Geology topics

Research about Mexico

Source-linked reports with geographic coverage including Mexico.

At least 55 records · Page 3Linked to original sources

USGS2021

Late Holocene environmental change in Celestun Lagoon, Yucatan, Mexico

Epikarst estuary response to hydroclimate change remains poorly understood, despite the well-studied link between climate and karst groundwater aquifers. The influence of sea-level rise and coastal geomorphic change on these estuaries obscures climate signals, thus requiring careful development of paleoenvironmental histories to interpret the paleoclimate archives. We used foraminifera assemblages, carbon stable isotope ratios (δ 13 C) and carbon:nitrogen (C:N) mass ratios of organic matter in sediment cores to infer environmental changes over the past 5300 years in Celestun Lagoon, Yucatan, Mexico. Specimens (> 125 µm) from modern core top sediments revealed three assemblages: (1) a brackish mangrove assemblage of agglutinated Miliammina and Ammotium taxa and hyaline Haynesina (2) an inner-shelf marine assemblage of Bolivina , Hanzawaia , and Rosalina, and (3) a brackish assemblage dominated by Ammonia and Elphidium . Assemblages changed along the lagoon channel in response to changes in salinity and vegetation, i.e. seagrass and mangrove. In addition to these three foraminifera assemblages, lagoon sediments deposited since 5300 cal yr BP are comprised of two more assemblages, defined by Archaias and Laevipeneroplis, which indicate marine Thalassia seagrasses, and Trichohyalus, which indicates restricted inland mangrove ponds. Our data suggest that Celestun Lagoon displayed four phases of development: (1) an inland mangrove pond (5300 BP) (2) a shallow unprotected coastline with marine seagrass and barrier island initiation (4900 BP) (3) a protected brackish lagoon (3000 BP), and (4) a protected lagoon surrounded by mangroves (1700 BP). Stratigraphic (temporal) changes in core assemblages resemble spatial differences in communities across the modern lagoon, from the southern marine sector to the northern brackish region. Similar temporal patterns have been reported from other Yucatan Peninsula lagoons and from cenotes (Nichupte, Aktun Ha), suggesting a regional coastal response to sea level rise and climate change, including geomorphic controls (longshore drift) on lagoon salinity, as observed today. Holocene barrier island development progressively protected the northwest Yucatan Peninsula coastline, reducing mixing between seawater and rain-fed submarine groundwater discharge. Superimposed on this geomorphic signal, assemblage changes that are observed reflect the most severe regional wet and dry climate episodes, which coincide with paleoclimate records from lowland lake archives (Chichancanab, Salpeten). Our results emphasize the need to consider coastal geomorphic evolution when using epikarst estuary and lagoon sediment archives for paleoclimate reconstruction and provide evidence of hydroclimate changes on the Yucatan Peninsula.

Yucatan, Celestun Lagoon↗

USGS2021

First documentation of long-distance travel by a Florida manatee to the Mexican Caribbean

West Indian manatees ( Trichechus manatus ) are separated into two allopatric subspecies: the Florida manatee ( T. m. latirostris ) and the Antillean manatee ( T. m. manatus ). In the winter of 2020–2021, an adult manatee was sighted off the coast of Cancun, Quintana Roo, Mexico, in areas where Antillean manatees are not typically seen. The individual had distinct watercraft scars on its body, which were matched using photo-identification to a known male Florida manatee (PE424) that had been repeatedly photographed in Florida since 1998. This is the first record of a Florida manatee visiting the Mexican Caribbean. Previous reports of individuals from this subspecies in Cuba, combined with genetic evidence, suggest some level of connectivity among geographically separated manatee populations. We present the first evidence of along-distance movement by amanatee from Florida to the Yucatan Peninsula (Quintana Roo, Mexican Caribbean). This case, previous reports of Florida manatees in Cuba, and genetic evidence, suggest acertain degree of genetic mixture among the two subspecies.

Yucatan Peninsula↗

USGS2021

The North American Breeding Bird Survey in Mexico, 2008 to 2018—A status report

Collection of avian population data has repeatedly been identified as a high priority for bird conservation in Mexico. To meet this need, in 2008 the North American Breeding Bird Survey (BBS), a volunteer-based survey, was expanded to include northern Mexico. The BBS in Mexico (Mexican BBS) is managed by the North American Bird Conservation Initiative (NABCI), Mexico’s National Coordination Office inside the Comisión Nacional para el Conocimiento y Uso de la Biodiversidad (CONABIO). During 2008–18, 252 surveys were conducted along 68 routes in Mexico, with geographic coverage varying from year to year. Of these 68 routes, 36 were surveyed three or more times. Thirty-one observers conducted the surveys, and 21 of these observers conducted two or more surveys. Just two observers conducted more than one-third of the 252 surveys, and both observers were paid to conduct the surveys. The low availability of local observers who are qualified, willing, and able to volunteer their services to conduct BBS surveys may prove to be the biggest obstacle to the success of the Mexican BBS program, especially in the context of Mexico’s ongoing safety and security concerns. Apart from the amount of data collected, many surveys did not adhere to pre-established quality-control requirements, and this would result in the exclusion of a large percentage of the data from potential trend analyses. Only 31 percent of the surveys met all the quality-control criteria. Additional observer training may help resolve this issue. Of greater concern is the selection of region-specific sampling date windows during which the surveys are conducted. Observers consistently conducted surveys outside the preliminarily prescribed sampling date window, reflecting the need to re-evaluate the regional appropriateness of this date window. Regardless of the quality of the data, the quantity of data available from 2008 to 2018 is insufficient for trend analysis using methods typically employed by U.S. Geological Survey BBS analysts. Reaching minimum sample size thresholds for statistical analysis will require a substantial increase in effort. During 2008–18, no strata (defined as the intersection of State and Bird Conservation Region boundaries) reached the suggested minimum of 14 sampled routes, and most routes were not run consistently. This report provides information needed for an evaluation of the merits of continuing to invest in the Mexican BBS program in its current form. Such an evaluation should consider the likelihood of achieving the primary project goal of producing reliable long-term population trend estimates, a projected timeline for meeting this goal, and include an assessment of the potential value of any additional data products.

Baja California, Baja California Sur, Chihuahua, C↗

USGS2021

Environmental conditions associated with occurrences of the threatened Yaqui Catfish in the Yaqui River Basin, Mexico

The Yaqui Catfish Ictalurus pricei is an understudied species, with limited information available on its ecology, distribution, and local habitat use. Native to the southwestern United States and northwestern Mexico, Yaqui Catfish populations are declining, which has prompted listing of the species as threatened in the United States and as a species of concern in Mexico. Water overallocation, habitat degradation, invasive species introductions, and hybridization with nonnative Channel Catfish I . punctatus have caused the populations in Mexico to decline. The United States population collapsed after years of low recruitment. To better focus conservation efforts as well as define habitat associated with Yaqui Catfish occurrences, we assessed the distribution in the Yaqui River basin of Mexico by using historical data at a landscape scale. Yaqui Catfish were historically found across the watershed among a diversity of environments but were most frequently associated with small, intermittent streams. Basin land cover was dominated by forest, shrubland, and grassland, and Yaqui Catfish generally occurred in stream segments at similar proportions. However, a small number of Yaqui Catfish occurrences were associated with urban and cropland land cover types in proportions greater than the availability of those categories on the landscape. With the species facing declines in the region, this work will help to inform future conservation efforts aimed at securing the Yaqui Catfish, protecting suitable habitat, and better defining its current status in Mexico.

Yaqui River Basin↗

USGS2021

PumaPlex100: An expanded tool for puma SNP genotyping with low-yield DNA

The original PumaPlex is a high-throughput assay developed to genotype 25 single nucleotide polymorphisms (SNPs) in pumas ( Puma concolor ). Here, we describe the development of PumaPlex100 – an expanded version of the original assay that now genotypes > 100 SNPs. We tested 142 candidate SNPs and developed a panel of 101 polymorphic loci, which are spread across four multiplexes and suitable for genotyping of non-invasive samples. This panel will provide researchers a set of standardized markers, that can be analyzed with minimal bioinformatic skills, for the assessment of population structure and genetic diversity. These SNPs will serve as an important resource for the continued genetic monitoring of this species, especially monitoring through non-invasive sampling.

Sonora↗

USGS2021

Summary of available data from the monarch overwintering colonies in central Mexico, 1976–1991

Historical estimates of the area occupied by overwintering Danaus plexippus (monarchs) in central Mexico (between winters of 1976 and 1991) were published in García-Serrano and others (2004) and more recently in Mawdsley and others (2020). Our primary objectives were to identify the specific data that informed those estimates and, importantly, determine the degree to which the reported estimates reflect the total size of the overwintering monarch population during that period. Understanding how historical estimates of the overwintering area relate to total population size is necessary to ensure that inferences about population abundance and temporal trends are reliable, particularly as the U.S. Fish and Wildlife Service is in the process of determining if the species should be listed under the U.S. Endangered Species Act.

Open-File Report↗

USGS2021

Distribution of tiger salamanders in northern Sonora, Mexico: Comparison of sampling methods and possible implications for an endangered subspecies

Many aquatic species in the arid USA-Mexico borderlands region are imperiled, but limited information on distributions and threats often hinders management. To provide information on the distribution of the Western Tiger Salamander ( Ambystoma mavortium ), including the USA-federally endangered Sonoran Tiger Salamander ( Ambystoma mavortium stebbinsi ), we used traditional (seines, dip-nets) and modern (environmental DNA [eDNA]) methods to sample 91 waterbodies in northern Sonora, Mexico, during 2015-2018. The endemic Sonoran Tiger Salamander is threatened by introgressive hybridization and potential replacement by another sub-species of the Western Tiger Salamander, the non-native Barred Tiger Salamander ( A. m. mavortium ). Based on occupancy models that accounted for imperfect detection, eDNA sampling provided a similar detection probability (0.82 [95% CI: 0.56-0.94]) as seining (0.83 [0.46-0.96]) and much higher detection than dip-netting (0.09 [0.02-0.23]). Volume of water filtered had little effect on detection, possibly because turbid sites had greater densities of salamanders. Salamanders were estimated to occur at 51 sites in 3 river drainages in Sonora. These results indicate tiger salamanders are much more widespread in northern Sonora than previously documented, perhaps aided by changes in land and water management practices. However, because the two subspecies of salamanders cannot be reliably distinguished based on morphology or eDNA methods that are based on mitochondrial DNA, we are uncertain if we detected only native genotypes or if we documented recent invasion of the area by the non-native sub-species. Thus, there is an urgent need for methods to reliably distinguish the subspecies so managers can identify appropriate interventions.

Sonora↗

USGS2021

Local fruit availability and en route wind conditions are poor predictors of bird abundance and composition during fall migration in coastal Yucatán Peninsula

In migratory stopover habitats, bird abundance and composition change on a near daily basis. On any given day, the local bird community should reflect local environmental conditions but also the environments that birds encountered previously along their migratory route. For example, during fall migration, the coast of the Yucatán Peninsula in Mexico receives birds that have just crossed the Gulf of Mexico and their abundance and composition may be associated with regional factors such as wind conditions experienced on previous dates. Other factors, such as local fruit availability, may also influence daily variation in bird abundance and composition. Using mist net data from 2 coastal national parks in the Yucatán Peninsula during fall migration in 2016 and 2017, we did not find a strong association between daily changes in bird abundance or community composition with wind conditions and ripe fruit availability. Thus, despite wind and fruit being known to be important to individual birds (influencing stopover duration and departure decisions), their effects might not scale up to be drivers of population and community level variation. On the other hand, we found that the 2 sites shared only about half of their species and those shared species had different temporal abundance patterns at each site. Site and year differences in temporal patterns of migration might arise because populations of the same species are on different migration routes and schedules. While bird arrival is not timed to hit peaks in fruit production in our study sites, whether bird–resource mismatch is a general characteristic of tropical coastal stopover habitats requires further research. If birds on migration have adapted to seasonal variation in food availability, they might be equipped to deal with the additional variability in food supply that is expected to occur with climate change.

Yucatan Peninsula↗

USGS2021

Augmentation of natural prey reduces cattle predation by puma (Puma concolor) and jaguar (Panthera onca) on a ranch in Sonora, Mexico

Retaliatory killing of large carnivores due to livestock predation is one of the major threats for the conservation of many declining populations of predators. According to empirical observations, there is a higher incidence of livestock predation when native prey abundance is low. In this study, we applied a treatment consisting of augmentation of prey abundance by translocation of peccaries ( Pecari tajacu ) and placement of four feed stations for white-tailed deer ( Odocoileus virginianus ) on a cattle ranch in Sonora, Mexico, with verified calf predation by puma ( Puma concolor ) and jaguar ( Panthera onca ). We quantified and compared consumed prey over two periods—phase I (8 months before the augmentation of prey) and phase II (8 months after the augmentation of prey)—through investigation of kill sites from Global Positioning System–collared jaguar and puma, prey identification from analyzed scat using molecular DNA techniques, and opportunistic discoveries of recently killed animal remains by either predator. We calculated the relative abundance of species (17 mammals [one species with two distinct age classes] and 1 bird species) through camera traps and for the most relevant prey species for this study (deer, calf, and peccary), we also estimated prey use by the predator, based on their availability during each period (prey preference). In the prey composition analyses of scat, we observed a significant reduction in the consumption of bovids and a significant increase in the consumption of peccaries during phase II. In the analyses of prey use, during phase I, predators consumed peccaries and calves at a higher proportion in relation to their availability. During phase II, consumption of calves declined from being preferred, to being consumed at the same proportion as their availability. Application of these results can contribute to the decrease of livestock predation and therefore conservation of pumas and jaguars.

Sonora↗

USGS2021

Oases: Finding hidden biodiversity gems in the southern Sonoran Desert

In the arid southern Sonoran Desert, the rugged canyons of the Sierra El Aguaje contain numerous freshwater oases. These habitats are supported by small springs which are usually located along geologic faults in volcanic and granitic bedrock. Genetic evidence from freshwater-obligate species (e.g., fish and frogs) suggests these or similar spring-fed habitats have persisted for thousands to millions of years. Though biologists are just beginning to study these habitats, at least 210 species of aquatic invertebrates have been documented, along with several species of fishes, amphibians, and semi-aquatic reptiles. Additionally, euryhaline fishes occasionally colonize freshwater habitats when hurricane-induced floods connect oases with the sea. At least six new, potentially endemic, species of aquatic invertebrates have been found in recent years, but much work remains to be done to fully document the biota of these oases. Groundwater pumping, introductions of nonnative species, and unmanaged human recreation all threaten the biodiversity of these desert oases. We hope this chapter will draw attention to these beautiful habitats and promote conservation of their unique biota

Sonora↗

USGS2020

Novel circoviruses detected in feces of Sonoran felids

Sonoran felids are threatened by drought and habitat fragmentation. Vector range expansion and anthropogenic factors such as habitat encroachment and climate change are altering viral evolutionary dynamics and exposure. However, little is known about the diversity of viruses present in these populations. Small felid populations with lower genetic diversity are likely to be most threatened with extinction by emerging diseases, as with other selective pressures, due to having less adaptive potential. We used a metagenomic approach to identify novel circoviruses, which may have a negative impact on the population viability, from confirmed bobcat ( Lynx rufus ) and puma ( Puma concolor ) scats collected in Sonora, Mexico. Given some circoviruses are known to cause disease in their hosts, such as porcine and avian circoviruses, we took a non-invasive approach using scat to identify circoviruses in free-roaming bobcats and puma. Three circovirus genomes were determined, and, based on the current species demarcation, they represent two novel species. Phylogenetic analyses reveal that one circovirus species is more closely related to rodent associated circoviruses and the other to bat associated circoviruses, sharing highest genome-wide pairwise identity of approximately 70% and 63%, respectively. At this time, it is unknown whether these scat-derived circoviruses infect felids, their prey, or another organism that might have had contact with the scat in the environment. Further studies should be conducted to elucidate the host of these viruses and assess health impacts in felids

Sonoran Desert↗

USGS2020

A new species of freshwater mussel in the genus Popenaias Frierson, 1927, from the Gulf coastal rivers of central Mexico (Bivalvia: Unionida: Unionidae) with comments on the genus

The Gulf coastal drainages of central Mexico are a faunal transition zone between North and South America and harbor a unique assemblage of freshwater mussels (Bivalvia: Unionida). However, little information is available regarding the taxonomy, distribution, and evolutionary history of the Mexican mussel fauna due to limited sampling over the last 100 years. To address these knowledge gaps, we evaluated species-level diversity in the genus Popenaias Frierson, 1927, in Mexican Gulf coastal drainages as part of a larger effort to inform conservation efforts for members of this genus both in Mexico and the United States of America. Based on our analyses, we describe Popenaias berezai n. sp. from the Río Valles of the Río Pánuco basin, San Luis Potosí, Mexico. We also provide presumptive distributional range, phylogenetic structure, and molecular and morphological diagnoses of the new species and provide comments on the other species currently in Popenaias . Our findings highlight the high levels of endemism among freshwater mussels in Mexican Gulf coastal drainages and will help guide impending conservation actions for P. popeii , which is listed as “endangered” in the United States.

Gulf Coast↗

USGS2020

Prioritizing water security in the management of vector borne diseases: Lessons from Oaxaca, Mexico

Changes in human water use, along with temperature and rainfall patterns, are facilitating habitat spread and distribution of Aedes aegypti and Aedes albopictus mosquitoes, the primary vectors for the transmission of Dengue, Chikungunya, and Zika viruses in the Americas. Artificial containers and wetspots provide major sources of mosquito larval habitat in residential areas. Mosquito abatement and control strategies remain the most effective public health interventions for minimizing the impact of these vector borne diseases. Understanding how water insecurity is conducive to the establishment and elimination of endemic mosquito populations, particularly in arid or semi-arid regions, is a vital component in shaping these intervention strategies.

Oaxaca↗

USGS2020

An analysis of the factors that control fault zone architecture and the importance of fault orientation relative to regional stress

The moment magnitude 7.2 El Mayor−Cucapah (EMC) earthquake of 2010 in northern Baja California, Mexico produced a cascading rupture that propagated through a geometrically diverse network of intersecting faults. These faults have been exhumed from depths of 6−10 km since the late Miocene based on low-temperature thermochronology, synkinematic alteration, and deformational fabrics. Coseismic slip of 1−6 m of the EMC event was accommodated by fault zones that displayed the full spectrum of architectural styles, from simple narrow fault zones (<100 m in width) that have a single high-strain core, to complex wide fault zones (>100 m in width) that have multiple anastomosing high-strain cores. As fault zone complexity and width increase the full spectrum of observed widths (20−200 m), coseismic slip becomes more broadly distributed on a greater number of scarps that form wider arrays. Thus, the infinitesimal slip of the surface rupture of a single earthquake strongly replicates many of the fabric elements that were developed during the long-term history of slip on the faults at deeper levels of the seismogenic crust. We find that factors such as protolith, normal stress, and displacement, which control gouge production in laboratory experiments, also affect the architectural complexity of natural faults. Fault zones developed in phyllosilicate-rich metasedimentary gneiss are generally wider and more complex than those developed in quartzo-feldspathic granitoid rocks. We hypothesize that the overall weakness and low strength contrast of faults developed in phyllosilicate rich host rocks leads to strain hardening and formation of broad, multi-stranded fault zones. Fault orientation also strongly affects fault zone complexity, which we find to increase with decreasing fault dip. We attribute this to the higher resolved normal stresses on gently dipping faults assuming a uniform stress field compatible with this extensional tectonic setting. The conditions that permit slip on misoriented surfaces with high normal stress should also produce failure of more optimally oriented slip systems in the fault zone, promoting complex branching and development of multiple high-strain cores. Overall, we find that fault zone architecture need not be strongly affected by differences in the amount of cumulative slip and instead is more strongly controlled by protolith and relative normal stress.

Baja California↗

USGS2020

Clinical presentation and serological responses to natural outbreaks of rabies in a captive colony of common vampire bats

We report mortality events in a group of 123 common vampire bats ( Desmodus rotundus ) captured in México and housed for a rabies vaccine efficacy study in Madison, Wisconsin. Bat mortalities occurred in México and Wisconsin, but rabies cases reported herein are only those that occurred after arrival in Madison (n = 15). Bats were confirmed positive for rabies virus (RABV) by the direct fluorescent antibody test. In accordance with previous reports, we observed long incubation periods (more than 100 days), variability in clinical signs prior to death, excretion of virus in saliva, and changes in rabies neutralizing antibody (rVNA) titers post-infection. We observed that the furious form of rabies (aggression, hyper-salivation, and hyper-excitability) manifested in three bats, which has not been reported in vampire bat studies since 1936. RABV was detected in saliva of 5/9 bats, 2–5 days prior to death, but was not detected in four of those bats that had been vaccinated shortly after exposure. Bats from different capture sites were involved in two separate outbreaks, and phylogenetic analysis revealed differences in the glycoprotein gene sequences of RABV isolated from each event, indicating that two different lineages were circulating separately during capture at each site.

Tropical Medicine and Infectious Disease↗

USGS2020

Introgression obscures lineage boundaries and phylogeographic history in the western banded gecko, Coleonyx variegatus (Squamata: Eublepharidae)

The geomorphological formation of the Baja California peninsula and the Gulf of California is a principal driver of diversification for the reptiles of North America’s warm deserts. The western banded gecko, Coleonyx variegatus , is distributed throughout the Mojave, Sonoran and Peninsular deserts. In this study we use multilocus sequence data to address deep phylogeographic structure within C. variegatus . Analyses of mtDNA data recover six divergent clades throughout the range of C. variegatus . Topology of the mtDNA gene tree suggests separate origins of peninsular populations with an older lineage in the south and a younger one in the north. In contrast, analyses of multilocus nuclear data provide support for four lineages, corresponding to the subspecies C. v. abbotti , C. v. peninsularis , C. v. sonoriensis and C. v. variegatus . Phylogenetic analyses of the nuclear data recover C. v. abbotti and C. v. peninsularis as a clade, indicating a single origin of the peninsular populations. Discordance between the nuclear and mtDNA data is largely the result of repeated episodes of mtDNA introgression that have obscured both lineage boundaries and biogeographic history. Dating analyses of the combined nuclear and mtDNA data suggest that the peninsular clade diverged from the continental group in the Late Miocene.

Baja California↗

USGS2020

Water quality and ecological risk assessment of intermittent streamflow through mining and urban areas of San Marcos River sub-basin, Mexico

Intermittent rivers are becoming more ecologically stressed worldwide. Flow cessation occurs naturally and spatiotemporally in these systems and anthropogenic activities such as wastewater discharges can have considerable impacts. Public entities mostly monitor water quality in permanent streams, leading to insufficient monitoring of intermittent streams and consequently to their potentially inadequate management.. This study analyzed spatiotemporal patterns of water quality and associated ecological risk through the quantification of physicochemical and microbiological pollutants in the intermittent river system of El Novillo and San Marcos in Northeast Mexico. Results showed that water quality varied geographically and seasonally. Based on national and international criteria, annual averages of water quality parameters analyzed suggested that streamflow in these river systems is of poor quality and poses high ecological risk to aquatic life. In the urban area, annual mean concentrations of Cd and Pb (0.14 and 0.4 mg/L) were 77- and 10-fold higher than their respective water quality criteria (<0.0018 and 0.04 mg/L). Statistically significant (q < 0.05) correlations were identified in concentrations of cyanide, Cd, Cu and Pb between wastewater seeping into the river and streamflow within the urban area. These observations highlight the unique sensitivity of intermittent urban streams to anthropogenic activities and may provide useful information to enhance current water management plans for the El Novillo-San Marcos River system for the protection of ecosystem integrity and human health.

El Novillo Canyon↗

USGS2020

Methane oxidation dynamics in a karst subterranean estuary

Chemical gradients between fresh, brackish and saline waters shape biogeochemical reactions and organic matter transformation within subterranean estuaries. In the Yucatán Peninsula’s karst subterranean estuary (KSE), methane and dissolved organic matter generated during the anaerobic decomposition of tropical forest vegetation are transported into flooded cave networks where microbial consumption greatly reduces their concentrations in the groundwater. To test the hypothesis that chemoclines associated with salinity gradients of the KSE are sites of methane oxidation, we obtained methane concentration and δ 13 C profiles of unprecedented vertical resolution from within a fully-submerged cave system located 6.6 km inland from the coastline using the ‘OctoPiPi’ (OPP) water sampler. Along a 12–24 cm thick low-salinity-halocline at ∼4.5 m water depth, salinity increased from fresh to brackish (0.2–1.8 psu), methane concentrations decreased, and δ 13 C values increased, as expected for microbial methane oxidation. The underlying brackish water had elevated oxygen concentrations compared to the always anoxic freshwater, suggesting that aerobic methane oxidation is the dominant process facilitating methane consumption. By contrast, as salinity increased from 1.8 to 36 psu through a 24–36 cm thick high-salinity-halocline between the meteoric lens and the saline groundwater at ∼20 m water depth, methane concentrations and δ 13 C values were constant. Conservative mixing and kinetic isotope models incorporating the methane data confirm a hotspot for microbial methane oxidation at the low-salinity-halocline. At least 98% of methane originating in the anoxic freshwaters was removed before its transport via channelized flow towards the coastline. These findings provide novel insight into the spatial constraints of methane dynamics within a karst subterranean estuary.

Ox Bel Ha cave system↗