Layout 1 SPECIAL SECTION Potentially toxic cyanobacteria blooms in the Southern Alps and the Italian peninsula Introduction In the last twenty years, the frequency of cyanobacterial blooms in northern Italian lakes has increased, affecting all lake types. Climate change and increasing trophic levels of water bod- ies were shown to favour cyanobacterial blooms (Hilt et al., 2017; Sterner et al., 2020). In this note, we report and briefly discuss an overview of cyanobacterial blooms recorded in northern Italy since 2000. Specifically, the purpose of this contribution is to highlight the temporal evolution of blooms, and the methods of investigation used for their characterization in relation to anthro- pogenic pressure factors. In this study, we define blooms as cell population growth of all potentially toxic cyanobacterial species in surface water ex- ceeding the thresholds provided by the Italian law (100,000 cells/mL). Lakes were grouped according to their altitude, alka- linity, average and maximum depth, and surface area. The geo- graphical area covered by this study includes the whole southern Alpine chain, from Piedmont to Friuli Venezia Giulia. For each lake, we reported the trophic status, the ecological status derived from physical-chemical parameters and the phytoplankton index IPAM (Wolfram et al., 2014), and the species list and cell density of potentially toxic cyanobacteria (Chorus and Welker, 2021). Fur- thermore, we reported the presence of cyanotoxins and their con- centration. Data reported have been retrieved from laboratory reports and environmental agencies websites. Data were collected by regional Environmental Protection Agencies (EPAs) and local authorities as part of: i) monitoring ac- tivities aiming at the assessment of the ecological status of water bodies according to the Water Framework Directive (2000/6) (WFD); ii) evaluation of water bodies as suitable for bathing, ac- cording to the EU 2020/2184 directive; iii) monitoring of drinking water supplies. The chemical and physical parameters of the lakes were determined using the sampling protocol published by the National Agency for Environmental Protection (ISPRA, 2014) and the manual of analytical methods published by the National Research Council (APAT CNR-IRSA, 2003). The monitoring of cyanobacteria in bathing waters was carried out according to the Italian national legislation and the Italian National Institute of Health report (Funari et al., 2014). The detection of microcystins was carried out using either ELISA tests (concentrations expressed as µg/L MC-LR) (Gurbuz et al., 2012) or by LC-MS (concentra- tion expressed as µg/L of each MC variant). MC data reported in the text have been obtained by ELISA test, if not stated otherwise. Cyanobacterial blooms in the Po River basin and the eastern Alps Fabio Buzzi,1 Chiara Agostinelli,1 Renata Alber,2 Andrea Beghi,1 Eugenia Bettoni,1 Enrico Bressan,3 Ketty Caraffini,1 Elisa Carena,1 Francesca Caviglia,4 Matteo Dossena,2 Pier Luigi Fogliati,4 Riccardo Formenti,1 Giorgio Franzini,5 Romana Fumagalli,1 Federica Giacomazzi,5 Francesca Lazzeri,2 Manuela Marchesi,1 Eleonora Masala,6 Paola Montanari,1 Mariano Nasello,6 Francesco Nastasi,1 Franca Pandolfi,1 Francesca Vietti,4 Damiano Virgilio,3 Samuel Vorhauser,2 Chiara Zampieri,5 Elisa Zanut3 1ARPA Lombardia, Milano; 2APPA Bolzano; 3ARPA Friuli-Venezia Giulia, Palmanova; 4ARPA Piemonte, Torino; 5ARPA Veneto, Padova; 6ATS Monza-Brianza, Monza, Italy ABSTRACT In this paper, we briefly describe episodes of cyanobacterial blooms that have occurred in lakes of northern Italy since 2000. In ad- dition to listing the species involved in these blooms, we provide information on the trophic and ecological status of the water bodies and the presence of algal toxins. Furthermore, we report an example of a risk assessment effort aimed at developing a quality control system for water intended for human consumption. The use of high-frequency monitoring techniques, integrated with predictive mod- elling, remote sensing, and molecular analysis for species identification, is becoming increasingly important in the context of the effects of ongoing climate change. Corresponding author: Fabio Buzzi, ARPA Lombardia, Milano, Italy. E-mail: f.buzzi@arpalombardia.it Key words: cyanobacteria, blooms, toxins, modelling, high-fre- quency monitoring. 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: 18 April 2024. Accepted: 19 December 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), 2025 Licensee PAGEPress, Italy Advances in Oceanography and Limnology, 2025; 16:12584 DOI: 10.4081/aiol.2025.12584 This work is licensed under a Creative Commons Attribution- NonCommercial 4.0 International License (CC BY-NC 4.0). F. Buzzi et al.2 Results We reported the results of our study presenting the northern Italy lakes grouped by category and from the westernmost to the easternmost (Table 1). According to the Italian legislation, large and deep lakes are categorized as water bodies of type AL-3; lakes with a maximum depth of less than 125 meters are categorized as water bodies of type AL-6; shallow lakes with an average depth equal to or lower than 15 m are categorized as AL-5. All these categories include lakes that provide essential and valuable serv- ices to the communities and economic activities located in their surroundings; among these, AL-6 lakes will increasingly be im- portant as major water sources in the near future (Sterner et al., 2020; Sterner, 2021). Lake Maggiore is the most oligotrophic of the Italian deep subalpine lakes, and its phytoplankton community is characterized by low production levels (CNR IRSA, 2024). According to the WFD, its ecological quality status ranks as “Good”. Moderate sur- face algal blooms, dominated by Dolichospermum lemmermannii (Richter) P.Wacklin, L.Hoffmann & J.Komárek, were recorded in 2011 (Callieri et al., 2014). Another moderate event was recorded in 2023, which was instead dominated by Microcystis spp. (Figure 1a). Microcystin concentrations were below the Limits of Quan- tification (LOQ) in both cases. Table 1. List of lakes and cyanobacterial species that caused the blooms described in the text. Lake Lake type Trophic status Ecological status Species Surface bloom/ Toxins cellular density (MC, µg L–1) (cells mL–1) Maggiore AL-3 oligotrophic Good D. lemmermannii Yes 30 Como AL-3 Mesotrophic Moderate M. aeruginosa Yes <20 D. lemmermannii Iseo AL-3 Eutrophic Moderate P. rubescens Yes N.A. Garda AL-3 Oligotrophic Good D. lemmermannii Yes <20 Idro AL-6 Eutrophic Moderate P. rubescens Yes N.A. A. flos aquae Grande d’Avigliana AL-6 Meso-eutrophic Moderate Microcystis spp. 177,358 <20 Sirio AL-6 Meso-eutrophic Moderate Aphanothece/Aphanocapsa 129,000 <20 Varese AL-5 Eutrophic Moderate P. rubescens Yes 16.7 Microcystis spp. L. robusta W. Naegeliana Alserio AL-5 Eutrophic Poor A. flos-aquae Yes N.A. Annone est AL-5 Eutrophic Moderate W. naegeliana Yes N.D. Annone ovest AL-5 Eutrophic Moderate W. naegeliana Yes N.D. Montorfano AL-5 Eutrophic Moderate Microcystis spp. Yes N.A. Piano AL-5 Eutrophic Moderate A. flos-aquae Yes N.A. Pusiano AL-5 Meso-eutrophic Moderate Microcystis spp. Yes 12.8 D. lemmermannii L. robusta Segrino AL-5 Meso- oligotrophic Good Microcystis spp. Yes N.D. Candia AL-5 Eutrophic Moderate Microcystis spp. Yes 4.45 Planktolyngbya sp. Anabaena sp. Del Frassino AL-5 Eutrophic Poor D. planctonicum Yes N.D. C. raciborskii A. gracile Ragogna AL-4 Mesotrophic Moderate D. planctonicum* No N.D. Sartirana AL-4 Hypereutrophic Bad D. crassum Yes N.D. Caldaro AL-4 Oligo-mesotrophic Moderate Aphanocapsa sp. 117,495 0.609 Aphanothece sp. Microcystis sp.* Fiè AL-4 Eutrophic N.A. Aphanocapsa sp. 220,663 1.5 Aphanothece sp. Cyanodiction sp. Costalovara AL-4 Mesotrophic N.A. Aphanocapsa sp. 93,314 0.204 Cyanodiction sp. N.A., not available; N.D., not determined; LOQ, limits of quantification. Cyanobacterial blooms in the Po River basin and the eastern Alps 3 The northern basin of Lake Lugano is eutrophic and meromic- tic (Lepori et al., 2022), and its ecological quality status ranks as “Moderate.” An extensive bloom of Microcystis spp. lasting two months was recorded in 2023. Microcystin LR concentration was as high as 30 µg L–1. Lake Como is mesotrophic (Rogora et al., 2018) and, since 2000, has been affected by several large-scale surface blooms cov- ering up to tens of square kilometers. The species causing the blooms were Microcystis aeruginosa (Kützing) Kützing and D. lemmermannii (Figure 1b). However, concentrations of algal tox- ins were lower than LOQ. Lake Iseo is eutrophic and meromictic (Leoni et al., 2019). In 2013, a significant population of Planktothrix rubescens (De Candolle ex Gomont) Anagnostidis & Komárek developed in the metalimnetic area, giving rise to a bloom in April 2013. There is no information on the presence of cyanotoxins in this episode. Lake Garda is oligotrophic, and its ecological quality status ranks as “Good”. Four surface blooms caused by D. lemmerman- nii were observed in 2011, 2012, 2017, and 2020 (Figure 1c). However, blooms observed in this lake were less widespread in comparison with the blooms of D. lemmermannii observed in Lake Como. In each bloom, cyanotoxins concentrations were lower than the LOQ, regardless of the most recent technological improvements adopted by EPA Veneto (implementation of a LC- MS analysis). The appearance, ecology, and toxicity of D. lem- mermannii populations in Lake Garda were investigated by Salmaso et al. (2015a, 2015b, 2024a) and Capelli et al. (2017). Regarding the AL-6 typology, Lake Idro is meromictic and eutrophic (Viaroli et al., 2018). Its ecological quality status is “Moderate.” In winter 2004, a surface bloom of P. rubescens de- veloped, whereas, in 2010 and 2023, blooms were dominated by Microcystis spp. and Aphanizomenon flos-aquae Ralfs ex Bornet & Flahault, respectively. No algal toxins were detected on all these occasions. The Piedmontese water bodies type AL-6, Lake Grande of Avigliana, which was subjected to hypolimnetic aeration for over ten years, and Lake Sirio, developed surface blooms in June 2023. In the first case, blooms were dominated by Microcystis spp., whereas in the second case by Aphanothece spp. and Aphanocapsa spp. During these blooms, algal toxins were not de- tected. Both lakes have “Moderate” ecological status. Among the AL-5 lakes typology, Lombardy has many morainic lakes in the eutrophic state, which favoured the devel- opment of several blooms of potentially toxic cyanobacteria. Lake Varese, which has a “Moderate” ecological quality status, has the highest Total Phosphorus (TP) concentration (60 µg L–1). This lake developed intense blooms of A. flos-aquae, P. rubescens, and Microcystis spp. More recently, it developed blooms of Woroni- chinia naegeliana (Unger) Elenkin and Limnoraphis robusta (Paracutty) J.Komárek, E.Zapomelová, J.Smarda, J.Kopecký, E.Rejmánková, J.Woodhouse, B.A.Neilan & J.Komárková (Fig- ure 1d). In 2022, Microcystis spp. produced microcystins at con- centrations as high as 16.7 µg L–1. The Lakes Alserio, East Annone (Figure 1e), West Annone (Figure 1f), Montorfano, Piano, and Segrino are between olig- otrophic and eutrophic state with spring TP concentrations rang- ing between 15 and 45 µg L–1. Their ecological quality status is reported in Table 1, with cyanobacteria blooms that have af- fected them. Among the Piedmont water bodies AL-5, Lake Candia is eu- trophic and has “Moderate” ecological quality status. In 2023, it developed a surface bloom of “Anabaena” sp. (Figure 1g); during this episode, the microcystins concentration reached 4.45 µg L–1. Lake Frassino is a small eutrophic lake of glacial origin in the province of Verona. The lake, which has “Poor” ecological quality status, occasionally developed summer blooms of Dolichosper- mum planctonicum (Brunnthaler) Wacklin, L.Hoffmann & Komárek and Cylindrospermopsis raciborskii (Wołoszyńska) Seenayya & Subba Raju. The presence of cyanotoxins was not investigated. Among the Friulian water bodies type AL-5, Lake Ragogna developed relatively high summer-autumn abundances of D. planctonicum. Abundances were within the range expected for mesotrophic lakes during a seasonal ecological succession, from 100 to 1000 cells/mL. The small and polymictic Lake Sartirana showed the highest trophic level (Table 1). In this water body, blooms of Dolichos- permum crassum (Lemmermann) P.Wacklin, L.Hoffmann & J.Komárek were associated with fish die-off following a very strong heatwave and a storm that stirred up sediments and con- tributed to oxygen consumption. Among the Sudtyrol water bodies of type AL-5, the polymic- tic lakes Caldaro, Fiè, and Costalovara showed high cell densities of Aphanothece sp., Aphanocapsa sp., and Cyanodiction sp., which did not produce surface blooms. Consequently, algal toxins were detected but at very low concentrations. Integrating conventional and innovative approaches in the study of algal blooms The increase in frequency of cyanobacterial blooms requires the use of integrated methods employing modern technologies to characterize and possibly predict the events with high accuracy. High-frequency data acquisition enables the implementation of tools for bloom prediction (meteorological index, Ndong, 2014; machine learning, Rousso, 2019) and of "early warning" systems to signal potential bloom initiations, associated with the use of re- mote sensing (Bresciani et al., 2017; Giardino et al., 2019). Data collected from buoys deployed in the middle of the lakes are also crucial for calibrating process-based models (QWET) used to as- sess changes in nutrient loads in relation to climate change (Fenocchi et al., 2019; Fenocchi et al., 2020). In Lakes Maggiore, Como, and Varese, buoys equipped with sensors for both meteor- ological and continuous measurement of chemical, physical, and biological parameters in surface layers have already been de- ployed (SIMILE Interreg Italy Switzerland Project; AQST Action program for the protection of Lake Varese financed by Regione Lombardia). Thermistor chains extend up to 50 meters in deep lakes, while in Lake Varese, they reach the bottom. Continuous temperature data acquisition allows the characterization of tem- perature evolution profiles and the study of internal lake waves. A simple example of an early warning system is the one imple- mented in Lake Varese, which uses a combination of three thresh- old levels, one for dissolved oxygen, one for pH, and the last for phycocyanin concentration. In case two out of three thresholds are exceeded, the system sends an alert to the health authorities. In Piedmont and Lombardy, measures for reducing internal loads through hypolimnetic withdrawal during stratification were adopted in some small and medium-sized eutrophic lakes, namely lakes Alserio, Annone Est, Varese, Grande d'Avigliana, and Sirio. F. Buzzi et al.4 Moreover, washout of dissolved nutrients will be adopted for nu- trient dilution in Lake Sartirana. Some lakes monitored by the EPA are used as drinking water supply and connected to aquifers and wells. In some of these cases (e.g., Lake Pusiano; Figure 1h), in compliance with the drinking water directive, the health authority of Brianza has started assess- ments aimed at the identification of potentially toxic cyanobacte- ria and their toxins (specifically microcystins) in some of the water intake points surrounding the lake. Benthic cyanobacteria are pioneer organisms adapted to ex- treme conditions. In the Alpine lakes and rivers, these microor- ganisms are important components of periphyton in benthic substrates and submerged vegetation (Salmaso et al., 2024b) and have also been found in high-altitude melting lakes (Metz et al., 1998). In our investigations, low nutrient levels and high con- centrations of suspended solids were found. For example, float- ing mats consisting of a mixture of Spirogyra sp. and Oscillatoria limosa C.Agardh ex Gomont were detected in Lake Montorfano (Figure 1i). ELISA tests conducted by the local health authority detected concentrations of microcystin LR as high as 2.8 µg L–1. In perspective, also considering the high toxic potential of cyanobacterial populations that form mats on a va- riety of substrates, this component should be closely monitored in lakes and rivers. Conclusions The increase in temperature observed in recent years has been accompanied by an increase in the frequency and duration of cyanobacterial blooms. Trophic state paradigms have shifted, due to the warming effect, more prominently in average and small-sized water bodies. Molecular techniques, aimed at the identification of certain species and the detection of genes en- coding for cyanotoxins, would be of great help for a more pre- cise bloom characterization. For surveillance and bloom episode characterization, the adoption of multidisciplinary approaches that use high-frequency monitoring (cytometry), remote sensing, and predictive modelling approaches would be also particularly useful, especially in the case of extreme events. In terms of pre- vention, it is essential to determine nutrient loads and implement measures for their reduction to the lowest possible level, while remediation efforts could be facilitated by the complementary use of predictive modelling. Figure 1. Cyanobacterial blooms recorded in northern Italy during the last 20 years. a) Lake Maggiore during a bloom of Microcystis sp. b) Surface bloom of D. lemmermannii in Lake Como. c) Summer bloom of D. lemmermannii in Lake Garda. d) L. robusta in Lake Varese. e) W. naegeliana in Lake Annone-eastern basin. f) A. flos-aquae in Lake Annone -western basin. g) Surface accumulation of “Anabaena” sp. in Lake Candia. h) P. rubescens in Lake Pusiano. i) Accumulation of O. limosa mats along the shore in Lake Montorfano. Cyanobacterial blooms in the Po River basin and the eastern Alps 5 References APAT IRSA-CNR, 2003. Metodi analitici per le acque. Available from: https://www.irsa.cnr.it/wp/wp-content/uploads/2022/04/ Vol1_ Sez_1000_Indice_ParteGenerale.pdf Bresciani M, Giardino C, Lauceri R, et al., 2017. Earth observa- tion for monitoring and mapping of cyanobacteria blooms. Case studies on five Italian lakes. J. Limnol. 76:127-39. Callieri C, Bertoni R, Contesini M, Bertoni F, 2014. 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