## 3 Results
For the entire study period in LN, GPP values were 5.6, 8.4 and 6.6 g O2 m-2 24 h-1 in A0, A2 and A+, respectively, and 9.7, 10.9 and 13.3 g O2 m-2 24 h-1 in HN, respectively. For ER, the figures were 5.2, 7.8 and 6.2 g O2 m-2 24 h-1 in LN and 8.5, 10.9 and 12.0 g O2 m-2 24 h-1 in HN. Accordingly, NEP was 0.4, 0.6 and 0.4 g O2 m-2 24 h-1 in LN and somewhat higher in HN: 1.2, 1.0 and 1.3 g O2 m-2 24 h-1. The figures for BACPR were 6.6, 9.6, 14.8 cells \(\times\) 107 mL-1 h-1 in LN and 35.6, 88.6 and 55.0 cells \(\times\) 107 mL-1 h-1 in HN. Figure 2: Box plot (10%, 25%, 75% and 90% percentiles) of 24 h mean temperature, oxygen concentration (O2) and oxygen percentage (O2 (%)) in the low (LN) and high (HN) nutrient mesocosms before, during and after the heat wave (the 4 replicates pooled). The three temperature scenarios are: A0 (green), A2 (blue) and A2+ (red). Also shown is the irradiation (PAR) in the three periods (upper right). Note the different scale on the y-axes of the LN and HN treatments. We found a significant positive effect of temperature on ER and BACPR, but not on GPP and NEP (Table 3). The HW had a positive effect on GPP, ER and BACPR and a negative effect on NEP compared with the pre/post HW period, but when taking the changes in the A0 mesocosms into account, only the HW effect on GPP and ER was significant (Table 3). When dividing the data into low and high nutrient concentrations, temperature had a significant positive effect on GPP, ER and BACPR in both LN and HN, while NEP was not significantly related to temperature (Table 3). In the LN mesocosms, there was a positive HW effect on GPP, ER and BACPR compared with the pre/post HW period, but when considering the changes in A0, only the HW effect on GPP and ER was significant. In HN, there was a positive HW effect on GPP, ER and BACPR compared with the pre/post heat wave period, but when considering the changes in A0, no HW effect was found (Table 3). Figure 3: Box plot (10%, 25%, 75% and 90% percentiles) of total phosphorus (TP), phosphate (PO4), total nitrogen (TN), nitrite+nitrate (NO3) and chlorophyll a (Chl a) in the low and high nutrient mesocosms before, during and after the heat wave (the 4 replicates pooled). The three temperature scenarios are: A0 (green), A2 (blue) and A2+ (red). Note the different scale on the y-axes of the LN and HN treatments. We also ran the models excluding the cold period 9-18 July, as it may preclude disclosure of significant results of the HW. However, only minor changes appeared. A significant HW effect on GPP now emerged also when accounting for changes in A0, and the HW led to significantly lower O\({}_{2}\) and O\({}_{2}\)% (all \(p\) < 0.05) in LN, but otherwise no effect of removing the cold period data was observed. Figure 4: Changes in 24 h gross primary production (GPP), ecosystem respiration (ER)and net ecosystem production (NEP) in the low and high nutrient mesocosms before, during and after the heat wave. The three temperature scenarios are: A0 (green), A2 (blue) and A2+ (red). Values are means of all four replicates. Variation—see Figure 5. Grey indicates the experimental heat wave period. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Data Set** & **Treatment** & **GPP** & **ER** & **NEP** & **BACPR** \\ \hline \multirow{4}{*}{Full} & Nutrient & *** (+) & *** (+) & * (+) & *** (+) \\  & Temperature & ns & * (+) & ns & ** (+) \\  & Heat wave & *** (+)1 & *** (+)1 & *** (-)0 & *** (+)0 \\ \hline \multirow{2}{*}{LN} & Temperature & * (+) & *** (+) & ns & ** (+) \\  & Heat wave & *** (+)1 & *** (+)1 & ns & *** (+)0 \\ \hline \multirow{2}{*}{HN} & Temperature & * (+) & * (+) & ns & * (+) \\  & Heat wave & *** (+)0 & *** (+)0 & ns & *** (+)0 \\ \hline \hline \end{tabular}
\end{table}