Ecosystem shifts at two mid-Holocene tipping points in the alpine Lake Son Kol (Kyrgyzstan, Central Asia)

Tipping points can be defined as critical ecosystem thresholds that start self-enforced dynamics pushing systems into new stable states. Many lake ecosystems of arid Central Asia are sensitive to hydrological changes as they are located at the intersection of the influence of the dry Siberian Anticyclone and the relatively humid mid-latitude Westerlies, and their sediment records can be used to study past tipping points. We studied subfossil chironomid remains preserved in a ca. 6000-year-long sediment record from the Central Asian lake Son Kol (Central Kyrgyzstan) to reconstruct past ecosystem dynamics. Our results show abrupt transitions from a chironomid fauna dominated by macrophyte-associated, salinity-indicating taxa, to a vegetation-independent fauna, and subsequently to a macrophyte-associated, freshwater-indicating fauna. A comparison of the chironomid-based environmental reconstruction to other proxy indicators from the same record suggests a phase of increased Westerly strength starting about 4900 cal. yr BP. This increase led to enhanced precipitation and sediment fluxes into the lake, which in turn led to high turbidity levels and consequently to a macrophyte collapse causing abrupt changes in the chironomid fauna. At 4300 cal. yr BP, a weakening of the Westerlies in combination with higher lake levels led to lower turbidity and ultimately to the recovery of the macrophyte population and associated changes in the chironomid assemblage. These two sequences of events show how the occurrence of a gradual change in an external trigger (Westerlies) can trigger a cascade of within-lake processes (turbidity, macrophyte density) and may ultimately lead to an abrupt reorganization of the ecosystem (chironomid fauna), providing models for tipping points.

Distribution of subfossil chironomids (Diptera, Chironomidae) along a water depth gradient in the shallow Lake Spore, northern Poland

Subfossil chironomid (Diptera, Chironomidae) remains are often used as indicators of lake level changes in palaeolimnological studies. However, their usefulness as a water depth proxy can vary between the sites, depending on the lake morphology, mode of taphonomic processes or amplitude of past water level fluctuations, among other factors. In this study, we have examined the distribution of subfossil chironomids in the shallow Lake Spore (northern Poland) to assess the influence of water depth on the fauna. Our aim was to evaluate the site-specific utility of subfossil chironomids for lake level reconstruction at Lake Spore. The subfossil chironomid assemblages in Lake Spore have heterogeneous distribution, suggesting they are predominately composed of remains deposited close to the sampling location. A strong relationship between the water depth and the chironomids is marked by the 25.12% variance explained by water depth in the taxonomic data. Moreover, according to generalized linear models (GLMs) out of 44 dominant taxa, 12 have significant relationships with water depth. However, the sensitivity of our chironomid fauna to water depth changes is not continuous along the entire depth gradient. The most abrupt assemblage change occurs at 2.6–3.7 m water depth, in proximity to the depth where macrophytes become less dense and finally disappear. We conclude that, despite these strong chironomid-water depth relationships, only major water level fluctuations can be satisfactorily reconstructed due to the limited turnover rates of the fauna along a depth gradient and relatively small amplitude of the lake level variations characteristic for East-Central Europe.

Summer temperature variability inferred from subfossil chironomid assemblages from the south-east margin of the Qinghai–Tibetan Plateau for the last 5000 years

We present a quantitative mean July temperature record spanning the last c. 5000 years from an alpine lake in south-western China. The reconstruction is based on the application of an established chironomid-based inference model using 100 lakes from the region. The reconstructed summer temperature changes are within 2.4°C of modern throughout the record. The results suggest that the summer temperature changes in south-east margin of the Qinghai–Tibetan plateau (QTP) predominantly responds to Asian Summer Monsoon influence, forced by summer insolation until c. 3200 cal. BP. Four cooling events, each separated by c. 500 years (between 3200 and 1600 cal. BP), were observed and these may correspond to the 500-year quasi-periodic solar fluctuation. The most recent cooling period, that is, ‘the Little Ice Age’, appears robust in the Heihai Lake record, providing further evidence that a hemisphere-wide forcing mechanism is possible for this climate event.

Chironomid communities as indicators of local and global changes in an oligotrophic high mountain lake (Enol Lake, Northwestern Spain)

<p>The benthos of the high mountain Enol Lake (Picos de Europa National Park, Spain) was analyzed in order to understand the spatiotemporal factors and patterns controlling its current Chironomidae community. In total, more than 14,000 chironomid larvae were identified, belonging to 27 taxa. The results have pointed out the presence of 3 main chironomid assemblages in the lake: i) littoral community, which is mainly controlled by temperature and oxygen seasonal changes, ii) <em>Chara</em>-dominant community, which is mainly controlled by the presence and abundance of Characeae in the lake, and iii) profundal community, which is affected by low oxygen levels caused by sediment and organic matter discharge to the lake due to human pressures in the lake basin. We provide valuable insights for the managers to understand the current ecological status of Enol Lake and to evaluate which measures should be implemented to preserve or improve it. Moreover, our results constitute an essential step forward to improve the interpretation of the past changes of the lake by means of the subfossil chironomid community. </p>