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 sedimantation 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.

Biomass and structure of planktonic communities along an air temperature gradient in subarctic Sweden

“…We found no indication in our study that water temperature was a strong regulator of plankton community structure, not even for the diversity of the phytoplankton as expressed by the Simpson index (Table 3). In that respect, our result is similar to the outcome of experimental temperature manipulation of an unproductive Norwegian lake (Lydersen et al. , 2008).…”

Effects of stratification depth and dissolved organic matter on brackish bacterioplankton communities

“…The observed differences in primary production are not unexpected, as incubations to assess primary production were carried out at variable depths to directly account for the differences in average effective light climate experienced by phytoplankton in mesocosms with shallow and deep stratification depths, respectively. These effects of variable stratification depth are similar to the findings of Lydersen et al (2008), although their study did not target bacteria. In the latter study, manipulation of the thermocline in Lake Breisjøen resulted in few and small changes in biota.…”

Thermocline deepening and mixing alter zooplankton phenology, biomass and body size in a whole‐lake experiment

“…In conjunction with conclusions from other recent studies based on time series observations, mesocosm experiments and other types of whole‐lake manipulations, results point to important effects of climate change on the zooplankton through changes to phenology, species composition and overall biomass (e.g. Adrian et al ., ; Lydersen et al ., ; Berger et al ., ; Cantin et al ., ). In addition to a switch from large cladocerans to smaller species, including copepods, our study demonstrates a clear reduction in overall mean body size.…”

“…Recent evidence indicates that the features of summer thermal stratification in temperate lakes can have consequences for planktonic community structure (Lydersen et al ., ; Berger et al ., ; Cantin et al ., ). In particular, a change in thermocline depth is a likely outcome of climate change in north temperate regions through a number of direct and indirect mechanisms (Schindler, ).…”