Sub-fossil chironomids as indicators of hydrological changes in the shallow and high-altitude lake Shen Co, Tibetan Plateau, over the past two centuries

Abstract

Understanding climate and monsoonal dynamics on the Tibetan Plateau is crucial, as recent hydrological changes, evidenced by rising lake levels, will be accelerated by current global warming and may alter aquatic habitats and species inventories. This study combines chironomid assemblages with sedimentological, mineralogical and geochemical data of a short sediment core (37.5 cm) from the high-altitude (> 4,733 m asl), saline (9 g L-1) and shallow (~5 m water depth) Shen Co, located in the southern part of the central Tibetan Plateau. The predominantly littoral, species-poor (10 chironomid morphotypes) chironomid assemblages are dominated by salt-tolerant taxa, that are highly sensitive to lake level fluctuations and macrophyte vegetation dynamics, making them ideally suited for tracking lake level changes over time. Results indicate a period (from ca. 1830 to 1921 CE) of drier conditions with low runoff and high evaporation rates in the Shen Co catchment, as indicated by a dominance of low-Mg calcite and dolomite and increased Ca/Fe and Sr/Rb ratios. This resulted in a decline in lake levels, an increase in salinity and the periodic occurrence of desiccation events at the sampling site. The first chironomid morphotype to appear after the dry period is Acricotopus indet. morphotype incurvatus, which indicate still low (<2 m) but rising lake levels after 1921 CE due to increasing runoff and a lower evaporation/precipitation ratio, as reflected by coarser grain size, higher quartz content and increased TN, TOC and Al/Si ratios. A replacement of A. indet. morphotype incurvatus by Procladius is observed as lake level rise continued after 1950 CE. The highest lake level is proposed for the period since 2006 CE. From 1955 to 1960 CE and from 2011 to 2018 CE, the presence of the phytophilic taxon Psectrocladius sordidellus-type supported abundant macrophyte growth. These changes are consistent with climate reconstructions from the northern and central Tibetan Plateau, indicating warmer and wetter climate conditions since the beginning of the 20th century, which have led to an increase in lake level in a number of Tibetan lakes. Our study specifically highlights 1920 and 1950 as years with enhanced precipitation. This can be attributed to the strong, with overlapping multidecadal cycles of Westerlies and monsoon systems. This study demonstrates the significance of studying small, shallow lakes, as they frequently contain aquatic communities that respond more rapidly to the changes in the lake system. In addition, this study expands our understanding of the ecology of Tibetan chironomid morphotypes, highlighting this group’s potential as paleolimnological proxies for investigating past environmental and climatic changes.

Chironomid (Insecta: Chironomidae) community structure response to hydrological changes in the mid‐1950s in lake Nam Co, Tibetan Plateau

“…This trend has been observed in different records such as ice cores, pollen, geochemistry and chironomids from sites relatively close to Nam Co that suggested a cold‐dry climate (e.g. Thompson et al, 1989, 1997; Chen et al, 2009; Pu et al, 2013; Yang B. et al, 2014; Cui et al, 2021; Rigterink et al, 2022).…”

“…(Zhang et al, 2017), where Tanytarsini morphotypes were highly abundant. In contrast, low species richness ( S ) was reported from sediment cores from Sugan Lake ( S = 5 non‐rare taxa) (Chen et al, 2009) and Heihai ( S = 17 non‐rare taxa) (Chang et al, 2017), northern TP, Shen Co, a shallow lake ( S = 7 non‐rare taxa) about 9 km north of Nam Co (Rigterink et al, 2022), and Son Kol, Kyrgyzstan ( S = 10 non‐rare taxa) (Laug et al, 2020). In Lake Tiancai, southwestern China, the richness of the most abundant chironomid taxon ( S = 41) was the highest reported from a sediment core collected in high‐altitude Central Asia (Zhang et al, 2017).…”

“…However, in the multiproxy analysis presented here, we have detected considerable changes since 1956 ce . In Shen Co, a shallow lake located about 9 km north of Nam Co, chironomid composition as well as mineralogical and geochemical data from a short sediment core highlighted that after 1950 ce the water level rose (Rigterink et al, 2022). The increase in precipitation was attributed to a strong overlap of moisture supply from Westerly winds and the monsoon system (Rigterink et al, 2022).…”

“…In Shen Co, a shallow lake located about 9 km north of Nam Co, chironomid composition as well as mineralogical and geochemical data from a short sediment core highlighted that after 1950 ce the water level rose (Rigterink et al, 2022). The increase in precipitation was attributed to a strong overlap of moisture supply from Westerly winds and the monsoon system (Rigterink et al, 2022). The climate data recorded from Qiantang, a meteorological station on the southern TP, show increasing trends of precipitation from 1961 to 2005 ce (Lu and Liu, 2010).…”

“…Most of these studies focused on modern training sets to develop transfer functions with the goal to infer mean summer temperatures. Only one study has addressed hydrological changes in a small, shallow lake (Rigterink et al, 2022). None of these studies, however, has considered the consequences that hydrological changes may have had for chironomid communities and lake productivity on the south‐central TP in recent years.…”