## 3 Results  
### Physical and Nutrient Dynamics  
During summer 2018, high pressure (>1013 mbar) dominated over Ireland and, as a consequence, the country experienced unusually dry and warm conditions from the middle of May to the 12 th June compared to the previous 15-year average during this time period (Figure 2A,B). Between the 13 th and the 19 th June, Storm Hector brought heavy rain and strong winds across Ireland as well as a significant drop in mean daily air temperatures (Figure 2). There was an accumulation of 64 mm of precipitation during these 7 days, which occurred after 11 days and was followed by 20 days with no rain (0 mm-day\({}^{-1}\)) (Figure 2B). On the 14 th June 2018, coinciding with the arrival of Storm Hector, wind speed reached a daily mean value of 9.6 m-s\({}^{-1}\) (Figure 2C), with maximum gusts of 26.91 m-s\({}^{-1}\)[75]. After Storm Hector, from the 21 st June to the 12 th July, high pressure stretching from the Azores to Scandinavia brought very warm and dry conditions, during which a maximum air temperature of 29.8 \({}^{\circ}\)C on the 27 th June was recorded at the study site (Newport Furnace, Met Eireann weather station) (Figure 2A). Thermal stratification was recorded in Feeagh from mid-May as a result of the increasing air temperatures and calmer weather conditions (low precipitation and wind speeds) recorded at that time. This resulted in a sudden increase in Schmidt stability (St), reaching values above 400 J-m\({}^{-2}\) from the 1 st June and a maximum of 548.25 J-m\({}^{-2}\) recorded on the 7 th June (Figure 3A). With the arrival of Storm Hector on the 14 th June, strong winds and a decrease in surface heat input from the atmosphere led to a decline in St values to 192.52 J-m\({}^{-2}\) (three-fold reduction). After two weeks, stable stratification was again observed in the Feeagh water column, with the maximum St value of 669.41 J-m\({}^{-2}\) reached on the 30 th June. Schmidt stability remained well above the 15-year average throughout July 2018 (>510 J-m\({}^{-2}\)) (Figure 3A). The mixed layer deepened from 6.5 m before the storm to 16 m on the 21 st of June, highlighting a deepening of the mixed surface layer to a maximum of 9.5 m during the intermediate period (Figure 3B). From July onwards, the mixed layer gradually deepened from 2 m on the 3 rd July to 42 m by the end of September, when the lake became fully mixed again and there was a complete breakdown of the water column stratification (Figure 3B). Figure 2: **(A)** Daily mean air temperature (\({}^{\circ}\)C) and (**B**) daily accumulated precipitation (mm) in Newport Furnace weather station. Black lines indicate 2018, the average for all years from 2004 to 2018 is shown with a grey line, and the grey shaded area indicates the 95% confidence interval around that average. **(C)** Daily 2018 mean wind speed (m\(\cdot\)s\({}^{-1}\)) recorded at Feeagh AWQMS, 2.3 m above lake surface level. The red dotted vertical lines mark the 14 th June 2018 (Storm Hector). Daily estimates of epilimnetic (0.9 m) lake metabolism (GPP, R and NEP) were calculated for each of the respective periods. Reliable estimates were produced for 27 days out of a possible 49 days (55.10%) within the sub-period under study (28 th May-15 th July) and between 7 and 10 observation days (n) for each specific period. Pre- and post-storm values of R and NEP were significantly different from the intermediate period (Kruskal-Wallis test and [PERSON]'s post hoc test, \(p<0.05\)) (Figure 4). Respiration increased while NEP decreased, corresponding to a decrease in GPP during the mixing event linked with extreme-weather-related conditions. The resulting NEP estimates were nearly always negative, but clearly higher during the periods with calm and hot sunny days (early June and early July) (Figure 4). Figure 3: **(A)** Schmidt stability (J-m\({}^{-2}\)) of Lough Feeagh. Black line indicates 2018 values. The mean \(\pm\) SD for the 2004–2018 period is shown by the grey line and area.