Subterranean Waters of Yucatán Peninsula, Mexico Reveal Epigean Species Dominance and Intraspecific Variability in Freshwater Ostracodes (Crustacea: Ostracoda)

The Yucatán Peninsula is a karstic region, rich in subterranean environments with a diverse crustacean stygobiont fauna. In order to gain insights into the biological evolution of the subterranean environments of this region, we evaluated the ostracode species composition of caves and cenotes in five independent sampling campaigns (2008, 2013, 2017–2019). Using morphometric analyses, we evaluated inter-population morphological variability; using molecular analysis based on mitochondrial COI and nuclear 18S rDNA, we evaluated genetic differentiation in selected species. The observed fauna is composed of 20 (epigean) species, presenting a lack of strict stygobionts. Morphometric analyses discriminated up to three morphotypes in each of the three most abundant species: Cytheridella ilosvayi, Alicenula sp. and Cypridopsis vidua. High intraspecific morphological variability was found either in shape or size. Phylogenetic analysis based on COI demonstrated the existence of three lineages on C. ilosvayi, with high support (>0.9). The 18S rDNA sequences were identical among individuals of different populations. A lack of congruence between the genetic markers precluded us from postulating speciation in subterranean environments. It is likely that Late Pleistocene—Early Holocene climate variation related to sea level and precipitation was forcing agent for epigean ostracode dominance in subterranean environments of the Peninsula

Influence of late Quaternary climate on the biogeography of Neotropical aquatic species as reflected by non-marine ostracodes

We evaluated how ranges of four endemic and non-endemic aquatic ostracode species changed in response to long-term (glacial–interglacial cycles) and abrupt climate fluctuations during the last 155 kyr in the northern Neotropical region. We employed two complementary approaches, fossil records and species distribution models (SDMs). Fossil assemblages were obtained from sediment cores PI-1, PI-2, PI-6 and Petén-Itzá 22-VIII-99 from the Petén Itzá Scientific Drilling Project, Lake Petén Itzá, Guatemala. To obtain a spatially resolved pattern of (past) species distribution, a downscaling cascade is employed. SDMs were reconstructed for the last interglacial (∼120 ka), the last glacial maximum (∼22 ka) and the middle Holocene (∼6 ka). During glacial and interglacial cycles and marine isotope stages (MISs), modelled paleo-distributions and paleo-records show the nearly continuous presence of endemic and non-endemic species in the region, suggesting negligible effects of long-term climate variations on aquatic niche stability. During periods of abrupt ecological disruption such as Heinrich Stadial 1 (HS1), endemic species were resilient, remaining within their current areas of distribution. Non-endemic species, however, proved to be more sensitive. Modelled paleo-distributions suggest that the geographic range of non-endemic species changed, moving southward into Central America. Due to the uncertainties involved in the downscaling from the global numerical to the highly resolved regional geospatial statistical modelling, results can be seen as a benchmark for future studies using similar approaches. Given relatively moderate temperature decreases in Lake Petén Itzá waters (∼5 ∘C) and the persistence of some aquatic ecosystems even during periods of severe drying in HS1, our data suggest (1) the existence of micro-refugia and/or (2) continuous interaction between central metapopulations and surrounding populations, enabling aquatic taxa to survive climate fluctuations in the northern Neotropical region

Geodiversity influences limnological conditions and freshwater ostracode species distributions across broad spatial scales in the northern Neotropics

Geodiversity is recognized as one of the most important drivers of ecosystem characteristics and biodiversity globally. However, in the northern Neotropics, the contribution of highly diverse landscapes, environmental conditions, and geological history in structuring large-scale patterns of aquatic environments and aquatic species associations remains poorly understood. We evaluated the relationships among geodiversity, limnological conditions, and freshwater ostracodes from southern Mexico to Nicaragua. A cluster analysis (CA), based on geological, geochemical, mineralogical, and water-column physical and chemical characteristics of 76 aquatic ecosystems (karst, volcanic, tectonic) revealed two main limnological regions: (1) karst plateaus of the Yucatán Peninsula and northern Guatemala, and (2) volcanic terrains of the Guatemalan highlands, mid-elevation sites in El Salvador and Honduras, and the Nicaraguan lowlands. In addition, seven subregions were recognized, demonstrating a high heterogeneity of aquatic environments. Principal component analysis (PCA) identified water chemistry (ionic composition) and mineralogy as most influential for aquatic ecosystem classification. Multi-parametric analyses, based on biological data, revealed that ostracode species associations represent disjunct faunas. Five species associations, distributed according to limnological regions, were recognized. Structural equation modeling (SEM) revealed that geodiversity explains limnological patterns of the study area. Limnology further explained species composition, but not species richness. The influence of conductivity and elevation were individually evaluated in SEM and were statistically significant for ostracode species composition, though not for species richness. We conclude that geodiversity has a central influence on the limnological conditions of aquatic systems, which in turn influence ostracode species composition in lakes of the northern Neotropical region

Geodiversity influences limnological conditions and freshwater ostracode species distributions across broad spatial scales in the northern Neotropics

Geodiversity is recognized as one of the most important drivers of ecosystem characteristics and biodiversity globally. However, in the northern Neotropics, the contribution of highly diverse landscapes, environmental conditions, and geological history in structuring large-scale patterns of aquatic environments and aquatic species associations remains poorly understood. We evaluated the relationships among geodiversity, limnological conditions, and freshwater ostracodes from southern Mexico to Nicaragua. A cluster analysis (CA), based on geological, geochemical, mineralogical, and water-column physical and chemical characteristics of 76 aquatic ecosystems (karst, volcanic, tectonic) revealed two main limnological regions: (1) karst plateaus of the Yucatán Peninsula and northern Guatemala, and (2) volcanic terrains of the Guatemalan highlands, mid-elevation sites in El Salvador and Honduras, and the Nicaraguan lowlands. In addition, seven subregions were recognized, demonstrating a high heterogeneity of aquatic environments. Principal component analysis (PCA) identified water chemistry (ionic composition) and mineralogy as most influential for aquatic ecosystem classification. Multi-parametric analyses, based on biological data, revealed that ostracode species associations represent disjunct faunas. Five species associations, distributed according to limnological regions, were recognized. Structural equation modeling (SEM) revealed that geodiversity explains limnological patterns of the study area. Limnology further explained species composition, but not species richness. The influence of conductivity and elevation were individually evaluated in SEM and were statistically significant for ostracode species composition, though not for species richness. We conclude that geodiversity has a central influence on the limnological conditions of aquatic systems, which in turn influence ostracode species composition in lakes of the northern Neotropical region.</p

Mercury Pollution History in Tropical and Subtropical American Lakes: Multiple Impacts and the Possible Relationship with Climate Change

Sediment cores obtained from 11 tropical and subtropical American lakes revealed that local human activities significantly increased mercury (Hg) inputs and pollution levels. Remote lakes also have been contaminated by anthropogenic Hg through atmospheric depositions. Long-term sediment-core profiles revealed an approximately 3-fold increase in Hg fluxes to sediments from c. 1850 to 2000. Generalized additive models indicate that c. 3-fold increases in Hg fluxes also occurred since 2000 in the remote sites, while Hg emissions from anthropogenic sources have remained relatively stable. The tropical and subtropical Americas are vulnerable to extreme weather events. Air temperatures in this region have shown a marked increase since the 1990s, and extreme weather events arising from climate change have increased. When comparing Hg fluxes to recent (1950-2016) climatic changes, results show marked increases in Hg fluxes to sediments during dry periods. The Standardized Precipitation-Evapotranspiration Index (SPEI) time series indicate a tendency toward more extreme drier conditions across the study region since the mid-1990s, suggesting that instabilities in catchment surfaces caused by climate change are responsible for the elevated Hg flux rates. Drier conditions since c. 2000 appear to be promoting Hg fluxes from catchments to lakes, a process that will likely be exacerbated under future climate-change scenarios

Imprints of the Little Ice Age and the severe earthquake of AD 2001 on the aquatic ecosystem of a tropical maar lake in El Salvador

Using a 530-year sediment record from the maar Lake Apastepeque, El Salvador, and based on diverse geochemical and biological (cladocerans, chironomids, diatoms, ostracods, testate amoebae) indicators, we estimated climatic and environmental alterations during the Little Ice Age (LIA) and reconstructed the recent history of the lake. Results demonstrate relatively humid conditions in the mid-elevations (500 m a.s.l.) of El Salvador during most parts of the LIA, resulting in high lake levels. Contrarily, the first part of the LIA was characterized by drier climates comparable to studies from Mexico and Belize, which correlated this phase with the Spörer minimum. Regional comparison with palaeorecords from the northern Neotropics reveals a high heterogeneity in local expressions of the LIA in Central America, likely connected to the high topographic heterogeneity of the region. Since the beginning of the 20th century, Lake Apastepeque has experienced enhanced human impact expressed as increased nutrient supply. The most recent period was characterized by significant environmental disturbance, which we relate to an upper-crustal earthquake, one of the strongest over the last 500 years, that affected the region on 13th February 2001 (Mw = 6.6, epicentre at 10 km depth, 30 km from the lake). The release of toxic bottom components such as hydrogen sulphide and high turbidity and turbulence of water caused major species turnover in the lake ecosystem, resulting in a massive fish kill and colonization by large cladocerans. Modern sediments still show slightly altered biota communities compared to pre-earthquake assemblages, indicating that the ecosystem has still not fully recovered

Tropical freshwater ostracodes as environmental indicators across an altitude gradient in Guatemala and Mexico

Ostracodes are bivalve microcrustaceans with calcium carbonate shells that preserve well in lake sediment. They are very sensitive to environmental variables and are therefore powerful tools in paleoclimate and paleoenvironmental studies that cover time periods from decades to millions of years. Detailed knowledge of species ecological preferences and robust taxonomy are prerequisites for such studies. Such information, however, is still lacking for many areas of the world, including the Neotropics. Previous studies in the northern Neotropics were conducted mainly in the karst lowlands of the Yucatán Peninsula, but higher-altitude areas remained relatively poorly investigated. This study was designed to expand our knowledge of the modern, Neotropical freshwater ostracode fauna, across an altitudinal gradient from the karst lakes in the lowlands of El Petén, Guatemala (~100 - 500 m asl), to the mid-elevation water bodies of the Lacandón forest (~500 - 1000 m asl), to the higher-altitude lakes of Montebello, Chiapas, México (~1000 - 1500 m asl). Eighteen ostracode species were identified in 24 lakes. Ostracodes were absent in Lakes Amarillo and Lacandón (mid-altitude), and San Diego (lowlands). Statistical analysis indicated that the most abundant species, Cypridopsis vidua, Cytheridella ilosvayi, Pseudocandona antillana, and Darwinula stevensoni have a continuous distribution along the entire altitudinal gradient. Other species display more restricted distributions, determined by temperature, precipitation and conductivity. For example, Eucypris sp. is restricted to the lowlands, Vestalenula sp. and Cypria sp. were found only at middle elevations, and a Cyprididae species was restricted to the highlands. Species diversity is slightly greater in warm lakes at middle altitudes (Haverage = 1.09) than in water bodies in the lowlands (Haverage = 0.94) and in cooler lakes in the highlands (Haverage = 0.94). LOESS regressions provided ecological preference information for the four most frequent and widely distributed species, with respect to temperature, conductivity, bicarbonate (HCO3-) concentration, precipitation, and pH. Cypria petenensis, Heterocypris punctata, and Paracythereis opesta display higher abundances in lowland lakes, whereas, Cytheridella ilosvayi, and Pseudocandona antillana prefer lowland and mid-elevation lakes. Environmental conditions in the higher-elevation lakes of Montebello favor the presence of Darwinula stevensoni. Such quantitative ecological information will improve ostracode-based paleoenvironmental reconstructions in southern México and northern Guatemala, and our approach serves as a model for future paleoecological studies that employ other aquatic bioindicators, such as testate amoebae, cladocerans, and chironomids

Ostracode relative abundances in surface sediments in Northern Neotropics (southern Mexico, Yucatán Peninsula to Nicaragua)

The following database contains freshwater ostracode relative abundances from littoral and sediment surface samples of 76 aquatic ecosystems of the Northern Neotropical region. Samples were collected during July-October 2013, coinciding with the rainy season in the region. Sampling sites are located on the Yucatán Peninsula Mexico (n=28), Guatemala (n=26), El Salvador (n=14), Honduras (n=5) and Nicaragua (n=3). Ostracode species were used to test their potential as bioindicators of (paleo) climate change and anthropogenic impact in aquatic ecosystems of the region. At littoral areas, we sampled between submerged vegetation using a 250 µm mesh hand net, whereas sediment samples were collected from the deepest bottom using an Ekman grab. Ostracode extraction and counting was carried out using 15 cm3 of each sample. Species identification was undertaken using individual adult specimens with complete soft parts. When identification down the species level was not possible, we assigned a putative species name (e.g., “Cyprideis sp 1”)

Climate ultrastructure and aquatic community response to Heinrich Stadials (HS5a-HS1) in the continental northern Neotropics

Highlights • Evaluation of environmental variability induced by Heinrich Stadials (HS5a-HS1) in continental northern Neotropical region. • Multiproxy evidence reveals mild temperature decreases and drastic fluctuations in precipitation during HSs. • Ultrastructure of HSs suggests individual environmental response of each Stadial making them contrasting from each other. • Most drastic climate changes induced by HSs exerted positive effects on diversity of aquatic communities. Abstract We reconstruct environmental conditions of the period 53-14 kyr BP in the continental northern Neotropical region. We evaluate in detail the magnitude of climatic fluctuations and their effects on aquatic communities during six Heinrich Stadials (HS1-HS5a), using sediments from Lake Petén Itzá, Guatemala, and a multiproxy approach. In Lake Petén Itzá typical Heinrich Stadials (HSs) are recorded in sediments as alternations of gypsum and clay, and abrupt changes in magnetic susceptibility, CaCO3 and biological compositions. This suggests that HSs were periods of hydrological unbalance, characterized by dry spells, punctuating the predominant humid conditions characterizing the period 53-14 kyr BP. The ultrastructure of HSs allows us to identify four different types of climatic conditions associated to HSs: 1) prevailing dry conditions but changing to humid (HS5, HS3); 2) predominantly humid conditions but changing to arid (HS2); 3) fluctuating humid-dry-humid (HS4, HS1); and 4) arid with high lake water conductivity (HS5a). The continuous presence of tropical ostracode species during HSs suggests that lake water temperatures were not drastically lowered. Ostracode-based transfer functions indicate that during HSs, epilimnetic water temperatures decreased by 1–3 °C compared to mean modern temperatures. Lake solute composition and conductivity were strongly affected by HSs. During HS5a and HS1 we estimate conductivity values > 800 μS cm−1. Diversity indices show significant differences (F5,70 = 3.74, p = 0.004) of ostracode species composition among HSs. Highest diversities occurred during HS5a, HS4 and HS1, which display greater climatic alterations than the other HSs. Fluctuating climates seem to have exerted positive effects on diversity of aquatic communities by producing an increase in habitat heterogeneity

Limnological, sedimentological, mineralogical and geological data of 76 aquatic ecosystems of the northern Neotropics (from Yucatán Peninsula Mexico to Nicaragua) sampled in 2013

Here, we present 23 water physical (temperature, dissolved oxygen, pH, conductivity) and chemical (major anions Cl-, SO42-, CO32-, HCO3- and cations Ca2+, K+, Mg2+, Na+), sedimentological (total carbon (TC), total inorganic carbon (TIC), total organic carbon (TOC), total nitrogen (TN)), mineralogical (quartz, carbonate, phyllosilicates and feldspars) and geological (altitude, bedrock type and age of sediments) variables of aquatic systems of the Northern Neotropical region. Sampling was conducted in 76 aquatic systems during July-October 2013, coinciding with the rainy season in the region. Sampling sites are located on the Yucatán Peninsula Mexico (n=28), Guatemala (n=26), El Salvador (n=14), Honduras (n=5) and Nicaragua (n=3). We aim to identify limnological regions based on the measured variables and to infer the influence of geodiversity in observed patterns. Water physical and chemical variables were measured in situ with a WTW Multi Set 350i multiparameter probe at a water depth of 0.5 m. Water samples for analysis of major anions and cations were collected at water depths of 0.5 m below surface. TC and TN in sediments contents were determined by combustion with a LECO TruSpec Macro CHN analyzer. TIC was quantified with a Woesthoff Carmhograph C-16. TOC was calculated by subtracting TIC from TC. Qualitative and semi-quantitative mineralogical compounds were examined by x-ray diffraction with a RIGAKU Miniflex600. ArcGIS software was used to identify geological attributes of sampling sites such as bedrock and age of sediments. Altitude, latitude, and longitude were determined with the navigator Garmin GPSmap 60c