No abstract

Phytoplankton response to a heat wave in deep prealpine Lake Mondsee, Austria, was minor overall, although temperature significantly affected the epilimnetic phytoplankton assemblage at the deepest site of the lake. We detected no significant effect of nutrients. Using 3 complementary optical methods-light microscopy (Utermöhl technique), FlowCAM, and acoustic flow cytometry (AFC)-we found relatively low horizontal variation in the epilimnion but significant changes in phytoplankton community composition integrated over the 0-20 m water layer at the central station of the lake. These changes were mainly caused by a vertical shift in the Planktothrix rubescens peak from 11 to 16 m during the heat wave. Air temperatures reaching or exceeding 30 °C were measured in the area during 6 consecutive days at the end of June to the beginning of July, the first of 3 heat waves recorded during summer 2015; in-shore surface water temperature exceeded 27 °C. We sampled 9 stations across Lake Mondsee on 4 occasions during the heat wave and analysed temperature, nutrient levels, conductivity, pH, and the phytoplankton community. In addition to reporting the implications of increasing water temperatures for the algal assemblage in a deep stratified lake, we discuss the pros and cons of the different optical methods for phytoplankton identification and counting. For future field campaigns similar to the present study, we recommend using light microscopy to assess large or rare species, such as Ceratium hirundinella, and AFC, FlowCAM, or similar semi-automated devices for abundant small-to medium-sized species.

Many European lakes are monitored according to the EU Water Framework Directive (WFD), with focus on phytoplankton biomass and species composition. However, the low-frequency WFD monitoring may miss short-term phytoplankton changes. This is an important issue because short-term extreme meteorological events (heat waves and heavy rain) are predicted to increase in frequency and intensity with climate change. We used records from Lake Mondsee (Austria) from 2009 to 2015 to test if a reduction from monthly to seasonal sampling affected the average annual phytoplankton biovolume. Furthermore, we combined inverted light microscopy, FlowCAM and flow cytometry to estimate the effect of sampling during extreme events on average phytoplankton biovolume. Relative to monthly sampling, seasonal sampling significantly overestimated phytoplankton biomass. A heat wave in 2015 and two episodes of heavy rain in 2015 and 2016 caused species-specific changes; biovolumes of chlorophytes and the filamentous cyanobacterium Planktothrix rubescens (De Candolle ex Gomont) Anagnostidis & Komárek increased significantly during the heat wave. Using live material with FlowCAM and flow cytometry, we detected small and fragile cells and colonies that were either ignored or underrepresented by analysing fixed samples with light microscopy. We suggest a modified sampling and analysis strategy to capture short-term changes within the phytoplankton community.

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.

Moderate weather extremes are predicted to increase in frequency and intensity as a function of climate change. These extremes can result in sudden changes in temperature, light and nutrient levels, which may strongly affect the phytoplankton community in many lakes. We conducted a microcosm experiment with eight freshwater algal species, representing different functional groups, to investigate the interactive effects of these environmental variables on a phytoplankton community typical of an oligo‐mesotrophic temperate lake. We manipulated temperature (6, 16 and 26°C), light (18, 56, 132 μmol m−2 s−1) and soluble reactive phosphorus (0, 1, 5 μg/l) concentrations in a 3 × 3 × 3 factorial design and analysed treatment effects on algal biomass, species diversity (SDiv) and functional diversity (FDiv). Strong temperature variations affected phytoplankton biomass and diversity more strongly than moderate changes in nutrient and light levels. In particular, SDiv and FDiv decreased significantly with increasing temperatures. Our results support earlier findings that lake warming favours heterotrophic bacteria, relative to phytoplankton. We conclude that sudden temperature increases as they typically occur in temperate lakes during heat waves may decrease phytoplankton diversity, leading to altered ecosystem functioning. In contrast, heavy rain events usually characterised by decreasing levels of temperature and light intensities, concurrent with increases in nutrient concentrations, may sustain high phytoplankton diversity.