Abstract

Eutrophication and warming are key drivers of cyanobacterial blooms, but their combined effects on microcystin (MC) concentrations are less studied. We tested the hypothesis that warming promotes cyanobacterial abundance in a natural plankton community and that eutrophication enhances cyanobacterial biomass and MC concentrations. We incubated natural seston from a eutrophic pond under normal, high, and extreme temperatures (i.e., 20, 25, and 30 °C) with and without additional nutrients added (eutrophication) mimicking a pulse as could be expected from projected summer storms under climate change. Eutrophication increased algal- and cyanobacterial biomass by 26 and 8 times, respectively, and led to 24 times higher MC concentrations. This effect was augmented with higher temperatures leading to 45 times higher MC concentrations at 25 °C, with 11 times more cyanobacterial chlorophyll-a and 25 times more eukaryote algal chlorophyll-a. At 30 °C, MC concentrations were 42 times higher, with cyanobacterial chlorophyll-a being 17 times and eukaryote algal chlorophyll-a being 24 times higher. In contrast, warming alone did not yield more cyanobacteria or MCs, because the in situ community had already depleted the available nutrient pool. MC per potential MC producing cell declined at higher temperatures under nutrient enrichments, which was confirmed by a controlled experiment with two laboratory strains of Microcystis aeruginosa. Nevertheless, MC concentrations were much higher at the increased temperature and nutrient treatment than under warming alone due to strongly promoted biomass, lifting N-imitation and promotion of potential MC producers like Microcystis. This study exemplifies the vulnerability of eutrophic urban waters to predicted future summer climate change effects that might aggravate cyanobacterial nuisance.

Highlights

  • The incidence and intensity of cyanobacterial blooms are on the rise worldwide [1,2,3]

  • Run-off is primarily restricted to enhanced average winter precipitation, yet a rise in short intense summer storms during periods of droughts is expected as a result of climate change [10]

  • There was a clear response of adding nutrients (14 mg·N·L−1 as NaNO3 and 1.4 mg·P·L−1 as K2 HPO4 ), on the total- and cyanobacterial chlorophyll-a concentrations as well as on phytoplankton cell concentrations (Figure 1)

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Summary

Introduction

The incidence and intensity of cyanobacterial blooms are on the rise worldwide [1,2,3]. Toxins 2017, 9, 64 toxin-producing cyanobacteria [9] In those scenarios, run-off is primarily restricted to enhanced average winter precipitation, yet a rise in short intense summer storms during periods of droughts is expected as a result of climate change [10]. Run-off is primarily restricted to enhanced average winter precipitation, yet a rise in short intense summer storms during periods of droughts is expected as a result of climate change [10] Such events will fuel receiving waters with a pulse of nutrients during the growing season that may further promote cyanobacteria [11]

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