Effects of Consecutive Extreme Weather Events on a Temperate Dystrophic Lake: A Detailed Insight into Physical, Chemical and Biological Responses

Abstract

Between May and July 2018, Ireland experienced an exceptional heat wave, which broke long-term temperature and drought records. These calm, stable conditions were abruptly interrupted by a second extreme weather event, Atlantic Storm Hector, in late June. Using high-frequency monitoring data, coupled with fortnightly biological sampling, we show that the storm directly affected the stratification pattern of Lough Feeagh, resulting in an intense mixing event. The lake restabilised quickly after the storm as the heatwave continued. During the storm there was a three-fold reduction in Schmidt stability, with a mixed layer deepening of 9.5 m coinciding with a two-fold reduction in chlorophyll a but a three-fold increase in total zooplankton biomass. Epilimnetic respiration increased and net ecosystem productivity decreased. The ratio of total nitrogen:total phosphorus from in-lake versus inflow rivers was decoupled, leading to a cascade effect on higher trophic levels. A step change in nitrogen:phosphorus imbalances suggested that the zooplankton community shifted from phosphorus to nitrogen nutrient constraints. Such characterisations of both lake thermal and ecological responses to extreme weather events are relatively rare but are crucial to our understanding of how lakes are changing as the impacts of global climate change accelerate.

Impacts of Extreme Weather Events on Bacterial Community Composition of a Temperate Humic Lake

“…Previously, Shade et al (2010) [16] found a significant relationship between the Schmidt stability of a lake in Taiwan and the differences in bacterial communities in the epilimnion and hypolimnion. Calderó et al (2020) [58] saw a decrease in the biovolume of Cyanobacteria and an increase in diatoms in Lough Feeagh on May 28 in the same study year. Both the high stability and the changes in phytoplankton biomass presumably have also influenced the shift in the free-living bacterial community.…”

“…The availability of DO was associated with samples taken in the spring, when DO was highest and the lake was fully mixed and slowly began to stratify [58]. High values for Chl a and Schmidt stability defined the bacterial community in the summer, when the lake was unusually stable and phytoplankton biomass reached a relatively high peak [58]. Color was correlated with the samples taken in the autumn.…”

“…The environmental variables correlated with the both the free-living and particle-associated bacterial populations of Lough Feeagh were DO, Chl a, Schmidt stability, and color, while the particle-associated samples were also correlated with TP. The availability of DO was associated with samples taken in the spring, when DO was highest and the lake was fully mixed and slowly began to stratify [58]. High values for Chl a and Schmidt stability defined the bacterial community in the summer, when the lake was unusually stable and phytoplankton biomass reached a relatively high peak [58].…”

“…These events consisted of two named storm events (high wind speed and precipitation), six high discharge events, and one drought period. The year 2018 was also extreme for Lough Feeagh due to a heatwave [63] and increased Schmidt stability compared to earlier years [58]. Previously, Jones et al (2008) [14] found that the bacterial community in a lake in Taiwan was surprisingly resilient to disturbances caused by typhoons.…”

“…Extreme weather events and mixing events have been found to trigger shifts in lake processes, including biological processes [7,65]. Calderó et al (2020) [58] also described a cascade effect observed after Storm Hector in Lough Feeagh in 2018 and it is likely that the free-living bacterial population was impacted as well. Following Storm Hector, the second half of the drought period continued until July 14 and was characterized by the highest temperatures of the study period, up to 29.8 • C, and high stability.…”