Future climate in temperate regions is projected to get warmer and in many cases wetter. This poses questions about how phytoplankton in our lakes will respond. A dynamic modeling approach based on an allometric description of phytoplankton characteristics was used to investigate how the biomass of different functional groups of phytoplankton will respond to a changed future climate. Simulations based on a warmer future climate scenario suggest that we will experience an increase in phytoplankton biomass in northern temperate lakes. Moreover, phytoplankton groups are projected to shift towards a dominance of cyanobacteria at the expense of diatoms. Climate may affect phytoplankton, either via in-lake changes in temperature and stratification, or due to altered processes at the watershed level, which influence rates of nutrient export and water discharge. This study found that changes in lake temperature and stratification are the major causes of the projected increase in phytoplankton biomass, but that changes in the timing of nutrient export did influence the succession of diatoms. Variation in SPIM (suspended particulate inorganic matter) can have an important role in influencing the depth of the euphotic zone in a turbid lake, and hence the light climate experienced by phytoplankton. Wind and river discharge were found to regulate SPIM in this study, not only wind as in many other studies. Variations in SPIM could be adequately described by a few governing equations. This thesis suggests that, as a result of climate change, lakes close to the limit of becoming eutrophied may be pushed past a threshold beyond which water quality problems will become more prevalent. Finally it is important to bear in mind that all models are simplifications of the reality as we understand it. Still, the use of models can often give a good indication as to what might be expected in the future.

<p>This thesis presents predictive models for important variables concerning eutrophication effects in lakes. The keystone is a dynamic phosphorus model based on ordinary differential equations. By calculating mass fluxes of phosphorus into, within and out from a lake, the concentrations of different forms of phosphorus in different compartments of the lake are estimated.</p><p>The dynamic phosphorus model is critically tested and several improvements are presented, including two new compartments for colloidal phosphorus, a sub-model for suspended particulate matter (SPM) and new algorithms for lake outflow, water mixing, diffusion, water content and organic content of accumulation sediments are implemented. Predictions with the new version show good agreement against empirical data in five tested lakes.</p><p>The sub-model for SPM uses the same driving variables as the basic phosphorus model, so the inclusion of this model as a sub-model does not require any additional variables. The model for SPM may also be used as a separate model giving monthly predictions of suspended particulate matter in two water compartments and one compartment with SPM available for resuspension in ET-sediments.</p><p>Empirical data from Lake Erken (Sweden) and Lake Balaton (Hungary) are used to evaluate the variability in settling velocity of SPM. It is found that the variability is substantial and may be accounted for by using a dimensionless moderator for SPM concentration. Empirical data from accumulation area sediments in Lake Erken are used to develop a model for the dynamics of phosphorus sedimentation, burial and diffusion in the sediments. The model is shown to provide reasonable monthly predictions of four functional forms of phosphorus at different sediment depths.</p><p>Simulations with the lake phosphorus model using two different climate scenarios indicate that lakes may respond very differently to climate change depending on their physical character. Lake Erken, with a water retention time of 7 years, appears to be much more sensitive than two basins of Lake Mälaren (Sweden) with substantially shorter retention times. The implication would be that in eutrophic lakes with long water retention times, eutrophication problems may become serious if the future becomes warmer. This will be important in contexts of lake management when remedial measures against lake eutrophication have to be taken.</p>

This thesis presents predictive models for important variables concerning eutrophication effects in lakes. The keystone is a dynamic phosphorus model based on ordinary differential equations. By calculating mass fluxes of phosphorus into, within and out from a lake, the concentrations of different forms of phosphorus in different compartments of the lake are estimated. The dynamic phosphorus model is critically tested and several improvements are presented, including two new compartments for colloidal phosphorus, a sub-model for suspended particulate matter (SPM) and new algorithms for lake outflow, water mixing, diffusion, water content and organic content of accumulation sediments are implemented. Predictions with the new version show good agreement against empirical data in five tested lakes. The sub-model for SPM uses the same driving variables as the basic phosphorus model, so the inclusion of this model as a sub-model does not require any additional variables. The model for SPM may also be used as a separate model giving monthly predictions of suspended particulate matter in two water compartments and one compartment with SPM available for resuspension in ET-sediments. Empirical data from Lake Erken (Sweden) and Lake Balaton (Hungary) are used to evaluate the variability in settling velocity of SPM. It is found that the variability is substantial and may be accounted for by using a dimensionless moderator for SPM concentration. Empirical data from accumulation area sediments in Lake Erken are used to develop a model for the dynamics of phosphorus sedimentation, burial and diffusion in the sediments. The model is shown to provide reasonable monthly predictions of four functional forms of phosphorus at different sediment depths. Simulations with the lake phosphorus model using two different climate scenarios indicate that lakes may respond very differently to climate change depending on their physical character. Lake Erken, with a water retention time of 7 years, appears to be much more sensitive than two basins of Lake Mälaren (Sweden) with substantially shorter retention times. The implication would be that in eutrophic lakes with long water retention times, eutrophication problems may become serious if the future becomes warmer. This will be important in contexts of lake management when remedial measures against lake eutrophication have to be taken.