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The effect of mixotrophic chrysophytes on secondary productivity in pelagic food webs

Periodic Reporting for period 1 - CHRYSOWEB (The effect of mixotrophic chrysophytes on secondary productivity in pelagic food webs)

Reporting period: 2016-02-01 to 2018-01-31

Plankton occupy a key position in aquatic trophic webs, therefore, understanding its global change-mediated shifts is of high importance. While changes in the plankton of eutrophic lakes (e.g. increased dominance of cyanobacteria) are a widely discussed topic in the current literature, the consequences of global change for oligotrophic lakes receives little attention. Mixotrophic chrysophytes (golden algae – Chrysophyceae) are important elements of the phytoplankton in lakes with a low nutrient content, such as alpine lakes. Their contribution is predicted to increase with global change-associated phenomena, such as enhanced thermal stratification and increased input of terrestrial carbon (‘brownification’). Though interactions between chrysophytes and zooplankton have hitherto received little attention, some studies clearly indicated detrimental dietary effects. According to this, an increase in chrysophytes could impair secondary production with far-reaching consequences for the whole ecosystem, including changes in the fish stock. Alpine lakes represent multiple ecosystem services. Salmonid fish populations of many mountain lakes are economically important. If golden algae impair zooplankton composition and production, their increase may imply changes in the fish stock. Besides, chrysophyte blooms can cause unpleasant fishy odour, decreasing the recreational value of their habitats.
The aim of CHRYSOWEB was to specifically address the consequences of the increasing share of golden algae for consumers in lake food webs. To this end, we applied an integrative approach (laboratory experiments, field observations, biochemical analyses) to get a comprehensive understanding of this key group on aquatic ecosystem functioning.
By using an array of chrysophyte species as food, we found multiple effects on zooplankton, which were generally negative compared to a high-quality reference food. The mechanisms include feeding inhibition, nutritional deficiency and toxicity. The results of CHRYSOWEB imply that the dominance of chrysophytes is able to significantly alter zooplankton secondary production and community composition, and therefore energy flow in aquatic food webs.
During the project, all the major aims of CHRYSOWEB were fulfilled. At the beginning, Dr Vad learned the basics and gained practice of algae and zooplankton isolation and cultivation. Afterwards, multiple experiments with algae and zooplankton were implemented under the supervision of Dr Ptacnik. Chrysophytes were fed to zooplankters to test ingestion rates, growth, fitness, and toxicity. A master student and a number of trainees were also involved in the feeding experiments, co-supervised by Dr Vad. Furthermore, growth of multiple chrysophytes was tested with and without the addition of dissolved organic carbon and was related to their biochemical and stoichiometric composition. The results derived from the experiments were supplemented with observations from the field.
The nutritional quality of chrysophytes was found to be intermediate to detrimental for zooplankton. The responses of different higher taxa of zooplankton to chrysophyte diet varied considerably. Water fleas (cladocerans) were especially sensitive to blooms of large colonial algae, which reduced their food uptake by physical interference, while copepods were more successful in handling them. On the other hand, copepods seem to be more sensitive to food quality, being unable to reach adult stage on any golden algae we provided as food. Based on short-term toxicity tests, however, only a few chrysophyte species proved to be toxic, implying that negative effects generally lie in their low nutritional value, which was furthermore supported by their biochemical composition.
The results of CHRYSOWEB were communicated to the wide scientific community at international conferences, including the ASLO Aquatic Sciences Meeting, which is the largest meeting of researchers in aquatic sciences. Press releases were published to inform the broader public about the project, including an article in ‘Der Standard’, which is one of the largest daily newspapers in Austria. The results were also disseminated to Austrian and international university students with different backgrounds in forms of seminars and lectures. The results provide a basis for at least three separate papers, which will be submitted to leading journals in limnology and ecology (publications expected in 2018/2019). Besides, the results of CHRYSOWEB will provide a basis for a master thesis (Claudia Schneider, University of Vienna).
CHRYSOWEB was designed to allow for predictions how aquatic trophic webs in alpine and other nutrient-poor lakes may change in the near future in relation to global change. As these systems provide numerous ecosystem services, the results of the project are important to raise public awareness and gain attention of key stakeholders to pressing concerns. CHRYSOWEB gained considerable public interest, was highlighted in the national media (see dissemination), and a number of people visited the host institute to discuss the potential consequences of global change on alpine lakes.
The project provided an excellent training to Dr Vad, with several opportunities to improve his skills both conceptually and methodologically. He was well-integrated in a highly international research team, allowing him to expand his scientific network. He showed great capacity to work independently, performing the task and project management largely on its own. Furthermore, he was a co-supervisor of a master student and a number of trainees, which greatly improved his leadership skills. CHRYSOWEB has undoubtedly contributed to the development of his scientific independence.
The topic of CRYSOWEB is timely and novel and therefore, the results have a great potential for higher education. They were already used at university lectures and seminars, and will be used in the future as well.