The Red Harmful Plague in Times of Climate Change: Blooms of the Cyanobacterium Planktothrix rubescens Triggered by Stratification Dynamics and Irradiance

Planktothrix rubescens is a harmful planktonic cyanobacterium, forming concentrated metalimnetic populations in deep oligo- and mesotrophic lakes, even after successful restoration. In Lake Zurich (Switzerland), P. rubescens emerged as a keystone species with annual mass developments since the 1970s. Its success was partly attributed to effects of lake warming, such as changes in thermal stratification and seasonal deep mixing. However, recent observations based on a biweekly monitoring campaign (2009–2020) revealed two massive breakdowns and striking seasonal oscillations of the population. Here, we disentangle positive from negative consequences of secular lake warming and annual variations in weather conditions on P. rubescens dynamics: (i) despite the high survival rates of overwintering populations (up to 25%) during three consecutive winters (2014–2016) of incomplete deep convective mixing, cyanobacterial regrowth during the following stratified season was moderate and not overshooting a distinct standing stock threshold. Moreover, we recorded a negative trend for annual population maxima and total population size, pointing to a potential nutrient limitation after a series of incomplete winter mixing. Thus, the predication of steadily increasing blooms of P. rubescens could not be confirmed for the last decade. (ii) The seasonal reestablishment of P. rubescens was strongly coupled with a timely formation of a stable metalimnion structure, where the first positive net growth in the following productive summer season was observed. The trigger for the vertical positioning of filaments within the metalimnion was irradiance and not maximal water column stability. Repetitive disruptions of the vernal metalimnion owing to unstable weather conditions, as in spring 2019, went in parallel with a massive breakdown of the standing stock and marginal regrowth during thermal stratification. (iii) Driven by light intensity, P. rubescens was entrained into the turbulent epilimnion in autumn, followed by a second peak in population growth. Thus, the typical bimodal growth pattern was still intact during the last decade. Our long-term study highlights the finely tuned interplay between climate-induced changes and variability of thermal stratification dynamics and physiological traits of P. rubescens, determining its survival in a mesotrophic temperate lake.

Controlling metalimnetic bloom of Planktothrix rubescens by a novel water withdrawal strategy: a modelling study

<p>Aggregations of cyanobacteria in lakes and reservoirs are commonly associated with surface blooms, but may also occur in the metalimnion as subsurface or deep chlorophyll maxima. Metalimnetic cyanobacteria blooms are of great concern when potentially toxic species, such as Planktothrix rubescens (P. rubescens), are involved. Apparently, metalimnetic blooms of P. rubescens have increased in frequency and severity in recent years so there is a strong need to establish the external factors controlling its growth. We hypothesized that P. rubescens blooms in reservoirs can be managed by modifying the water withdrawal strategy and altering the light climate experienced by the algae. We tested our hypothesis in Rappbode Reservoir by establishing a series of withdrawal and light scenarios based on a calibrated water quality model (CE-QUAL-W2). Our scenarios demonstrated that metalimnetic water withdrawal reduced P. rubescens biomass in the reservoir. According to the simulation results we defined an optimal withdrawal volume to control P. rubescens blooms in the reservoir as approximately 10 million m<sup>3</sup> during its blooming period. The numerical results also indicated that P. rubescens growth can be most effectively suppressed if the metalimnetic withdrawal is applied in the early stage of its rapid growth (i.e. before the occurrence of blooms). Additionally, the results showed that P. rubescens biomass gradually decreased with increasing light extinction and nearly disappeared when the extinction coefficient exceeded 0.55 m<sup>-1</sup>.  Our results indicated that close linkages among in situ measurements, model simulations, empirical growth rate and flushing rate calculations could inform management strategies to minimise the harmful impacts of P. rubescens in water supplies. Such a strategy could be used in reservoir operational strategies as an adaptation way to offset the rise in P. rubescens populations that has been linked to climate change.</p>

Die hard in Lake Bourget! The case of Planktothrix rubescens

Blooms of Planktothrix rubescens have been recorded for 15 years in Lake Bourget (France), from 1995 to 2009. Then, the presence of this filamentous and toxic cyanobacterium became anecdotic between 2010 and 2015 and it was clearly thought that such a proliferation was over. However, against all odds, blooms occurred again in 2016 and 2017 despite apparent very low phosphorus concentrations in surface waters of the lake. Aims of this study were thus to explain the reasons of this come back in order to propose scenarios likely to be helpful to stakeholders who need to know if such proliferations may occur again in the future. We show that phosphorus input, both from the main tributaries to the lake and possibly from the sediments, were likely the triggers of the new development of the cyanobacterium since a minimum autumn/winter inoculum of P. rubescens was detected the year before. Then, the bloom, that was observed deeper than previous years, was associated to a conjunction of factors already well-known to favour the development of this very competitive species (i.e. mild winter temperature, water column stability, available light at depth, surface water transparency, low predation, etc…). Although many factors and processes could account for the occurrence and bloom of the cyanobacterium, not observed, measured or taken into account here, a plausible scenario could be proposed and may be useful to deciders. One thing remains unclear: where do the cyanobacterium hides when it is not observed during the routine monitoring survey and thus from which place it could initiate its development (nearshore, in the pelagic zone, from the sediment?), unless it is simply not sampled and observed due to methodological bias.

Die hard in Lake Bourget! The case of Planktothrix rubescens reborn

Blooms of Planktothrix rubescens have been recorded for 15 years in Lake Bourget (France), from 1995 to 2009. Then, the presence of this filamentous and toxic cyanobacterium became anecdotic between 2010 and 2015 and it was thought that its proliferation was over. However, blooms occurred again in 2016 and 2017 despite apparent low phosphorus concentrations in surface waters of the lake. We have attempted to explain the reasons for this come back in order to develop scenarios helpful to stakeholders who are concerned such proliferations may occur in the future. We show that phosphorus input, both from the main tributaries to the lake and possibly from the sediments, were likely the triggers of the new development of the cyanobacterium provided a minimum autumn/winter inoculum of P. rubescens was detected the year before. The subsequent bloom was observed deeper than previous years and associated with a conjunction of factors known to favour the development of this species (i.e., mild winter temperature, water column stability, available light at depth, surface water transparency, low predation, etc.). Although many factors and processes could account for the occurrence and bloom of the cyanobacterium, a plausible scenario is proposed. One thing remains unclear: where does this cyanobacterium “hide” when it is not observed during the routine monitoring surveys and from which place it could initiate its development (nearshore, the pelagic zone, or from the sediment?).

Removal of Positively Buoyant Planktothrix rubescens in Lake Restoration

The combination of a low-dose coagulant (polyaluminium chloride—‘Floc’) and a ballast able to bind phosphate (lanthanum modified bentonite, LMB—‘Sink/Lock’) have been used successfully to manage cyanobacterial blooms and eutrophication. In a recent ‘Floc and Lock’ intervention in Lake de Kuil (the Netherlands), cyanobacterial chlorophyll-a was reduced by 90% but, surprisingly, after one week elevated cyanobacterial concentrations were observed again that faded away during following weeks. Hence, to better understand why and how to avoid an increase in cyanobacterial concentration, experiments with collected cyanobacteria from Lakes De Kuil and Rauwbraken were performed. We showed that the Planktothrix rubescens from Lake de Kuil could initially be precipitated using a coagulant and ballast but, after one day, most of the filaments resurfaced again, even using a higher ballast dose. By contrast, the P. rubescens from Lake Rauwbraken remained precipitated after the Floc and Sink/Lock treatment. We highlight the need to test selected measures for each lake as the same technique with similar species (P. rubescens) yielded different results. Moreover, we show that damaging the cells first with hydrogen peroxide before adding the coagulant and ballast (a ‘Kill, Floc and Lock/Sink’ approach) could be promising to keep P. rubescens precipitated.

Vertical and horizontal distribution of the microcystin producer Planktothrix rubescens (Cyanobacteria) in a small perialpine reservoir

Among cyanobacteria, Planktothrix rubescens (De Candolle ex Gomont) Anagnostidis & Komárek is a species that is well adapted to develop in moderately nutrient rich and deep lakes. In this typology of waterbodies, the competitive abilities of this species rely in its capacity to stand and growth in the dimly illuminated metalimnetic layer during the warmer months. I this work, we have studied the seasonal development and distribution of this species in Lake Ledro, a meso-oligotrophic reservoir located in the Eastern Alps. During the last decade, this species has given rise to numerous and extended surface bloom episodes, causing the reddening of vaste areas of the lake. In summer, the light intensities in the zone of the greater development of this cyanobacterium (in the metalimnion, between the euphotic depth and the layer of maximum development of the species) were bewteen 2 and 20 µmol m-2 s-1, i.e. values that were well within the light intensities required to sustain the optimal growth of filaments. The formation of the autumn and winter blooms was triggered by the cooling of surface waters and increase of the mixed layer, which, eroding the metalimnion, entrained the filaments of Planktothrix in the surface mixed layers. The formation of the surface blooms was associated with the presence of high amounts of microcistins, which in a few occasions reached concentrations between 10 and 22 µg L-1, posing potential problems for the exploitation of water resources.