# Effect of Extreme Climate Events on Lake Ecosystems
###### Abstract
Lower biomass of large Cladocera and lower zooplankton:phytoplankton ratios indicated higher fish predation in these tanks as well. None of these metrics were affected by temperature. However, the zooplankton community structure indicated a higher predation pressure from fish at low nutrient concentrations during the heatwave, while the opposite pattern was found at high nutrient loading at the highest temperature, the latter likely reflecting observed fish kills in these tanks. Stability analyses also suggested modest effects of the heatwave as total zooplankton biomass showed overall high resistance. The metabolism metrics were also affected by nutrients but also by long-term temperature adaptation compounded by the heatwave [24]. Nutrients had a substantial effect on both gross primary production (GPP), ecosystem respiration (ER), net ecosystem production (NEP), and bacterioplankton production (BACPR), and positive effects of temperature on ER and BACPR were also found. The heatwave had a positive effect on GPP and ER. All of these metabolic process variables also responded to a cool and cloudy-low irradiance event in the middle of the heatwave, resulting in a severe drop in O2, not least in the heated mesocosms. In the same experiment, substantial effects associated with the heatwave were also found on the biomass of bacterioplankton, heterotrophic flagellates, and ciliates, respectively [25], and to some degree also on the greenhouse gas (GHG) dynamics [26]. The results from this experiment collectively indicated that microbial communities and ecosystem processes are more sensitive to a short-term heatwave than larger-bodied and more slow-growing organisms such as zooplankton [24]. Modelling experiments allow exploration of the impacts of extreme climatic events under current and future conditions. [PERSON] et al. [27], using a weather generator, constructed 30-year scenarios that included no change in climate conditions, a gradual progressive change, increased frequency of heatwaves, and a merging of the latter two. They elucidated the effect on a lake's physical properties applying an ensemble of one-dimensional (1-D) hydrodynamic lake models. Such 1-D models have been shown recently to reproduce the overall direction and magnitude of change during extreme events with accurate timing and little bias, although with some increase in uncertainty during events [28]. The study of [PERSON] et al. [27] revealed that the gradual increase scenario had the largest impact on annual water temperatures and on the length of the stratification period; however, increased heatwaves had a large effect on the summer lake conditions and introduced a larger degree of variability in water temperature. They also highlighted that the use of an ensemble of models provided results with a higher degree of confidence, and they therefore advocate for the need to include heatwaves in climate studies to better understand the possible consequences for lake ecosystems. [PERSON] et al. [29] used a 1-D hydrodynamic ecosystem-model to determine the influence of extreme climate events on a temperate and temporarily summer stratified lake in Denmark. After calibration and validation, they ran a range of extreme warming scenarios that affected the air temperature inputs to the model and were based on data from the heatwave seen over northern Europe in May-July 2018, mimicking situations of extreme warming returning every year, every three years, every five years in summer, and all year round, respectively. They found only modest impacts of the extreme climate events on internal nutrient loading; however, there were much stronger effects on the summer average of chlorophyll \(a\) and peak cyanobacteria biomass, which were up to 39% and 58% higher than during the baseline, respectively. The phytoplankton blooms occurred up to 15 days earlier and lasted up to 15 days longer during the heatwave years relative to the baseline, illustrating the strong effect that more frequent and longer-term heatwaves may have on temperate lakes.