Abstract
Recurring blooms of filamentous, red-pigmented and toxin-producing cyanobacteria Planktothrix rubescens have been reported in numerous deep and stratified prealpine lakes, with the exception of Lake Constance. In a 2019 and 2020 Lake Constance field campaign, we collected samples from a distinct red-pigmented biomass maximum below the chlorophyll-a maximum, which was determined using fluorescence probe measurements at depths between 18 and 20 m. Here, we report the characterization of these deep water red pigment maxima (DRM) as cyanobacterial blooms. Using 16S rRNA gene-amplicon sequencing, we found evidence that the blooms were, indeed, contributed by Planktothrix spp., although phycoerythrin-rich Synechococcus taxa constituted most of the biomass (>96% relative read abundance) of the cyanobacterial DRM community. Through UPLC–MS/MS, we also detected toxic microcystins (MCs) in the DRM in the individual sampling days at concentrations of ≤1.5 ng/L. Subsequently, we reevaluated the fluorescence probe measurements collected over the past decade and found that, in the summer, DRM have been present in Lake Constance, at least since 2009. Our study highlights the need for a continuous monitoring program also targeting the cyanobacterial DRM in Lake Constance, and for future studies on the competition of the different cyanobacterial taxa. Future studies will address the potential community composition changes in response to the climate change driven physiochemical and biological parameters of the lake.
Highlights
Multiwavelength fluorometer profiles were taken along the water column, using a Moldaenke FluoroProbe, and were evaluated directly on the ship to determine the depths of the maxima of green pigment and red pigment concentrations
We examined the phylogenetic relationship of the main Synechococcus Amplicon sequence variants (ASVs) that were detected in 2019 and 2020, with the reference sequences of (i) all the cultivated phycoerythrin-rich Synechococcus spp. of Lake Constance, as established by Ernst and colleagues in 2003 [45], and (ii) Synechococcus rubescens and phycoerythrin-rich Cyanobium gracile reference sequences, as established by NCBI (Figure 3)
The deep water red pigment maxima (DRM) in Lake Constance is not dominated by the phycoerythrin-rich, filamentous and often toxin-producing Planktothrix rubescens
Summary
The formation of cyanobacterial blooms involves the complex interplay of regional and biological variables, and blooms have been reported worldwide with an increasing frequency [1,2]. It has been shown that climate change and eutrophication are, in many cases, major contributors to bloom formation, the mechanisms through which nutrients and temperature interact to amplify blooms varies extensively between cyanobacterial groups. The filamentous cyanobacterial genus, Planktothrix, occurs preferentially in prealpine and alpine lakes in temperate regions, and has been responsible for many blooms in the past (for example, in Lake Zurich [4,5], Lake Bourget [6], Lake Garda [7] and Lake Mondsee [8]), where its mass occurrence can evolve to become a major influence on the food web [9,10]
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