Effects of thermocline deepening on lake plankton communities

Abstract

Theory predicts, and some evidence demonstrates that in lakes, the depth of the thermocline can have a large structural influence on the spatial distribution, and strongly influences the composition of plankton communities. However, experimental assessments of responses of the planktonic food web to thermocline depth have not yet been done at the whole-basin scale. We conducted an experiment wherein we artificially lowered the thermocline in an isolated basin of a three-basin lake, maintaining another isolated basin as a control. The vertical distribution and taxonomic composition of both phytoplankton and zooplankton were monitored throughout the summer months. Greater phytoplankton production, especially in the epilimnion, attributable mainly to increases in the chlorophytes was observed with thermocline deepening, but at the deepest thermoclines, production was limited. Total zooplankton biomass was unaffected by thermocline depth, suggesting top-down control by predators. Zooplankton biomass peaks were less pronounced in the manipulated basin, but tended to follow the thermocline whether at its normal position or as it was deepened. Zooplankton composition was significantly altered by large increases in densities of predatory cyclopoid copepods and rotifers; taxa commonly found in turbulent environments. Overall, both phytoplankton and zooplankton communities demonstrated important shifts in structure and composition in response to thermocline deepening.

Zooplankton in Ancient and Oligotrophic Lake Ohrid (Europe) in Association with Environmental Variables

“…The abundance of zooplankton in the metalimnion could be argued from several points of view. The peculiar environmental conditions in this transitional layer have also been observed in other studies (Cantin et al, 2011;Karpowicz and Ejsmont-Karabin, 2017). For instance, the non-significant difference in vertical distribution of the nutrients was in compliance with the orthograde regime in oligotrophic lakes.…”

Thermal structure and response to long‐term climatic changes in Lake Qiandaohu, a deep subtropical reservoir in China

“…Thermal structure and stratification in lake ecosystems are physical features that exert important controls on inlake vertical fluxes of dissolved and particulate material (Aeschbach-Hertig et al 2007), and on lake ecosystem structure and function (O'Reilly et al 2003;Kaiblinger et al 2007;Cantin et al 2011). Stratification is facilitated by the thermal expansion properties of water, which create a stable vertical density gradient, resulting from heating (or cooling if below 3.98uC) of surface waters.…”

Biomass and productivity responses of zooplankton communities to experimental thermocline deepening

“…Indeed, the thermal structure of north temperate lakes has been demonstrated to be sensitive to variation in temperature and subtle changes in water clarity (Snucins and Gunn 2000). Several studies have explored the response of plankton communities to the first thermocline deepening scenario as part of whole-lake manipulations in which the thermocline was mechanically lowered (Lydersen et al 2008;Forsius et al 2010;Cantin et al 2011) or the water column partially or completely destratified (Heo and Kim 2004;Berger et al 2007). To date, however, there have been no basin-scale manipulations testing the response of planktonic communities to a depressed thermocline in the absence of active mixing (Scenario 2).…”

“…An efficient transfer of energy from primary producers to metazoan zooplankton can be linked to higher productivity of young-of-year fish in particular (Mehner and Thiel 1999). Several lines of evidence suggest that this central role of zooplankton communities will be disrupted by changes in the thermal structure of the water column because of changes in their phenology and community composition (Thackeray et al 2010;Cantin et al 2011). Thus, it is reasonable to assume that these responses will be accompanied by changes in food web processes.…”

“…In recognition of the need to clarify our understanding of how planktonic communities respond to variable thermocline depths, several studies have manipulated stratification and/or thermocline depth in mesocosms (Berger et al 2007) and whole-lake experiments (Heo and Kim 2004;Forsius et al 2010;Cantin et al 2011) using a variety of approaches that include aeration and turbulent mixing and water displacement using propeller-type devices. For those studies reporting on zooplankton, both positive and negative effects on total zooplankton biomass have been observed.…”