Ecosystem effects of thermal manipulation of a whole lake, Lake Breisjøen, southern Norway (THERMOS project)

Abstract

Abstract. We conducted a 3-year artificial deepening of the thermocline in the dimictic Lake Breisjøen, southern Norway, by means of a large submerged propeller. An adjacent lake served as untreated reference. The manipulation increased thermocline depth from 6 to 20 m, caused a significant increase in the heat content, and delayed ice-on by about 20 days.There were only minor changes in water chemistry. Concentrations of sulphate declined, perhaps due to greater reduction of sulphate at the sediment-water interface. Concentrations of particulate carbon and nitrogen decreased, perhaps due to increased sedimentation velocity. Water transparency increased. There was no significant change in concentration of phosphorus, the growth-limiting nutrient.There were few significant changes in principal biological components. Phytoplankton biomass and productivity did not change, although the chlorophyll-a concentration showed a small decrease. Phytoplankton species richness increased, and the species composition shifted. Growth of periphyton increased. There was no change in the macrophyte community. The manipulation did not affect the zooplankton biodiversity, but caused a significant shift in the relative abundance (measured as biomass) in the two major copepod species. The manipulation did not affect the individual density, but appeared to have changed the vertical distribution of zoobenthos. Fish populations were not affected.The lake is oligotrophic and clearwater and the manipulation did not change the supply of phosphorus, and thus there were only minor changes in lake chemistry and biology. Effects might be larger in eutrophic and dystrophic lakes in which internal processes are stronger.

Physical and chemical consequences of artificially deepened thermocline in a small humic lake – a paired whole-lake climate change experiment

“…Increases in DOC concentrations have been observed in surface waters over large areas in northern Europe and North America, and processes related to both climate and deposition changes have been suggested as likely causes (Tranvik et al, 2002;Vuorenmaa et al, 2006;Monteith et al, 2007). Using the dynamic model INCA-C, Futter et al (2009) were able to simulate present day (1990)(1991)(1992)(1993)(1994)(1995)(1996)(1997)(1998)(1999)(2000)(2001)(2002)(2003)(2004)(2005)(2006)(2007) increasing trends in DOC in the lake and catchment outflow of Valkea-Kotinen as functions of observed climate and modelled SO 4 deposition.…”

Challenges and opportunities for integrating lake ecosystem modelling approaches

“…In particular, incorporation of ice and snow dynamics based on physical processes (Leppäranta 1993;Saloranta 2000;Salonen et al 2009) gives the model code additional utility for boreal lakes. MyLake has been applied to lakes in Norway (Lydersen et al 2003;Saloranta 2006) and Finland (Kankaala et al 2006;Saloranta et al 2009). …”

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).…”