Thermocline deepening boosts ecosystem metabolism: evidence from a large‐scale lake enclosure experiment simulating a summer storm

Abstract

Extreme weather events can pervasively influence ecosystems. Observations in lakes indicate that severe storms in particular can have pronounced ecosystem-scale consequences, but the underlying mechanisms have not been rigorously assessed in experiments. One major effect of storms on lakes is the redistribution of mineral resources and plankton communities as a result of abrupt thermocline deepening. We aimed at elucidating the importance of this effect by mimicking in replicated large enclosures (each 9 m in diameter, ca. 20 m deep, ca. 1300 m in volume) a mixing event caused by a severe natural storm that was previously observed in a deep clear-water lake. Metabolic rates were derived from diel changes in vertical profiles of dissolved oxygen concentrations using a Bayesian modelling approach, based on high-frequency measurements. Experimental thermocline deepening stimulated daily gross primary production (GPP) in surface waters by an average of 63% for >4 weeks even though thermal stratification re-established within 5 days. Ecosystem respiration (ER) was tightly coupled to GPP, exceeding that in control enclosures by 53% over the same period. As GPP responded more strongly than ER, net ecosystem productivity (NEP) of the entire water column was also increased. These protracted increases in ecosystem metabolism and autotrophy were driven by a proliferation of inedible filamentous cyanobacteria released from light and nutrient limitation after they were entrained from below the thermocline into the surface water. Thus, thermocline deepening by a single severe storm can induce prolonged responses of lake ecosystem metabolism independent of other storm-induced effects, such as inputs of terrestrial materials by increased catchment run-off. This highlights that future shifts in frequency, severity or timing of storms are an important component of climate change, whose impacts on lake thermal structure will superimpose upon climate trends to influence algal dynamics and organic matter cycling in clear-water lakes.

A fast and sensitive method for the continuous in situ determination of dissolved methane and its δ13C‐isotope ratio in surface waters

“…Temporal variations in CH 4 concentration and δ 13 C‐CH 4 values at Lake Stechlin might therefore be either controlled by local methane production/oxidation or physical variations in the thermocline. Turbulence and internal seiching is a common phenomenon in stratified lakes and reported for Lake Stechlin (Kirillin and Engelhardt ; Kirillin et al ; Giling et al ). Lake hydrological dynamics dominated by internal seiches may have partly caused upwelling of colder deep water with lower CH 4 concentration and more negative δ 13 C‐CH 4 ( see Fig.…”

A Small Temperate Lake in the 21st Century: Dynamics of Water Temperature, Ice Phenology, Dissolved Oxygen, and Chlorophyll a

“…In deep waters of Harp Lake, because water temperature is predicted to be stable and net primary production is negligible (Figures and ), the simulated increase of DO depletion is mainly caused by the decline of water diffusivity and mixing and the earlier occurrence of water overturning. The slow recovery of deepwater DO in Harp Lake under RCP2.6 can be explained by the weak mass exchange between surface and deep waters in this deep stratified lake, as well as the increase of heterotrophic respiration due to thermocline deepening (Giling et al, ). Importantly, under climate warming, the processes described above are expected to drive the similar dynamics of DO in other oligotrophic temperate lakes with strong summer stratification because rapid surface water warming, increased ecosystem respiration, and weakened water convection may also occur there (Fang & Stefan, ; Yankova et al, ; Zwart et al, ).…”

“…To date, our understanding of the future dynamics of these state variables in small temperate lakes is still limited (Couture et al, ; Fang & Stefan, ; Obertegger et al, ; Oliver et al, ; Yao et al, ). First, although the processes that control the dynamics of water temperature, ice phenology, DO, and chlorophyll a are closely interacted (Couture et al, ; Giling et al, ; Obertegger et al, ), the modeling studies for small temperate lakes usually only focused on one or two of these variables (Vincent et al, ; Yao et al, ) and few modeling studies have predicted the chlorophyll a dynamics (Fang & Stefan, ; Joehnk & Umlauf, ; Yao et al, ). Second, the difference of DO dynamics in surface and deep waters was less explored and the impacts of catchments on the future changes of lakes were sometimes ignored (Fang & Stefan, ).…”

Storm impacts on alpine lakes: Antecedent weather conditions matter more than the event intensity

“…increase in lake surface water temperature (Austin & Colman, 2007;O'reilly, et al, 2015;Woolway et al, 2017), shorter ice cover (Magnuson et al, 2000), longer stratification length (Kraemer et al, 2015), warming-induced shift in biological assemblages (Yvon-Durocher, Montoya, Trimmer, & Woodward, 2011) or abundances (Kraemer, Mehner, & Adrian, 2017). The contribution of extreme events to the realized and projected modifications to lakes under climate change are still poorly considered (Jones, Kratz, Chiu, & Mcmahon, 2009) despite a growing body of evidence of the disproportional role of episodic meteorological disturbances and fluctuations on the physics, biogeochemistry and ecology of lakes (de Eyto et al, 2016;Giling, Nejstgaard, et al, 2017;Jennings et al, 2012;Kasprzak et al, 2017;Klug et al, 2012).…”