Cyanobacteria are common inhabitants of lake plankton from oligotrophic pristine environments to heavily-impacted eutrophic lakes (Whitton and Potts 2012). Although they can be very important in oligotrophic lakes, especially through unicellular small-size species (Stockner et al. 2007), they are especially dominant in eutrophic lakes, where filamentous and larger colonial species form frequent blooms that can cause surface scums and unpleasant odours (Reynolds and Petersen 2000). Furthermore, several bloom-forming cyanobacteria species produce toxins that can cause an impact on the ecosystem biodiversity, as well as on the safety of water resources for several uses (Watson et al. 2015). Several factors may contribute to the dominance of cyanobacteria in lakes, including higher temperature and lower TN/TP optima compared to other phytoplankton groups, low light-energy requirements, superior uptake kinetics for inorganic carbon, the ability to fix molecular nitrogen and buoyancy, amongst others (Dokulil and Teubner 2000).

In the last decades of the 20th century, several Azorean lakes started to show signs of eutrophication (Porteiro 2000), with increased cyanobacteria abundance and the occurrence of blooms (Santos et al. 2005). This caused generalised concern amongst local authorities and population and monitoring programmes to access lake trophic status and ecosystem health were implemented in the region.

The Azores are a nine-island archipelago in the middle of the North Atlantic Ocean, distant roughly 1500 km from Europe and 1900 km from North America, divided into three groups according to their geographical position. Aligned along 615 km in WNW-ESE trend, the western group includes Flores and Corvo islands, the central group being composed by Graciosa, Faial, Pico, São Jorge and Terceira islands, while the eastern group comprises Santa Maria and São Miguel islands. Despite their geographical proximity, islands have distinct features that distinguish them, such as their geological setting (Moore 1990, Azevedo and Portugal Ferreira 2006, Cole et al. 2008) and their age (Ávila et al. 2016).

The diversity of habitats amongst islands differs naturally due to their geological setting and origin, allowing a great diversity of inland habitats, such as freshwater lakes (Porteiro 2000, Cruz and França 2006, Morton 2014). According to Porteiro 2000, the Azores have 88 lakes, located in São Miguel, Terceira, Pico, São Jorge, Flores and Corvo islands. The lakes can be classified into two types, according to their origin: i) lakes within volcanic depressions and ii) lakes in topographically depressed areas (Constância et al. 2000, Antunes 2004). Azorean lakes presented great differences in their size, elevation and depth. The largest lake is in São Miguel island (358.7 ha - Lagoa Azul), while the small lakes < 1 ha are present in São Miguel, Pico, Terceira and São Jorge (Porteiro 2000). Most of the lakes are located between 600 and 800 m altitude, while the range is from 230 m (São Miguel island) and 1,050 m altitude (Pico island). Most of the lakes are shallow with maximum depths ranging from 0.4 to 7 metres. These lakes are permanently mixed and cyanobacteria rarely dominate their phytoplankton. Deeper lakes, with maximum depth between 14 and 115 m, have a monomictic warm thermal regime, presenting stratification normally between April and October. Contrary to shallow lakes, deep, stratified lakes, located at lower altitudes where human activities are more intense, have undergone eutrophication and their phytoplankton is dominated by cyanobacteria, especially during the stratification period.

Here we present detailed data on the distribution of species belonging to several groups of cyanobacteria in Azorean lakes from 22 years (1996-2018) of lake phytoplankton monitoring for trophic and ecological status assessment (Luz et al. 2020).

Part of this study was financed by Portuguese National Funds, through FCT − Fundação para a Ciência e a Tecnologia, the European Union, QREN, FEDER, COMPETE, by funding the CIBIO/InBIO (project UID/BIA/50027/2013 and POCI-01-0145-FEDER-006821) and P. M. Raposeiro (DL57/2016/ ICETA/EEC2018/25). R. Luz and R. Cordeiro were financed by the European Interreg MAC 2014-2020 projects REBECA (MAC/1.1a/060) and FRCT - Fundo Regional de Ciência e Tecnologia (SRMCT - Secretaria Regional do Mar, Ciência e Tecnologia, Governo Regional dos Açores) grant refª M3.1.a/F/017/2015, respectively. We thanks DRCT (Azores Regional Government) for co-financing Interreg projects REBECA and for funding Azorean branch of CIBIO/InBIO (CIBIO-Açores). The surveys performed comply with the current laws of Portugal.

Each author contributed equally to manuscript preparation.