## 3 Results  
Time series of water temperature oxygen concentration (O\({}_{2}\)) and percent saturation (O\({}_{2}\)%) are shown in Figure 1 and boxplots of the same variable plus photosynthetic active radiation (PAR) are shown for the different treatments and periods in Figure 2. Linear mixed effect model results are given in Table 2. The maximum of the 24 h daily mean water temperatures during the HW were 22.8 \({}^{\circ}\)C in A0, 30.2 \({}^{\circ}\)C in A2 and 31.5 \({}^{\circ}\)C in A2+ (Figure 1), while the corresponding minimum values were 14.2, 19.7 and 17.9 \({}^{\circ}\)C, respectively. The period with the HW was unusually warm (see A0 curve in Figure 1), and warmer than the periods before and after the HW. In addition, there was a sudden drop in water temperature in mid-July during a rainy and cold period, a period that also experienced less irradiation (Figure 1). The highest temperature in A2 and A2+ during the artificial HW was substantially (3.6 \({}^{\circ}\)C) higher than the values recorded on a single day in the previous years of the experiment in these mesocosms, being 26.6 and 27.9 \({}^{\circ}\)C, respectively (authors' unpublished results). The water temperature decline had implications for O\({}_{2}\) and O\({}_{2}\)%, as both decreased substantially in all mesocosms during the rainy/cold period, not least in the HNs; for both HN and LN, larger decrease occurred in the heated mesocosms than in the ambient mesocosms. During this period, the 24 h mean oxygen percentage in LN dropped to an average of 8.0, 5.1 and 5.8 mg O\({}_{2}\) L\({}^{-1}\) in A0, A2 and A2+, respectively, and to 8.2, 4.5 and 4.5 mg O\({}_{2}\) L\({}^{-1}\) in HN (Figure 1), while the same figures for minimum concentrations at night averaged 7.0, 3.3 and 4.3 mg O\({}_{2}\) L\({}^{-1}\) in LN and 6.3, 2.2 and 2.1 mg O\({}_{2}\) L\({}^{-1}\) in HN, being as low as 0.6 and 0.9 mg O\({}_{2}\) L\({}^{-1}\) in two of the heated (A2+) HN mesocosms (data not shown). Overall, maximum and minimum O\({}_{2}\) and O\({}_{2}\)% showed larger day-to-day variations in HN than in LN over the study period (Figure 1), and the water was mostly supersaturated with O\({}_{2}\) in both LN and HN, but with higher maximum percentages in HN. Thus, O\({}_{2}\) and O\({}_{2}\)% were strongly positively related to the nutrient level (Table 2, Figures 1 and 2). The 24 h daily mean O\({}_{2}\) values in A0, A2 and A2+ for the entire study period were 10.4, 9.9 and 9.2 mg L\({}^{-1}\), and 13.0, 11.5 and 11.7 mg L\({}^{-1}\) in LN and HN, respectively. For O\({}_{2}\)%, the figures were 112, 116 and 111% in LN and 138, 134 and 141% in HN. However, we found no significant temperature or HW effect on O\({}_{2}\), and O\({}_{2}\)% using the full data set (Table 2). When LN and HN data were separately analyzed, we found a positive HW effect on O\({}_{2}\), but this was not significant when taking into account the warming effect of the natural HW in A0 (Table 2). effects were recorded. We found a significant HW effect on Chl-a, TN and PO4, all showing an increase, but not more than in A0, reflecting the fact that the temperature in A0 during this period also was higher than before and after the HW due to a natural HW during July (Table 2). After analyzing LN and HN data separately, we found increasing Chl-a and decreasing PO4 with increasing temperature in LN, while PO4 increased in HN (Table 2). No temperature treatment effect was recorded for the other variables. During the HW, Chl-a was higher in LN, but not significantly so when the warming effect of the natural HW in A0 was considered (Table 2). Time series (24 h means) of GPP, ER and NEP are shown in Figure 4, while boxplots of the same variables and BACPRO for the six treatments and three periods are given in Figure 5 and linear mixed effect models' results are in Table 3. On the full dataset, the models revealed a significant positive effect of nutrients on GPP, ER, NPP and BACPR.