva-387547</a>). From this dataset we extracted only the maximum CH4 formation rates and are reported in the file <a href="https://zenodo.org/api/files/39e74df0-e0c7-41a6-88c6-d37bcc1ffb49/CH4Formation_BrazilianReservoirs.csv">CH4Formation_BrazilianReservoirs.csv</a></p> <p>CH4 formation rates used to calculate CH4 formation decay rate with age for Swedish lakes (<a href="https://zenodo.org/api/files/39e74df0-e0c7-41a6-88c6-d37bcc1ffb49/CH4Formation_SwedishLakes_forDecayRate.csv">CH4Formation_SwedishLakes_forDecayRate.csv</a>)</p> <p>Extraction of PLS analysis results <a href="https://zenodo.org/api/files/39e74df0-e0c7-41a6-88c6-d37bcc1ffb49/PLS_List.xlsx">PLS_List.xlsx</a></p&gt

Predicting Methane Formation Rates of Freshwater Sediments in Different Biogeographic Regions

These datasets uploaded here contain: CH4 formation rates of sediment layers incubated from nine Swedish lakes (CH4Formation_SwedishLakes.csv), their sediment characteristics measured in different layers and land use data CH4 formation rates retrieved from Isidorova et al. (2019,urn:nbn:se:uu:diva-387547). From this dataset we extracted only the maximum CH4 formation rates and are reported in the file CH4Formation_BrazilianReservoirs.csv CH4 formation rates used to calculate CH4 formation decay rate with age for Swedish lakes (CH4Formation_SwedishLakes_forDecayRate.csv) Supplementary information SupplementaryInfo.pdf related to CH4 formation rates and sediment age Extraction of PLS analysis results PLS_List.xls