Paula Echeverría-Galindo

Aquatic ecosystems have changed dramatically, but the relative roles of external forcings and internal atmospheric variability remain unclear. Here, using Tibetan lake ecological reconstructions and Earth system simulations, we reveal how shared external forcings shaped Tibetan Plateau limnoecology over the past millennium through two distinct pathways. In the temperature-centric pathway, cooling episodes driven by volcanic activity and internal variability likely regulated preindustrial lake conditions. This baseline was disrupted as recent forced warming has shortened lake ice-cover and increased meltwater input, altering lake resources and triggering unprecedented diatom shifts. In the freshening-centric pathway, salinity-tolerant diatoms tracked monsoon-driven precipitation changes and lake freshening, both governed by shifts in the intertropical convergence zone. Preindustrial shifts likely reflected hemispherically asymmetric orbital and volcanic forcings, whereas modern changes have been altered remarkably by Northern Hemisphere industrial aerosol fluctuations and warming-induced meltwater. As multifaceted stressors intensify, Tibetan lake ecosystems may continue diverging from their natural variability.

A new ostracod species, Heterocypris exodonta Bonilla-Flores & Karanovic, sp. nov. is described from a high-altitude temporary pond near Nam Co, Southern Tibetan Plateau. Detailed morphological analyses of valves, soft body, and the partial mitochondrial COI (cytochrome c oxidase subunit I) sequence distinguish this species from its close congeners, Heterocypris incongruens and Heterocypris salina . Key taxonomic features include the morphology of the female genital lobe and the structure of the internal opening of normal pores, particularly variations in the female genital aperture at the intersection of the seminal duct and the oviduct. Pore openings in H. exodonta Bonilla-Flores & Karanovic, sp. nov. and H. incongruens exhibit a turbine shape, while in H. salina they display a simple aperture. Phylogenetic analysis based on the COI dataset supports the new species description and suggests potential synonymy and cryptic species within the H. salina and H. incongruens complexes. Ecologically, these species mainly inhabit temporary water bodies with high oxygen levels and likely follow an r-type ecological strategy, being opportunistic in colonizing new habitats, reproducing via parthenogenesis, and undergoing rapid population growth. This study enhances the understanding of Southern Tibetan Plateau ostracod biodiversity, providing crucial morphological and molecular data for species identification, particularly for parthenogenetic species lacking standard male morphological characters. Additionally, this research offers methods to prevent misidentifications and misinterpretations in (paleo-)ecological studies, contributing significantly to the broader knowledge of taxonomy and ecology of ostracods from the Southern Tibetan Plateau. Due to the high morphological plasticity of Heterocypris incongruens , our findings highlight the need for caution when identifying similar species, as external resemblance may conceal genetic divergence.

Taxonomic clarification of Tonnacypris stewarti comb. nov. (= Herpetocypris stewarti), a non-marine ostracod first described by Daday in 1908, was required due to the existence of various synonyms without detailed morphological descriptions. Our study examined specimens from Mang-tsa, Nam Co (Tibetan Plateau) and Lake Band-e Amir (Afghanistan). We identified seven definitive synonyms (Herpetocypris stewarti Daday 1908, Ilyodromus estonicus Järvekülg 1960, Eucypris afghanistanensis Hartmann 1964, E. gyirongensis Yang 1982, E. rischtanica Schneider 1963 in Mandelstam & Schneider 1963, Tonnacypris estonica (Järvekülg 1960), and T. gyirongensis (Guo et al. 2016). After a detailed study of the morphology of valves and soft parts supplemented by a literature review, we propose combining the genus Tonnacypris with the specific epithet stewarti, resulting in the new name Tonnacypris stewarti. We documented the ontogenetic series of T. stewarti in surface sediment samples from Nam Co. Only female specimens were found, consistent with observations in Mongolia, where they were considered asexual populations. Sexual populations were reported from Afghanistan and the southern Tibetan Plateau, suggesting geographic parthenogenesis in the Palearctic region. Because of widely missing males, we explored the use of female genital lobes as a distinguishing feature for taxonomy. Considering current ecological knowledge, T. stewarti could serve as a (paleo)—bioindicator of cold, oligotrophic, and fresh to slightly brackish habitats, including rivers, lagoons, and lakes. This study emphasizes the importance of resolving taxonomic issues for future paleolimnological research. Moreover, our investigation highlights the potential use of sclerotized internal female genital lobes as distinctive characteristics for species differentiation, especially in asexual populations.

The recent rise in air temperatures detected at high altitudes of the Tibetan Plateau has accelerated glacier melt and retreat. Moreover, enhanced monsoonal precipitation has increased runoff and transport of allochthonous material to the lakes. Consequently, water levels are rising, modifying the spatial distribution and composition of local aquatic biota. To infer these environmental and biological changes in recent decades, a 30‐cm‐long sediment core, representing the past ~160 years, from Nam Co, an endorheic lake, was analyzed for subfossil chironomid assemblages and sediment geochemistry. In total, 25 chironomid morphotypes were identified. Nineteen were considered as non‐rare taxa (abundances ≥2%) and six as rare taxa (abundances S = 19) is evident compared to the previous 100 years. The simultaneous decrease in the abundance of profundal Micropsectra radialis‐type and increase of both Chironomus and Procladius, taxa adapted to more eurytopic and slightly warmer water bodies, indicate increasing water temperatures and intensified primary productivity. The dominance of littoral chironomid assemblages reflects increasing lake water levels, flooded shorelines and expansion of littoral areas driven by increased precipitation and glacial meltwater input both resulting from the increase in air temperatures. This scenario is confirmed by increases in total nitrogen and Zr/Rb ratios, indicating higher productivity and coarser grain size as a consequence of increased runoff via the Niya Qu. These hydrological changes have resulted in a positive water balance that can be linked to an increase in moisture supply from the Indian summer monsoon and glacier melt, reflecting increasing temperatures and precipitation since 1956 ce, ultimately driven by anthropogenic warming.

Lake Tzibaná is one of the largest (1.27 km²) and deepest (Z_max = 52 m) karstic lakes in the UNESCO’s Biosphere Reserve “Nahá-Metzabok” and in the Lacandon Forest, southeastern Mexico. It archives sediments from multiple sources and the inflowing Nahá River forms deltaic deposits. In 2019, the water level in Lake Tzibaná declined by ~ 15 m, persisting for 4 months and exposing the Nahá River Delta. A geophysical profile on the exposed delta revealed an accumulation of ~ 20 m of such deposits. Three sediment outcrops from an inactive channel in the Nahá River Delta, which ranged in height from 0.6 to 1.43 m, were sampled and a multi-proxy analysis of biological remains and geochemical variables was conducted. Four facies were observed: (1) massive-coarse sand, (2) fine sand, (3) dark leaf litter and (4) massive silty clay, each characterized by specific microcrustacean, testate amoebae and diatom taxa. Six dark leaf litter horizons were radiocarbon dated and revealed a complex depositional history including inverted ages making the establishment of an age model difficult. Nevertheless, past lake-level changes and the formation of the four facies match three characteristic water-level stages, which can also be observed on recent satellite images: (1) Massive-coarse sand deposits, with compositional and sedimentological characteristics of a shoreline environment and fluvial lateral banks, were formed during large-magnitude reductions in the lake level, similar to the one in 2019, (2) Interbedded layers of fine sand and dark leaf litter, currently found in low-energy fluvial environments, were formed during shorter and less pronounced decrease intervals, and (3) Massive silty clay, with distinctive microorganisms from low-energy lacustrine environments, is deposited during high water-level stages, when the delta is covered by water. Our findings illustrate how hydrological changes alter sedimentary dynamics in deltaic areas of lakes. Despite the complexity of their depositional processes, deltaic records can serve as a complementary source of paleolimnological information to records from distal zones due to their sensitivity to variations in water level, especially during extreme and prolonged desiccation events. Future research should attempt to combine evidences from deltaic and sediment sequences from deeper zones of Lake Tzibaná to reconstruct water-level variations during the entire Holocene. Understanding past lake-level reductions is not only relevant for the local indigenous communities but also crucial for the conservation of this ecosystem of international importance.

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

Lakes on the Tibetan Plateau are especially vulnerable to global warming and increasing temperatures but are also sensitive to changes in the atmospheric circulation such as the Westerlies and the Asian Summer Monsoon, which are main drivers of precipitation on the Plateau. Shallow lake environments in such high-altitudinal areas, which are not directly influenced by meltwater supply, are excellent study sites to determine changes in precipitation and evaporation. Here, we present a 300-year high-resolution chironomid record from the high-altitude (> 4,733 m a.s.l.), saline (9 g L-1) and shallow (~ 5 m water depth) lake Shen Co (N 31° 00’, E 90°29’), situated on the southern part of the central Tibetan Plateau. We combined chironomid assemblages with sedimentological, mineralogical and geochemical data from a short sediment core (37.5 cm) to detect hydrological changes since 1830 CE. Our study revealed three different periods in Shen Co: (1) from 1830 until 1920 CE sediments were void of chironomids, suggesting dry conditions leading to low lake levels, high salinity resulting from low runoff and high evaporation rates, supported by increasing Mg/Ca and Sr/Rb ratios of the sediments; (2) a humid phase characterized by the appearance of Acricotopus indet. morphotype incurvatus from 1920 until 1950 CE, indicating rising lake levels caused by higher runoff and decreased evaporation, also supported by sediment analysis with increasing TOC and Ti as well as a decreasing Ca/Ti ratio; and (3) a continuous water level rise from the 1950s onwards with a lake level maximum as well as high macrophyte growth since the beginning of the 21th century, supported by the dominance of Procladius and the phytophilic taxon Psectrocladius sordidellus-type. We compared our results with paleoclimate records from the Tibetan Plateau, based on e.g. ice core δ18O, pollen, tree rings, indicating warmer and wetter climate conditions on the central Tibetan Plateau during the last ~ 100 years. Our findings highlight that chironomid records from shallow lake environments are excellent indicators of lake level variations as well as changes in macrophyte vegetation.

Dwelling in a variety of aquatic habitats, one of the most abundant groups of microcrustaceans, ostracodes, are widely used indicator organisms in paleolimnological research. Typically, they are identified via traditional methods using morphological features but this may be excessively time‐consuming and prone to inter‐investigator variation. DNA barcoding and metabarcoding have become important tools for specimen identification, with a great impact in the field of taxonomy, (paleo‐)ecology and evolution. Despite its potential, metabarcoding has been rarely used to analyze the community structure of ostracodes. Here, we evaluate the performance of a metabarcoding approach for ostracode identification in surface sediment samples collected from Lake Nam Co on the Tibetan Plateau. We tested six different primer pairs amplifying fragments of three different genes, and compared their success in inferring ostracode communities, coupled with morphological identification of ostracodes from the same sediment samples. In total, depending on the primer pair used, seven to nineteen ostracode amplicon sequence variants (ASVs) were identified. Via microscopy, eight morphospecies were identified. We found considerable differences between primer pairs in yielding ostracode sequences via metabarcoding. In general, the highest proportions of ostracode reads and ASVs were found with primers amplifying fragments of the 18S rRNA gene, whereas primers for COI gene had the highest in silico amplification success and highest sequencing depth per sample but only contained <1% of ostracode sequences. As a consequence, the metabarcoding results with 18S rRNA gene were more consistent with the morphological data compared to those obtained with COI or mitochondrial 16S rRNA primers. No significant effects of treatment with different sediment quantities for DNA extraction (10 g vs. 0.5 g) were found on ostracode ASVs community composition. These results indicate that DNA metabarcoding can serve as an efficient tool for ostracode‐based environmental reconstructions but requires an informed decision on primers and target gene, as well as extending the barcoding database for improved accuracy.

High altitudinal aquatic ecosystems are subject to environmental change due to global warming and increasing solar radiation. The Nam Co catchment is part of the highest and largest alpine plateau on Earth, where the effects of climate change are expressed stronger than the global average. Thus, this area has experienced rapid changes in biodiversity. Fluctuations between wetter and drier periods during the last 2,000 calibrated (cal.) years were detected. These changes may alter the dynamics in ecosystems and therefore their resilience to climate change.A ~65 cm sediment record from Nam Co spanning the late Holocene, was analyzed to evaluate the assemblage composition of three of the most abundant and diverse benthic taxa (Arcellinidae, Ostracoda and Chironomidae) and the diverse family of small bivalves (Sphaeriidae). In general, the presence of the bivalve Pisidium stewarti, together with a high abundance of black-coated ostracod shells, and high Ca/Ti and Zr/Rb ratios correspond to the driest period (~ 1,000 - 1,860 cal. years BP) detected in our sediment record. For the last 256 cal. years, higher lake levels were inferred from aquatic fauna composition and geochemical analysis (XRF and XRD) suggesting a more humid environment. This period was characterized by higher temperatures and a higher input of organic matter. Species not previously reported for Lake Nam Co such as Arcellinida species, the ostracod Ilyocypris angulata, several chironomid species, and the bivalve P. stewarti, were observed. These new records, as well as the detection of varieties in morphological structures (e.g. spines, aggregate material, valve ornamentations, etc.) highlight the probable existence of cryptic species in the ecosystem, which is an important factor to take into account for biodiversity evaluation and paleoenvironmental inferences, due to potential misleading ecological interpretation.Therefore, emphasis should be placed on combining ecology, morphology and DNA analysis to corroborate the taxonomy of species already described, and determine the accurate richness and distribution of the species in an environment where endemism is expected. This is essential in order to evaluate possible losses or gains in terms of diversity that climate change may exert on aquatic ecosystems in the future.