## 4 Discussion
No significant difference for NO\({}_{3}\) is also reasonable, since NO\({}_{3}\) levels in shallow lakes often decrease in the summer due to uptake in organisms (higher TN in HN) or loss by denitrification [50]. No temperature treatment effects were recorded for the four variables; however, Chl-a and PO\({}_{4}\) significantly increased during the HW, but not when accounting for the changes in A0, which is more likely a reflection of the naturally higher temperatures (Figure 1) during the HW than before and after. This suggests that eutrophication was intensified during this overall natural warmer period occurring during the period that we applied the artificial HW (Figure 3), but the effect of the 5 \({}^{\circ}\)C extra temperature increase in A2 and A2+ could not be discerned. When dividing the data into the two nutrient levels (treatments), we found that PO\({}_{4}\) increased with temperature in HN but decreased in LN (Table 2). The higher PO\({}_{4}\) in HN likely reflects increased internal loading under eutrophic conditions, as has been observed for eutrophic lakes in both experimental studies [51] and when conducting lake mass balances [52]. We also found higher Chl-a in LN with increasing temperature, which is likely a result of the overall lower PVI of plants in the warmer mesocosms [35] and thus is a diminished constraining effect of plants on phytoplankton development [53]. During the HW, Chl-a increased relative to the pre/post-HW period in LN, but not when accounting for changes in A0, indicating that the extra heating during the HW was not of importance. The mixed effect models revealed a strong significant positive effect of nutrient treatments on the metabolism variables, GPP, ER, NPP and BACPR (Figures 4 and 5, Table 3). That higher nutrient loading leads to increased metabolic rates and higher net productivity is well-known [13; 14]. It is also well-established that BACPR increases with eutrophication (e.g., [17]). We found a significant positive effect of temperature on ER and BACPR, both when the dataset was analyzed in full and when it was divided into LN and HN, but no effect was found on GPP and NEP. While the positive effects on GPP, ER and BACPR were expected because these processes are temperature dependent ([7; 8; 9; 10], see Equation (1), we anticipated a lower NEP with increasing temperature as ER, in theory, is more dependent on temperature than on GPP [9; 10]. While the 24 h daily mean NEP varied between positive and negative values during the course of the experiment in all temperature scenarios (Figure 4), the average for the study period was positive and similar across the three temperature treatments (A0, A2 and A2+) for both LN and HN, indicating organic matter accumulation during summer irrespective of temperature and HW variances. However, NEP was about 2-3 times higher in HN than in LN, which suggests an overall higher net accumulation of organic matter at high nutrient levels. We also related the temperature-corrected GPP, ER and NET (20 \({}^{\circ}\)C selected, GPP20, ER20 and NEP20, using Equation (1)) to explore the effect of other environmental variables (Table 4). We found that GPP20 and ER20 were related to nutrients in the full dataset and in HN. By contrast, PVI was the most important variable in LN, emphasizing the key role that macrophytes and associated periphyton may play for the metabolism in clear water shallow lakes with abundant macrophytes (e.g., [54]). BACPR was not affected by PVI but was affected by Chl-a and/or nutrients and temperature, suggesting that the bacterioplankton was mainly fuelled by pelagic resources, even at high PVI. In both LN and HN, HW effects on GPP, ER and BACPR appeared, but only the effect on GPP and ER in LN remained significant when accounting for the changes in the A0 mesocosms during this period (Table 3).