Stream Margins Research Articles

Subglacial hydrology and ice stream margin locations

AbstractFast‐flowing ice streams in West Antarctica are separated from the nearly stagnant ice in the adjacent ridge by zones of highly localized deformation known as shear margins. It is presently uncertain what mechanisms control the location of shear margins and possibly allow them to migrate. In this paper we show how subglacial hydrological processes can select the shear margin location, leading to a smooth transition from a slipping to a locked bed at the base of an ice stream. Our study uses a two‐dimensional thermomechanical model in a cross section perpendicular to the direction of flow. We confirm that the intense straining at the shear margins can generate large temperate regions within the deforming ice. Assuming that the melt generated in the temperate ice collects in a drainage channel at the base of the margin, we show that a channel locally decreases the pore pressure in the subglacial till. Therefore, the basal shear strength just outside the channel, assuming a Coulomb‐plastic rheology, can be substantially higher than that inferred under the majority of the stream. Results show that the additional basal resistance produced by the channel lowers the stress concentrated on the locked portion of the bed. Matching the model to surface velocity data, we find that shear margins are stable when the slipping‐to‐locked bed transition occurs less than 500 m away from a channel operating at an effective pressure of 200 kPa and for a hydraulic transmissivity equivalent to a basal water film of order 0.2 mm thickness.

Shear heating and weakening of the margins of West Antarctic ice streams

AbstractIce streams are fast flowing bands of ice separated from stagnant ridges by shear margins. The mechanisms controlling the location of the margins remain unclear. We use published ice deformation data and a simple one‐dimensional thermal model to show that West Antarctic ice stream margins have temperate ice over a substantial fraction of their thickness, a condition that may control their width. The model predicts a triple‐valued relation between the thickness‐averaged lateral shear stress and the lateral shear strain rate. Observed strain rates at the margins imply that they support slightly less lateral shear stress than adjacent ice within the stream. This requires enhanced basal resistance near the margin. We suggest, in agreement with the limited observations, the presence of a channelized drainage system at the margin that reduces the pore fluid pressure at the ice‐till interface, thus increasing the shear stress acting on the yielding Coulomb plastic bed.

Beetles (Coleoptera) of Peru: A Survey of the Families. Hydraenidae

Diversity in Peru: 2 genera, 13 species. Recognition: Hydraenids, commonly called minute moss beetles, may be recognized by their small size (generally ,3 mm), very short nine-segmented antennae with a five-segmented club, and six or seven abdominal ventrites. Habitat: These beetles are usually found along margins of streams (especially sandy streams), but sometimes along the margins of lentic habitats (including rock pools). Adults often live interstitially among sand grains and gravel, or occupy the region just below the water surface along the shore or on half-submerged rocks. Most species can be collected by the “shore washing” method, in which water is splashed on the shoreline, and flows back into the stream or pond. Beetles are dislodged from the sand and gravel, and float to the surface, where they can be collected with a fine mesh net. Standard aquatic nets often have meshes that are too large, and the hydraenids pass through. Notes: The following list of Hydraenidae is derived from Perkins (1980, 2011). Blackwelder (1944–1957) did not list any species for Peru. There are certainly more species of Hydraenidae to be discovered in Peru.

Combined LM and SEM study of the middle Miocene (Sarmatian) palynoflora from the Lavanttal Basin, Austria: part III. Magnoliophyta 1 – Magnoliales to Fabales

Previous studies on the palynoflora from the Lavanttal Basin show that it contains a rich assemblage of spores and gymnosperm pollen. Present and ongoing investigations of dispersed angiosperm pollen suggest a high diversity within this group, and due to the excellent preservation of the material, some rare pollen types are recognised. The Magnoliales to Fabales pollen record documented here contains 30 different taxa. Only a few pollen types are assigned to Magnoliids (four taxa); these are rare in the pollen record. Similarly, the Commelinids comprise five taxa and are also rare. Most of the angiosperm pollen originates from Eudicots (21 taxa). Of the angiosperm taxa documented here, Magnolia, Carex, Ranunculaceae, Platanus, Trochodendron, Buxus, Cercidiphyllum, Daphniphyllum, Distylium, Fortunearia, Parrotia, Parthenocissus, Vitis, Euphorbia, Salix and Papilionoideae are recorded for the first time from the Lavanttal Basin. This also includes the first fossil pollen record of Trochodendron worldwide and the first reliable pollen record of Daphniphyllum. Several of the taxa described here had a wide Northern Hemispheric distribution from the Eocene until the end of the Miocene. Also, key relatives of the fossil taxa are presently confined to humid warm-temperate environments, suggesting a very mild climate during the middle Miocene (Sarmatian) of the Lavanttal area. Some of the taxa encountered also support previous observations that the sediments of the Lavanttal Basin accumulated in a lowland wetland environment. This is based on pollen from aquatic taxa thriving in lakes, streams and swamps, and pollen from terrestrial plant taxa occupying margins of lakes and streams, backswamps, floodplains, river plains and hummocks. Other angiosperm pollen clearly originates from plants thriving on drier substrates, reflecting various vegetation units of the mixed evergreen/deciduous broad-leaved/conifer forests surrounding the wetland basin.

Aspectos biométricos, morfoanatômicos e histoquímicos do pirênio de Bactris maraja (Arecaceae)

Bactris maraja (marajá) é uma palmeira frequente nas margens de lagos e igarapés amazônicos, cujos frutos comestíveis são consumidos pelo homem e pela fauna. Os estudos morfoanatômicos das estruturas reprodutivas podem contribuir com informações que subsidiarão estudos taxonômicos, fisiológicos e ecológicos. O objetivo deste estudo foi determinar características biométricas e morfoanatômicas do pirênio, bem como características histoquímicas da semente. Frutos maduros foram coletados e foram determinadas as dimensões, a massa fresca, e o teor de água, e descritas a forma, coloração e textura. O estudo anatômico foi feito pelo método de inclusão em resina. Foram aplicados testes histoquímicos para detecção dos principais metabólitos. Tanto os pirênios quanto as sementes isoladas apresentaram pequena variação biométrica, exceto para a massa fresca. O pirênio varia de obovado a elíptico, com endocarpo espesso, rígido, formado por células esclerificadas de paredes espessas. A semente é obovada depressa a elíptica, com tegumento fino, cartáceo, marrom-claro, aderido ao endocarpo, constituído de células parenquimáticas espessadas e taníferas; endosperma volumoso, sólido, homogêneo, branco, formado por células oblongas, elípticas e ovais transversalmente, e arredondadas longitudinalmente, com reserva proteica e lipídica; embrião lateral, cônico, pequeno, branco-leitoso, com conteúdo proteico e lipídico, polo radicular indiferenciado e polo caulinar com três primórdios foliares.

Atividade antibacteriana de extratos de folhas de Montrichardia linifera (Arruda) Schott (Araceae)

RESUMO O uso de plantas medicinais no tratamento de doenças é uma estratégia antiga utilizada por praticamente todas as populações do mundo, e, embora novos antibióticos tenham sido desenvolvidos para o controle de micro-organismos infecciosos, às vezes são ineficazes. Diversos extratos de plantas medicinais têm efeitos antimicrobianos, principalmente quando associados à antibióticos de uso clínico, representando alternativa terapêutica para doenças infecciosas. Montrichardia linifera, conhecida popularmente como aninga, é espécie macrófita, aquática emergente de hábito herbáceo, pertencente a família Araceae e ocorre em áreas alagáveis. A utilidade farmacológica desta espécie é diversificada tendo sido relatada como cicatrizante, antirreumático, antidiurético e expectorante. Devido à relevância no campo etnofarmacológico, ampla utilização na medicina popular e escassez de trabalhos relacionados à atividade antibacteriana desta espécie, objetivou-se com este trabalho avaliar a atividade antibacteriana de extratos alcoólicos de folhas de Montrichardia linifera, coletadas na margem do rio Igaraçu, Parnaíba-PI. O extrato foi testado em oito cepas de bactérias: Staphylococcus aureus, Staphylococcus epidermidis, Enterococcus faecalis, Escherichia coli, Klebsiella pneumoniae e Pseudomonas aeruginosa. Foram utilizadas as técnicas de verificação da formação de halos de inibição e determinação das concentrações inibitórias e bactericidas mínimas. Os testes antibacterianos evidenciaram como principais resultados que o extrato metanólico seco (EMS), extrato metanólico fresco (EMF), e o extrato etanólico seco (EES), apresentaram ação antibacteriana, enquanto o extrato etanólico fresco (EEF) não apresentou atividade para as bactérias testadas. O EMS foi o mais eficiente, inibindo o crescimento bacteriano na concentração de 200 μg/mL para E. faecalis, 400 μg/mL para S. aureus, 400 μg/mL para S. epidermidis e 2.000 μg/mL para P. aeruginosa. O EMF obteve CIM de 2.000 μg/mL para E. faecalis e EES obteve CIM de 250 μg/mL para E. faecalis. Os resultados demonstraram que M. linifera constitui fonte eficiente de compostos bioativos antibacterianos. Os estudos sobre as propriedades farmacológicas de plantas da família Araceae são escassos, e os resultados deste trabalho são pioneiros em relação a atividade antibacteriana desta espécie.

Life in the Main Channel: Long-Term Hydrologic Control of Microbial Mat Abundance in McMurdo Dry Valley Streams, Antarctica

Given alterations in global hydrologic regime, we examine the role of hydrology in regulating stream microbial mat abundance in the McMurdo Dry Valleys, Antarctica. Here, perennial mats persist as a desiccated crust until revived by summer streamflow, which varies inter-annually, and has increased since the 1990s. We predicted high flows to scour mats, and intra-seasonal drying to slow growth. Responses were hypothesized to differ based on mat location within streams, along with geomorphology, which may promote (high coverage) or discourage (low coverage) accrual. We compared hydrologic trends with the biomass of green and orange mats, which grow in the channel, and black mats growing at stream margins for 16 diverse stream transects over two decades. We found mat biomass collectively decreased during first decade coinciding with low flows, and increased following elevated discharges. Green mat biomass showed the greatest correlations with hydrology and was stimulated by discharge in high coverage transects, but negatively correlated in low coverage due to habitat scour. In contrast, orange mat biomass was negatively related to flow in high coverage transects, but positively correlated in low coverage because of side-channel expansion. Black mats were weakly correlated with all hydrologic variables regardless of coverage. Lastly, model selection indicated the best combination of predictive hydrologic variables for biomass differed between mat types, but also high and low coverage transects. These results demonstrate the importance of geomorphology and species composition to modeling primary production, and will be useful in predicting ecological responses of benthic habitats to altered hydrologic regimes.

A new species and lectotype designations for New Zealand Hydora (Elmidae: Larainae) with a world checklist

The family Elmidae is represented by a single genus Hydora Broun in New Zealand; they are the most abundant Coleoptera encountered in streams there. There are nine previously described species of Hydora, six from New Zealand, to which we add Hydora musci sp. nov. based on larvae and adults. Hydora musci can be distinguished from all remaining New Zealand species by the following characters: adult with pronotum with weak lateral transverse depression, pronotal lateral carina incomplete extending from posterior margin approximately two-thirds anteriorly, sublateral longitudinal carina less than one-third of the average carina length; elytral punctate striae poorly developed, elytral setae no longer than one-third width of eye, outer surface of mesotibia polished, surface of metatibia not polished; larva with thoracic and abdominal segments bearing single dorsal and lateral protrusions (often reduced), tooth absent on anterior margin of head, laterotergites complete on abdominal segments I and II, complete to incomplete on segments III to IV. Most specimens of H. musci were found in stable tributary streams containing bryophytes in the Arnold River Catchment 31 km inland from Greymouth on the West Coast of the South Island. Larvae are grazers that co-exist with adults at the stream margins and in splash zones associated with bryophytes attached to large boulders. A catalogue of the world Hydora species is included, and lectotypy is established for the following New Zealand species: Hydora subaenea Broun, Hydora picea (Broun), Hydora obsoleta Broun, Hydora nitida Broun, Hydora lanigera Broun and Hydora angusticollis (Pascoe).http://zoobank.org/urn:lsid:zoobank.org:pub:8EB6FA4E-BD27-4EAD-8113-977518118CBD

Modeling the elastic transmission of tidal stresses to great distances inland in channelized ice streams

Abstract. Geodetic surveys suggest that ocean tides can modulate the motion of Antarctic ice streams, even at stations many tens of kilometers inland from the grounding line. These surveys suggest that ocean tidal stresses can perturb ice stream motion at distances about an order of magnitude farther inland than tidal flexure of the ice stream alone. Recent models exploring the role of tidal perturbations in basal shear stress are primarily one- or two-dimensional, with the impact of the ice stream margins either ignored or parameterized. Here, we use two- and three-dimensional finite-element modeling to investigate transmission of tidal stresses in ice streams and the impact of considering more realistic, three-dimensional ice stream geometries. Using Rutford Ice Stream as a real-world comparison, we demonstrate that the assumption that elastic tidal stresses in ice streams propagate large distances inland fails for channelized glaciers due to an intrinsic, exponential decay in the stress caused by resistance at the ice stream margins. This behavior is independent of basal conditions beneath the ice stream and cannot be fit to observations using either elastic or nonlinear viscoelastic rheologies without nearly complete decoupling of the ice stream from its lateral margins. Our results suggest that a mechanism external to the ice stream is necessary to explain the tidal modulation of stresses far upstream of the grounding line for narrow ice streams. We propose a hydrologic model based on time-dependent variability in till strength to explain transmission of tidal stresses inland of the grounding line. This conceptual model can reproduce observations from Rutford Ice Stream.

Descriptions of Immature Stages of Three Species of Laccocorinae (Hemiptera: Heteroptera: Naucoridae) from India

Abstract Adults and first through fifth instars of three species of Naucoridae in the subfamily Laccocorinae were collected from localities in Tamil Nadu and Karnataka states in southern India. Specifically, Heleocoris indicus (Montandon) and Heleocoris vicinus (Montandon) were collected from the vegetated margins of streams that contained no congeners. Pogonocaudina indica Sites & Zettel was collected from vertical rock surfaces at the type locality. All instars of these three species are described here. First instars of both species of Heleocoris are immaculate, whereas subsequent instars are densely punctate. The first instar of P. indica has pronounced dorsal tubercles with stout spines, whereas subsequent instars lack these structures. All instars of P. indica have a deep terminal cleft and dense fringe of hairs on the perimeter of the posterior abdominal segments. Second through fifth instars of these three species can be distinguished based on relative length of the mesonotal wing pad compared with the exposed part of the metanotal lateral margin.

Larval burrow morphology and groundwater dependence in a mire-dwelling dragonfly,Petalura gigantea(Odonata: Petaluridae)

Most species of petalurid dragonflies have a fossorial larval stage, which is unique in the Odonata. Larvae typically excavate burrows in soft peaty soils in mires, seepages or along stream margins, which are occupied by a single larva throughout the long larval stage. This paper reports on a study of burrow morphology in Petalura gigantea, with the objectives of describing their burrows, documenting any variability in burrow morphology across the hydrogeomorphic range of habitats used by the species, identifying factors contributing to any such variability, resolving questions in relation to the single previous illustration of a burrow system and identifying the level of groundwater dependence of larvae. The species was found to be an obligate, groundwater dependent, mire-dwelling species with well-maintained and sometimes complex burrows. Burrow complexity and morphological variation are inferred to be a response by larvae to the hydrogeomorphic characteristics of the habitat and substrate attributes. A...

Deformation-induced melting in the margins of the West Antarctic ice streams

Flow of glacial ice in the West Antarctic Ice Sheet localizes in narrow bands of fast-flowing ice streams bordered by ridges of nearly stagnant ice, but our understanding of the physical processes that generate this morphology is incomplete. Here we study the thermal and mechanical properties of ice-stream margins, where flow transitions from rapid to stagnant over a few kilometers. Our goal is to explore under which conditions the intense shear deformation in the margin may lead to deformation-induced melting. We propose a 2-D model that represents a cross section through the ice stream margin perpendicular to the downstream flow direction. We limit temperature to the melting point to estimate melt rates based on latent heat. Using rheology parameters as constrained by laboratory data and observations, we conclude that a zone of temperate ice is likely to form in active shear margins.

Density, Demography, and Microhabitat of Campeloma Decampi (Gastropoda: Viviparidae)

Campeloma decampi, the Slender Campeloma, is a federally endangered snail endemic to the Tennessee River drainage in Alabama, U.S.A. We studied a population in Round Island Creek, Limestone County, in July, 2010, to obtain information about density, microhabitat, and demography. The overall mean density at the site was 49.2/m2 (± 14.4 SE), but the distribution was highly clumped. We used generalized linear models and multi-model inference to examine the response of snail density to seven microhabitat explanatory variables. The greatest densities were associated with shallow, low-flow areas with silt and clay near the stream margin. Shell heights ranged from 4.3–34.7 mm, and the size distribution appeared to be composed of three cohorts possibly representing age 0+ recruits, age 1+ individuals, and individuals ≥2 years of age. The population was dominated by small individuals (4-12 mm; modal size class = 6 mm), and individuals >20 mm made up only 7% of the population. This size distribution suggests that parturition occurs over a protracted period from late winter to summer and that most individuals produce only one or two broods in their lifetime; however, additional sampling and information about life span are needed to more conclusively describe the reproductive strategy.

Observations on Oriental Weatherfish (Misgurnus anguillicaudatus), an Exotic Species in the Hudson River Valley, New York

Abstract We collected data on the distribution, sex, size, fecundity, and food habits of a newly discovered population of Misgurnus anguillicaudatus (Oriental Weatherfish) in the Hudson Valley, NY. Oriental Weatherfish were distributed throughout the Dwaar Kill in Orange and Ulster counties and at least 7 km of the Wallkill River, principally in the soft substrates of stream margins. Males were smaller than females and apparently had shorter life spans. Sexual maturity and spawning attributes were evident. Mature males had pronounced dorsolateral ridges on at least the posterior third of the body, and females exhibited post-spawning abrasions dorsal and anterior to the vent. Female Oriental Weatherfish are batch-spawners producing up to 18,000 eggs at one time. Oriental Weatherfish in the Dwaar Kill consumed a variety of aquatic macroinvertebrates and exhibited generalist feeding habits. This species does not seem to be affecting other vertebrates where currently found, but potential negative interactions...

Changing climate and the altitudinal range of avian malaria in the Hawaiian Islands – an ongoing conservation crisis on the island of Kaua'i

Transmission of avian malaria in the Hawaiian Islands varies across altitudinal gradients and is greatest at elevations below 1500 m where both temperature and moisture are favorable for the sole mosquito vector, Culex quinquefasciatus, and extrinsic sporogonic development of the parasite, Plasmodium relictum. Potential consequences of global warming on this system have been recognized for over a decade with concerns that increases in mean temperatures could lead to expansion of malaria into habitats where cool temperatures currently limit transmission to highly susceptible endemic forest birds. Recent declines in two endangered species on the island of Kaua'i, the 'Akikiki (Oreomystis bairdi) and 'Akeke'e (Loxops caeruleirostris), and retreat of more common native honeycreepers to the last remaining high elevation habitat on the Alaka'i Plateau suggest that predicted changes in disease transmission may be occurring. We compared prevalence of malarial infections in forest birds that were sampled at three locations on the Plateau during 1994-1997 and again during 2007-2013, and also evaluated changes in the occurrence of mosquito larvae in available aquatic habitats during the same time periods. Prevalence of infection increased significantly at the lower (1100 m, 10.3% to 28.2%), middle (1250 m, 8.4% to 12.2%), and upper ends of the Plateau (1350 m, 2.0% to 19.3%). A concurrent increase in detections of Culex larvae in aquatic habitats associated with stream margins indicates that populations of the vector are also increasing. These increases are at least in part due to local transmission because overall prevalence in Kaua'i 'Elepaio (Chasiempis sclateri), a sedentary native species, has increased from 17.2% to 27.0%. Increasing mean air temperatures, declining precipitation, and changes in streamflow that have taken place over the past 20 years are creating environmental conditions throughout major portions of the Alaka'i Plateau that support increased transmission of avian malaria.

Aligned glaciotectonic rafts on the central Barents Sea seafloor revealing extensive glacitectonic erosion during the last deglaciation

AbstractErosion rates on glaciated continental shelves are remarkably high, especially within ice stream troughs. Although glaciotectonic erosion may have considerably contributed to enhanced glacial erosion of these landscapes, entrainment mechanisms of glaciotectonically emplaced megablocks and rafts remain little understood. Here we report a northeast‐southwest trending chain of over 1300 glacial rafts, observed on the seafloor in Bjørnøyrenna, a paleo‐ice stream trough with particularly high erosion rates. The landform assemblage adjacent to the glacial rafts includes streamlined bedforms and crevasse‐squeeze ridges and may represent a lateral shear zone where a paleo‐ice stream met cold stagnant ice during the last deglaciation. Individual glacial rafts along the chain are up to 30 m high, tend to be elongated with lengths and widths on the order of tens of meters, and are oriented from northeast‐southwest to north‐south. The north‐southern rearrangement of rafts may reflect a compressional strain regime along the ice stream margin.

Glacial geomorphology of terrestrial-terminating fast flow lobes/ice stream margins in the southwest Laurentide Ice Sheet

Glacial geomorphological mapping of southern Alberta, Canada, reveals landform assemblages that are diagnostic of terrestrial-terminating ice streams/fast flowing outlet glaciers with lobate snouts. Spatial variability in features that comprise the landform assemblages reflects changes in (a) palaeo-ice stream activity (switch on/off); and (b) snout basal thermal regimes associated with climate sensitive, steady state flow. Palaeo-ice stream tracks reveal distinct inset sequences of fan-shaped flowsets indicative of receding lobate ice stream margins. Former ice lobe margins are demarcated by (a) major, often glacially overridden transverse moraine ridges, commonly comprising glacitectonically thrust bedrock; and (b) minor, closely spaced recessional push moraines and hummocky moraine arcs. Details of these landform types are well exhibited around the former southern margins of the Central Alberta Ice Stream, where larger scale, more intensive mapping identifies a complex glacial geomorphology comprising minor transverse ridges (MTR types 1–3), hummocky terrain (HT types 1–3), flutings, and meltwater channels/spillways. The MTR type 1 constitute the summit corrugation patterns of glacitectonic thrust moraines or major transverse ridges and have been glacially overrun and moderately streamlined. The MTR type 2 sequences are recessional push moraines similar to those developing at modern active temperate glacier snouts. The MTR type 3 document moraine construction by incremental stagnation because they occur in association with hummocky terrain. The close association of hummocky terrain with push moraine assemblages indicates that they are the products of supraglacial controlled deposition on a polythermal ice sheet margin, where the HT type 3 hummocks represent former ice-walled lake plains. The ice sheet marginal thermal regime switches indicated by the spatially variable landform assemblages in southern Alberta are consistent with palaeoglaciological reconstructions proposed for other ice stream/fast flow lobes of the southern Laurentide Ice Sheet, where alternate cold, polythermal, and temperate marginal conditions associated with climate sensitive, steady state flow sequentially gave way to more dynamic streaming and surging activity.

Vertical distance from drainage drives floristic composition changes in an Amazonian rainforest

Background: Plant composition changes with topography and edaphic gradients that correlate with soil-water and nutrient availability. Data on soil water for the Amazon Basin are scarce, limiting the possibility of distinguishing between soil and soil-water influences on plant composition. Aim: We tested a new proxy for water table depth, the terrain height above nearest drainage (HAND), as a predictor of composition in trees, lianas, palms, shrubs, and herbs and compared HAND to conventional measures of height above sea level (HASL) and horizontal distances from nearest drainage (HDND). Methods: Plant-species composition in 72 plots distributed across 64 km2 of lowland evergreen terra firme forest was summarised using non-metric multidimensional scaling (NMDS). NMDS scores were regressed against estimates of HAND, HASL and HDND. Results: Plant composition was highly correlated with the vertical distance from water table, capturing up to 82% of variation. All life forms showed highest turnover rates in the zone with seasonally water-saturated soils, which can extend 350 m from stream margins. Conclusions: Floristic composition is closely related to water table depth, and HAND appears to be the most robust available topographical metric of soil-water gradients. Brazilian conservation laws protecting 30-m-wide riparian buffers are likely to be too narrow to encompass the full zone of highest floristic turnover and may be ineffective in safeguarding riparian plant diversity.

Desiccation Resistance in Embryos of the Killifish, Fundulus heteroclitus: Single Embryo Measurements

Northern killifish, Fundulus heteroclitus macrolepidotus, spawn in estuaries at high tides. Embryos may be stranded in air at stream margins as the water recedes. These aerially incubated embryos are exposed to desiccation stress and may survive and develop normally to hatching at ∼14 days post-fertilization (dpf). We developed a technique to quantitatively measure the kinetics of water loss at various developmental stages from single embryos in controlled relative humidities (RHs). Embryos were able to tolerate short periods (2 hr) of severe desiccation and survive to hatching. Mid-stage (7 dpf) embryos showed the highest degree of desiccation tolerance compared to early-stage (2 dpf) and late-stage (14 dpf) embryos. We classified the patterns of water loss into four phases, the perivitelline space (PVS) phase, the resistance phase, the desiccation phase, and the equilibration phase. In the PVS phase, water loss was rapid at all developmental stages and all RHs (∼25% of total embryo weight). The water loss rate was slower during the resistance phase. It decreased as RH increased and length of this phase was longer in mid-stage than in early- and late-stage embryos. The water loss rate and length of the desiccation phase also depended on RH. These data support the hypothesis that low permeability embryonic compartment surface membranes retard water loss significantly and promote prolonged survival of these embryos during desiccation. We also show this mechanism cannot completely account for the survival of severely desiccated embryos (especially in 23% RH) and that there must also be complementary cellular responses.

Shallow groundwater systems in a polar desert, McMurdo Dry Valleys, Antarctica

The McMurdo Dry Valleys (MDVs), Antarctica, exist in a hyperarid polar desert, underlain by deep permafrost. With an annual mean air temperature of −18 °C, the MDVs receive <10 cm snow-water equivalent each year, collecting in leeward patches across the landscape. The landscape is dominated by expansive ice-free areas of exposed soils, mountain glaciers, permanently ice-covered lakes, and stream channels. An active layer of seasonally thawed soil and sediment extends to less than 1 m from the surface. Despite the cold and low precipitation, liquid water is generated on glaciers and in snow patches during the austral summer, infiltrating the active layer. Across the MDVs, groundwater is generally confined to shallow depths and often in unsaturated conditions. The current understanding and the biogeochemical/ecological significance of four types of shallow groundwater features in the MDVs are reviewed: local soil-moisture patches that result from snow-patch melt, water tracks, wetted margins of streams and lakes, and hyporheic zones of streams. In general, each of these features enhances the movement of solutes across the landscape and generates soil conditions suitable for microbial and invertebrate communities.