| Blooms of cyanobacteria are increasing globally, linked to anthropogenic pressures as warming and eutrophication. Many taxa produce toxins, which affects the surrounding food web both directly and indirectly, resulting in beach closures and negative health effects on human as well as drinking water resources. Their toxin regulation is however not fully understood and the effect of global warming still needs to be resolved. Toxic taxa are common in the Baltic Sea and hypothesized to be favored by high temperatures, and even with reduced nutrient loads, their abundance is still high. Monitoring data since 1980 also suggests that local environmental conditions can promote or limit cyanobacterial responses to warming. Their poor nutritional quality also has effects for zooplankton grazers, resulting in lower energy transfer, but evidence of such co-evolution in response to environmental change is scarce.
Generally, studies on climate warming effects on organisms in natural food webs depend on comparisons across space, sometimes combined with the study of temporal trends. Although such studies are valuable and often the only possible empirical approach to study relationships between warming and organisms in natural food webs, it requires that other factors that vary among study sites can be controlled for. We propose to overcome this limitation using a unique setup with artificial heating of an enclosed coastal bay, which has received warm water discharge from a nearby nuclear power plant since 1980, and its adjacent reference area. This paired design gives opportunity to disentangle warming effects on organisms already adapted to high temperatures. In the proposed study, we will develop and combine chemical, ecological, and genetic approaches to study adaptation in cyanobacterial and zooplankton populations from these contrasting environments.
We will test three hypotheses:
I) Cyanobacteria and zooplankton hatched from both warming and ambient sediment archives will indicate local adaptation, reflected by a difference in growth rates and genomic composition, as well as in cyanobacterial toxicity.
II) Cyanobacteria and zooplankton communities from warming and ambient summer blooms will reflect locally adapted populations with differences in species and sub-species composition and their genomic profiles, and cyanobacterial toxicity.
III) There will be a seasonal shift towards earlier cyanobacterial blooms in the warmer as compared to the ambient coastal region, affecting on community composition and toxicity.
The studies will provide understanding about the effects of warming on toxicity of bloom-forming filamentous cyanobacteria and consequences for zooplankton in the Baltic Sea. This knowledge will be useful in management and improve predictions on bloom formation and toxin production. It will be based on toxin measurements, whole-genome sequencing, and transcriptomics of natural communities, in relation to environmental variables. This is pivotal in predicting how cyanobacteria will respond to climate change and in the understanding of where and when they produce toxins, which is key knowledge in drinking water cleaning, recreational site management, and how it affects zooplankton, a key prey type for fish.
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