Drought response and anthropogenic impacts as recorded in a high desert terminal lake, northern Great Basin USA, during the last millennium

INTRODUCTION

Summit Lake, a high-altitude terminal lake located in the Black Rock Range of Nevada, USA (Fig. 1), provides a unique archive of environmental and climate history within the northern Great Basin. The lake (elevation = 1,780 m, Z = ∼10 m, Lake Area = 2.8 km²⁾ has ice cover during winter, is primarily fed by Mahogany Creek with intermittent inflow by Snow Creek, and supports the growth of diverse macrophytes in the warmer months (Simmons et al., 2020). Formed at the end of the Pleistocene, the lake originated from a large landslide that blocked the flow of Soldier and Snow creeks (Curry and Melhorn, 1990), creating a sedimentary basin with potential insights into both the natural and climatic history of the Great Basin. Summit Lake has shown recent sensitivity to precipitation deficits in that the lake surface elevation decreased ∼4 m during the 2012–2016 Western US drought and may be under threat based on future warming scenarios (Simmons et al., 2020; Rzyska-Filipek et al., 2018).

Summit Lake's ecological significance extends beyond its sediment record. The lake supports a unique subpopulation of Lahontan cutthroat trout, a genetically distinct adfluvial fish that moves between the lake and upper watershed through Mahogany Creek during its spawning and rearing cycle. This population holds cultural significance for the Summit Lake Paiute Tribe, historically known as “Agai Panina Ticutta” or the “Summit Lake Trout Eaters,” reflecting the longstanding role of the lake in indigenous traditions (Coffin and Cowan, 1995; Vinyard and Winzeler, 2000). Recognized as a crucial component of the Northwestern Lahontan basin population segment, Summit Lake's trout population is central to regional recovery efforts for the Lahontan cutthroat trout (Gerstung, 1988; Dickerson and Vinyard, 1999).

Despite the ecological and cultural importance of Summit Lake, comprehensive studies on its paleolimnology have been limited, with much of the lake's history remaining undocumented. Wigand's (2004) unpublished study is one of the few attempts to reconstruct environmental changes in the lake. This study, which used pollen, non-pollen palynomorphs, charcoal, and sediment analysis to explore climate and vegetation history, represents a preliminary step toward understanding the lake's ecological history over the past 5,000 years. However, that study relied on a simplified linear age model constructed with only two bulk sediment radiocarbon dates and left significant gaps in the lake's long-term environmental record.

One subject not revealed in previous work is the effect of the Medieval Climate Anomaly (MCA) on Summit Lake. The MCA was manifested as a multi-centennial period of intense aridity in the Western US, whose effects were centered from 800–1300 CE but locally persisted as late as 1600 CE (Cook et al., 2016). We employ diatom subfossils as a chief biological proxy in this study, as they are abundant in the Summit Lake core and have been shown to be useful in reconstructing past ecological change, including fluctuations in nutrient availability and lake levels, that may be tied to broader climate events such as drought (Smol and Stoermer, 2010). The presence of diatom assemblages within Summit Lake's sediments offers an opportunity to reconstruct the lake's long-term ecological dynamics, contributing to a deeper understanding of environmental shifts in a region where paleoclimate data remain sparse. Herein we present a diatom analysis from the upper 128 cm of lake core sediment, representing approximately the last 1000 years, and use it to contextualize future climate scenarios and threats to this valuable cultural resource.

MATERIAL AND METHODS

Modern sampling.— To obtain ecological information on the present-day diatom flora, a series of diatom samples were taken of both planktonic and benthic habitats and compared with limnological sampling that was conducted quarterly by the field staff at Summit Lake. Diatom samples include both vertical and surface plankton tows taken in the vicinity of the core site in July of 2021 and 2022, and additional discrete water depths using a Van Dorn water sampler taken on three dates: 4/21/23, 7/25/23, and 9/11/23. Discrete water samples collected via a Van Dorn sampler were preserved, settled and counted with a Sedgewick rafter, and the number of cells per volume of water were calculated to determine absolute abundance at the various depths following the methods described in Noble et al., (2013). Benthic diatoms were sampled from the littoral zone capturing rock, sediment, and macrophyic substrates during July of 2021 and 2022 (Fig. 1). Wet mounts of fresh material were made of the benthic samples to note the most prominent species, attachment of epiphytes, and community associations. Limnological data were collected from the epilimnion by the Summit Lake biological staff from lake stations SP2, SP3, and SP4 (Fig. 1) on 4 dates: 9/20/22, 5/16/23, 7/25/23, and 9/18/23 using a 20 cm Secchi disc and YSI multiparameter water quality meter (ProPlus model on the first two dates, and the ProDiss model on the last two). The measurements recorded by the water quality meter were temperature, turbidity, specific conductance, pH, and dissolved oxygen. Collectively these stations provided useful spatial data on the variability of limnological parameters in the lake over a 1-year period.

Core suite.— During the summers of 2021 and 2022 Summit Lake was cored from the deepest spot in the lake, at roughly 10 m water depth (Fig.1). At this site, multiple long cores were collected through repeated drives with a square rod Livingstone-type corer (Wright, 1967) with a 3 cm diameter 1 m long steel barrel, and extruded whole into PVC sleeves. In order to capture the lower density surface sediment, the Livingstone coring head was replaced with a Bolivia-type head attached to a 7 cm diameter 1.5 m long polycarbonate tube that preserved the sediment-water interface with minimal disruption. The part of the cores sampled and used in this study include two of the Bolivia cores (SUM 21-2B, SUM 21-2C) and the uppermost drive of the 2022 Livingstone core (SUM 22-2A-1L) with additional drives in the upper ∼2.5m used in ¹⁴C dating (Fig. 2). The Bolivia core SUM 21-2B, was used for both diatom analysis and ²¹⁰Pb dating, and reached 112 cm in length, having bottomed out on the top of a higher density layer. This high-density layer was encountered in all subsequent cores from this locality at ∼115 cm below lake floor and interpreted in the field to be the top of a desiccated interval, with the sediment below 115 cm exhibiting a crumbly, pelleted texture composed of hard mud lumps.

The upper 40 cm of SUM 21-2B was sectioned in the field using an extruder; the upper 20 cm was sectioned into 0.5-cm slices, and the 20–40 cm interval into 1-cm slices based on estimations of sedimentation rates. The lower 72 cm was sealed and returned to the lab where it was later split, imaged and subsampled for diatoms and additional ²¹⁰Pb samples to help refine the age model. A companion Bolivia core 115 m long (SUM21-2C) was taken approximately one meter away from SUM21-2B and entered the high density layer for the lower ∼1cm. This core was left intact for sedimentological analysis, then later opened in the lab, photographed, described, and sampled for organic geochemical analyses. Diatom samples were also taken from the uppermost drive of the Livingstone core sequence, SUM22-2A-1L, down to a sediment depth of 128 cm, penetrating the high-density layer and providing complete overlap of the transition. The Livingstone drives were extruded intact, wrapped, and returned to the lab for imaging, description and subsampling for diatoms across the density transition. All core sections, both Bolivia and Livingstone, were examined and sampled for terrestrial plant macrofossils suitable for ^14C dating.

All Summit Lake cores were correlated and the cumulative depths of each sample established through visual alignment of core images using the open-source software Corelyzer 2.1.2 provided by the Continental Scientific Drilling facility at the University of Minnesota, with additional alignment constraints informed by sedimentologic properties. In particular, the density transition encountered at ∼115 cm below lake floor provided a highly visible correlation marker (Fig. 2). After diatoms were analyzed, the alignment was again rechecked and adjusted by moving SUM22-2A-1L 2 cm lower to better align the floral peaks in the ∼10 cm interval of overlap. The adjusted depths in SUM22-2A-1L are reflected in the tables and figures, and sample depth is reported using the median depth of a section. Thus, a 1-cm thick sample taken at a depth of 90–91 cm is given the depth of 90.5 cm. All core depths discussed represent cm below lake floor, based on correlations. Coordinates for all core segments used in the study are found in Table 1, including segments that were subsampled for ¹⁴C dating to develop the age model discussed in the following section. Correlation of the core suite is illustrated in Figure 2a, and Figure 2b shows photos of the core sections sampled for diatoms and a magnified view of the density transition.

Dating.— An age model for the composite core was constructed using ²¹⁰Pb-dating. In addition, ¹³⁷Cs and ²³⁴Am depth profiles were established to use the 1963 peak fallout from nuclear weapons testing in the northern hemisphere as a stratigraphic datum to cross-check the ²¹⁰Pb age model (Pennington et al. 1978; Appleby, 2002). Samples for ²¹⁰Pb dating were sectioned, their volume noted, and both the wet and dry weight determined. They were then analyzed at Servicio Académico de Fechado (SAF), from Instituto de Ciencias del Mar y Limnología at Universidad Nacional Autónoma de México (UNAM) to determine the activities of total ²¹⁰Pb (²¹⁰Pb_total, 46.5 keV), ²²⁶Ra (²¹⁰Pb_supported, through ²¹⁴Pb in secular equilibrium, 351.9 keV), ²⁴¹Am (59.5 keV), and ¹³⁷Cs (661.7 keV) using gamma-ray spectrometry (HPGe well detector, Ortec-Ametek) following Díaz-Asencio et al. (2020). Gamma-ray spectrometry was conducted to conserve sediment for future analyses. Sediment samples were measured for two to three days to obtain a counting uncertainty of <10% for ²¹⁰Pb. The ²¹⁰Pb_excess activities were calculated as the difference between ²¹⁰Pb_total and ²¹⁰Pb_supported. The age model and mass accumulation rate (MAR, reported in g cm^-2 yr^-1) were estimated from the ²¹⁰Pb data using the Constant Flux method (Robbins, 1978; Sanchez-Cabeza and Ruiz-Fernández, 2012) as described in Sanchez-Cabeza et al. (2014). Although the initial run of this model provided satisfactory results, it seemed that there was still some ²¹⁰Pb_excess at 40 cm depth, so additional samples were analyzed to a depth of 60 cm to ensure that the equilibrium depth (i.e. where excess and supported ²¹⁰Pb activities are equal) had been reached. Since a nearly constant excess of ²¹⁰Pb persisted even in the deepest layers, we applied the PIRLA method (Binford, 1990) to calculate the mean ²¹⁰Pb_total activity where concentrations remain approximately constant within measurement uncertainty, identifying the deepest section with measurable ²¹⁰Pb_excess at 36–37 cm.

Fig. 1.

(above) Locality map of Summit Lake (NV, USA) showing locations of the coring, modern phytoplankton sampling (circles), and benthic algal sampling (triangles) sites.

Fig. 2.

a. Schematic diagram showing the core sections used in this study, and their correlation based on the position of a distinct sharp transition ∼115 cm from low density homogeneous mud to higher density mud with a crumbly to pelleted texture. b. Photos of the Bolivia and Livingstone core sections sampled for diatoms illustrating a color transition ∼15-17 cm, the water-rich nature of the upper sediment in the Bolivia core, and the notable textural change below ∼115 cm. An enlarged view of the transition at ∼115 cm appears on the far right.

¹⁴C dating was used to extend the age model further back in time. Four samples of terrestrial plant material (e.g., wood fragments, seeds) selected for AMS ¹⁴C were isolated from the sediment cores at the Sedimentology and Paleoclimate Laboratory, Occidental College and pretreated and measured at the W.M. Keck Carbon Cycle AMS Laboratory, University of California, Irvine (Table 2). Of these samples, three were collected from the upper 115 cm, above the density transition, and one from below at 232.5 cm. The R package Plum (Aquino-López et al., 2018) was used to cross validate and integrate the ²¹⁰Pb and ¹³⁷Cs, data with three ¹⁴C dates obtained from above the density transition, extending the age model down to 114 cm. The date at 232.5 cm was not incorporated into the Plum model due to concerns that there may be an erosional gap at 115 cm, or highly variable sediment accumulation history in the lower sediment package that would not be handled well by the Plum software. Instead, a linear interpolation of age using the package CLAM (Blaaw, 2010) was done with the oldest age returned from the Plum model at 114 cm and the age of the sample at 232.5 cm. Additional age model development is underway to aid in interpretation of the lower sections of the core.

Table 1.

Geographic coordinates of cores and modern sample localities in decimal degrees using a WGS84 datum.

Diatom preparation.— A total of 43 sediment samples were processed for diatom analysis from the SUM 21-2B and SUM 22-2A-1L sample suite generally following the method in Stoermer et al. (1995). Each of the samples was digested and boiled in 30% Hydrogen Peroxide (H₂O₂) for an hour to remove any organic materials present in the samples and dislodge clays adhering to the diatom subfossils. Deionized (DI) water was added to fill the diatom suspensions, left to settle overnight, and decanted with a suction pipe. A few mg of nitric acid was added to each sample to remove any carbonate fraction, additional DI water added, and the decantation process was repeated five times until the pH of the water was neutral. The cleaned diatom slurry for each sample was then transferred and concentrated into glass vials and strewn slides were made by pipetting a diluted volume of homogenized diatom slurry onto a coverslip and left overnight to air dry. Permanent mounts were then made using ZRAX (refractive index ∼ 1.7, Prof. W.P Dailey's Formula).

Identification and enumeration.— A voucher flora was constructed as an internal identification reference, documenting specimens through images, assigning suites of specimens to operational taxonomic units (OTUs) that were given species codes, and subsequently reconciling the OTUs to known taxa in the literature (Hamilton et al., 2023). Use of a voucher flora allowed for consistent identification during the enumeration process, grants reinterpretation of names applied to specimens, and as noted by other authors (e.g., Bishop et al., 2017; Alers-Garcia et al., 2021) facilitates taxonomic continuity in identification over time, especially in long-term ecological studies. The voucher flora was made as comprehensive as possible through the examination of each slide to observe rare diatom taxa. Diatoms were digitally photographed using an OLYMPUS BX51 microscope paired with an OLYMPUS DP71 camera using a 100× oil immersion objective lens with DIC.

Initial taxonomic assignments of OTUs in the voucher flora were made using the Diatoms of North America curated taxonomic database (Spaulding et al., 2021) and Cantonati et al. (2017). The OTUs that could not be assigned to a published species were left in open nomenclature and ‘SUM’ appears in their reconciled name to show they were from Summit Lake (e.g., Stephanodiscus SUM sp. 1, Pseudostaurosira SUM sp. 3, etc.). The details of each OTU including the species code, reconciled taxonomic name, basionym, references used in the identification, and identification comments and measurements were recorded in a spreadsheet ( Supplemental Table S2 (11_Supplemental_Drought_response_and_anthropogenic_impacts.zip)). Once the voucher flora was constructed, the slides were enumerated at the species level.

Enumeration procedures followed a transect-based approach to avoid bias in species composition and density estimates by systematically scanning across transects until a minimum of 300 diatom valves were observed and recorded. When possible, diatoms were enumerated at the level of the OTUs in the voucher flora. Only specimen fragments >50% were enumerated, and elongate forms were counted when part of the central area was present in the fragment. Some OTUs were combined for counting purposes because they could not be consistently separated during a transect. Additional specimens were enumerated at higher taxonomic levels when they could not be resolved. Commonly, many Epithemia and gomphonemoids occurred in girdle view and were not resolvable to the species level with confidence. These specimens were enumerated at the genus level.

Table 2.

¹⁴C ages used to construct the age model in Plum. Note the oldest age was not incorporated into the Plum model. Instead, a linear interpolation with the base of the model at 110 cm was used to estimate the age of the lowermost 17 cm of core.

Geochemistry.— Particulate organic carbon (C) and nitrogen (N) concentrations and δ¹³C, were analyzed from bulk sediment samples taken at 10-cm intervals in core SUM21-2C-1B to establish relative proportions of organic source matter and identify any long-term patterns that might inform past lake history. These analyses were performed in the Nevada Stable isotope Lab at the University of Nevada Reno. Samples were first fumigated with HCl for several weeks to remove any carbonate fraction following the method of Yamamuro and Kayanne (1995). The analyses were then conducted using a Eurovector EA 3000 elemental analyzer interfaced to a Micromass IsoPrime stable isotope ratio mass spectrometer with an acetanilide standard, using the method of Kornexl et al. (1999). The δ¹³C results are reported relative to the Vienna PeeDee Belemnite (VPDB) standard. Analytical error for wt.%N, wt.% C, and δ¹³C was estimated from calculating the percent difference of replicate samples, taking the absolute value of the difference between replicate values and dividing by their mean value. The analytical error as a percent difference was calculated as 4 wt.% N, wt.% C, and 1% δ¹³C.

Data analysis.— Principal Components Analysis (PCA) was conducted on the enumerated diatom data using in the software PAST version 4.15 (Hammer et al., 2001) to establish relationships between sites and samples. (Hammer and Harper, 2006). Non-specific counting categories (i.e. unknown centric) were rare in the count totals and were removed from the dataset, except for the fragilarioid chain counting group described under enumeration methods. Following the PCA, a stratigraphically constrained cluster analysis was generated using the hierarchical agglomeration method CONISS in the R package Rioja (Juggins, 2017) to determine the positions of diatom zones, using an incremental sum of squares to minimize the within-cluster dispersion and optimize the cluster groupings (Grimm, 1987). Within Rioja, the number of statistically significant clusters were determined by running a broken-stick model (Birks, 2012). The software C2 (Juggins, 2007) was then used to create a stratigraphic chart of diatom enumerations for the most common taxa. For this diagram, some species counts were combined at the genus level, including, Cymbella, Epithemia, Surirella, and Gomphonema to either preserve low abundance species counts in the data analysis, or to include high numbers of girdle view counts not resolvable at the genus level.

RESULTS

Modern sampling.— All the modern diatom phytoplankton observed were assigned to Stephanodiscus SUM sp. 1. Additional soft-bodied algae were noted but not identified nor enumerated. The first planktonic sampling date, 21 April 2023, was shortly after ice-out and showed few diatoms. The maximum abundance was noted during the second campaign, July 25, 2023, with maximum cell abundance at 5 m depth, and high abundance also at 9 m. Stephanodiscus concentrations dropped substantially by the Sept 11, 2023 sampling date (Fig. 3). Benthic diatom samples from the nearshore showed that the most abundant forms associated with aquatic plant material, either epiphytic or entangled, were Cocconeis placentula, Epithemia spp. (largely E. gibba and E. sorex), Ulnaria sp., Nitzschia spp., araphid chains (likely Staurosira), and Fragilaria mesolepta. Cocconeis placentula was found in its highest concentrations attached to floating milfoil specimens. A few gomphonemoids were also seen attached to milfoil but were not as abundant as the other taxa observed.

Table 3.

Limnological data collected by the Summit Lake Paiute Tribe scientific staff, 2022-2023.

Limnological data reported from three stations in SummitLake(Table3),togetherwithpreviousobservations (Simmons et al., 2020) allow the lake to be characterized as a moderately eutrophic alkaline system with high conductivity and low to moderate turbulence. Due to a change in sondes, the specific conductance data were not reliable from the first two sampling dates of September 20, 2022 and May 16, 2023. On the July 25, 2025 date, specific conductance values showed a range of 359–620 µS/cm, depending on the station, and high specific conductance values persisted through to the fall sampling on September 19, 2023. Turbidity showed some variation on the dates collected, ranging from slightly turbid (4 NTU) to cloudy (max 13 NTU), and Secchi depths ranged from 1.5–2.5 m, both varying with season and station (Table 3). It should be noted, however, that water measurements were typically taken on calm weather days, and turbidity measurements may be biased to reflect the sampling conditions.

Fig. 3.

Counts of live Stephanodiscus SUM sp. 1 from Van Dorn samples at 0.5, 5, and 9 m depth during three sampling dates in 2023.

Core chronology and mass accumulation rate (MAR).— The ²¹⁰Pb_total depth profile exhibited activities that steadily declined with increasing core depth, indicating good preservation of the sediment layers, and suggesting that post-depositional mixing was negligible. The ²¹⁰Pb_supported activities ranged between 38.0 and 55.8 Bq kg^-1, and ²¹⁰Pb_excess between 17.6 and 265 Bq kg^-1. The ²¹⁰Pb_excess flux was determined to be 236±38 Bq m^-2 year^-1, in good agreement with the mean ²¹⁰Pb depositional fluxes of around 200 Bq m^-2 year^-1, reported by Zhang et al. (2021) for the latitudinal bands of 30–40° and 40–50° North. This observation supports that the ²¹⁰Pb accumulated in the lake is mostly derived from atmospheric precipitation. The ¹³⁷Cs activities ranged between 2–123 Bq kg^-1, with detectable values almost along the whole sediment core, whereas the ²⁴¹Am activities (1.5-5.0 Bq kg^-1) showed detectable values between 8 and 26 cm depth; both artificial radionuclides showed a clear maximum at the section 20–21 cm (Fig. 4).

Fig. 4.

Down-core activity profiles for a. ²²⁶Ra and ²¹⁰Pb and b. ¹³⁷Cs and ²⁴¹Am, and c. mass accumulation rate (MAR) calculated from the ²¹⁰Pb CF age model. Horizontal error bars depict one standard deviation uncertainty in the measured activities. Samples are centered on mid-depth for each of the core interval thicknesses.

The CF model indicates that the upper 36 cm of the core contains sediments accumulated since the end of the 1800s (Fig. 4). The ¹³⁷Cs and ²⁴¹Am depth maxima (found at 20–21 cm depth) correspond to the year 1963±1, aligning with the peak of atmospheric nuclear weapons testing fallout, thereby corroborating the sediment dating and confirming the accuracy of the chronological framework. The temporal profile of MAR (Fig. 4) shows a steady increase from the early 1900's until ∼1950 (from ∼0.01 to ∼0.06 g cm^-2 yr^-1), and a sharp increase that lasted a decade between the late 1950s and late 1960s before dropping again sharply to the early 1950s value, where it remained until ∼2010, when the MAR began another gradual increase to present day values of ∼0.1 g cm^-2 yr^-1.

The Plum age model was also corroborated using the ²⁴¹Am maximum at 20–21 cm (the range of the section was 1958–1963) and it shows good congruency between the CF age model for the upper 40 cm of the sediment core (Fig. 5), which added confidence to the Pb-derived chronologies (Appleby, 2001). The age model below 40 cm is weaker because of the ¹⁴C uncertainties. The age at 110 cm derived near the base of the Plum model is 1367 CE with a ∼100-year uncertainty envelope at the 95% confidence level (1287–1395 CE). In the part of the section where age was interpolated using CLAM, between 114 and 128 cm, the uncertainty increased further. The interpolated age at 128 cm is 923 CE with a ∼200 year envelope at the 95% confidence level (809-1027 CE). The interpolated ages were rounded to 5-year intervals for plotting purposes. The median age for the lowest sample depths of diatom samples, adjusted depth based on diatom correlations, and assigned ages can be found in the supplemental data ( Table S1 (11_Supplemental_Drought_response_and_anthropogenic_impacts.zip)).

Geochemistry.— Stable isotope geochemistry taken from 2 to 112 cm showed that samples contain between 3–15 wt% C and 0.3–1.3 wt% N. The cores show a slight lightening in color below 15 cm (Fig. 2b), although it was not associated with a substantial decrease in wt.% C. δ¹³C values ranged from -20 to -23, falling into the range of those reported previously for benthic algae, macrophytes and soil carbon (Finlay and Kendall, 2007). C:N values ranged from 8–11, falling into the range for aquatic plants and algae (Kendall et al., 2001), and there is no strong trend that would indicate an increase in either terrestrial input (generally enriched in C) nor aquatic input (generally more enriched in N). A plot of C:N vs. δ¹³C, adapted from Finlay and Kendall (2007), shows the position of the Summit Lake samples relative to the major sources of particulate organic matter in freshwater systems (Fig. 6) with the organic matter in the upper 112 cm of the Summit Lake core reflective of aquatic sources, notably macrophytes and benthic algae.

Fig. 5.

Age-depth model produced in Plum. Model parameters appear in red text in the top panels which show a. the MCMC series; b-d prior (green) and posterior (gray) distributions of accumulation rate, accumulation variability (i.e. memory), and ²¹⁰Pb flux; e. levels of supported ²¹⁰Pb by depth. Blue shading on the main plot represents the total ²¹⁰Pb values estimated by the model; the blue boxes represent measured values of total ²¹⁰Pb for each section. Purple boxes are the estimated level of supported ²¹⁰Pb. The gray area is the age-depth model with the red dashed line representing the mean age and the gray dashed lines representing the uncertainty envelope at the 95% confidence level. Uncertainty envelopes for three ¹⁴C ages appear in blue on the right side of the main panel below 60 cm depth.

Sedimentary diatom flora.— A total of 70 operational taxonomic units (OTUs) were identified, then later reconciled using available literature. Of the OTUs, 30 were assigned to formally described species known in the published literature and an additional 38 species were left in open nomenclature with (e.g., Stephanodiscus SUM sp.1). To provide clarity, a subset of the flora is illustrated in Figures 7 and 8, with a focus on the more common taxa, especially those that are common and left in open nomenclature. The full reconciled voucher flora can be found in the supplemental data (S2 Table, S3 Plates). Enumeration of OTUs showed that just six were common (defined by relative abundance >2% in at least 1 sample). Benthic species accounted for 74% of the specimens enumerated throughout the core record, with Cocconeis placentula, Amphora copulata, and Nitzchia amphibia dominating the assemblages. These three benthic taxa comprise 2–40% of the total abundance in all samples combined. Planktonic species account for 24% of the diatom assemblages and are dominated by Stephanodiscus SUM sp.1 and Stephanodiscus minutulus.

Ordination analysis and constrained cluster analysis of species data from the core sequence identified predominant species drivers of core biostratigraphy and divided the core assemblages into four stratigraphic zones (Figs. 9, 10). The first two PCA axes explained 91% of the variance in species data. A large part of the variance can be attributed to the six abundant taxa: Stephanodiscus SUM sp 1., Stephanodiscus minutulus, Cocconeis placentula, Amphora copulata, Nitzschia amphibia, and fragilarioid chains. These six species vectors have the greatest magnitude in driving sample separation in a PCA species biplot (Fig. 9). The top 14.5 cm of the core plot discretely in a single grouping, with some separation of the top-most sample (Fig. 9). The remaining samples form a gradient, with Amphora- and Nitzschia-rich samples, plotting lower right quadrant, largely samples from the base of the core, and those with higher abundances of Cocconeis placentula and Stephanodiscus minutulus in the upper right quadrant, largely samples from the middle part of the core (Fig 9).

Fig. 6.

Plot of major sources of freshwater particulate matter in Summit Lake sediments (blue dots) and their associated values for C:N vs d¹³C. Boxes delineating source contributions adapted from Finlay and Kendall (2007)

A stratigraphic plot of the diatom relative abundances illustrates assemblage distinctions temporally (Fig. 10). The broken stick model indicates two statistically significant breaks in the cluster analysis, at the boundaries between Zones II and III, and Zones III and IV (Fig. 10). In addition, the lowest two samples at 123.5 and 127.5 cm show a high distance score on the dendrogram and are sufficiently distinct to be considered a separate zone (Zone I). These zones are also apparent in ordination space (Fig. 9). Briefly, Zone I (128-123 cm) is distinguished by high abundances of Stephanodiscus SUM sp. 2, Zone II (123-108 cm) by a benthic suite rich in nitzschioids and Amphora copulata, Zone III (108-15 cm) by an increase in Cocconeis placentula and Stephanodiscus minutulus, and Zone IV (15-0cm) by a dominance of Stephanodiscus SUM sp. 1. A more detailed description of these floral changes, combined with their interpreted environmental significance is provided in the discussion section below.

DISCUSSION

Core stratigraphy.— Major changes from base to top of the core are discussed below with respect to sedimentological changes, diatom abundance zones, and limnological data.

The Medieval Climate Anomaly (MCA) = 800–1300 CE

Zone I: Early MCA, 128-123 cm (935-1030 CE)—This interval represents the lowest 2 samples within the desiccated sediments (127.5 and 123.5 cm), which are distinct in that they contain comparatively high abundances of the planktonic diatoms Stephanodiscus SUM sp. 2 and Stephanodiscus minutulus. S. minutulus is a small diatom common in nutrient-rich lakes and large rivers and has been used as an indicator of highly productive waters (Cumming et al., 1995). Stephanodiscus SUM sp. 2 has a similar appearance to S. oregonicus, except that it has greater fascicle density (Fig. 7), and its ecology is unknown. It was not observed in the lake today. Epiphytes are less abundant in this zone than in those above, and the eutrophic indicator S. minutulus may suggest a phytoplankton-dominated system with low clarity limiting macrophyte growth (Scheffer et al., 1993), differing from the current state where there is considerable macrophyte coverage in the benthic environment, despite the moderate turbidity and Secchi depths of 1-2 m observed in the monitoring data (Table 3). The age of these samples is less constrained than higher in the core; the lower sample's median age is 935 CE with a 200-year uncertainty envelope. This age places these samples within the MCA.

Fig. 7.

Light micrographs of common diatoms. 1, 2. Stephanodiscus SUM sp. 1; 3. Stephanodiscus SUM sp. 2; 4. Stephanodiscus minutulus; 5. Lindavia ocellata; 6. Ulnaria SUM sp.1; 7. Ulnaria SUM sp. 2; 8. Fragilaria mesolepta; 9. Fragilaria SUM sp.2; 10. Pseudostaurosira parasitica; 11. Pseudostaurosira brevistriata; 12,13. Pseudostaurosira SUM sp. 2; 14, 15. Staurosira SUM sp. 2; 16. Staurosirella martyi; 17, 18. Nitzschia SUM sp. 2, 19, 20. Nitzschia SUM sp. 1, 21. Nitzschia SUM sp. 7; 22. Tryblionella hungarica; 23. Hantzschia amphioxys; 24. Surirella undulata; 25. Surirella SUM sp. 1; 26. Cocconeis placentula sensu lato; 27. Planothidium frequentissimum; 28. Gogorevia exilis; 29. Placoneis SUM sp. 1; 30. Pinnularia SUM sp. 1; 31. Pinnularia SUM sp. 3; 32 Pinnularia borealis. Scale bars in all figures 10 µm.

Fig. 8.

Light micrographs of common diatoms. 1. Brachysira SUM sp. 1; 2. Navicula SUM sp. 2; 3,4. Navicula SUM sp. 1; 5. Epithemia turgida; 6. Epithemia adnata; 7. Epithemia sorex; 8. Epithemia gibba; 9, 10. Cymbella mexicana species group; 11. Halamphora SUM sp. 1; 12, 13. Amphora copulata; 14, 15. Rhoicosphenia abbreviata; 16, 17. Gomphonema SUM sp. 1; 18, 19. Gomphonema SUM sp. 2; 20, 21. Gomphonema SUM sp.7; 22, 23. Gomphonema SUM sp. 3; 24, 25. Gomphonema SUM sp. 4; 26. Gomphonema SUM sp. 6.; 27,28. Gomphonema SUM sp. 5. Scale bars in all figures 10 µm.

Zone II: late MCA, 123-108 cm (935-1375 CE)—The base of this zone is marked by a decrease in phytoplankton and increase in benthic species, the most prominent of which are Amphora copulata, Fragilaria mesolepta, Gomphonema spp., and Epithemia spp. (Figs. 7, 8). Maximum abundance of this benthic suite occurs in the middle of the zone, decreasing towards the top while the benthic epiphyte Cocconeis placentula and the phytoplankter Stephanodiscus minutulus are increasing (Fig. 10). Both Amphora copulata and Nitzschia amphibia are solitary motile species, Gomphonema species produce mucilage stalks and are epiphytic, as are Epithemia which grows prostrate on substrates (Spaulding & Edlund, 2009; Spaulding, 2010). Epithemia spp. are additionally associated with N-fixing cyanobacteria (DeYoe et al., 1992; Stancheva et al., 2013) and hint at a shift in nutrient dynamics towards N-limitation in this interval. This subfossil assemblage suggests a lake environment characterized by higher benthic habitat area, and the near absence of phytoplankton suggests this is likely the result of lowered water levels (Reinemann et al., 2009; Hobbs et al., 2011). This zone straddles the top of the desiccated interval at 115 cm and fits with the interpretation that the maximum precipitation deficits occurred towards the end of the MCA, ca. 1200 CE, and were sufficient to dry up the lake in its deepest spot. Interestingly, the shallow benthic flora associated with this zone persists for another ∼175 years above the top of the desiccated sediments, after the lake's return, until a new lacustrine state is established ca. 1375 CE at the end of the MCA.

Fig. 9.

Principal Component Analysis (PCA) biplot of components 1 and 2 based on species relative abundance values showing sample sites (dots numbered with sample depth, cm) and selected species (vectors). Sample depths are given in top depth (i.e. 0–0.5 cm labeled as 0). Shaded fields correspond to the diatom zones shown in Fig. 10.

Post Medieval Climate Anomaly (MCA) = 1300–2022 CE

Zone III: Macrophyte phase, 108–14.5 cm (∼1375–1986 CE)—This zone is defined by Cocconeis placentula, which reaches its peak abundance in this interval. Cocconeis placentula is a prostrate epiphyte that can establish on rocks, macrophytic algae, and aquatic plants. It is abundant on the filamentous algae Cladophora (Kingston, 2003) and is known to be tolerant of high nutrient levels (Paul et al., 2020). Modern epiphytic and eplithic samples taken along the shoreline and in the littoral zone were rich in Cocconeis placentula, and it was the most common species growing epiphytically on Summit Lake milfoil samples. The phytoplankton component in this interval consists largely of S. minutulus. This suggests that there may have been eutrophic waters, and sufficient water depth to support a phytoplankton community, but with sufficient clarity to support abundant macrophytes in the benthic habitat. The decreased presence of Epithemia spp. indicates changing nutrient ratios in this zone, moving away from N-limited conditions.

Zone IV. Phytoplankton phase, 14.5–0 cm (∼1986–2022 CE).—This zone reflects a significant transformation in diatom community structure, with the phytoplankter Stephanodiscus SUM sp.1 emerging rapidly as the dominant species (Fig. 10), attaining a relative abundance of 69%. This zone has comparatively few benthic species and suggests a shift in lake ecology to a plankton dominated ecosystem, with a reduced macrophyte population and therefore a reduced substrate base for Cocconeis placentula, compared to 50 years ago. This environment persists today, although the uppermost sample shows a recent shift towards increased benthic algae that may be attributable to the amount of macrophyte growth seen today in the littoral areas of the lake. Examination of the core shows a color change coinciding with the base of this zone (Fig. 2b), but no significant change in organic matter concentrations nor source material.

Fig. 10.

Stratigraphic plot of most abundant diatom taxa (planktonic and benthic taxa by percent abundance) showing sediment core depth (cm) on the primary y-axis and age (yrs CE) on the secondary y-axis. Stratigraphic zones (I - IV) identified by constrained cluster analysis. The cluster dendrogram appears on the far right and its x axis (distance) is total sum of squares.