## 2 Materials and Methods
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Abbreviation** & **Definition** & **Abbreviation** & **Definition** \\ \hline
**A0** & Ambient treatments & **HN** & High nutrient treatments \\ \hline
**A2** & Ambient + 2–4 \({}^{\circ}\)C treatments & **IPCC** & Int. Panel on Climate Change \\ \hline
**A2+** & Ambient + 3–6\({}^{\circ}\)C treatments & **LN** & Low nutrient treatments \\ \hline
**BACPR** & Bacteriaplankton Production & **NEP** & Net Ecosystem Production \\ \hline
**Chl-a** & Chlorophyll-a & **NEP20** & NEP standardized to 20 \({}^{\circ}\)C \\ \hline
**DO** & Dissolved Oxygen & **NO\({}_{3}\)** & Nitrite+nitrate \\ \hline
**DO\%** & DO percent saturation & **PO\({}_{4}\)** & Orthophosphate \\ \hline
**ER** & Ecosystem Respiration & **O\({}_{2}\)** & Oxygen concentration \\ \hline
**ER20** & ER standardized to 20 \({}^{\circ}\)C & **O\({}_{2}\)\%** & Oxygen percent saturation \\ \hline
**GPP** & Gross Primary Production & **PAR** & Photosynthetic Active Radiation \\ \hline
**GPP20** & GPP standardized to 20 \({}^{\circ}\)C & **PVI** & Plant Volume Inhabited \\ \hline
**HW** & Heat wave & **TCA** & Trichoroacetic acid \\ \hline
**HW0** & pre- and post- HW period & **TEMP** & Temperature in the water \\ \hline
**HW1** & HW period & **TP** & Total phosphorus \\ \hline
**HW2** & Ambient summer period & **TN** & Total nitrogen \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Abbreviations used. was determined on a spectrophotometer after extraction with ethanol [32]. Standard methods were used for determination of chemical variables [33]. Macrophyte abundance was quantified as percent volume of the water column inhabited by plants (plant volume inhabited, PVI, %). Percentage cover and height of the submerged plants were assessed, allowing estimation of the proportion of the water column occupied by submerged plants (for further details, see [PERSON] et al. [34]. Data are presented in [35]). Gas transfer velocities for O\({}_{2}\) were measured in three mesocosms in December 2009 [36] and corrected for the water temperature [37]. There were only little variations in the \(k_{O_{2}}\) estimates based on the recovery of artificial reduction in O\({}_{2}\) in three of the mesocosms (0.0080, 0.0086 and 0.0100 m h\({}^{-1}\) at 20 \({}^{\circ}\)C) [36]. As the mixing in all mesocosms are done with the same set-up (paddles), we assumed similar piston velocities and used an average of these three estimates for all mesocosms (\(k_{O_{2}}=0.0088\) m h\({}^{-1}\) at 20 \({}^{\circ}\)C). Wind speed alters piston velocity; however, at low wind speed (<3 m s\({}^{-1}\)) the effect is negligible [38], which was the case in our study as the mesocosms are relatively well sheltered from the wind. GPP and ER was estimated using the 30 min interval records of dissolved oxygen (DO), light intensity and water temperature according to the method of [39; 40] (see [PERSON] et al. [41] for details) seen in Equation (1):  
\[ \begin{split}\left(\frac{dDO}{dt}\right)=\left(K_{2}\cdot 1.0241^{(T_{w}- 20)}\cdot(DO_{sat}-\ DO_{t})\right)-\ \left(p_{20}\cdot 1.07^{(T_{w}-20)}\right)+\ \left(\tau_{20}\cdot 1.035^{(T_{w}-20)}\cdot \frac{l_{t}}{y+l_{t}}\right)\\ (O_{2}\ change)=(Air\ water\ exchange)-(Temperature\ dependent\ respiration)\\ +(Light\ and\ temperature\ dependent\ production)\end{split}  \tag{1}\]  
where: \(T_{W}\): water temperature in \({}^{\circ}\)C, \(DO_{sat}\): dissolved oxygen saturation concentration in mg L\({}^{-1}\), \(DOt\): dissolved oxygen concentration in mg L\({}^{-1}\), \(I_{t}\): light intensity--Photosynthetic Active Radiation (PAR) mol m\({}^{-2}\) 30 min\({}^{-1}\)  
Constants \(\rho_{20}\), \(\tau_{20}\) and \(\eta\) were estimated using the secant non-linear method, PROC NLIN, in the statistical software SAS 9.3.