## 2 Materials and Methods  
The experiment was conducted in lake mesocosms established in August 2003 in Jutland, Denmark, for studying the effects of climate change at contrasting nutrient levels [29]. In brief, the facility includes 24 outdoor flow-through mesocosms (diameter 1.9 m, water depth 1 m, retention time of about 2.5 months). At the start of the experiment in 2003,a 20 cm layer of a mixture of washed and nutrient-rich sediment from a nearby freshwater pond was added to each mesocosm (see details in [PERSON] et al. [29]). The mesocosm facility has three temperature treatments: unheated (A0, abbreviations are provided in Table 1), treatment A2 (A2) according to the International Panel on Climate Change's (IPCC) predicted temperature increase [24] and treatment of A2 + 50%, which is A2*1.5 (A2+). The A2 and A2+ mesocosms are warmed by heating elements placed about 10 cm above the sediment. To avoid stratification, the water column is fully mixed by paddles [29]. The water temperature in A0 mesocosoms fluctuates with the air temperature, while the two heated treatments are 2-4 \({}^{\circ}\)C (A2) and 3-6 \({}^{\circ}\)C (A2+) warmer than the A0 mesocosms depending on the season based on future climatic projections for Central Jutland, Denmark downscaled to monthly resolutions using 1961-1990 as the reference period [30]. The temperature treatments are crossed with two nutrient treatments (low (LN) and high (HN)), i.e., six treatments for each run with four replicates. LN only receives nutrient input from tap water, while HN receives additional nutrients weekly (7 mg P m\({}^{-2}\) d\({}^{-1}\) and 27.1 mg N m\({}^{-2}\) d\({}^{-1}\)). The two nutrient loadings mimic conditions in clear water and turbid water shallow lakes, respectively [31]. Accordingly, submerged macrophytes generally dominate in the LN mesocosms, whereas the HN mesocosms are mainly turbid with abundant phytoplankton. Temperature is measured continuously in all mesocosms. Oxygen is recorded approximately 0.4 m below the surface using a two-wire oxygen probe (OxyGuard International A/S, Birkerod, Denmark) [29]. Our study was conducted in summer 2014, i.e., 11 years after the initiation of the mesocosoms experiment. During July, a heat wave (HW) was simulated in A2 and A2+ by increasing the water temperature by 5 \({}^{\circ}\)C, while the A0 mesocosms remained unheated. Hence, during the heat wave the heated mesocosms were about 7-9 \({}^{\circ}\)C (A2) and 9-12 \({}^{\circ}\)C (A2+) warmer than in A0. As for the long-term temperature treatments, the water temperatures in A2 and A2+ were continuously adjusted during the heat wave following the temperature variation in A0. Thus, we studied the effects of a heat wave occurring in shallow lakes by the end of this century, when lakes are expected to be warmer, and compared the results with the present-day situation. Water for nutrient analysis and chlorophyll-a (Chl-a) was collected 2 times before, 6 times during and 2 times after the HW. Sampling was conducted with a 1 m long tube sampler (covering the whole water column) at 6 randomly selected sites and pooled. A 100-1000 mL subsample was filtered through GFC filters and the Chl-a concentration