A palaeoecological study investigating the impacts of multiple tephra depositions on a lacustrine ecosystem in Northeast China, using diatoms as environmental indicators

doi:10.21203/rs.3.rs-2226497/v1

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A palaeoecological study investigating the impacts of multiple tephra depositions on a lacustrine ecosystem in Northeast China, using diatoms as environmental indicators

https://doi.org/10.21203/rs.3.rs-2226497/v1

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published 02 Mar, 2023

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Tephra layers are common in lake sediments and although they have often been used as chronological controls, few studies have investigated the impacts of past tephra depositions on lake ecosystems (Tephropalaeoecology). For the first time we systematically assess how different types of tephra layers vary in their ecological impact on the same lacustrine system. We use a diatom-based tephropalaeoecological approach to infer the impacts of five tephra deposits on Lake Sihailongwan, a well-studied volcanic lake in Northeast China, over the past 30,000 years. The five tephra layers (including two micro-tephras) have varying thicknesses and were deposited in time periods with different climatic conditions. Changes in diatom communities and chrysophyte cyst concentrations between pre- and post-tephra samples were used to infer changes in lake conditions and highlight the importance of lake background conditions in mediating the impact of tephra. While the two micro-tephra layers did not cause observable changes, the three thicker tephras induced pronounced changes in lake conditions and thus diatom communities. The two thick tephras deposited in more eutrophic and warmer lake conditions caused larger responses from diatoms. We argue that water column phosphorous decreased due to reduced sediment-water phosphorous loading as thick tephra layers formed an impermeable layer at the lake bottom. This is supported by a decrease in total diatom concentration and a decline in high phosphorous-requiring taxa such as Discostella stelligeroides and Stephanodiscus minutulus as well as modern limnological observations which showed that groundwater influxes from the lake bottom are the main source of nutrients to the lake. By contrast, the thick tephra deposited in more oligotrophic and colder lake conditions caused less conspicuous changes. When the lake was already low in phosphorous, diatoms did not respond to a further decline in phosphorous but rather responded to the minor increase in silica from the dissolution of tephra particles in the water column. This was inferred from the slight increases in overall diatom concentration and opportunistic taxa such as Cyclotella comensis fo. minima. Diatom analysis of the post-tephra sediments above the three thick tephras showed that the aquatic ecosystem did not completely recover, indicating the long-lasting effects of these thick tephras and shifts to new lake ecosystem equilibria.

Diatoms

Tephra

Palaeoecology

Palaeolimnology

Maar lake

Volcanic eruption impacts

Palaeolimnology as an interdisciplinary science and method has a wide range of applications including the assessment of long-term natural climate variability, anthropogenic pollution and environmental change as well as tracking animal populations (Douglas 2013). The commonality between these is that they deal with changes in the past with information inaccessible by direct measurements or instrumental monitoring (Guilizzoni 2012). The majority of palaeolimnological studies are concerned with longer-term perturbations and very few try to deal with transient environmental disturbances such as volcanic eruptions (Payne and Egan 2019). Palaeolimnology provides a useful tool to investigate past eruptions that can be recorded in lake sediments as tephra layers. Tephra deposits are commonly found in lake sediments around the world and are used frequently as marker layers in sediment cores and when they can be dated, they are the basis for tephrochronology (Lowe 2011), but their impacts on the ecology and functioning of the lakes themselves have often been overlooked (Urrutia et al. 2007). However, we can use the long-term records in lake sediments to identify and reconstruct the impacts of volcanic eruptions. This is valuable for understanding the eruptive history of a region and assessing the response, recovery, equilibrium, sensitivity and adaptability of a lake system to abrupt perturbations (Payne and Egan 2019).

Tephra is unconsolidated pyroclastic material blown into the air by volcanic eruptions (Arnalds2013). Once in the air, tephra can disperse and deposit on different environments proximal or distal to the volcanic source. Tephra depositions in water bodies can alter the chemical and physical properties of aquatic systems, impacting aquatic organisms by tephra-induced changes in nutrients, habitats and living conditions (Ayris and Delmelle2012; Egan2016). In volcanically active regions, tephra depositions can be the dominant drivers of change in lacustrine ecosystems (Michelutti et al. 2015). Physically, tephra can directly blanket and smother habitats and organisms. Tephra particles in lakes can increase water turbidity (Mayr et al. 2019), reducing light penetration and autochthonous photosynthesis (Burwell 2003). The formation of a water-sediment interface barrier in the bottom of lakes can occur (Eastwood et al. 2002), as tephra tends to cement and form an impermeable layer when reacting with water (Egan et al. 2018). This disturbs nutrient exchange between the sediment and the water, particularly reducing the possibility for nutrients to diffuse into the water column from the bottom sediment (Harper et al. 1985; Lotter et al. 1995; Hutchinson et al. 2019). On the other hand, silica (SiO₂) can leach from the tephra particles into the water column (Barker et al. 2000), fertilising siliceous biological production (Hickman and Reasoner 1994; Hardardóttir et al. 2001).

Diatoms are cosmopolitan unicellular algae found in almost every aquatic ecosystem (Jones 2013). As one of the main aquatic primary producers, diatoms are sensitive to changes in their environments (Mackay et al. 2003). This together with their often excellent preservation in sediments arising from their siliceous frustules, make them robust biological indicators for reconstructing past aquatic conditions and tracking ecosystems' responses to perturbations (Lowe and Walker 2015). Accordingly, diatoms are ideal proxies for tracking the impacts of tephra depositions on lacustrine systems. Studies have demonstrated that tephra depositions can induce changes in diatom populations and lake systems in physical and chemical ways (Fig. 1).The more widely documented changes include increase in water turbidity (Mayr et al. 2019), benthic habitat smothering (Hickman and Reasoner 1994), increased in lake SiO₂ from tephra particle dissolutions (Barker et al. 2000), as well as occurrence of a water-sediment interface barrier which restricts P-loading (Eastwood et al. 2002; Egan et al. 2018; Harper et al. 1985; Lotter et al. 1995; Hutchinson et al. 2019). Chrysophytes, another widespread group of algae that produce siliceous resting cysts which are also well preserved in sediments (Adam and Mahood 1981; Sandgren 1991; Pla et al. 2003), although less extensively used than diatoms, can also be employed as indicators of a lake’s condition, in complement to diatoms.

There is a lack of consensus on diatoms and lakes’ responses to tephra depositions and the impact duration and time required for a lake system to recover to the pre-perturbation state (Wutke et al. 2015). Some studies have suggested very brief impact durations in which ecosystems were able to return to their pre-tephra states in 5–20 years (Lotter and Birks 1993; Hutchinson et al. 2019), while other studies have indicated much longer recovery periods (200–500 years) (Hickman and Reasoner 1994; Egan et al. 2018). Thus, the impacts of tephra on lakes are likely to be site-specific. Additionally, there are studies that have found no correlations or significant changes between diatom communities and tephra deposition. For instance, Hickman and Reasoner (1994) examined diatoms in some shallow Canadian lakes and only found minor diatom concentration decreases not large enough to be attributed to tephra depositions. Similarly, Abella (1998) found no clear signals in total diatom abundance and productivity following tephra deposition in Lake Washington (USA). The reason for this could be that these tephras were deposited in a quantity not large enough to induce ecological responses (Telford et al. 2004), or because the lake background conditions were already experiencing other non-volcanic fluctuations.

This study investigated the impacts of five temporally and physically (as measured by thickness) distinct tephras on a lake in northeast China (Lake Sihailongwan) during the past 30,000 years, using diatoms and chrysophytes as indicators for changing aquatic conditions. By comparing across multiple tephra layers from the same lake, site consistency can be ensured as the lake catchment geology and morphology have been constant throughout, although climate change will have occurred. Accordingly, the aims were to:

1)Identify the presence or absence of significant changes in the diatom assemblages pre- and post-tephra deposition.

2) Examine differences in lake response and recovery to tephras of different thicknesses and under different climatic regimes.

Study site

Northeast China is an area with widespread Cenozoic volcanism. Due to the deep subduction of the Pacific plate slab, a number of intra-plate volcanic belts were created in this region (Duan et al. 2009). The most intense one being the Changbai Mountain Volcanic Belt. Located in the middle of this volcanic belt are the Changbai Mountains and the Longgang Volcanic Field (LVF) (Wang et al. 2001). The LVF (Fig. 2) is a volcanic field located in the south of Jilin Province characterised by alkali-basaltic rocks covering an upper Archean basement (Frank 2007). It contains 164 volcanic cones and 8 maar lakes (Liu and Taniguchi 2002). The maar lakes are meso-oligotrophic, remote and have water supplied mainly from groundwater, runoff and precipitation. They are also relatively well-buffered by abundant bicarbonate ions (HCO₃⁻) (Yan 1998).

Sihailongwan (SHLW) (42°17'13.8’N 126°36’07.0’E) is one of the 8 crater lakes in the LVF. It has a U-shaped basin (Zhao and Hall 2015) and is located ~ 20km southeast of the Jinlongdingzi volcano (Liu et al. 2009) (Fig. 2). SHLW has a water surface area of 0.5km², a maximum water depth of 50m (Chu et al. 2005) and a small non-calcareous 0.7km²catchment delimited by the maar rim, which rises 120m above the lake water surface (Mingram et al. 2018). The lake water level is controlled by precipitation and groundwater with no surface inflow/outflow (Rioual et al. 2016; Mingram et al. 2018). SHLW overturns twice a year in spring and autumn (Chu et al.2005) and has a distinct thermal stratification during the summer (Mingram et al. 2004). A thick ice cover forms on the lake surface in winter (Chu et al.2005). The modern regional climate exhibits strong Asian monsoon-controlled seasonality, manifested by cold dry winters and warm wet summers (Chu et al. 2005; Stebich et al. 2009). SHLW has laminated lake sediments with little bioturbation (Zhao et al. 2017; Wu et al. 2019) due to the presence of lake bottom anoxia arising from the distinct maar morphology (Mingram et al. 2004).

The dominant nutrient (Si and P) influx into SHLW is via groundwater (Schettler et al. 2006a), shown by the nutrient concentration measurements in groundwater and the vertical lake nutrient profiles (Fig. 3). Si and P are the main limiting factors currently affecting primary/photosynthetic productivity in the lake (Schettler et al. 2006a). SHLW’s nutrient condition is highly dependent on the climate. When warmer and wetter conditions prevail, groundwater influx increases, supplying more nutrients to the lake. The dissolved nutrients in groundwater mostly come from local organic and siliciclastic materials originated within the lake catchment (especially from the weathering of alkali- basaltic pumice tuff). When colder conditions prevailed, remote aeolian detrital material deposited from beyond the LVF constituted the majority of the lake nutrient input (Schettler et al. 2006a; Zhu et al. 2013).

Much of the autochthonous sediment fraction constitutes of biogenic silica (bSiO₂) from diatom production (Schettler et al. 2006a). Sediment trap data have revealed that in the present conditions, diatom flux peaks in autumn after the summer maximum precipitation (Chu et al. 2005; Schettler et al. 2006b), due to the increased epilimnion nutrients from groundwater diffusion and lake overturn (Schettler et al. 2006b). Diatoms in this lake are highly diverse and, Staurosira longwanensis, has been described as a new species from material collected in this lake (Rioual et al. 2014; Rioual et al. 2016).

The SHLW sediment composite profile is combined from three overlapping adjacent cores to produce a continuous sequence spanning the last 65,000 calibrated years BP (cal. yr BP). The three cores were retrieved from SHLW in 2001 using a high-precision Usinger corer (Parplies et al. 2008; Mingram et al. 2009; Mingram et al. 2018). The age of the sediment core is based on varve chronology corrected with AMS ¹⁴C dates together with several well-dated and widespread tephra layers which have been used as tephrochronology tie-points and age controls (Mingram et al. 2009). Details of the age-depth model were given in Mingram et al. (2018) and summarised in Fig. S1.

Seven major tephra layers and numerous micro-tephra layers have been identified in the SHLW sediment sequence (Mingram et al. 2018). Five tephra layers named T1, T2, BT, T4 and AT, with thicknesses ranging from > 18cm to micro-tephra < 1cm have been chosen for investigation in this study. The tephra layers span across a large section of the sediment core occurring in different depths, time intervals and climatic conditions. Four of the five tephra layers are basaltic in nature and one tephra layer is rhyolitic (Table 1; Fig. S2).The five tephras can be broadly classified into three categories according to their thicknesses: Very thick tephra > 15cm (T2); thick tephra 5-15cm (T1 and T4); micro-tephra < 1cm (BT and AT).

To investigate the diatom and the lake’s responses to the five tephras, three adjacent 1cm samples above and below each tephra layer were selected for investigation. Additionally, samples containing the tephra layers were also analysed (Fig. S2). Higher resolution sampling was not possible because the entire sequence had already been sliced in contiguous 1-cm thick samples. A total of 39 samples were analysed for diatoms. Diatom slide preparation largely followed Battarbee et al. (2001). 30% H₂O₂and HCl were added to each sample (0.01g of sediment placed in a test tube) to remove organic materials and carbonates. The tubes were then filled with distilled water and the suspensions left to settle overnight. The resultant supernatant liquid was removed. Microspheres were added for the calculation of diatom concentrations (Battarbee and Kneen 1982) that were expressed in number of valves per gram of dry sediment. Lastly, the samples were pipetted onto coverslips and mounted with Naphrax® on a hotplate onto microscope slides. Diatom valves were counted under a light microscope with oil immersion at ×1000 magnification. A minimum of 500 valves were counted for each sample, except for those diatom-barren ‘within-tephra’ samples. Diatom species were identified mainly using Krammer and Lange-Bertalot (1986, 1988, 1991a, b) and Lange-Bertalot et al. (2017). Some regional floras from Mongolia, Korea and Russia: Metzeltin et al. (2009), Joh (2010, 2011 and 2012), Lee (2011 and 2012) and Kulikoskiy et al. (2016) were also employed. Chrysophyte cysts and their scales were also counted in conjunction. However, no attempt was made to identify different cyst morphologies due to the lack of knowledge and the current difficulties in assigning chrysophyte cysts to taxonomic names (Pla et al. 2003).

Using Canoco 5.0 (ter Braak and Šmilauer 2012), detrended correspondence analysis (DCA) was first performed on a reduced dataset including diatom taxa whose abundances were > 2% in all samples from all samples spanning the five tephra layers investigated. Diatoms data were square root transformed and rare taxa downweighed to reduce noise. For individual tephra, a non-metric multidimensional scaling (nMDS) analysis using Bray-Curtis similarity index was carried out for both species relative abundances and concentrations to examine the potentially different patterns. nMDS was chosen over other multivariate techniques because it does not take absolute distances between samples into account and only computes the relative distances between samples (Hammer et al. 2001). It also does not assume any underlying variable-environmental relationships (Hutchinson et al. 2019). This method is appropriate for this study as it is concerned with the degree of tephra- induced changes relative to the background condition fluctuations between a small number of samples.

Down-core variations of all samples

The DCA analysis on all samples presents an overall picture of down-core variations in background conditions (Table S1 shows the summary statistics). An axis 1 gradient of 3.65 (> 3) was obtained, suggesting that the use of DCA was appropriate. However, the cumulative explained variation of axes 1 and 2 is only 36.22%. The DCA ordination plot indicates distinct clusterings between samples associated with different tephras (Fig. S2). ‘Within-tephra’ samples have not been included due to their very low diatom counts that biased their relative percentages.

Diatom and chrysophyte cyst analysis before and after T1

Planktonic diatom species make up most of the assemblages, with Discostella stelligeroides being especially dominant (Fig. 4) representing ≥ 50% of the assemblages in all samples and occurring at exceptionally high concentrations before the deposition of T1. Asterionella formosa also occurs at a relatively high concentration. After T1 deposition, the number of benthic species increases with a decrease in the planktonic/benthic diatom (P/B) ratio. The most significant change is the decrease (from ~ 45×10⁸ to ~ 10×10⁸ valves/g) in Discostella stelligeroides concentration. Other planktonic species such as Asterionella formosa and species of Fragilaria also display reduced concentrations. Total diatom concentration decreases dramatically after T1 deposition from ~ 75×10⁸ to ~ 10×10⁸ valves/g. Chrysophyte cyst concentration decreases from ~ 8×10⁷ to ~ 3×10⁷cysts/g after T1 and the diatom/chrysophyte cyst (D/C) ratio decreases by almost half. The nMDS on diatom concentrations indicates a large shift after T1 deposition. The degree of change within the background pre-tephra samples is negligible compared to the change before and after tephra deposition (Fig. S4).

Diatom and chrysophyte cyst analysis before and after T2

The planktonic species Stephanodiscus minutulus is the most abundant species before T2 deposition (Fig. 5) and decreases from ~ 90×10⁷valves/g to almost zero immediately post-tephra. Additionally, all benthic species decrease in concentration to almost zero after tephra deposition. Total diatom concentration decreases after T2 deposition from ~ 16×10⁸ valves/g to almost zero. The P/B ratio does not exhibit a change immediately after T2 deposition, but an increase of ~ 40 is observed in the subsequent sample 363.5-364.5cm. Chrysophyte cyst concentration decreases substantially from ~ 8×10⁷to ~ 1×10⁷ valves/g after T2 deposition. However, a larger magnitude of increase in chrysophyte cyst concentration is seen between pre-tephra samples (384.5cm and 383.5cm). D/C ratio decreases from ~ 20 to less than 10 after tephra deposition, this ratio peaks in the bottom-most within-tephra sample to a value of 120. nMDS of diatom concentrations in samples associated with T2 shows a relatively large degree of change that can be attributed to T2 deposition. However, a large shift can also be seen between two post-tephra samples (364.5cm and 353.5cm) (Fig. S4).

Diatom and chrysophyte cyst analysis before and after BT

An increase in the concentrations of Discostella stelligeroides together with ‘other benthic’ species is observed in the two samples where BT is located (642-643cm and 643-644cm) relative to samples deposited before this tephra. For other taxa, variations in concentrations are observed in this time interval but without an apparent direction of change. Total diatom concentration increases by ~ 2×10⁸ valves/g upon deposition of BT. However, the same magnitude of change is also observed in samples before BT deposition. The P/B ratio increases in general throughout this period. Chrysophyte cysts show some degree of variations, although their concentration seems to be stable in samples immediately before and after tephra deposition. The D/C ratio increases by almost 2/3 after BT deposition from ~ 15 to ~ 25 (Fig. 6). The nMDS plot shows that the immediate samples associated with BT (643cm and 642cm) exhibit a very small degree of variations compared to other non-tephra samples (Fig. S4).

Diatom and chrysophyte cyst analysis before and after T4

Cyclotella comensis fo. minima and Cyclotella rossii shows high concentrations with their highest at sample 1289.5-1290.5cm, having values of ~ 200×10⁶ and ~ 80×10⁶valves/g, respectively (Fig. 7). The most distinct changes regarding this tephra are the clear increase in Cyclotella comensis fo. minima and Cyclotella rossii concentrations after T4 deposition. Cyclotella comensis fo. minima increases from ~ 20×10⁶ valves/g pre-tephra to ~ 200×10⁶ valves/g post-tephra, with a subsequent decrease. Similarly, Cyclotella rossii increases from ~ 20×10⁶valves/g pre-tephra to ~ 80×10⁶ valves/g post-tephra, then also experiences a subsequent decrease. Benthic species with relatively high concentrations such as Staurosira longwanensis, Staurosira venter and Staurosirella pinnata exhibit variations but without clear direction of change. Total diatom concentration increases in the immediate post-tephra sample (1289.5-1290.5cm) from ~ 1.2×10⁸ to ~ 4×10⁸ valves/g, then decreases to ~ 2×10⁸ valves/g. The P/B ratio also increases after T4 deposition by a value of ~ 3, then decreases to values like those in pre-tephra samples. Chrysophyte cysts in general increase in concentration after T4 deposition. The D/C ratio increases slightly from ~ 14 to ~ 17. nMDS on diatom concentrations shows that the strongest variation is between the samples immediately pre- and post-tephra as represented by the longest relative distance (Fig. S4).

Diatom and chrysophyte cyst analysis before and after AT

Cyclotella comensis fo. minima dominates in most samples (Fig. 8) together with relatively high concentrations of Lindavia balatonis. All species regardless of their planktonic or benthic habitat, exhibited some degree of variations in this period (29,697 − 29,799 cal years BP). Fluctuations in total diatom concentration is observed, however it is uncertain whether changes in total diatom concentration relate to AT deposition. A small decreasing trend can be seen in the P/B ratio, but likewise changes cannot be linked to the tephra unambiguously. Chrysophyte cyst concentration exhibited a considerable degree of background variations. Similarly, the D/C ratio showed a certain level of fluctuation, however any linkage to AT deposition is ambiguous. nMDS on diatom concentrations suggest small or similar degrees of change in samples directly associated with AT (1482cm and 1481cm) to those from background samples (Fig. S4).

Overall diatom trends from the Last Glacial Maximum (LGM) to the Holocene

The five tephra layers were deposited in different time intervals characterised by different climatic conditions (Table 1), which is emphasised by the DCA plot for all diatom samples (Fig. S3) illustrating distinct clusterings of samples associated with each tephra.

T1 (2012 cal years BP) was deposited in a climate characterised by relative warmer temperature and higher precipitation than present as inferred from pollen analysis (Stebich et al. 2007; Stebich et al. 2015). This is consistent with the very high overall diatom concentrations, implying high diatom productivities and high nutrient status in SHLW during this period. There was likely a high dissolved nutrient supply due to enhanced groundwater discharge and high terrestrial productivity (Schettler et al. 2006a).

T2 (10,422cal years BP) occurred shortly after the onset of the Holocene (abrupt warming) (Stebich et al. 2007; Stebich et al. 2015). This period was dominated by Stephanodiscus minutulus(Fig. 5), a species with high P requirements (Van Dam et al. 1994; Interlandi et al. 1999; Reavie et al. 2000). Similar to T1, this also indicates a period of elevated lake nutrients from groundwater and terrestrial inputs.

The overall diatom concentration at the time of BT deposition (between 15,686 cal years BP) was lower compared to T1 and T2, suggesting lower lake productivity under cooler and possibly drier conditions (Stebich et al. 2007; Parplies et al. 2008; Stebich et al. 2009). Relatively high abundances of Cyclotella and Discostella species (Fig. 8) are seen and Cyclotella comensis fo. minima and Cyclotella rossii both occur under low productivity conditions (Scheffler and Morabito 2003; Saros and Anderson 2015; Ossyssek et al. 2020). Discostella stelligeroides is often found in eutrophic conditions but it has wide ecological tolerance (Genkal 2015).

T4 was deposited during the Last Glacial Maximum (LGM), most probably under the coldest conditions of the five tephras (Mingram et al. 2018; Zhu et al. 2021). Similar to BT, this period shows low overall diatom production and was mainly dominated by Cyclotella comensis fo. minima and Cyclotella rossii but not Discostella stelligeroides (Fig. 7). This suggests even more oligotrophic lake conditions and low groundwater nutrient input as well as terrestrial biogenic productivity.

AT corresponds with the onset of the LGM and likely represented similar climate conditions to T4 (same minerogenic clastic varve type) but not as severe (Mingram et al. 2018; Zhu et al. 2021). This is manifested once more by the low diatom concentrations and the dominance of Cyclotella comensis fo. minima (Fig. 8).

Aquatic responses to different tephra thickness

Consequences of very thick tephra deposition (T2 – 15–19cm thick)

For the very thick tephra T2, diatom concentrations show significant tephra-related changes relative to background fluctuations as indicated by the nMDS plot. The sample immediately post-T2 had a significantly different diatom concentration from background samples as well as the two following samples. From the diatom concentration stratigraphy, the immediately post-T2 sample contained negligible diatom concentrations resembling those ‘within-tephra’ samples. The decrease in diatom productivity post-T2 is manifested across all species but especially by Stephanodiscus minutulus as it was the dominant species in background samples (accounting > 50%). Stephanodiscus minutulus is a eutrophic species with high P requirements (Reavie et al. 2000; Interlandi et al. 1999). This may suggest a decrease of P in SHLW after T2 deposition due to a lake bottom tephra barrier limiting P-loading (Telford et al. 2004). T2 has a thickness of 15–19 cm and was deposited in the early Holocene, a period of increased temperature, precipitation and high terrestrial productivity. Nutrient conditions within the lake were probably relatively high. Therefore, a decrease in P input would have had a profound effect on diatom productivity, especially in a volcanic setting such as that of SHLW where the nutrient condition is highly dependent on groundwater influx (Telford et al. 2004; Schettler et al. 2006a). This is a highly probable scenario considering the thickness of T2 (18cm), which would have likely covered the entire lake bottom. Nevertheless, the effect of reduced P seems to be short-lived. Overall diatom concentrations, especially planktonic diatoms (mainly Stephanodiscus minutulus) started to increase in the next consecutive sample after the immediate post-tephra sample. In their study on the Holocene sequence of SHLW, based on geochemical proxy indicators, Schettler et al. (2006a) recorded a rise in bSiO₂ flux rate after T2 and related this to increased diatom productivity due to the enhanced inflow of nutrients from eroded pumice tuff in the lake catchment. Our results imply that the release of nutrients from catchment pumice did not occur immediately after T2 but only started to inflow after 3–10 years.

It is assumed that T2 had a major burial impact owing to its thickness, destroying the diatom communities to an extent where the lake conditions turned to a state quasi-devoid of life. The sequential occurrences of Amphora, Gomphonema, Nitzschia and Sellaphora species after tephra deposition indicate the early recolonisation of diatoms in a harsh environment, as these benthic species were not present or only present in small percentages before the deposition of T2. In particular, Nitzschia species are highly motile and can avoid burial in the sediments (Lowe 2003; Kociolek 2011; Solak et al. 2019). Similarly, Sellaphora species also have an epipelic habitat and wide environmental tolerances (Van Dam et al. 1994; Wetzel et al. 2015).

Low concentrations of Stephanodiscus minutulus were found in the bottom-most ‘within-tephra’ sample, while several benthic species that were not present in any other samples were found in the top-most ‘within-tephra’ sample. The poor sorting of the material suggests that after the initial airfall a slump may have occurred with T2, in-washing tephra and sediments from the catchment and introducing benthic species into the lake centre where the core was taken. As such, T2 is probably composed of a mixture of airborne tephra material and in-washed/slump material. Another possible explanation is that the tephra boundaries were not determined accurately when the core was sliced into 1-cm thick samples. Accordingly, the species found in the topmost ‘within-tephra’ sample could represent early lake responses to tephra deposition, but they have been allocated into ‘within-tephra’ due to the ambiguous tephra boundaries (Payne and Egan 2019).

Chrysophyte cyst changes are in concordance with diatom changes. The decline in chrysophyte cyst concentration after T2 deposition suggests a decline in lake nutrients, especially Si (Fig. 5) (Douglas and Smol 1995; Pla and Anderson 2005). Chrysophyte cysts are highly silicified and have therefore high Si requirements. Si is also a limiting nutrient in SHLW supplied together with P through groundwater inflow (Schettler et al. 2006a). Thus, it is likely that the lake bottom tephra barrier also impeded Si diffusion into the water column.

Consequences of thick tephra depositions (T1 -7cm thick and T4- 6cm thick)

T1 and T4 have a similar thickness of 7 and 6 cm respectively, however they were deposited at different times under different local climate regimes (Table 1). T1 was deposited in the late Holocene with similar but more eutrophic conditions than T2 (early Holocene). T4 on the contrary was deposited during the LGM when climate conditions were the most unfavourable to diatom growth with especially low precipitation, temperature and groundwater nutrient flux (Stebich et al. 2007). Despite the similarity in tephras, the lake responded to tephra depositions in different ways due to differences in background climate and lake conditions.

Overall diatom production decreased after T1 deposition, likely due to the presence of a tephra barrier on the water-sediment interface, preventing P-loading from groundwater into the lake through a mechanism similar to what occurred with the very thick tephra, T2. Although Barker et al. (2000) suggested that only tephra layers with thicknesses > 10cm can significantly limit P diffusion, T1 was only 7cm in thickness but still had an effect. The eutrophic background conditions probably exacerbated diatoms’ responses to decreased P and furthermore, T1 was a basaltic tephra with very low SiO₂ content. A decline in concentration was observed mainly for planktonic species as suggested by the decline in P/B ratio (Fig. 4). This was especially so for Discostella stelligeroides which is a species with small cell size and fast growth rate that can respond quickly to nutrient enrichment (Saros and Anderson 2015). This may also be true for its response to nutrient depletion. Asterionella formosa, a eutrophic species (Lund 1950), also showed the same decreasing trend. Eutrophic species have narrower environmental tolerances than oligo-mesotrophic species, therefore even small fluctuations in the nutrient status can induce large alterations in their population abundances (Passy 2008).

In contrast to T1, an abrupt increase in overall diatom concentration was observed for T4, especially in planktonic species as indicated by the increase in P/B ratio (Fig. 7). Major increases in concentration were seen in Cyclotella comensis fo. minima and Cyclotella rossii. The enhancement in the concentrations of small centric planktonic diatom species after deposition of T4 possibly indicates an increase of lake nutrients. This is because smaller- sized diatoms have faster growth rates and lower nutrient use efficiency, therefore their populations tend to increase under abundant nutrient conditions (Saros and Anderson 2015; Wentzky et al. 2020). While both Cyclotella species showed concentration increase, Cyclotella comensis fo. minima responded more abruptly due to its smaller cell size, it is also referred to as an ‘opportunistic’ species (Scheffler and Morabito 2003; Ossyssek et al. 2020). Generally, Cyclotella species are believed to be good at taking advantage of an input of Si to develop large populations in oligotrophic lakes. Therefore, the increase in Cyclotella potentially indicates that the lake SiO₂ was elevated through the dissolution of tephra particles in the water column. T4 was deposited during the LGM when the climate conditions were the most unfavourable for diatoms to develop, with especially low precipitation, temperature and groundwater nutrient flux. The nutrient conditions in SHLW were possibly very low, thus even small increases in lake Si content could trigger ecosystem change. Additionally, if T4 had imposed a barrier effect for nutrient diffusion into the water column, this effect was not significant as the nutrient input was already low; the lake would not respond dramatically to any further decreases. The inference that the dissolution of SiO₂ from tephra caused the increased diatom concentrations observed in SHLW seems more probable.

T1 possibly also had a burial effect on littoral habitats as suggested by an increase in ‘deep benthic’ taxa that can be found in epipsammic habitats (for example small Staurosirella, Staurosira and Pseudostaurosira) after T1 deposition. However, these taxa did not appear in the sample immediately post-tephra, the immediate onset of T1 probably created conditions too harsh for an epipsammic assemblage to develop. As time passed, harsh burial conditions eased which created habitats suitable for these species. These observations are almost identical to those reported by Egan (2016) from a lake in Washington, USA, where she also attributed them to habitat alterations and new species colonisation. Another potential impact of T1 was sustained water column turbidity. Discostella stelligeroides has very high light requirements for photosynthesis (Saros and Anderson 2015), and its concentration remained relatively low for > 30 years after T1. Although the effect of light limitation is believed to only last for days (Barker et al. 2000), we cannot rule out the possibility of prolonged in-wash of fine tephra material from the lake catchment (Christensen 2011). Since SHLW was in proximity to Jinlongdingzi volcano (the source of T1), tephra was probably deposited in huge quantity on SHLW’s catchment. Together with the wet and warm climate of this period, substantial amounts of terrestrial tephra could be weathered and washed into the lake for years after the eruption. The spring-summer peak precipitation in this region also coincides with intra-annual diatom bloom (Schettler et al. 2006b), introducing water turbidity that could have profoundly limited Discostella stelligeroides growth.

Chrysophyte cyst changes broadly echo the diatom-inferred changes both for T1 and T4. The D/C ratio decreased after T1 (Fig. 4). The D/C ratio can be used as an inference for lake trophic change, where a decrease in this ratio often reflects decrease in nutrients (Douglas and Smol 1995; Pla and Anderson 2005). This is because chrysophytes tend to thrive more in oligotrophic conditions due to their high capabilities for nutrient sequestering and storage (Lotter et al. 1998). On the contrary, they tend to get outcompeted by diatoms in eutrophic conditions owing to their lower growth rate (Duff et al. 1997). T4 on the contrary, showed increased chrysophyte cyst concentrations after T4 deposition and the D/C ratio also decreased as a result (Fig. 7). Chrysophyte cysts are highly silicified, therefore increasing Si would boost their growth. Furthermore, chrysophytes could be more sensitive to nutrient changes than diatoms (Lotter et al. 1998), meaning that they could take advantage of even small elevations in Si introduced by a basaltic tephra.

Micro-tephras (BT – 0.05cm thick and AT – 0.05cm thick)

No significant diatom or chrysophyte-inferred changes can be associated with the two micro-tephras BT and AT. From the nMDS plots of diatom concentrations in samples associated with BT and AT, the background samples show large fluctuations and the degree of change between background samples was as large if not larger than the change between the pre- and post-tephra samples. This is also illustrated by overall diatom concentration, P/B ratio and chrysophyte cyst concentration (Figs. 6 and 8).

BT for example, exhibited slight rises in overall diatom concentration (Fig. 6), especially by Discostella stelligeroides and some benthic diatoms after deposition. However, these concentrations were already increasing before the deposition of BT. BT may have been too small to induce any identifiable chemical or physical alterations on the lake system (Telford et al. 2004). Additionally, any changes induced by BT may have been confounded by the strong background in climate fluctuations during the period of BT deposition as it coincided with Heinrich event 1, an abrupt climatic reversal to cold and dry conditions during the last deglaciation (Hodell et al. 2017).

Another plausible explanation for the inability to attribute changes to tephras is the low confidence about which of the samples analysed for diatoms actually contain the BT layer and whether the immediately pre- and post-tephra samples truly constitute the correct pre- and post-tephra diatom assemblages. Since the thickness of BT was < 1cm, thinner than the sample resolution, it is impossible to distinguish the potentially immediate effect of BT as both the pre- and post-tephra diatom assemblages are all contained within one 1-cm thick sample. Additionally, the photograph of the core shows that there was a slight bending in the lamination, an artefact due to the coring/extruding (Fig. S2), further complicating sample slicing.

Impact durations and recovery

Apart from the two micro-tephras (BT and AT), none of the other tephras exhibited complete recovery back to their background conditions through the intervals investigated in this study. After T4 there was a tendency of shifting towards the background state, but diatom assemblages never returned completely back to their initial composition (Fig. S4). There are two possible explanations for this. Firstly, the tephra (especially the very thick ones) likely caused permanent/long-term alterations of the lake ontogeny (Telford et al. 2004). This could especially be the case for small maar lakes like SHLW with simple hydrogeology. Lake system equilibriums can be shifted easily into new equilibrium states by disturbances, resulting in chronic ecosystem change (Barker et al. 2000). Another plausible explanation for the lack of recovery could be the ongoing climate changes experienced at SHLW. As the tephras were altering the lake system, extraneous changes in climate and catchment conditions were also imposing influences on the lake conditions (Lotter et al. 1995; de Klerk et al. 2008). Accordingly, any recovery signals would simply be confounded and concealed.

Limitations and implications for future tephropalaeoecological studies

While this study demonstrated the potentials of using palaeolimnology and palaeocology as methods of examining the impacts of past volcanic eruptions, major limitations were also revealed. This section outlines areas of possible improvements and some proposed principles that could inform future tephropalaeocological research:

1) High sampling-resolution (ideally with an annual resolution). This was not possible in this study as the core had been pre-sampled at 1cm intervals, each comprising ~ 30–40 years of sediment. High sampling-resolution helps to distinguish volcanic-induced changes from background environmental fluctuations. Additionally, this would allow the capture of more subtle, complex and short-lived impacts arising from transient volcanic events, which could have been overlooked by lower sampling-resolutions.

2) Examination of species flux. This study presented diatom species concentration instead of species flux rate due to the lower reliability of the age-controls around tephra layers (poor quality of the laminations causing larger counting errors) that prevented the calculation of sediment accumulation rates that are necessary to compute diatom fluxes. Species flux is potentially more robust in illustrating changes in diatom assemblages as it takes into account differences in sedimentation rates.

3) Accurate documentation of tephra horizons and ‘true’ pre- and post-tephra layers. One major concern of this study was the inaccuracy in attributing which samples included the tephra boundaries due to the low sampling resolution that was adopted when the core was sliced. Diatoms were observed in some within-tephra samples while some non-tephra samples appeared to be diatom-barren. It is not sufficient to determine horizons based on chronology alone (Payne and Egan 2019), one needs to incorporate other examinations such as changes in stratigraphic profile and sediment physio-geochemistry in order to accurately determine different layers.

4) Use of statistical analysis. The nMDS analysis of this study served as a useful tool to help inform whether changes in diatom assemblages could be attributed to tephra depositions or not. It is difficult to establish causations in tephro-palaeoecological studies as other longer-term environmental fluctuations could occur at the same time. Statistical analysis provides a more objective mean of data interpretation (Payne and Egan 2019).

This study investigated the impacts of five tephras on SHLW in northeast China during the past 30,000 years, through the changes in diatoms and chrysophyte cysts observed in the sediment sequence. Not all tephras induced significant shifts in the diatom communities. The two micro-tephra layers (BT and AT) did not cause significant change. The likely reason being not enough tephra material was deposited to induce lake system alterations to change the diatom communities. Although, the apparent lack of diatom response may also be due to the coarse sampling resolution. The effect of these two tephra layers could potentially be very short-lived (much shorter than the sampling resolution), therefore any changes in the lake system detected by diatoms were smoothed out. Conversely, the other three thicker tephras (T1, T2 and T4) showed significant pre- and post-tephra changes.

Diatom data from T1 and T2 showed significant declines in overall concentrations, signalling a decline in P concentrations, possibly from the presence of a lake bottom tephra barrier preventing P-loading. T4 on the other hand, showed a slight increase in diatom concentrations, indicating elevated Si concentrations. Under different background climates, diatoms and the lake responded differently. T1 and T2 were deposited in relatively warm and eutrophic conditions, whereas T4 was deposited during the LGM when cold climatic conditions prevailed, and the lake was oligotrophic, emphasising the importance of the background conditions in governing the lake’s responses to tephra depositions.

Chrysophyte cysts and scales, although not investigated comprehensively due to methodological and taxonomic limitations, largely echoed the same signals presented by diatoms. This indicates their potential application in complementing other proxies in palaeo- reconstructions (Pla et al. 2003). Chrysophytes, like diatoms, are widely found in diverse communities in many lakes. Future research should focus on developing methods to classify chrysophyte cysts and scales in order to utilise this proxy to its full potential (Duff et al. 1997).

In terms of lake system recovery from tephra disturbances, for none of the three tephra layers (T1, T2 and T4) with significant tephra-induced changes, the diatom communities showed a complete recovery back to background state. The tephras had likely caused long-term alterations on lake ontogeny, shifting the lake system to new equilibria.

Acknowledgments

We thank Prof. Guoqiang Chu and Dr. Luo Wang for their comments on tephras and diatoms, respectively and Prof. Anson Mackay for his advice on multivariate statistical analyses. Patrick Rioual is currently supported by the Strategic Priority Research Program of the Chinese Academy of Sciences (grant number XDB26000000).

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Table 1 Characteristics of the five tephra layers investigated in this study. The geochemical compositions and source of tephra data are from Zhao and Hall (2015), Miyairi et al. (2004), Mingram et al. (2009), Liu et al. (2009) and Zhao et al. (2007). The background climate information associated with tephra are from Stebich et al. (2007, 2009 & 2015), Schettler et al. (2006a & b), Mingram et al. (2018), Zhu et al. (2021) and Parplies et al. (2008). The climate and lake trophic status reconstructions were based on a range of proxies including pollen, branched glycerol dialkyl glycerol tetraethers (brGDGTs), geochemical and stable isotope analyses.

Tephra	Depth (cm)	Age (cal years BP)	Thickness (cm)	Overall chemistry	Geochemical compositions				Volcanic source	Background climate and inferred lake condition
Tephra	Depth (cm)	Age (cal years BP)	Thickness (cm)	Overall chemistry	SiO₂(wt%)	Na₂O (wt%)	K₂O (wt%)	P₂O₅ (wt%)	Volcanic source	Background climate and inferred lake condition
Tephra 1 (T1)	112.5-119.5	2012	7	Trachybasaltic	48.25	4.81	2.88		Jinlongdingzi Volcano	Late Holocene. Slight cooling trend and slight shift in forest cover to herbaceous drought-tolerant vegetation e.g. taiga and steppe. Shortened growing seasons.
Tephra 2 (T2)	365.5-383.5	10422	15-19	Trachybasaltic	46.44	3.22	2.13	0.52	Jinlongdingzi Volcano	Early Holocene. Increased temperature, precipitation and forest cover. Vegetation shift from birch- dominated pioneer species to temperate deciduous forests. Enhanced lacustrine productivity.
Basaltic micro- tephra (BT)	642.5	15686	~0.05	Trachybasaltic					Possibly local LVF eruption	Late glacial. Relatively cold conditions characterised with low lacustrine productivity. Coincided with Heinrich event 1.
Tephra 4 (T4)	1290.5-1296.5	24770	6	Trachybasaltic	49.59	5.05	2.80		Possibly local LVF eruption	Last Glacial Maximum. Low lacustrine productivity. Lake catchment likely affected by permafrost cover. Coldest conditions among 5 tephra layers. Lowered lake water level due to dryer and cooler climate. High aeolian clastic influx. Vegetation composed of steppe patches.
Aira-Tn micro- tephra (AT)	1481.5	28037	~0.05	Rhyolitic	74.51	2.94	3.10	0.02	Aira caldera, Japan	Transition into the Last Glacial Maximum. Cold and dry climate. Low lacustrine productivity. High aeolian clastic influx.

No competing interests reported.

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A palaeoecological study investigating the impacts of multiple tephra depositions on a lacustrine ecosystem in Northeast China, using diatoms as environmental indicators

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