P-SPACE - A safe operating space for the prevention of eutrophication in riverscapes under multiple stress
The third PhD college of the Thematic Area Water Resources and Environment
Our riverine landscapes can be under siege by excessive growth of both benthic and pelagic algae - a manifestation of eutrophication. Eutrophication can seriously threaten the health of aquatic ecosystems, and humans and impair water-related services through oxygen deficiency, H2S production, and fish kills. The primary cause of eutrophication in inland waters is human activity - especially excessive inputs of mineral nutrient, particularly phosphorous (P). Although nutrient inputs to aquatic systems have long been regulated by law in Europe, the problem of eutrophication persists and future trends are unclear. In other parts of the world, the problem even continues to grow. While mineral nutrients are clearly the primary control for eutrophication, critical concentrations that trigger massive algal blooms have been found to be system-specific, potentially driven by anthropogenic modifications of stream morphology, such as river straightening or reduced floodplain connectivity. However, to the best of our knowledge, there is neither an empirical basis nor a mechanistic framework to explain these system-specific nutrient-algae relationships for streams and rivers in the German and European context.
However, system-specific nutrient-algae relationships are challenging when it comes to defining a safe operating space (SOS) for nutrients to prevent uncontrolled eutrophication. In the P-Space project, we therefore specifically choose to take a holistic view of the riverscape, including the river channel, wetlands, and floodplain lakes, to examine the relationships between changes in the river network and the critical nutrient concentrations that define a SOS. Specifically, we ask:
- How does the degradation of river morphology, particularly under consideration of altered hydrological conditions due to climate change, shift critical P concentrations that trigger uncontrolled eutrophication?
- Can we predict the likelihood of leaving the SOS with scenarios of future climate and catchment P concentrations?
- What locations in a riverscape and stream network are most vulnerable to eutrophication, temporally and spatially?
We have launched four linked PhD projects to address these challenges and to define the boundaries of a SOS to prevent eutrophication in riverscapes. The projects encompass data analysis on large datasets from Germany, other European countries and the U.S. (PhD I, IV), targeted field and experimental work across different stream orders including floodplain lakes (PhD I, II), modelling of spatial and temporal patterns of eutrophication in river networks up to basin scale (PhD III, IV) extending to future eutrophication scenarios (PhD I, III, IV).
The structure of P-SPACE fosters exchange and joint research activities between PhD projects as well as with the PhD college DYNAMO and the Helmholtz International Research School TRACER.
Research context and knowledge gaps
PhD project I focuses on a SOS preventing eutrophication in river Elbe floodplain lakes and wetlands around the City of Magdeburg. At the moment it is unclear if an efficient hydrological coupling of these water bodies to the stream increases the resilience of floodplain lakes to drought, for example, through mixing, oxygen supply and low residence times preventing excessive algal growth. The focus of the project is on field work. We study the water quality in floodplain lakes which are connected with varying efficiency to the main stream either by surface water or by groundwater.
Key questions
- Unacceptable impairment of water quality in floodplain lakes increases with the length of time that the water bodies are disconnected from the river.
- Floodplain lakes with a more efficient connection to the stream via the groundwater are more resilient to drought compared to less efficiently coupled lakes.
- More frequent floods combined with longer and more severe periods of drought favor frequent redox changes in floodplain soils, thereby increasing the mobility of P and the load of P to the river.
PhD student
- Luisa Coder
Advisory team
Research context and knowledge gaps
Floodplains are crucial hotspots for nutrient cycling but are increasingly affected by eutrophication. Benthic sediments may release nutrients like phosphorus (P) into the water column, which can perpetuate eutrophication despite efforts to mitigate external nutrient loads. This sedimentary P release is frequently attributed to mechanisms like desorption, but microbial processes mediating organic matter mineralization may also increase P diffusive flux, promoting the risk of internal eutrophication. However, these microbial processes are rarely accounted for in P-release schemes. Furthermore, the higher frequency of droughts and the disconnection of surface water bodies to the main river may increase water residence time, water temperature and sediment surface desiccation, thus modifying benthic P release patterns in floodplain waterbodies. This project aims to quantify the dominant microbial processes in benthic biofilms driving internal eutrophication and how they are modified by drought and morphological degradation (i.e., disconnection from the river). We will perform field and laboratory experiments in a floodplain system to delineate the safe operating space for minimizing the risk of internal eutrophication.
Key questions
- Which microbial functions are the primary drivers of the sediment P buffering capacity and the internal eutrophication risk?
- How do varying intensities of drought impact microbial processes mediating the internal eutrophication risk in floodplain lakes with differing hydrological connectivity?
- What is the spatiotemporal safe operating space of floodplain lakes to minimize the risk of internal eutrophication under drought conditions?
PhD student
- Michele Meyer (only german)
Advisory team
Research context and knowledge gaps
Complex ecological dynamic systems often retain more than one regime of a stable equilibrium state, and thus regime shifts can be readily caused by external drivers with different speeds. To define SOS for rivers in order to mitigate eutrophication, regimes should be identified at any points within a river network. Although recent studies have shown temporal dynamics of river productivity regimes being dependent on geomorphological features, hydrological conditions, nutrient loading, and abiotic variables, understanding of the differentiated regimes for benthic and pelagic algal communities, which have opposite habitat preferences and compete for nutrient and energy resources, is still missing.
Key questions
- How differ the spatial patterns of eutrophication regimes among large river basins in Central Europe? And what are the relative roles of geomorphologic, hydrologic, and nutrient input conditions into the results?
- Will the threshold-based parsimonious approaches for abiotic factors (light, temperature, shear stress) facilitate to manifest the spatio-temporal variability of the competing algae along rivers?
- What is the role of river network structures and river connectivity in forming algal regime transient zones? And how will these regimes change in the future when current extreme conditions will become more frequent?
PhD student
- Niklas Heinemann
Advisory team
Research context and knowledge gaps
There is still a knowledge gap in our understanding of how the different types and degrees of human interference interact with natural changes across the river orders to modify the probability of severe algae blooms. At the local scale, this knowledge can be gained through experimental approaches. However, observational large-scale, large-sample empirical data analysis is needed to test the generality of local findings within and across river networks to define the SOS limits. Furthermore, to spatially and temporally explicitly predict the danger of algae blooms, particularly under future climate and land-use changes, all this knowledge needs to be synthesized into the SMART modelling framework.
Key questions
- What are the critical nutrient concentrations triggering algae blooms in German and European rivers, and how do those conditions differ between different degrees of human impacts within and across river networks?
- What is the optimal model structure - following the SMART philosophy - to capture the observed behavior of eutrophication states within the mQM (multiscale water quality model) framework?
- What landscape attributes govern the underlying model parameterizations that allow transferability to other catchments and changed boundary conditions; and how can such relationships be optimally established utilizing both process understanding and advanced (data-driven) approaches including knowledge-guided machine learning (KGML)?
PhD student
Advisory team