Ecosystem shift of a mountain lake under climate and human pressure: A move out from the safe operating space

Abstract

"A multiproxy approach including chironomid, diatom, pollen and geochemical analyses was applied on shortgravitational cores retrieved from an alpine lake (Lacul Bâlea) in the Southern Carpathians (Romania) to unveilhow this lakeresponded to natural and anthropogenic forcing over the past 500 years. On the basisof chironomidand diatom assemblage changes, and supported by sediment chemical data and historical information, we distin-guished two main phases in lake evolution. Before 1926 the lake was dominated by chironomids belonging toMicropsectra insignilobus-type and benthic diatoms suggesting well-oxygenated oligotrophic environment withonly small-scale disturbance. We considered this state as the lake's safe operational space. After 1926 significantchanges occurred:Tanytarsus lugens-type andT. mendax-type chironomids took over dominance and collectorfilterers increased until 1970 pointing to an increase in available nutrients. The diatom community showed themost pronounced change between 1950 and 1992 when planktonic diatoms increased. The highest trophic levelwas reconstructed between 1970 and 1992, while the indicator species of increasing nutrient availability,Asterionella formosaspread from 1982 and decreased rapidly at 1992. Statistical analyses evidenced that themain driver of thediatomcommunitychange wasatmospheric reactivenitrogen (Nr)fertilization that drasticallymoved the community towards planktonic diatom dominance from 1950. The transformation of the chironomidcommunitywas primarily driven by summermeantemperature increase that alsochanged the dominant feedingguild from collector gatherers to collectorfilterers. Our results overall suggest that the speed of ecosystemreorganisation showed an unprecedented increase over the last 100 years"," biological systems in many casesunderwent threshold type changes, while several system components displayed non-hysteretic change betweenalternating community composition. We conclude that Lake Bâlea is outside of its safe operating space today. Themain trigger of changes since 1926 was climate change and human impact acting synergically."