Introduction

The aim of this special issue is to present recent developments in the fields of paleolimnology and paleoecology that cover rapidly changing regions of Asia. We believe these papers are of broad global interest. Asia has a wide range of climate types and inland waters, and it is home to more than four billion people. Across this vast land mass, climate change (e.g., regional warming and extreme events) and atmospheric processes (e.g., nitrogen deposition) have altered the cycling of surface waters and biogeochemistry. With accelerating societal and catchment developments, however, inland waters in many parts of Asia have been increasingly affected, with shrinking aquatic resources, deteriorating water quality and decreasing ecosystem services (Pekel et al. 2016). Because of a general lack of long-term and systematic monitoring for the majority of Asian lakes, sediment records are particularly important for documenting environmental changes and ecosystem shifts that occurred in inland waters, and this information is critical for sustaining socio-economic development (Smol 2008, 2019; Sutherland et al. 2013).

With the research fields of paleolimnology and paleoecology expanding in many parts of Asia over the last decade (Yang et al. 2020), this special issue aims to highlight some of the latest results of sediment studies from Asia. Papers in this issue have a special focus on the temporal patterns and drivers of change in aquatic environments and their ecological components (e.g., communities, biodiversity and stability), as highlighted by Davidson et al. (2018). New analytical methods and cutting-edge ecological perspectives are increasingly being applied to sediment records within time scales of the Holocene (including the “Anthropocene”), as presented in this collected work.

Intriguingly, one of the co-editors of this issue (John P. Smol) notified the lead editor (Guangjie Chen) in August of 2017 that the first use of our cherished term Paleolimnology (aka. Palaeolimnology) may have appeared in a Chinese journal. With the help of Drs. Yuanyuan Liu and Rui Li, a photocopy of this paper entitled “Paleontological Terminology”, which was published in 1936 with a focus on a Chinese translation of English and German terminology (Zhang 1936), was obtained from the National Library of China in Beijing, and it enabled us to verify one of the first uses of this word. That said, we are honored to present this collection to highlight the fast-paced development of paleolimnological studies from this part of the world.

Special issue papers

A total of 10 papers is presented in this special issue. The papers cover multiple themes, including process-based proxy interpretations, sediment and modeling applications, sedimentary signatures of ecological shifts and human traces, as well as long-term lake development and regional changes in the East Asia monsoon climate. Geographically, these case studies include a variety of lakes from coastal, lowland, alpine, plateau and arid regions in East Asia.

Integrating processes with proxy interpretation and sediment applications

Li et al. (2022) compared the diatom assemblages and diversity indices of 95 surface sediment samples covering three different basin locations (i.e., deep, open-water zone; gently sloping shallow zone; and steeply sloping shallow zone) to examine diatom distributions along a water-depth gradient of ~ 3–20 m in a tropical maar lake from southern China. On a spatial scale, there existed a threshold depth of ~ 8 m for community structuring of diatoms, and the dominant taxa in surface samples from the water depths of < 8 m were dominated by epiphytic and episammic species, as was also found in a separate collection of lakeshore samples. However, the spatial changes in diatom diversity index values showed an inconsistent pattern along the gradient of water depth and among basin locations. Specifically, there existed significantly higher diatom diversity in the shallow-water zone with steep slopes when compared to the other two zones, suggesting that increased sediment movement and water action may enhance the re-deposition and accumulation of diatom taxa from different sources and thereby influence the diversity index. An implication of this study is that basin topography and lake bathymetry need to be considered in choosing coring sites for a reliable reconstruction of diatom diversity changes.

Lipid biomarkers have been applied increasingly to track lake changes linked to climatic and anthropogenic forcing. Pu and Meyers (2022) expanded on previous studies to test the use of a stanol (5α-stigmastanol) biomarker, commonly found in soils and sediments, for tracking human influences (e.g., mammalian feces) on a deep periglacial lake on the Tibetan Plateau. Over the last millennium, a rapid increase in the relative abundance of 5α-stigmastanol was consistent with a documented replacement of nomadic pastoralism by permanent settlement ca. 1700 CE. Thereafter, there was a gradual increase in the sediment concentration of this biomarker, which showed a temporal trajectory closely following those of the archived numbers of human inhabitants and livestock, as well as proxy records of anthropogenic influences from this region. Overall, the authors argued that the 5α-stigmastanol biomarker identified from the soil in the watershed and sediment is most plausibly a result of microbial activity linked to human population increases and animal husbandry in this study region.

With regard to the policy-making challenges of nutrient-loading reduction and climate-change mitigation for effective lake restoration, Qin et al. (2022) combined ecosystem modeling with paleolimnology to improve the certainty in predicting lake recovery time and ecological status for a poorly monitored shallow lake in eastern China. Both the default and modified models of PCLake (a process-based ecological model of shallow non-stratified lakes) were applied in this study, the latter of which was calibrated with seven parameters based on the diatom-inferred total phosphorus (TP) for the period 1983–1998. When both models were further validated with TP reconstructions for the period 1999–2018, model comparison results showed that the calibrated model performed better in fitting paleo-records with observed data. Furthermore, the simulated time series of lake changes with model output for five key variables during 1951–2019 enabled the authors to detect a significant shift in lake regime, with changes to a phytoplankton-dominated turbid state after ca. 1980. Under future scenarios of varying temperature (− 2 °C, current and + 2 °C) and nutrient loadings (reduction by 5%, 15% and 25%), the modified PCLake model results suggest that a reduction of nutrient loading by 25% can lead to a return of the lake regime to a clear state within 10 years under all temperature conditions, and that higher temperatures can decrease the nutrient reduction efficiencies. Meanwhile, the critical nutrient loading for lake restoration will be lowered under higher temperature scenarios, which can thus require a reconsideration of nutrient reduction targets. Overall, this study provides a novel perspective on predicting lake recovery time and ecological consequences through integrating paleolimnological data with ecosystem models.

Identifying anthropogenic signals and ecosystem shifts from lake sediments

Hu et al. (2022) examined cladoceran remains from a short core of a large, dimictic temperate lake from Xingjiang, northwestern China, over the last century. This lake has experienced multiple anthropogenic stressors, such as eutrophication, salinization, fish farming and river diversion in the last few decades. During the undisturbed, freshwater phase prior to ca. 1957, there was a relatively low level of cladoceran abundance and high species richness with a dominance of littoral taxa. With increasing salinity due to an irrigation-induced water-level drop in this arid area, salinity-tolerant planktonic species such as Bosmina longirostris increased significantly at the expense of littoral taxa. After ca. 1986, the increasing dominance of these small bosminids may have been enhanced by both lake salinization and eutrophication, as well as by an increasing yield of mostly introduced fish whose planktivorous feeding might favor the growth of small-sized bosminids. Meanwhile, there was a continuous decrease in the relative abundance of littoral taxa (e.g., ~ 5% at core top) over the last few decades, corresponding with a substantial loss of macrophyte coverage, which could be partly linked to lake salinization. Overall, this study provides an example of aquatic ecosystem shifts under the direct influence of anthropogenic disturbances over the last few decades in this arid region of Asia.

To tease apart the relative roles of climatic and anthropogenic forcing in driving lake ecological changes in climate-sensitive regions, Yan et al. (2022) focused on sediment signals of algal pigments, diatoms and stable isotopes from a semi-arid alpine lake within the margin of the East Asian Summer Monsoon (EASM) on the southwestern Loess Plateau of North China. Over the last two centuries, geochemical and biological proxies indicated a relatively stable lake environment despite documented deforestation and land-use expansion in this study region prior to ca. 1965. However, there was an abrupt shift towards a turbid lake state, as indicated by a spike in the generalist diatom Achnanthidium minutissimum as well as an increase in sediment grain size and SiO2 during ca. 1965–1980. This may reflect an interval of high rainfall and excessive soil erosion when the Palmer Drought Severity Index (PDSI) values were among the highest during the instrumental period of 1951–2008 in this region. Since ca. 1980, the largest diatom compositional shift was evidenced by the dominance of several planktonic taxa, possibly as a result of increased ice-free duration and enhanced thermal stability when a warmer climate and a weakening of the EASM were observed. This ecological shift occurred earlier than the clear declining trend of the sediment N isotopic signal from the 1990s, suggesting a limited impact (if any) of atmospheric N deposition. Therefore, the changes in regional climate and EASM intensity likely played a major role in driving ecological shifts in this small alpine lake despite the intensification of human influences over the last two centuries.

As the stratigraphic boundary between the Holocene and Anthropocene is still highly debated, partly due to regional heterogeneity in the history of human activities and catchment development, Yang et al. (2022) examined the sediment signals of metal pollution and soil erosion for a subtropical lake from the plateau region in southwestern China. This region is known for a long history of metallurgy, which can be dated back to the Neolithic and Bronze Ages. The sediment records revealed that both the concentration and anthropogenic Enrichment Factor (EF) of Pb showed a slow increase from ca. 950 CE before reaching a peak level at ca. 1450 CE, as well as an accelerating increase after ca. 1950 CE. In comparison, synchronous changes in the concentrations of Ti and several other metals (e.g., Cr, Cu and Fe), coeval with changes in magnetic susceptibility (MS), showed the dominant impact of soil erosion on the temporal variation of these terrigenous elements. The timing of the earlier increase in atmospheric Pb transport was consistent with regional industrial activities (e.g., silver smelting and ceramics production) during local kingdoms and Mongol occupation (i.e., 700–1450 CE). This increase predated the intensification of soil erosion by centuries, as was also found in neighboring lakes of this study region. Therefore, the anthropogenic signal, as inferred by sediment trace elements, was not only significant since the middle of the twentieth century, but it also displayed a 1000-year history of increasing catchment disturbances linked with regional industrial activities that could be considered in the boundary definition for the Anthropocene epoch.

Evaluating long-term climate signals and lake development in monsoon regions

In the tropical regions of East Asia, typhoons (i.e., tropical cyclones) can trigger catastrophic landslides and flooding, and their occurrence and intensity can leave identifiable sediment signals that can also reflect the strength of the East Asia summer monsoon (EASM). Wang et al. (2022) conducted sediment analyses for two lakes (i.e., one lowland, medium-sized lake and one alpine, small pond) in East Taiwan, with a focus on paleo-typhoon records over the last three millennia. Significant changes in sediment lithology and multi-proxy records suggested that the formation of this lowland lake was induced by landslides ca. 2850 cal. BP. After ca. 1600 cal. BP, the changes in both diatom and pollen assemblages indicated increased precipitation and lake levels as a result of a strengthened EASM. This might have caused a decrease in pH, as inferred by diatom assemblages in the alpine pond, through increased catchment runoff and acid export. Both lakes recorded an increase in typhoon intensity during the Little Ice Age, which was consistent with previous records from northeast Taiwan, but this pattern contrasted with an increased typhoon intensity often found during warmer periods (e.g., MWP) in northern coastal East Asia. It was hypothesized that anomalies in both global temperature and the El Nino-Southern Oscillation may have led to the asymmetric pattern of typhoon intensity in coastal areas of East Asia over the last millennium.

Because the warming-enhanced strength and duration of lake stratification can favor the growth of small-celled planktonic diatoms, Luo et al. (2022) were able to track a millennium-scale history of variations in the duration of ice-free seasons in a temperate maar lake from northeastern China using diatom records. The authors observed differences in diatom assemblages between two warm periods, the medieval climate anomaly (MCA) and the current warming. The summer duration in the warmest part of the MCA was longer than that of the current warm period in this area. They concluded that the summer duration of lake stratification might have been the longest over the last millennium because the relative abundance in the small-sized species of Discostella pseudostelligera reached a maximum during ca. 1020–1060 CE.

Based on studies of the deposits of Baiyangdian paleolake in northern China, Ni et al. (2022) conducted multi-proxy analyses of the lacustrine-fluvial sediments and reconstructed the Holocene history of climate change and lake-level fluctuations. Rising rainfall and lake levels were inferred for the interval ca. 10.0–2.9 kyr BP. The paleo-lake apparently expanded and reached > 4 m higher than the modern lake level during the intervals ca. 8.4 and 6.3–2.9 kyr BP. A general shift towards a drier climate occurred after ca. 2.9 kyr BP. Increasing evidence of human disturbances was indicated by an abrupt increase in sediment magnetic susceptibility, which was consistent with historical documents (e.g., Canal Records of Song Dynasty) over the last millennium. A further comparison with previous paleoclimatic proxy records led the authors to conclude that the EASM strongly impacted the Holocene hydroclimate fluctuations in this region in North China.

Hao et al. (2022) applied multi-proxy micropaleontological analyses (i.e., pollen, spores, freshwater algae and marine dinoflagellate cysts) to sediments from a 16-m borehole (0.9 m a.s.l.) from a coastal wetland of the Bohal Sea, North China. Three climatic stages were identified from the pollen results and from shifts in freshwater and marine indicators of regional sea-level fluctuations. During the early Holocene period (ca. 9910–7440 cal. years BP), a cool and dry climate was inferred from the dominance of steppes and coniferous and broad-leaved mixed forests when there existed lower sea levels and less precipitation in this region. Subsequently, an expansion of the coniferous and broad-leaved mixed forests was observed during the middle Holocene (ca. 7440–3650 cal. years BP), when a marine transgression was indicated by a rapid increase in the sediment concentrations of marine algae. The authors concluded that a relatively warm and moist climate was prevalent, which was in good agreement with the rising trend of relative mean sea level during the Holocene Climate Optimum. During the late Holocene (ca. 3650 cal. years BP to the present), a relatively cool and dry climate occurred with a moderate expansion of mixed forests, and with a slow rise of sea levels. Overall, this study provided evidence of Holocene vegetation development and marine transgressions in the coastal plain of North China.

Progress, problems and perspectives

Collectively, the papers in this special issue are examples of the broad breadth of recent paleolimnological studies being conducted in East Asia. Much progress has been made with evaluating the local patterns, regional heterogeneity and possible drivers of lake development through a combination of multi-proxy analyses, regional comparisons, and statistical applications. However, much work remains because an increasing number of lakes have been affected by the growing impacts of anthropogenic disturbances and a rapidly changing climate.

To date, many studies have inferred drivers and processes of lake changes in a descriptive or semi-quantitative way from sediment sequences, which are often characterized by mixed geochemical signals, incomplete preservation of indicators, and irregular time intervals. While paleo-data often provide evidence of the complex, long-term patterns of changes in the atmosphere, catchment, soil and water, the forcing and interaction of these processes may vary over time and space, possibly due to differences in lake typology, shifts in climate systems and variations in human influences. Within the lake basins, ecological processes from different habitats and trophic levels have rarely been examined or only weakly linked with modern surveys.

We suggest that future work needs to focus more on holistic and process-based perspectives for proxy interpretations and time-series analyses, and for the partitioning of natural and/or anthropogenic processes. Firstly, an assessment of mechanistic links of external forcing with in-lake processes should provide more complete and realistic insights into multiple and interactive processes. Secondly, an integrated analysis with contemporary surveys and controlled experiments can be especially helpful for teasing out in-lake processes that might largely drive long-term patterns, as recorded in lake sediments. Thirdly, using established patterns, processes and theories, specific hypothesis testing can be explored with appropriate site selection and proxy data. Finally, data concerning past lake changes can play a critical role in the evaluation and refinement of lake models, similar to how paleoclimate data have been used in the construction of climate models. With the development of more advanced statistical and simulation methods, there have been emerging efforts to incorporate paleolimnological data into processed-based lake models. Such collaborations should provide more reliable predictions of future ecosystem changes, which would enable policy makers and the public to take effective measures to avoid undesirable lake states and catastrophic shifts.