The sedimentation of salmonid spawning gravels in the Hampshire Avon catchment,UK: implications for the dissolved oxygen content of intragravel water and embryo survival

The accumulation of sediment within salmonid redd gravels can have a detrimental impact on the development of salmonid embryos; therefore, redd sedimentation represents a potential limiting factor for salmonid reproduction. The links between redd sedimentation, the dissolved oxygen content of intragravel water and salmonid embryo survival within the upper and middle parts of the Hampshire Avon catchment in southern England are explored. Measurements of surface and intragravel water quality and redd properties were undertaken for artificial redds constructed at known spawning sites. Salmonid embryos were also planted into artificial redds adjacent to the monitoring equipment. The rate of sedimentation of the newly cleaned redd gravels demonstrated a non‐linear decrease over time, which is attributed to a particle‐size‐selective depositional process. The results of the study confirm that low embryo survival and low dissolved oxygen concentrations in intragravel water can be attributed to the accumulation of sediment within the redd gravels. This was found to produce a reduction in redd permeability, which limited the interchange of surface and intragravel water and, therefore, the supply of dissolved oxygen to the intragravel environment. In view of the diminished status of salmonids within many of the UK's chalk rivers and streams, the results highlight the need for management initiatives aimed at reducing redd sedimentation and thereby optimizing salmonid embryo incubation success. Copyright © 2006 John Wiley & Sons, Ltd.     

Incubation,hatching and survival of eggs of Atlantic salmon (Salmo salar) in spawning redds influenced by groundwater

Many west coastal and northern Norwegian rivers run through deep, confined valleys with permeable layers of glacial and alluvial deposits. Groundwater flows through these permeable layers and enter lakes and rivers as underwater seepage and springs. Groundwater inflow to inland Norwegian rivers may constitute 40–100% of total water discharge during low flow periods in late summer and winter. Juvenile salmonids may take advantage of groundwater upwellings and actively seek out such patches. In regulated rivers groundwater influx may create refuges during low flow or hydropeaking episodes. The importance of groundwater for salmon redd site selection and egg survival is also clear, although less known and documented in regulated rivers.Eggs of Atlantic salmon (Salmo salar) are deposited in redds in river bed gravels lacking fine sediments and with high oxygen levels. Egg development is therefore dependent on the interaction of a number of environmental factors such as groundwater influx, oxygen and temperature. Atlantic salmon in the regulated River Suldalslågen, Western Norway, spawn relatively late compared to other Norwegian rivers, with a peak in early January. Newly emerged fry are found from the end of May to the beginning of June, i.e. “swim up” one month earlier than expected using models for egg and alevin development and river water temperatures. The most plausible explanation is that groundwater has a higher and more stable temperature than surface river water. In field experiments, fertilized salmon eggs were placed in boxes close to natural spawning redds in the river bed at sites influenced and those not influenced by groundwater. A difference of up to 40 days in 50% hatching was found, and “swim up” occurred at the end of May in boxes influenced by groundwater.Preliminary studies have revealed that groundwater also plays an important role in survival of salmon eggs in the River Suldalslågen when dewatered in winter. Eggs placed in boxes in groundwater seepage areas during winter in the dewatered river bed survived even when covered by ice and snow. The survival from fertilization until 30 April, one month before hatching, was 91%, the same survival as found for eggs placed in boxes in the wetted river bed. However, mortality from fertilization to hatching was higher compared to the eggs placed in wetted river bed, 57 and 91% respectively.Groundwater creates a horizontal and vertical mosaic of temperatures in spawning redd areas leading to potentially greater variation in spawning sites, time of hatching and “swim up”. This is likely to increase egg survival during low flow periods in regulated rivers. In conclusion, the interaction between groundwater and surface river water should therefore be considered when managing fish populations in regulated rivers.     

Sediment infiltration traps: their use to monitor salmonid spawning habitat in headwater tributaries of the Cascapédia River,Québec

Sediment infiltration can clog salmon nests and reduce egg survival. As a countermeasure, environmental managers often deploy infiltration traps to monitor sediment infiltration. Traps provide a repeatable means of measuring infiltration and enable comparisons to be made between sites. Results from infiltration rates measured with traps have also been used to estimate infilling rates into salmon nests. Application of these data is questionable, as the composition of the bed and the amount of fine sediment within the bed is known to affect infiltration rates. Thus, infiltration rates measured with infiltration traps may differ from the infiltration rates occurring in redd and riffle gravels. To examine how relationships between sediment infiltration rates varied between four watersheds, we continuously monitored suspended sediment transport, shear stress and infiltration rates at four sites over 5 months. We also compared infiltration rates measured with infiltration traps with changes in the hydraulic conductivity and subsurface grain size distribution of adjacent artificially constructed salmon nests and natural riffle gravels. Among the four watersheds, clear differences in sediment infiltration rates were observed. The differences correlated with the subsurface silt content but no strong relationship existed between land‐use or basin physiography/geology. Despite observing an average of 30 kg m⁻² of sediment finer than 2 mm being deposited in the infiltration traps during the study, no change in redd or riffle substrate was observed. If the deposition rates measured with the traps reflect the processes in redds, enough sediment would have been deposited to inhibit egg emergence. However, no reduction in egg survival to the eyed stage was observed. In summary, our results show that infiltration traps with clean gravels can be used to detect intersite differences in sediment transport regimes. Extrapolations of sediment infiltration rates measured with such collectors to estimate infiltration rates in redds or riffles is, however, flawed. Copyright © 2005 John Wiley & Sons, Ltd.     

High‐frequency logging technologies reveal state‐dependent hyporheic process dynamics: implications for hydroecological studies

This paper presents a novel method for assessing hyporheic water quality dynamics using advances in sensor technology. High‐resolution (15 min) dissolved oxygen (DO) and hydraulic head data were combined to assess groundwater–surface water (GW–SW) interactions in the hyporheic zone. DO concentrations varied at fine temporal and spatial scales, depending on the relative contributions of GW and SW. The effect of sample frequency on observed patterns of variability was assessed with reference to studies of the ecology of salmon spawning habitat. Conventional approaches fail to capture the full range of temporal variability in hyporheic water quality and demonstrate the need to reassess the interpretations of previous studies of the hyporheic zone. © Crown Copyright 2006. Reproduced with the permission of Her Majesty's Stationery Office. Published by John Wiley & Sons, Ltd.     

The influence of Pacific salmon decay products on near‐field streambed sediment and organic matter dynamics: a flume simulation

Pacific salmon are biogeomorphic agents shown to induce positive feedbacks on their natal watersheds. However, the literature documenting their ecological effects on in‐stream natal environments is more divisive. The disturbance salmon create during redd construction has the potential to reduce stream productivity. The pulse of salmon organic matter (SOM) and marine derived nutrients (MDNs) released during carcass decay has been reported as either stimulating in‐stream productivity or having no local effect. To evaluate the ecological costs and benefits of salmon spawning events, MDN delivery and storage processes need to be identified and quantified. A simulation was conducted in three flow‐through flumes (2 m × 2 m × 30 m) over a 33‐day period (consisting of 15 baseline, four MDN exposure, and 14 post‐exposure days) to assess near‐field sediment and organic matter dynamics during active and post‐spawn simulations. The objective of the study was to measure changes in the amounts and particle sizes of suspended and gravel‐stored fine sediment, in order to elucidate the process and significance of SOM recruitment to the gravel bed via sedimentation. Gravel beds in all flumes were enriched with SOM following treatments but the response was highest in the active spawn simulation. The more effective delivery in the active spawn simulation was attributed to its higher inorganic sediment concentration, which is known to enhance floc formation. Although the active spawn simulation delivered more SOM to the gravel bed, the post‐spawn phase may be equally important to natural streams because its decay phase is longer than the active spawn and consequently can provide SOM to the streambed as long as carcasses remain in‐stream. The delivery, and potential retention, of SOM to spawning streambeds and the intergravel environment may be particularly important for interior streams, which experience low flow conditions during the spawning phase and accordingly have the potential for hyporheic nutrient recruitment and storage. Copyright © 2014 John Wiley & Sons, Ltd.     

Tools for gauging the capacity of salmon spawning substrates

We present a set of river management tools based on a recently developed method for estimating the amount of salmon spawning habitat in coarse‐bedded rivers. The method, which was developed from a mechanistic model of redd building by female salmon, combines empirical relationships between fish length, redd area, and the sizes of particles moved by fish during spawning. Model inputs are the grain‐size indices D₅₀ and D₈₄ and an estimate of female fish length, which is used to predict the size of the redd that they will build and the size of the largest particle that they can move on the bed. Outputs include predictions of the fraction of the bed that the fish can use for redd building and the number of redds that they can build within the useable area. We cast the model into easy‐to‐use look‐up tables, charts, an Excel worksheet, a JavaScript web applet, and a MATLAB user interface. We explain how these tools can be used in a new, mechanistic approach to assessing spawning substrates and optimizing gravel augmentation projects in coarse‐bedded rivers. © 2016 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd.     

Impact Assessment of Pumping Parameters on Groundwater Bacterial Communities

Groundwater microbial community samples are traditionally collected using pumping techniques optimized for groundwater chemistry assessment, although the impact of groundwater pumping parameters on apparent bacterial community structures (BCSs) is not really known. We therefore studied the impact of pumping lift, flow regime, and tubing material on BCS, which were analyzed by terminal‐restriction fragment length polymorphism (T‐RFLP). Ruzicka dissimilarity coefficients were calculated between T‐RFLP profiles to assess disparities between BCS. Variations in pumping lift, flow regime, and tubing material did not affect the apparent BCS in experiments using a homogenous water system under laboratory conditions showing that the conditions within the tube had no detectable effect on BCS. However, pumping groundwater from aquifer monitoring wells at different flow rates in the field revealed a significant impact on the apparent BCS. Water samples collected from fine sediment were the most affected by the pumping flow rate.     

A field‐based assessment of oxygen supply to incubating Atlantic salmon (Salmo salar) embryos

Oxygen fluxes through artificially created salmon redds within four UK rivers were quantified and assessed against survival to hatching of Atlantic salmon embryos. All sites recorded high spatial variability in survival. Minimum survival to hatching was zero at all sites. Maximum survival to hatching ranged from 35% to 91%. Mean survival to hatching ranged from 8·7% to 71%. Intra‐ and inter‐site variations in rates of oxygen supply were observed. Generally, interstitial dissolved oxygen concentrations declined over the incubation period from a maximum recorded directly after redd creation, although localized fluctuations were recorded. Similarly, interstitial flow velocities declined over the incubation period from a maximum directly after redd creation to a minimum at hatching. With respect to the causes of embryo mortalities, oxygen supply was shown to be a stronger determinant of survival than interstitial oxygen concentration or interstitial flow velocity. To improve delineation of potential causes of embryo mortalities in the field, the statistical analysis was integrated within mass transfer theory of the processes controlling respiration to determine the likely mechanisms inhibiting respiration. Based on this analysis, mortalities were assessed to have resulted from periods of lethal oxygen concentrations, from periods of interstitial flow velocities that were insufficient to remove metabolic waste, or from combinations of oxygen concentration and interstitial flow that did not support respiratory requirements. A set of oxygen‐supply‐related thresholds for assessing incubation habitat quality are proposed. Copyright © 2007 John Wiley & Sons, Ltd.     

Long-term variability of oxygen and mineral phosphorus concentrations in Lake Dal’nee (Kamchatka)

Many-year data on the concentrations of O₂ and mineral phosphorus in a fattening-spawning water body for the Pacific salmon (red salmon) are presented; the seasonal and year-to-year dynamics of these con-centrations are analyzed. It is found that the lake water mass features a high concentration of O₂, the upper 20-m layer being most aerated. It is shown that the year-to-year variability of the concentration of O₂ dissolved in the lake water depends on the amount of organic matter coming with groundwater and surface runoff. The oxygen and phosphorus regimes of the lake at low and abundant entries of puberal red salmon for spawning are considered.     

Microbial Mineralization of Dichloroethene and Vinyl Chloride under Hypoxic Conditions

Mineralization of ¹⁴C‐radiolabled vinyl chloride ([1,2‐¹⁴C] VC) and cis‐dichloroethene ([1,2‐¹⁴C] cis‐DCE) under hypoxic (initial dissolved oxygen (DO) concentrations about 0.1 mg/L) and nominally anoxic (DO minimum detection limit = 0.01 mg/L) was examined in chloroethene‐exposed sediments from two groundwater and two surface water sites. The results show significant VC and dichloroethene (DCE) mineralization under hypoxic conditions. All the sample treatments exhibited pseudo‐first‐order kinetics for DCE and VC mineralization over an extended range of substrate concentrations. First‐order rates for VC mineralization were approximately 1 to 2 orders of magnitude higher in hypoxic groundwater sediment treatments and at least three times higher in hypoxic surface water sediment treatments than in the respective anoxic treatments. For VC, oxygen‐linked processes accounted for 65 to 85% of mineralization at DO concentrations below 0.1 mg/L, and ¹⁴CO₂ was the only degradation product observed in VC treatments under hypoxic conditions. Because the lower detection limit for DO concentrations measured in the field is typically 0.1 to 0.5 mg/L, these results indicate that oxygen‐linked VC and DCE biodegradation can be significant under field conditions that appear anoxic. Furthermore, because rates of VC mineralization exceeded rates of DCE mineralization under hypoxic conditions, DCE accumulation without concomitant accumulation of VC may not be evidence of a DCE degradative “stall” in chloroethene plumes. Significantly, mineralization of VC above the level that could reasonably be attributed to residual DO contamination was also observed in several nominally anoxic (DO minimum detection limit = 0.01 mg/L) microcosm treatments.     

Hydrological influences on hyporheic water quality: implications for salmon egg survival

The spatial and temporal variability of groundwater–surface‐water (GW–SW) interactions was investigated in an intensively utilized salmon spawning riffle. Hydrochemical tracers, were used along with high‐resolution hydraulic head and temperature data to assess hyporheic dynamics. Surface and subsurface hydrochemistry were monitored at three locations where salmon spawning had been observed in previous years. Temperature and hydraulic head were monitored in three nests of three piezometers located to characterize the head, the run and the tail‐out of the riffle feature. Hydrochemical gradients between surface and subsurface water indicated increasing GW influence with depth into the hyporheic zone. Surface water was characterized by high dissolved oxygen (DO) concentrations, low alkalinity and conductivity. Hyporheic water was generally characterized by high levels of alkalinity and conductivity indicative of longer residence times, and low DO, indicative of reducing conditions. Hydrochemical and temperature gradients varied spatially over the riffle in response to changes in local GW–SW interactions at the depths investigated. Groundwater inputs dominated the head and tail of the riffle. The influence of SW increased in the area of accelerating flow and decreasing water depth through the run of the riffle. Temporal GW–SW interactions also varied in response to changing hydrological conditions. Gross changes in hyporheic hydrochemistry were observed at the weekly scale in response to changing flow conditions and surface water inputs to the hyporheic zone. During low flows, caused by freezing or dry weather, hyporheic hydrochemistry was dominated by GW inputs. During higher flows hyporheic hydrochemistry indicated that SW contributions increased. In addition, high‐resolution hydraulic head data indicated that rapid changes in GW–SW interactions occurred during hydrological events. The spatial, and possibly the temporal, variability of GW–SW interactions had a marked effect on the survival of salmon ova. It is concluded that hyporheic dynamics and their effect on stream ecology should be given increased consideration by fisheries and water resource managers. Copyright © 2004 John Wiley & Sons, Ltd.     

Heterogeneity in ground water–surface water interactions in the hyporheic zone of a salmonid spawning stream

Spatial and temporal variability in ground water–surface water interactions in the hyporheic zone of a salmonid spawning stream was investigated. Four locations in a 150‐m reach of the stream were studied using hydrometric and hydrochemical tracing techniques. A high degree of hydrological connectivity between the riparian hillslope and the stream channel was indicated at two locations, where hydrochemical changes and hydraulic gradients indicated that the hyporheic zone was dominated by upwelling ground water. The chemistry of ground water reflected relatively long residence times and reducing conditions with high levels of alkalinity and conductivity, low dissolved oxygen (DO) and nitrate. At the other locations, connectivity was less evident and, at most times, the hyporheic zone was dominated by downwelling stream water characterized by high DO, low alkalinity and conductivity. Substantial variability in hyporheic chemistry was evident at fine (<10 m) spatial scales and changed rapidly over the course of hydrological events. The nature of the hydrochemical response varied among locations depending on the strength of local ground water influence. It is suggested that greater emphasis on spatial and temporal heterogeneity in ground water–surface water interactions in the hyporheic zone is necessary for a consideration of hydrochemical effects on many aspects of stream ecology. For example, the survival of salmonid eggs in hyporheic gravels varied considerably among the locations studied and was shown to be associated with variation in interstitial chemistry. River restoration schemes and watershed management strategies based only on the surface expression of catchment characteristics risk excluding consideration of potentially critical subsurface processes. Copyright © 2002 John Wiley & Sons, Ltd.     

Stabilization of fine gravels by net‐spinning caddisfly larvae

We examined the impact of Hydropsychidae caddisfly larvae on the incipient motion of two sizes of narrowly graded fine‐gravel (4–6 and 6–8 mm). This impact was assessed relative to the collective impact of other abiotic and biotic processes that are potentially important conditioning agents of fine‐gravels. Trays of gravel were placed in the River Soar, Leicestershire, UK, where they were colonized to natural densities by caddisfly larvae. Identical trays that were surrounded by a 250 µm mesh were also deployed, preventing colonization but allowing field conditioning of sediments, including minor reworking of grains and biofilm development. After 21 days in the river, trays were removed to a laboratory flume where grain entrainment stresses were established. In addition to the colonized and conditioned treatments, critical shear stresses were measured for identical sediments that were not placed in the river (laboratory gravels). Gravels that were colonized by Hydropsychidae required significantly greater shear stresses for entrainment than conditioned trays (p ≤ 0·002), however, there was no significant difference between conditioned and laboratory gravels. This implies that the presence of caddisfly can be a more important influence on fine‐gravel stability than some conditioning processes. Shields parameter was compared across treatments and across the two gravel size‐fractions using two‐way ANOVA. No significant differences or interactions were observed, indicating that 4–6 mm gravel was stabilized to a similar degree as 6–8 mm gravel by conditioning and colonization processes. Our results extend earlier studies in two important ways: (1) entrainment stresses were established for fine gravels that were colonized at natural densities, under natural stream conditions; and (2) the caddisfly effect was measured relative to both field‐conditioned and unconditioned laboratory controls. The temporal and spatial distribution of silk‐spinning caddisfly larvae suggests that they have the potential to influence fine‐sediment mobility in many rivers, worldwide. Copyright © 2008 John Wiley & Sons, Ltd.     

A field method for the concurrent measurement of fine sediment content and embryo survival in artificial salmonid redds

The infiltration cube method presented in this paper allows concurrent field measurement of fine sediment content and embryo survival in an incubating environment reproducing, as closely as possible, the morphology and grain size composition of a natural salmonid redd. An infiltration cube is made up of a rectangular (30 cm square and 20 cm high) steel frame with no walls, which has a folded plastic bag attached to its base. Each infiltration cube is buried in the substrate using a procedure aimed at reproducing, as closely as possible, the construction and morpho‐sedimentological structure of a natural salmonid redd. Using a tripod installed on the river bed or at the surface of the ice, the infiltration cube method provides a quick, easy and robust way to pull relatively large samples (approximately 65 kg) out of underwater substrate even during tough winter field conditions. The absence of walls on the cube also precludes bias in infiltration rates by both allowing lateral sediment transport within substrate and preventing loss of fine particles during retrieval from underwater. Copyright © 2006 John Wiley & Sons, Ltd.     

Field Observation of Diurnal Dissolved Oxygen Fluctuations in Shallow Groundwater

Dissolved oxygen (DO) concentrations influence many biogeochemical processes in groundwater systems but studies of temporal variability in DO are lacking. In this study, we used an optical DO probe to measure rapid changes in concentration due to plant‐groundwater interaction at an alluvial aquifer field site in Iowa. Diurnal DO concentrations were observed during mid‐ to late‐summer when soil conditions were dry, fluctuating approximately 0.2 to 0.3 mg/L on a daily basis. DO fluctuations in groundwater were out‐of‐phase with diurnal water table fluctuations, increasing during the day and decreasing at night. DO consumption at night is likely due to increased soil autotrophic and heterotrophic respiration linked with patterns of carbon supply derived from daytime photosynthetic activity, and consistent with available literature on diurnal soil respiration patterns. Although more work is needed to quantify specific processes, our results indicate the potential usefulness of the new optical DO technology to reveal insights regarding many ecohydrological processes.     

Thermal influence of urban groundwater recharge from stormwater infiltration basins

Groundwater warming below cities has become a major environmental issue; but the effect of distinct local anthropogenic sources of heat on urban groundwater temperature distributions is still poorly documented. Our study addressed the local effect of stormwater infiltration on the thermal regime of urban groundwater by examining differences in water temperature beneath stormwater infiltration basins (SIB) and reference sites fed exclusively by direct infiltration of rainwater at the land surface. Stormwater infiltration dramatically increased the thermal amplitude of groundwater at event and season scales. Temperature variation at the scale of rainfall events reached 3 °C and was controlled by the interaction between runoff amount and difference in temperature between stormwater and groundwater. The annual amplitude of groundwater temperature was on average nine times higher below SIB (range: 0·9–8·6 °C) than at reference sites (range: 0–1·2 °C) and increased with catchment area of SIB. Elevated summer temperature of infiltrating stormwater (up to 21 °C) decreased oxygen solubility and stimulated microbial respiration in the soil and vadose zone, thereby lowering dissolved oxygen (DO) concentration in groundwater. The net effect of infiltration on average groundwater temperature depended upon the seasonal distribution of rainfall: groundwater below large SIB warmed up (+0·4 °C) when rainfall occurred predominantly during warm seasons. The thermal effect of stormwater infiltration strongly attenuated with increasing depth below the groundwater table indicating advective heat transport was restricted to the uppermost layers of groundwater. Moreover, excessive groundwater temperature variation at event and season scales can be attenuated by reducing the size of catchment areas drained by SIB and by promoting source control drainage systems. Copyright © 2009 John Wiley & Sons, Ltd.     

Distribution of glaciofluvial sediment within and on the surface of a high arctic valley glacier: Marthabreen,Svalbard

Marthabreen is a 7·8 km long valley glacier in SW Spitsbergen. The glacier is partially covered by a layer of angular debris derived from rockfall in its accumulation area, pierced in places by pinnacles and ridges of glaciofluvial sediment. These concentrations of glaciofluvial sediment fall into three categories: (1) debris pinnacles; (2) longitudinal sediment dykes; (3) longitudinal ridge accumulations. Debris pinnacles are slabs of sediment (predominantly sands, gravels and cobbles) elevated to the glacier surface along thrusts. Longitudinal sediment dykes are low (<0·5 m high) ridges of debris melting out of vertical sediment dykes within the body of the glacier. They are composed of a range of facies including sands, granule gravels, pebble gravels and diamiction. These dykes are sub-parallel to the longitudinal foliation on the glacier and form during folding of the stratification. Longitudinal ridge accumulations are higher (>1 m high) ridges of sorted sand and gravels which are not associated with penetrative ice structures. Their occurrence downglacier of sediment dykes and debris pinnacles suggests that they originate as supraglacial or englacial channels or tunnels filled by sediment derived from the dykes or thrusts. The presence of large quantities of glaciofluvial sediment on the surface of Marthabreen does not imply englacial water flow at high levels within the glacier, but is related to ice deformational processes such as thrusting and folding of debris-rich stratification. Copyright © 1999 John Wiley & Sons, Ltd.     

Seasonal and short-term controls of riparian oxygen dynamics and the implications for redox processes

Riparian zones are highly-dynamic transition zones between surface water (SW) and groundwater (GW) and function as key biogeochemical-reactors for solutes transitioning between both compartments. Infiltration of SW rich in dissolved oxygen (DO) into the riparian aquifer can supress removal processes of redox sensitive compounds like NO₃⁻, a nutrient harmful for the aquatic ecosystem at high concentrations. Seasonal and short-term variations of temperature and hydrologic conditions can influence biogeochemical reaction rates and thus the prevailing redox conditions in the riparian zone. We combined GW tracer-tests and a 1-year high-frequency dataset of DO with data-driven simulations of DO consumption to assess the effects of seasonal and event-scale variations in temperature and transit-times on the reactive transport of DO. Damköhler numbers for DO consumption (DA_DO) were used to characterize the system in terms of DO turnover potential. Our results suggest that seasonal and short-term variations in temperature are major controls for DO turnover and the resulting concentrations at our field site, while transit-times are of minor importance. Seasonal variations of temperature in GW lead to shifts from transport-limited (DA_DO > 1) to reaction-limited conditions (DA_DO < 1), while short-term events were found to have minor impacts on the state of the system, only resulting in slightly less transport-limited conditions due to decreasing temperature and transit-times. The data-driven analyses show that assuming constant water temperature along a flowpath can lead to an over- or underestimation of reaction rates by a factor of 2–3 due to different infiltrating water temperature at the SW–GW interface, whereas the assumption of constant transit-times results in incorrect estimates of NO₃⁻ removal potential based on DA_DO approach (40%–50% difference).     

Cave sedimentation in the new Guinea highlands

This paper investigates the nature and processes of sedimentation of allogenic cave deposits in the high relief, everwet karsts of montane New Guinea. Under the high intensity rainfall regime, episodic mass movements in small karst catchments provide a wide range of sediment textures from clayey gravels to fine clays. These allogenic sediments are deposited into pools of water within the caves, giving sedimentary structures analogous to turbidites. Diamictons within the cave relate to episodic mudflows in the catchment. These deposits move as fluidized masses in a manner similar to some esker deposits. Cross-stratified sediments are formed by dumping of pulses of sediment laden water into deep pools. Extremely fine-grained clays and muds accrete parallel to underlying surfaces following flood pulses. These deposits represent the last phase of catchment instability, when a small amount of slopewash occurs. Catchment processes are dominated by solution and episodic mass movements. When the thick root mat which masks the ground is disrupted, some slopewash occurs but it is not a major component in catchment processes.     

Apparent redox potential discontinuity (aRPD) depth as a relative measure of sediment oxygen content and habitat quality

As hypoxic conditions spread in our oceans, indices that quickly and efficiently assess oxygen content in sediment pore water, and habitat quality are increasingly becoming desirable. Depth to the appa...