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MILJØFORSK-Miljøforskning for en grønn samfunnsomstilling

Lakes in Transition: a Biogeochemical Modelling Approach

Alternative title: Innsjøer i endring

Awarded: NOK 2.9 mill.

Compliance with water quality directives worldwide hinges on our ability to improve water quality and prevent further deterioration. The success of recent efforts to implement remediation measures in catchments, or perform lake geoengineering, is usually evaluated based on the response of ecological indicators, such as algal blooms or fish populations. In many cases based on these indicators, the outcomes of management actions are unclear, that is, managers and scientists alike have witnessed that indicators have behaved in unpredictable ways over the years, despite sustained measures. The context of this project is set by the question: Do lake remediation measures work? "Lakes in Transition" has worked to resolve this ambiguity. Our hypothesis was that the biogeochemical cycles of lakes can be mapped and acted upon. We filled knowledge gaps on how these cycles are linked in lakes, and on their resilience to external factors. Since the beginning of the project, we have further developed the process-based lake model MyLake was developed at NIVA in 2005-2007 and that has been since then adopted worldwide to predict ice formation, water temperature, algal biomass and the response of lakes to climate change. In Lakes in Transition we updated MyLake so that it can simulate biogeochemical cycles in lakes as well as sediment-water interactions. Over the course of 10 peer-reviewed publications we have improved the model so that it can capture the cycling of carbon, oxygen, phosphorus, nitrogen, iron, sulfur, and trace metals in boreal lakes. With the new tools developed, we mapped the thresholds in the biogeochemical cycles. We use the "response-surfaces" approach developed by our colleagues at the Center for Ecology and Hydrology (UK). Having mapped the response of the biogeochemical cycles to external drivers, opens the door for scrutinizing the behaviour of the models as they attempt to reproduce those thresholds and paves the way for further research. We tested the real-world limitation of the ecological theory that we had plan to test. Namely, we concluded that the statistical feature called flickering, which according to ecological theory would be different before and after a regime shift, could not be found using data from monitoring program. The low frequency of the data from monitoring program, compounded model uncertainly and with the noise from chaotic weather, a causing this. However, we found that recent time series of high-frequency data from sensor shown promises, but those times series are currently too short ?because of the novelty of the technology ? to used to explore regime shifts in long-term time series. The applications of the new version of MyLake, both for applied and fundamental lake research, were very promising and gave rise to several publications in high-impact journals. We studied, for instance, the effect of sulfur deposition and increasing carbon concentrations. Our findings show that acid deposition has changed the chemistry of lake sediments, such that lake sediments are now acting as a source of sulfur to the water column. This has implications for the retention of phosphorus in sediments, because sulfur is known to reduce the iron in minerals to which P is attached, thus freeing the P to the overlying water. In a collaboration with researchers from China, we have contributed to interpreting status and trends in Chinese lakes impacted by external phosphorus loads over the past decades. We have revealed that changing phosphorus sources were responsible for the observes diminutions in lake TP concentrations. We have also demonstrated that climate change is improving oxygenation conditions in boreal lakes. This is because shorter ice coverage allows oxygen to enter the lakes earlier than in the past. In contrast, we have shown in Estonian lakes that climate change will modify the ecological balance of the lakes, favouring the growth of potentially harmful blue-green algae and ultimately depleting bottom-water oxygen. On the more applied side, our work at Llake Vansjø ? in conjunction with the EU project MARS showcased that good river-basin and lake management are still a sound strategy to overcome detrimental effects of climate change. Together, these examples showcase the interest in using the multiple-stressors framework to explore critical factors in the biogeochemical cycles of lakes. Lakes in Transition ends in 2018. This project has paved the way for a better understanding of the response of lakes to climate and land-use change and to a stronger contribution of lakes models to climate research. Promising avenues for further work lies in using the model to better understand how lakes contribute to climate feedback, via their control on the carbon cycle. The use of lake model to better plan lake remediation measures, such as aluminum amendments to control eutrophication, is now a possibility based on the progress we?ve made in Lakes in Transition.

Lakes in Transition has allowed the field of lake modeling to progress significantly. Previous investment made in the MyLake code were leveraged and led knowledge generation, new tools, and international collaborations. Specifically, Lakes in Transition has led to 10 peer-reviewed publications on the response of coupled biogeochemical cycles in lakes and lake sediments. It has also lead to a comprehensive state-of-the-art code base on lake modelling, publicly available on GitHub. 61 researchers worldwide have requested access the code base so far. The updated model has been instrumental in leveraging other projects with funding external to Norway, totaling 8 M NOK in funds from FORMAS (Sweden), JPI (Water-JPI PROGNOS, EU evel) and ERA-Net (WateXr, EU level).

Timely compliance with the European Water Framework Directive hinges on our collective ability to improve lake water quality and prevent further deterioration. The success of recent efforts to implement remediation measures in catchments, or perform lake geoengineering, has been evaluated based on the response of ecological indicators, such as algal blooms or fish populations. Based on these indicators, the outcomes of management actions are unclear, that is, managers and scientist alike have witnessed that indicators have behaved in unpredictable ways over the years, despite sustained measures. The context of this project is set by the question: Do lake remediation measures work? We propose a research project to resolve this ambiguity. Ecological theory informs that the seemingly unpredictable behavior of the ecological indicator is a sign that the lake systems are undergoing regime-shifts, and that it can be averted. This behavior is known as flickering, and is an early warning sign. We hypothesize that flickering is underpinned by critical factors in the biogeochemical cycles. We will use novel biogeochemical modelling techniques and recent advances in tipping-point theories to provide lake managers with targeted, precise measures to tip flickering systems towards good quality status. This project, led by a young researcher, targets our knowledge gaps in the critical factor of limnic biogeochemical cycles. For effective management of lakes and reservoir, the interactions and feedbacks in biogeochemical cycles, and their impact in biological indicators, will be thoroughly explored. We will build on our expertise in the development and use of mechanistic models for lakes, and adopt novel model analysis techniques. Our goals are to: 1) with the help of models, 2) understand site-specific biogeochemical factors causing regime shifts, and 3) identify the underlying biogeochemical cause of flickering before impending regime-shifts, and use it as an early warning.

Publications from Cristin

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MILJØFORSK-Miljøforskning for en grønn samfunnsomstilling