Eutrophication

Eutrophication is the process by which an entire body of water, or parts of it, becomes progressively enriched with minerals and nutrients, particularly nitrogen and phosphorus. It has also been defined as "nutrient-induced increase in phytoplankton productivity". Water bodies with very low nutrient levels are termed oligotrophic and those with moderate nutrient levels are termed mesotrophic. Advanced eutrophication may also be referred to as dystrophic and hypertrophic conditions. Eutrophication can affect freshwater or salt water systems. In freshwater ecosystems it is almost always caused by excess phosphorus. In coastal waters on the other hand, the main contributing nutrient is more likely to be nitrogen, or nitrogen and phosphorus together. This depends on the location and other factors. When occurring naturally, eutrophication is a very slow process in which nutrients, especially phosphorus compounds and organic matter, accumulate in water bodies. These nutrie

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3D water quality modelling of a eutrophic lake

Christel Marie Chappuis

In an environment subject to anthropogenic pressures, the understanding of lakes is crucial as they are essential for ecosystems and represent a source of the scarce accessible freshwater existing on Earth. Water quality management is especially designed to protect this precious resource and relies on monitoring programs and model simulations. Case study of this project, Greifensee is a small eutrophic lake located in the northern plateau of Switzerland (ZH). For decades, it has been subject to nutrient-‐rich runoff from agricultural activities in its catchments, consequence of the fast population growth in the area. These considerable human-‐related pressures have led to a strong process of eutrophication and significantly altered its water quality. In recent years, although nutrient inputs have been reduced, the lake has still not fully recovered and remains classified as eutrophic. This project developed a 3D coupled hydrodynamic-‐biological model for Greifensee using the software Delft-‐3D from Deltares in the Netherlands. Delft-‐3D has been widely applied in various situations and validated, which makes it an excellent tool for such project. Moreover, the development of the model also relies on two monitoring campaigns, from the Eawag institute and Zürich Kanton. Finally, while the model provides an almost full spatio-‐temporal representation of the processes happening in this lake, final results simulated fairly well the general trends of chlorophyll-‐a, dissolved oxygen, orthophosphate and ammonium in terms of mass content and depth distribution. Understanding the ecological dynamics of a eutrophic lake using such kind of simulation will hopefully contribute, on a longer-‐term perspective, to the better management of the lake towards the recovery of its former ecological status.

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Nutrient Availability Does Not Affect Community Assembly in Root-Associated Fungi but Determines Fungal Effects on Plant Growth

Bing Bai, Florian Frédéric Vincent Breider

Nonmycorrhizal root-colonizing fungi are key determinants of plant growth, driving processes ranging from pathogenesis to stress alleviation. Evidence suggests that they might also facilitate host access to soil nutrients in a mycorrhiza-like manner, but the extent of their direct contribution to plant nutrition is unknown. To study how widespread such capacity is across root-colonizing fungi, we surveyed soils in nutrient-limiting habitats using plant baits to look for fungal community changes in response to nutrient conditions. We established a fungal culture collection and used Arabidopsis thaliana inoculation bioassays to assess the ability of fungi to facilitate host’s growth in the presence of organic nutrients unavailable to plants. Plant baits captured a representation of fungal communities extant in natural habitats and showed that nutrient limitation has little influence on community assembly. Arabidopsis thaliana inoculated with 31 phylogenetically diverse fungi exhibited a consistent fungus-driven growth promotion when supplied with organic nutrients compared to untreated plants. However, direct phosphorus measurement and RNA-seq data did not support enhanced nutrient uptake but rather that growth effects may result from changes in the plant’s immune response to colonization. The widespread and consistent host responses to fungal colonization suggest that distinct, locally adapted nonmycorrhizal fungi affect plant performance across habitats.

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Relationships between key functional traits of the waterlily Nuphar lutea and wetland nutrient content

Little attention has been paid to how aquatic habitat characteristics affect the traits of plant species. Nuphar lutea (L.) Sm. is a keystone species distributed across temperate regions of Europe, northwest Africa and western Asia. Its apparently low phenotypic variability compared to other aquatic plants led us to test whether the species exhibited significant phenotypic variability and whether trait values correlated to environmental parameters. The hypotheses were that (1) the environmental variation within our set of wetlands (both water and sediment characteristics) led to significant variation among four sets of traits related respectively to growth, reproduction, defence and storage and (2) that nutrient limitation (nitrogen and especially phosphorus) should affect plant traits towards a higher investment in storage and defence and a lower investment in growth and reproduction, thereby negatively affecting the success of N. lutea. To test these hypotheses, 11 populations of N. lutea were sampled in wetlands differing in physicochemical characteristics and spread along three rivers. A total of 15 traits, grouped into four sets (growth, reproduction, storage and defence), were measured during the growing season. Most N. lutea traits were related to the environmental characteristics of wetlands. The growth and reproduction traits were mostly positively related to habitat resource conditions, whereas the defence traits were positively correlated with both ammonium concentration and temperature, outlining possible anoxic stress (habitat adversity). Nitrogen or phosphorus limitation led to the variation of only a few traits: the rhizome starch content was higher in phosphorus-limited wetlands, while the rhizome length and volume, and the number of flowers were higher in nitrogen-limited wetlands.

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