Climate-driven limnological changes determine ecological thresholds in an alpine lake

Abstract

The consequences of climate warming may be dramatic in lakes because changes in surface heating can affect physical and biological processes. We examined sedimentary remains of Cladocera and Chironomidae from a climatically sensitive lake in the Austrian Alps to test the hypothesis that changes in surface water temperature cause direct and indirect shifts in species composition. Contrary to the expectation that oxygen availability in lakes generally decreases under climate warming, our records showed that the assemblages experienced a succession towards a state with increased oxygen availability. Among the benthic fauna, an ecological threshold was crossed around the year 1850, when taxa tolerant of low oxygen concentrations disappeared and oxyphilous taxa began to dominate. In the zooplankton community, the keystone grazer Daphnia was replaced by Bosmina. These ecological changes, especially during the past 20 yr, were most likely caused by improved oxygen conditions following the decrease in depth of the thermocline. As the faunal succession was strongly correlated with alpine air temperature trends, the change in thermocline depth has likely been driven by the increasing air temperatures, which have caused warming of the epilimnion and consequently, in combination with wind-induced mixing, deepening of the thermocline. These recent changes occur alongside further increasing alpine temperatures and may be reflected in further deepening of the thermocline. We conclude that, in addition to direct climate influences, changes in summer stratification and mixing depth may cause significant changes in aquatic community compositions through changes in oxygen availability under the present climate warming conditions.

Long-term water temperature reconstructions from mountain lakes with different catchment and morphometric features

“…Contrary to the general expectation that limnoecological conditions in lakes generally decrease under climate warming2021, there are indications that invertebrate communities in Twenger Almsee have experienced a succession towards a state of increased oxygen availability13. Apparently, there has been a change in the lake's thermal structure, as summer mixing depth has deepened, causing a crossing of an ecological threshold.…”

“…The cold summertime epilimnetic water temperatures, which hardly exceed 10°C1112, make Moaralmsee less stratified than lakes with warmer summertime temperatures, such as Twenger Almsee. Contrary to Moaralmsee, Twenger Almsee and Oberer Landschitzsee receive their water input mostly through precipitation and direct surface runoff, while the share of groundwater input is relatively small1314. Oberer Landschitzsee was thermally stratified between ~1850 and 1950 AD, but it has apparently become a polymictic basin during the past decades14.…”

Solar and atmospheric forcing on mountain lakes

“…2). This is not surprising since the changes in species composition in UGIG have been very subtle and only small changes have occurred in their relative abundances (Luoto and Nevalainen, 2013b), while in TWA a complete faunal turnover has occurred in both cladoceran and chironomid communities (Luoto and Nevalainen, 2013a). The turnover at~1850 CE, which is also seen in the PCoA scores (Fig.…”

“…Fossil assemblages of microcrustaceans (Cladocera) and macrobenthos (Chironomidae) from short sediment cores (17-25 cm) of four Austrian alpine lakes; Moaralmsee (MOAR; Luoto and Nevalainen, 2012), Oberer Landschitzsee (OBLAN; Nevalainen and Luoto, 2012), Twenger Almsee (TWA; Luoto and Nevalainen, 2013a) and Unterer Giglachsee (UGIG; Luoto and Nevalainen, 2013b) were used in the present study. The sediment profiles, including the chronologies (Suppl.…”

Impacts of Climate Warming on Alpine Lake Biota Over the Past Decade

“…Among the most abundant taxa in these lakes are the profundal Tanytarsus group lugens, Micropsectra radialis and the common and abundant Chironomini genus Sergentia, all inhabitants of mountain lakes in Alpine Central Europe (e.g., Lotter et al, 1997;Luoto and Nevalainen, 2013). The high abundance of Paracladius in two of the lakes (TWA and GIG) is noteworthy, as it has rarely been recorded (and usually at low abundances) in European mountain lakes.…”