Effects of climate change on boreal lake ecosystems: Productivity and community responses (completed)
This proposal addresses address the ecosystem effects on nutrient-poor (oligotrophic) boreal lakes in Norway and Fenno-Scandia, under the combined stresses of climate change, atmospheric deposition of nitrogen (N) and sulphur (S), and change in land vegetation cover. Export from land to water of dissolved organic carbon (DOC) along with the elements nitrogen (N), phosphorus (P) and silicate (Si) is expected to respond to changing climate patterns and nitrogen loading on a time scale from years to centuries. These elements are key drivers of light attenuation and productivity, which in turn will affect species composition.
Lake ecosystem effects will focus on lake productivity, phytoplankton size and community composition, and the success or failure of several indicator organisms. Societal impacts of these changes include reduced lake productivity, proliferation of nuisance algae and “browning” of drinking water – all related to DOC. The focus will be on lake ecosystem responses in the boreal zone by use of various downscaled climate scenarios, and these will again be aimed for implementation of general, boreal models. The role of freshwaters as sentinels of climate change and terrestrial effects in this context will also be highlighted.
About the project
Scientific approach and hypotheses. We have three main hypotheses related to ecosystem responses in boreal lakes.
- Lake productivity will decrease despite higher temperature due to browner waters (less light for photosynthesis) and lower levels of inorganic P (due to elevated retention in catchments). These two drivers will decrease primary production and increase the net heterotrophy of the systems and in concert with increased N intensify the P-limitation. Increased temperature (also related to browner waters) will promote smaller cells.
- Changes in DOC, light and nutrients will promote certain species, and particularly algal species that is undesirable both from a ecosystem and human point of view.
- Stenothermal (cold-loving) organisms will lose habitat, even more so because DOC is increasing (increasing the temperature)
Productivity and species composition in boreal lakes is sensitive to climate warming in combination with changed water chemistry and nutrient loading. Changed nitrogen (N) inputs (due to changing N deposition and catchment N retention under climate and land cover change) can tip primary productivity over from P limitation to N-limitation leading to substantial shifts in the phytoplankton community. Increased catchment export of dissolved organic matter (DOM, usually measured as dissolved organic carbon, DOC), driven by climate, deposition and land cover changes, will affect the light and warming regime of boreal lakes with likely impacts on algal productivity, and species and community composition. This project will consider the chain of drivers, catchment processes and lake responses in oligotrophic boreal lake catchments at various temporal and spatial scales. Our aim is to provide a larger understanding of boreal lake ecosystem responses to interacting drivers of change (climate, N+S deposition, forest cover). We will do so by linking temperature and depositions to catchment models and lake models to assess lake responses in terms of productivity and community effects. We will largely base this project on existing models, as well as Norwegian and Nordic climate, hydrology, catchment and lake datasets. We will study the response of several sensitive species to changing DOC and temperature in experimental settings and field surveys. We will use national infrastructure (the climate research station Langtjern, a highly instrumented boreal lake and catchment) to test, validate and further develop process-based catchment models - invaluable tools for predicting combined effects of stressors on lake chemistry - and ultimately on ecosystem responses.
The project will to a large extent be based on existing data, sampled under different projects and for various purposes, yet not analysed for this kind of multidisciplinary purpose. We will also collect new data. Most of the statistical, GIS-related and process-based modelling tools have been developed, yet there will be a need for extended calibration, refinement and integration of the models. The experimental facilities and set-ups are mostly tested and ready for implementation. This is an ambitious project, but given the expertise and the solid base of methods and data, we feel confident that the final deliveries of this project will bring fundamental new insights to climate change responses in boreal lakes.
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- Climatic and other stresses. Aim: Establish data for climatic and other stresses that will be used modelling of catchment processes (WP2), physical lake modelling (WP3), ecosystem production modelling (WP4) and in interpretation and analysis of ecosystem responses (WP5). Data will be established for past and future decades.
- Catchment processes and export of DOC and nutrients. Aim: Process-understanding of climatic drivers of DOC and nutrients in boreal catchments, upscaled in time (future climate) and space (Norway/Nordic scale) to serve as input to the physical lake models (WP3), lake productivity and size structure (WP4) and biological community response studies (WP5).
- The lake as integrator of catchment processes – model representations and extrapolations. Aim: To develop and parameterize a dynamical model for assessing the direct and indirect effects of climate change on lake ecosystem productivity
- Ecosystem response: Productivity and size structure. Aim: To identify and quantify the combined role of DOM (and light), temperature and nutrients (NO3 and P) on phytoplankton size structure and production. This will then subsequently be linked to climate prediction and the physical lake model for predicting responses under different scenarios
- Ecosystem response: species shifts. Aim: To assess the impacts of increased DOC and temperature on sensitive species and systems. We choose two planktonic species that have opposite responses to rising temperature and DOC concentration. On the one hand, the invasive nuisance alga Gonyostomum semen (hereafter referred to as Gonyostomum) is at present expanding its range in Norway to the north and west, probably due to regional warming and DOC increases. In contrast the arctic crustacean relict Branchinecta paludosa (fairy shrimp), once common in large parts of alpine south Norway, is now shrinking its range to higher and higher altitudes.