Insight into how environmental change determines the production and distribution of cyanobacterial toxins is necessary for risk assessment. Management guidelines currently focus on hepatotoxins (microcystins). Increasing attention is given to other classes, such as neurotoxins (e.g., anatoxin-a) and cytotoxins (e.g., cylindrospermopsin) due to their potency. Most studies examine the relationship between individual toxin variants and environmental factors, such as nutrients, temperature and light. In summer 2015, we collected samples across Europe to investigate the effect of nutrient and temperature gradients on the variability of toxin production at a continental scale. Direct and indirect effects of temperature were the main drivers of the spatial distribution in the toxins produced by the cyanobacterial community, the toxin concentrations and toxin quota. Generalized linear models showed that a Toxin Diversity Index (TDI) increased with latitude, while it decreased with water stability. Increases in TDI were explained through a significant increase in toxin variants such as MC-YR, anatoxin and cylindrospermopsin, accompanied by a decreasing presence of MC-LR. While global warming continues, the direct and indirect effects of increased lake temperatures will drive changes in the distribution of cyanobacterial toxins in Europe, potentially promoting selection of a few highly toxic species or strains.

Under ongoing climate change and increasing anthropogenic activity, which continuously challenge ecosystem resilience, an in-depth understanding of ecological processes is urgently needed. Lakes, as providers of numerous ecosystem services, face multiple stressors that threaten their functioning. Harmful cyanobacterial blooms are a persistent problem resulting from nutrient pollution and climate-change induced stressors, like poor transparency, increased water temperature and enhanced stratification. Consistency in data collection and analysis methods is necessary to achieve fully comparable datasets and for statistical validity, avoiding issues linked to disparate data sources. The European Multi Lake Survey (EMLS) in summer 2015 was an initiative among scientists from 27 countries to collect and analyse lake physical, chemical and biological variables in a fully standardized manner. This database includes in-situ lake variables along with nutrient, pigment and cyanotoxin data of 369 lakes in Europe, which were centrally analysed in dedicated laboratories. Publishing the EMLS methods and dataset might inspire similar initiatives to study across large geographic areas that will contribute to better understanding lake responses in a changing environment.

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SUMMARYIn this study, we have analyzed superoxide dismutase (SOD), ascorbate peroxidase (APX) and glutathione reductase (GR) activities, biomass accumulation and chlorophyll-a content in the Arthrospira platensis-M2 strain grown at different concentrations of zinc (Zn), tin (Sn) and mercury (Hg). We found that there is a close relationship between chlorophyll-a content and biomass accumulation in A. platensis-M2 strain as a result of Zn, Sn and Hg exposures. Sn was found to be the most toxic heavy metal among others because of the continious inhibition of both biomass and chlorophyll-a accumulation at 500 and 1000 μg mL −1 concentrations after the third day of the study, while they represented continuous increases at each Zn and Hg concentration over 7 days. Lower concentrations of Zn and Sn stimulate SOD and GR activities remarkably, probably due to oxidative stress caused by heavy metal toxicity. APX activity was significantly lowered by higher concentrations of the three metals used in this study. Our results suggest that higher heavy metal concentrations inhibited SOD, APX and GR activities but biomass and chlorophylla accumulation endured in a time-dependent manner, possibly due to some different defence mechanisms, which remain to be investigated.

IntroductionShallow lakes in Mediterranean climates, which are generally situated in lowland areas, due to their high evaporation/precipitation ratios, low geographic relief, and dense human population have long water residence times and are becoming more eutrophic (Allan et al., 1980;Borics et al., 2013). Moreover, elevated release of phosphorus from sediments or greater loads from the catchment area and destruction of submerged vegetation may trigger an increase of phytoplankton density and a related decrease in water clarity (Moss et al., 2009;Dokulil and Teubner, 2011). The increased biomass of phytoplankton and frequently occurring toxic algal blooms triggered the reassessment of lake management strategies (Borics et al., 2013). The Water Framework Directive was deigned to assess the ecological quality of surface waters through the analysis of various characteristics of aquatic flora and fauna, and to declare management plans in European countries (EC, 2009). Investigation of the functional traits of phytoplankton of shallow lakes was found to be important to estimate ecological quality and to understand the operation of these systems (Borics et al., 2012).Many attempts have been made to categorize traits and functions of phytoplankton (Reynolds et al., 2002;Borics et al., 2007;Padisák et al., 2009). At present, 40 phytoplankton functional groups (FGs) have been described, identified by numeric character codes (codons) (Padisák et al., 2009). Padisák et al. ( 2006) developed an index (Q index) using FGs to estimate the ecological status of lakes. The index combines the relative weight of FGs in the total biomass and considers a factor number for each assemblage for each type of water body. It was tested on water bodies significantly differentiated by origin, altitude, salinity, mixing, and stratification in the world (e.g., Crossetti and Bicudo, 2008;Pasztaleniec and Poniewozik, 2010) and in Turkey (e.g., Demir et al., 2014;Çelik and Sevindik, 2015).Lake morphometry and hydrology are criteria for the composition of lake biota (Murray and Pullar, 1910), and may favor distinct life strategies. Nevertheless, even taking into account the hydromorphology, substantial differences are recognizable when considering the effects of latitude and climate on phytoplankton composition and abundance (Pollingher, 1990). Moreover, lakes that are located in the same geographic region and have similar hydromorphologies could be composed of diverse phytoplankton assemblages as a result of different nutrient content and light availability (Scheffer, 1998;Naselli-Flores, 2000). In addition, Borics et al. (2014) found