Effects of climate change on lake functioning

Atmospheric carbon dioxide levels have increased from a baseline level of 280 ppm about 150 years ago to more than 360 ppm at present, and are predicted to rise further to 700 ppm in the course of this century. The increased levels of dissolved inorganic carbon as a result of an increased flux from atmosphere to water affect the carbon availability and thus the carbon : nitrogen stoichiometry of lakes.

The impact of rising pCO2 on harmful cyanobacteria
The phytoplankton of eutrophic lakes is often dominated by cyanobacteria (‘blue-green algae’). Cyanobacteria may produce a family of toxins, called microcystins, which are harmful to both humans and animals. As a result, waters dominated by such harmful cyanobacteria are often closed for recreation and cannot be used for agriculture or drinking water purposes. Microcystins are cyclic peptides based on carbon and nitrogen. We therefore hypothesize that changes in the carbon : nitrogen stoichiometry of lakes will affect microcystin production, either by physiological adaptation within cyano strains or by natural selection between different strains of cyanobacteria. Thus, the rise in atmospheric carbon dioxide might affect the toxicity of cyanobacteria.

Effects of increased pCO2 on freshwater food webs through physical-chemical coupling: a stoichiometric perspective
The elemental composition of phytoplankton is strongly affected by the nutrient stoichiometry of its environment. However, the availability of carbon, nitrogen and phosphorus in aquatic environments is influenced by current anthropogenic changes in the Earth’s climate. In particular, carbon dioxide levels in the atmosphere will be rising towards unprecedented levels at the end of this century, while the resultant global warming enhances stratification of aquatic ecosystems and thereby diminishes the nutrient supply to the surface mixed layer. These climate-driven processes will enrich planktonic food webs with carbon but suppress nutrient availability. Hence, at a global scale, climate change is likely to increase the carbon:nutrient stoichiometry of the plankton, which may alter the structure and functioning of entire aquatic food webs.

Climate induced shifts in freshwater ecosystems: models and experiments
The impact of current climate change on ecosystem functioning is beyond dispute, given the numerous proofs of ‘climate fingerprints’ or even ‘climate footprints’ on natural systems worldwide. Phenology, or the timing of the life cycle events in species, is directly linked to climatic cues such as temperature, precipitation and sunshine. Other biological responses include community shifts and range boundaries shifts. Species-specific responses to temperature change may affect the phenological coupling of trophic relationships within a food web. In freshwater systems, the phenological coupling of the key herbivore Daphnia and its algal food source may be affected when the build-up of a spring population of Daphnia is triggered by changes in photoperiod rather than temperature. We model these match-mismatch events using minimal seasonal forced models on zooplankton-phtyoplankton interactions. In our experiments, we focus on climate-induced shifts in plankton communities. We look at the impact of different ecosystems drivers, i.e. nutrient loading and climate on the competitive abilities of different algal groups. For a more comprehensive view of the impact of climate change that includes effects of nutrient loading as well as some feedback mechanisms within freshwater systems, we complement these experiments with analyses of the full-ecosystem model PCLake.
 

Climate change threatens water quality
Most of the lakes in the Netherlands are man-made and have preset water levels and poorly developed littoral zones. On basis of an extensive literature review we conclude that for shallow lakes in the Netherlands climate change will likely:

1. reduce the numbers of several target species of birds;
2. favour and stabilize cyanobacterial dominance in phytoplankton communities;
3. cause more serious incidents of botulism among waterfowl and enhances the spreading of mosquito borne diseases;
4. benefit invaders originating from the Ponto-Caspian region;
5. stabilize turbid, phytoplankton-dominated systems, thus counteracting restoration measures;
6. destabilize macrophyte-dominated clear-water lakes;
7. increase carrying capacity for primary producers, especially phytoplankton, thus mimicking eutrophication;
8. affect higher trophic levels as a result of enhanced primary production;
9. have a negative impact on biodiversity which is linked to the clear water state;
10. affect biodiversity by changing the disturbance regime.

We take these expectations as the starting point for our ongoing experimental and theoretical research on the impact of climate change on freshwater ecosystems. In this research we focus on spring dynamics and the phenology of algae and zooplankton because their interactions determines whether a clear water phase will occur and submerged macrophytes can develop.