Layout 1 Introduction Cyanobacteria or blue-green algae are prokaryotic oxygenic phototrophs that colonize all ecosystems. They have a great ability to adapt to changing environmental conditions. Some cyanobac- teria prefer cold waters, low light radiation, and low nutrient con- centrations. Others need abundant nutrients, high temperatures, and stable stratification of the water column. Cyanobacteria are characterized by great morphological variability, and have unicel- lular or colonial, globose, or filamentous forms (Komarek and Anagnostidis 2005, 2008; Komarek, 2013). Some genera of cyanobacteria can develop almost completely monospecific algal populations in their natural environment, with very high numbers of individuals per liter of water (algal blooms). The blooms are often accompanied by foams and superficial streaks or anomalous coloring. Since many species can produce toxins (cyanotoxins) as secondary metabolites, they are consid- ered pathogenic microorganisms to be controlled by public health institutions for bathing waters. Information relating to the presence of cyanobacteria in water comes from three sources: i) data from monitoring of the lakes that APPA must monitor according to Legislative Decree 152/06 (6 samplings per year), which provide the density (cell/l) and the biovolume (mm3/m3) of the algal species living the in the water column corresponding to the euphotic zone (Dlgs 152/2006, 2006; EU 2000); ii) data from bathing samples (5 collected in the May- September swimming period), collected in collaboration with Azienda Provinciale per i Servizi Sanitari (APSS) Trento, which coordinates the sampling and subsequent cyanotoxins analyses; these data provide the density (cells/ml) of cyanobacteria in sur- face water samples (Dlgs 116/2008, 2008): data obtained from sampling carried out following reports and alarms from citizens. All data involving algal blooms and monitoring are dissemi- nated through the water protection management plans and through the APPA website (https://www.appa.provincia.tn.it), which re- ports the most important bloom episodes and the main water col- oration and blooms described by citizens. The main cases of bloom recorded in the province of Trento in the past 15 years are described in the following timeline (Figure 1A). SPECIAL SECTION Potentially toxic cyanobacteria blooms in the Southern Alps and the Italian peninsula Potentially toxic cyanobacteria in Trentino lakes: bloom episodes detected by monitoring data over the last ten years Sabrina Pozzi, Silvia Costaraoss, Giovanna Pellegrini Provincial Agency for Environmental Protection of Trento, Italy ABSTRACT This short note illustrates some algal blooms caused by potentially toxic cyanobacteria in water bodies in the NE Alps, focusing in particular on the last ten years. The data illustrated come from monitoring activities, bathing health assessments, and analyses carried out on citizen reports. The most important blooms were caused by Planktothrix rubescens, Dolichospermum spp., and Microcystis aeruginosa, and affected Lakes Ledro, Caldonazzo, and Serraia. Experience in the Trentino region has highlighted the importance of effective and timely communication between the monitoring institutions and the users of the lake, to minimize any negative health effects and to increase the knowledge of the population. Corresponding author: Sabrina Pozzi, Provincial Agency for En- vironmental Protection of Trento, via Lidorno 1, 38123 Trento, Italy. E-mail: sabrina.pozzi@provincia.tn.it Key words: toxic cyanobacteria, algal bloom, Planktothrix rubescens, Microcystis aeruginosa, Dolichospermum spp., moni- toring data. Authors’ contributions: all the authors made a substantive intellec- tual contribution. All the authors have read and approved the final version of the manuscript and agreed to be held accountable for all aspects of the work. Conflict of interest: the authors declare no potential conflict of in- terest. Funding: none. Availability of data and materials: all data generated or analyzed during this study are included in this published article. Received: 28 June 2024. Accepted: 12 October 2024. Publisher’s note: all claims expressed in this article are solely those of the authors and do not necessarily represent those of their affil- iated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article or claim that may be made by its manufacturer is not guaranteed or endorsed by the publisher. ©Copyright: the Author(s), 2024 Licensee PAGEPress, Italy Advances in Oceanography and Limnology, 2024; 15:12752 DOI: 10.4081/aiol.2024.12752 This work is licensed under a Creative Commons Attribution- NonCommercial 4.0 International License (CC BY-NC 4.0). Non -co mmerc ial us e o nly Potentially toxic cyanobacteria in Trentino lakes: bloom episodes detected by monitoring data over the last ten years 21 Winter surface blooms in lakes Ledro and Caldonazzo Lake Ledro and Lake Caldonazzo (Figure 2) have stable pop- ulations of Planktothrix rubescens (De Candolle ex Gomont) Anagnostidis & Komárek that have given rise to winter and spring surface blooms over the years (Figures 1B, C). The two lakes are similar in many characteristics, but the total phosphorus concentrations during spring mixing are consistently higher in Lake Caldonazzo. Lake Ledro, however, has a signifi- cantly higher average annual biovolume of Planktothrix rubescens, further confirming that this species is better adapted to oligo-mesotrophic environments rather than in full mesotrophic conditions (Boscaini et al., 2017). Both lakes presented winter surface blooms over the years, when Planktothrix rubescens tends to be entrained to the surface by mixing processes. In contrast, during the summer months, Planktothrix rubescens is located at the depths where it finds the best temperature conditions while si- multaneously being able to make the most of nutrient availability. This species positions itself at the lower layer of the thermocline, where most of the nutrient accumulates, and also manages to take advantage of low light irradiance conditions (Walsby et al., 2004, 2006; Boscaini et al., 2017). This aspect may lead to an underes- timation of the risk not so much with regard to bathers but with regard to fish consumption and especially to the possible use of water for drinking (Manganelli et al., 2010), a situation not yet current in Trentino, but increasingly likely due to frequent drought episodes related to climate change. Summer blooms in lakes Levico, Canzolino and Serraia In September 2012, Lake Levico (Figure 2) showed an intense and anomalous bloom of Microcystis aeruginosa (Kützing) Kütz- ing. Lake Levico does not have high phosphorus concentrations at mixing and is classified as having good ecological status. After that year, no more massive blooms occurred. Lake Canzolino (Figure 2) showed frequent blooms caused by cyanobacteria. It is a small eutrophic basin characterized by excessive external nutrient loading, a large zone of summer hy- polimnetic anoxia, and very slow water exchange. In 2022 and 2023, it was affected by conspicuous blooms caused by the Figure 1. A) Timeline of the most important bloom events occurring from 2007 to 2023 in Trentino lakes. B) Distribution of Planktothrix rubescens in different layers of Lake Ledro. C) Surface bloom of Planktothrix rubescens in Lake Ledro in May 2015. D) Surface bloom caused by Microcystis aeruginosa in Lake Serraia in late summer 2023 and E) Accumulation caused by wind. F) Microcystis aeruginosa at 200x magnification (Lake Serraia). Non -co mmerc ial us e o nly S. Pozzi et al.22 cyanobacterium Woronichinia naegeliana (Unger) Elenkin in September and October, with values exceeding the normative limit for bathing water of 100,000 cells/ml. Lake Serraia (Figure 2) has been the focus of attention of provincial environmental services since the late 1990s, when marked phenomena of algal blooms began to become evident dur- ing the summer. These phenomena were the visible manifestation of eutrophication in the lake. The lake is included in the provincial monitoring network and is monitored and classified as required by Legislative Decree 152/06, but over the years, specific studies have been carried out, and sampling frequencies have been inten- sified. A large amount of data has been collected on this lake. Focusing only on the events of the last few years, in 2017 we observed a strong bloom of Dolichospermum spp. which was often identified in the samples with the coexistence of two (or more) different species, namely D. circinale (Rabenhorst ex Bor- net & Flahault) Wacklin, Hoffmann & Komárek and D. plancton- icum (Brunnthaler) Wacklin, L.Hoffmann & Komárek. In 2019, a new bloom of Dolichospermum spp. reached high concentration levels in early September. The scums were very vis- ible, and the coloration of the lake was particularly "unpleasant." During August and September 2020, the lake was again affected by an intense algal bloom, visible to the naked eye by the abnor- mal coloration of the water and the very apparent formation of yellow-green streaks. However, these bloom episodes, although unpleasant as a visual impact on the enjoyment of the lake, did not lead to the formation of toxins. Finally, in 2023, there was no evidence of Dolichospermum sp. throughout the summer season; however, a massive bloom of Microcystis aeruginosa developed in the second half of August and continued throughout September, leading to a ban on bathing, fishing, and recreational uses of the lake. Indeed, high levels of microcystins (over 100 μg/L in the su- perficial accumulation and over 40 μg/L in areas not affected by accumulations) were determined by APSS and Fondazione Ed- mund Mach (FEM) of S. Michele all’Adige in this circumstance. Information to the population on cyanobacterial blooms In the province of Trento there is not yet an established infor- mation system in case of toxic algal blooms. It is important that an adequate two-way information flow is built; the correct infor- mation must be provided to the users of the lakes through the man- agement institutions and at the same time the flow of information must also work in the opposite direction, through timely reports from citizens who are informed about the type of events. A better flow of information is therefore needed not only for people who directly use the water (swimmers, sportsmen, fisher- men, etc.), but also for the authorities responsible for managing the public resource (provincial and municipal governments). Also, because of increased blooms caused by climate change, informa- tion is very important so that people learn to recognize the danger. An example of correct information flow is reported on the occa- sion of the Planktothrix rubescens bloom that affected Lake Cal- donazzo in winter 2023. The analyses revealed the presence of toxins, and the local municipalities were promptly notified. To support the information to water users, panels with full explana- tions of the phenomenon and associated dangers were placed at access points to the lake. In this way, people were prevented from getting too close to the algal accumulations, which are also dan- gerous for animals, for example, for dogs that drink lake water (Fastner et al., 2018). Figure 2. Physical and trophic characteristics of the investigated Trentino lakes. Non -co mmerc ial us e o nly Potentially toxic cyanobacteria in Trentino lakes: bloom episodes detected by monitoring data over the last ten years 23 References Boscaini A, Brescancin F, Cerasino L, et al., 2017. 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