Mean Current Velocity Research Articles

Resolving combined wave-current fields from measurements using interior point optimization

Complex wave and wave-current conditions exist in the natural world, and are increasingly emulated in advanced experimental facilities to de-risk the deployment, operation and maintenance of offshore structures and renewable energy devices. This can include combinations of ocean swell, multi-directional wind-driven seas, and reflected wave conditions interacting with a current field. It is vital to understand the full nature of these potentially hazardous conditions so they can be properly simulated in numerical models, to contextualize measurements made in field, and experimental programmes. Here, a numerical framework is presented for isolating both the wave systems and the mean current velocities from measured data using an interior point optimizer.A developed frequency domain solver is used to resolve, from experimentally obtained wave gauge measurements, two opposing wave systems on a collinear current, and used to effectively isolate the wave systems and predict the current velocity using only wave gauge measurements. Thirty five test cases are considered; consisting of five wave spectra interacting with seven different current velocities ranging from −0.3m s-10.3m s-1. Comparisons between the theoretical and derived wave numbers and current velocities show good agreement and the performance of the method is similar to that of existing methodologies while requiring no a priori knowledge of the current velocity impacting the wave field required.Although results are presented for the collinear problem, the presented method can be applied to a wide range of wave and current combinations, and provides a useful tool for increasing understanding of both ocean and experimental conditions.

Effectiveness of the electric fish fence as a behavioural barrier at a pumping station

Dyke-based pumping stations have been linked with high fish mortalities during pumping events. Behavioural barriers like electric fish fences have been proposed as a promising solution to prevent entrainment of fish into pumps. In order to test the effectiveness of such barriers, the intake of a pumping station was equipped with a new generation electric fish fence while fish behaviour was observed with an adaptive resolution imaging sonar (ARIS) during non-electrified (reference) and electrified (treatment) operation modes. This study revealed the functionality of the fish fence as a behavioural barrier, with a fish turning rate of up to 72% at a mean water temperature of 4.3°C and a mean current velocity of 0.05ms–1. These field results suggest that new-generation electric fish fences may be a promising solution to reduce the effects of pumping stations on fish.

Sediment Resuspension Due to Near‐Bed Turbulent Effects: A Deep Sea Case Study on the Northwest Continental Slope of Western Australia

AbstractSediment transport equations often consider a mean velocity threshold for the initiation of sediment motion and resuspension, ignoring event‐based turbulent bursting processes. However, laboratory experiments have suggested that near‐bed sediment resuspension is influenced by intermittent turbulent coherent structures. In the field, accessibility constraints for deployment of easily operated equipment has largely prevented further identification and understanding of such processes, which may contribute to resuspension in the marine environment. Field experiments were conducted on the Northwest Slope, Australia, under conditions where the mean current velocities were below the estimated and measured time‐averaged critical velocity to investigate the relationship between near‐bed turbulent coherent structures and sediment resuspension. Results indicate that sediment resuspension occur even when velocities are below the estimated and measured mean critical values. The majority of turbulent sediment flux is due to ejection and sweep events, with lesser contributions from up‐acceleration and down‐deceleration (vertical flow) events. Spectral and quadrant analysis indicated the anisotropic and intermittent nature of Reynolds stresses, and wavelet transform revealed a group of turbulent bursting sequences associated with sediment resuspension. These observations, in flow conditions where resuspension was not expected to occur based on mean threshold concepts, reveal that intermittent turbulent events control sediment resuspension rather a single time‐averaged critical velocity. This highlights the need of considering turbulence as a significant factor in sediment resuspension and should be further investigated for inclusion into future sediment transport modeling.

Wave-current interactions in the continental shelf bottom boundary layer of the Australian North West Shelf during tropical cyclone conditions

The distant passage of an intense and large Category 3 Tropical Cyclone (TC) generated highly energetic surface waves in a large region across the Australian North West Shelf (NWS). Two sites on the continental shelf experienced maximum significant wave heights of up to 10 m with near-bed wave orbital velocities up to 0.7 m s−1 at 40 and 74 m depth. Concurrent current profiles were measured between 0.5 and 8.5 m above the seabed at these sites and therefore the observations presented in this study allowed a detailed analysis of the wave-current interactions in the continental shelf bottom boundary layer during highly energetic tropical cyclone conditions. These observations revealed a strong modification of the current profiles during the TC with an increase of the apparent bottom roughness by up to 2 orders of magnitude compared to the typical tide-dominated current conditions. This modification was predominantly due to wave-current interactions; however, the results also suggest that changes in bed forms during the TC conditions likely acted as a secondary mechanism contributing to the enhanced bottom roughness experienced by the mean currents. A comparison of the total mean bed shear stresses as a function of the waves and currents showed a somewhat larger enhancement due to the nonlinear wave-current interactions than predicted by common semi-empirical models. Overall, the enhancement of the mean bed shear stresses during the TC significantly reduced the mean current velocities in the bottom boundary layer, highlighting the importance of including wave-current interactions within ocean circulation models to accurately predict shelf currents during TC conditions.

Satellite observations of sub-mesoscale vortex trains in the western boundary of the South China Sea

Abstract This study uses high resolution MEdium Resolution Imaging Spectrometer (MERIS) satellite data to analyze sub-mesoscale ocean vortex trains (OVTs) in the western boundary of the South China Sea (SCS). Thirteen cases of vortex trains on the lee side of the Phu Quý Island were observed in the boreal summer from May to August of 2005−2010. The average diameter of the vortices is 28.1 ± 13.8 km. The average spacing distance estimated between two consecutive vortices is 66.3 ± 27.8 km. The dynamic parameters of a case observed on July 26, 2009 were extracted using a theoretical model. The aspect ratio is 2.8. The ratio of the propagation velocity of the vortex to the undisturbed flow velocity is 0.73. We use a nested high-resolution numerical model with the finest grid of 500 m, to analyze the dynamics of OVT induced by the local topography in the western SCS (WSCS). The results give a propagation velocity of vortex 0.49 of m s−1 and a mean current velocity of undisturbed flow 0.58 m s−1. Therefore, the ratio is 0.84. The shedding frequency is determined to be 1.14 × 10−5 s−1, corresponding to 1.02 day. The results also suggest that the sub-mesoscale cyclonic vortices along the OVT in the WSCS are characterized by low temperature and high Rossby number. The strong vertical vorticity and associated horizontal velocity shear of an individual vortex brings uplifted cold water on its leading side and warm water on its trailing side, which could drive an enhanced vertical exchange between the surface and oceanic interior.

Habitat Alterations and Fish Assemblage Structure in the Missouri River System, USA: is Ecomorphology an Explanation?

Sampling was conducted over a two-year period to determine if fish body morphology (as indicated by the Fineness Ratio (FR), an index of fish streamlining) and habitat alterations can interact to influence fish assemblage structure in three human-altered segments of the Missouri River. It was hypothesized that segments with more variability in depths, velocities, and substrates would have a fish assemblage characterized by more diversity in streamlining. Conversely, it was hypothesized that fish assemblages in more altered river segments would exhibit less diversity in streamlining, i.e., less variability from optimal values because of more uniform habitat conditions. In faster more uniform habitats, fewer variations from optimal streamlining would be adaptive. The three flowing segments studied encompassed the mouth of the Yellowstone River (YSS; moderately altered), the area below Garrison Dam, North Dakota (GOS; below dam-highly altered) and the segment from St. Joseph to Kansas City, Missouri (SKS; channelized-highly altered). The three segments exhibited greatly different fish assemblages. Small native minnows (Cyprinidae), particularly flathead chub (Platygobio gracilis), and deep-bodied suckers, such as bigmouth buffalo (Ictiobus cyprinellus), were common in the YSS. The GOS was dominated by the dorsally compressed fathead minnow (Pimephales promelas). Emerald shiner (Notropis atherinoides) and gizzard shad (Dorosoma cepedianum) dominated the fish assemblage in the SKS. When FR and physical conditions were analyzed across all habitat types, the least altered, more natural YSS was characterized by higher diversity of FR and higher variability in velocity and depth than the most altered SKS. Results for the GOS were more difficult to interpret. The fish assemblage in the highly channelized SKS exhibited the weighted mean FR that was closest to the optimal 4.5 value (FR = 4.42) with the smallest deviation from optimal (0.08). The highest mean and highest maximum current velocity use in the three segments was found for species such as sicklefin chub (Macrhybopsis meeki), sturgeon chub (Macrhybopsis gelida), and blue sucker (Cycleptus elongatus), that were nearly optimally streamlined. Deep-bodied species with FRs typically below 3.5, such as Ictiobus spp., river carpsucker (Carpiodes carpio), and centrarchids, tended to exhibit the lowest mean and lowest maximum current velocity use in the three segments . Results of this study can be useful in helping to understand fish assemblage composition in relation to river alterations in various geographical areas. Because the fineness ratio is only a simple index of streamlining and does not account for other anatomical (e.g., fins) and behavioral aspects of fishes, more studies on different species are needed to develop a stronger understanding of how fish cope with the current. Because of the complexity of interacting natural and human-caused factors affecting these highly altered large river systems, fish streamlining is only one of several factors that may influence the observed fish community structure of a large river system. Streamlining deserves consideration, however, for inclusion in more complex models explaining and predicting fish community structure in large rivers.

Predictive habitat modeling in two Mediterranean canyons including hydrodynamic variables

Madrepora oculata and Lophelia pertusa are the two main ecosystem engineering, scleractinian cold-water corals (CWC) found in Mediterranean canyons. Factors controlling CWC distribution in the Mediterranean Sea are not yet fully understood in spite of such ecosystems being recognized as sensitive habitats by the General Fisheries Commission for the Mediterranean. As they are threatened by fishery activity, they are subject to management and protection measures. In order to contribute towards identifying the major drivers governing CWC distribution at local scale, which is a prerequisite for proper management, we focused our attention on two canyons: (1) the Cassidaigne canyon, located in the eastern part of the Gulf of Lion, in which CWC ecosystems have settled in an upwelling environment and form large colonies, and (2) the Bari Canyon System, in the southwestern Adriatic, a site of coral growth that has been hypothesized to respond to hydrographic processes, including the cascading of North Adriatic Dense Water.The objective of our study was to combine several ecological variables to describe the environmental conditions in favor of CWC settlement and growth: (1) CWC observations, extracted from geo-referenced underwater videos, (2) seafloor characteristics derived from high-resolution bathymetry, (3) data on local hydrodynamic conditions (from high resolution hydrodynamic models). Habitat suitability models were used to identify the main variables driving CWC distribution. Models based on presence-only data (Maxent and ENFA) and on presence-absence data (GLMs) were fitted and compared.Seafloor ruggedness was identified to be the major factor driving CWC distribution in both canyons with the three methods. Two hydrodynamic variables (mean temperature and current velocity) were the second most important predictors for explaining CWC settlement and growth. Suitable areas for CWC habitat occurrence were mapped for both canyons. Spatial distributions were generally predicted at the same locations, although the GLM gave less realistic results in the Bari canyon system probably due to the limited range cover of the entire environmental conditions by the absence points, suggesting that the Maxent and ENFA models were more efficient.These theoretical distributions will help in the assessment of potential habitat extent in the deep-sea and also in the scheme of the Marine Strategy Framework Directive (MSFD).

Movement patterns and rheoreaction of larvae of a fluvial specialist (nase, Chondrostoma nasus): the role of active versus passive components of behaviour in dispersal

The dispersal of fish larvae in rivers might result from water movement but also from larval behaviour. Although potentially crucial for dispersion, knowledge of the role of behaviour is still fragmentary. This study intends to contribute to the question of how riverine fish larvae drift or move. All dispersal-relevant movement patterns of larvae of a characteristic rheophilic species were analyzed based on the parameters (i) swimming activity, (ii) direction of movement, and (iii) the orientation towards the current vector. Experiments were conducted in a novel flume mesocosm at three different flow scenarios covering the current velocity range of natural habitats. Mean current velocities in these scenarios were under, near, and over the “critical current velocity”, above which fish larvae are not able to constantly hold their position in the water column. Three consecutive larval stages were tested to account for possible ontogenetic shifts in movement behaviour, both during the day and at night. Our results strongly suggest that the assumption of mainly passively drifting larvae has to be refused; in total, 92.6% of all observed movement events were characterized by swimming activity and directed orientation, whereas only 7.4% could be assigned to passive drift. During downstream movement, a significant portion of movement events (57.1%) was attributed to larvae that orientated in an upstream direction and performed active swimming movements.

Development of Autonomous Boat‐Type Robot for Automated Velocity Measurement in Straight Natural River

AbstractThe present study describes an automated system to measure the river flow velocity. A combination of the camera‐tracking system and the Proportional/Integral/Derivative (PID) control could enable the boat‐type robot to remain in position against the mainstream; this results in reasonable evaluation of the mean velocity by a duty ratio which corresponds to rotation speed of the screw propeller. A laser range finder module was installed to measure the local water depth. Reliable laboratory experiments with the prototype boat robot and electromagnetic velocimetry were conducted to obtain a calibration curve that connects the duty ratio and mean current velocity. The remaining accuracy in the target point was also examined quantitatively. The fluctuation in the spanwise direction is within half of the robot length. It was therefore found that the robot remains well within the target region. We used two‐dimensional navigation tests to guarantee that the prototype moved smoothly to the target points and successfully measured the streamwise velocity profiles across the mainstream. Moreover, the present robot was found to move successfully not only in the laboratory flume but also in a small natural river. The robot could move smoothly from the starting point near the operator's site toward the target point where the velocity is measured, and it could evaluate the cross‐sectional discharge.

Pola Arus Permukaan dan Kondisi Fisika Perairan di Sekitar Pulau Selayar pada Musim Peralihan 1 dan Musim Timur

<strong>Surface Current Pattern and Physics Condition of Waters Around Selayar Island in the First Transitional and Southeast Monsoons. </strong> Seasonal observations of the flow of surface water and physics conditions around Selayar Island adjacent to Arlindo throughflow pathways of Makassar Strait have been conducted with a focus on the first transitional season and the southeast monsoon season. The purpose of this research is to obtain the pattern of seasonal surface current and physics characteristics of water column, i.e. temperature and salinity in Selayar Island waters during those seasons. The observations conducted on 22–27 May 2015 and 7–10 August 2015 illustrated the successive periods of the first transitional season and the southeast monsoon season. The methods used for taking oceanographic data such as temperature, salinity, and current were the stationary oceanographic measurement using CTD and currentmeter at 29 stations located in surrounding waters of Selayar Island. The surface current pattern generated from the interpolation process of the overall observation stations indicated that during the first transitional season the current moved eastward with an average velocity of 0.25 m/s. During the southeast monsoon season, the same pattern was still observed with a slightly higher average velocity of 0.26 m/s. The temperatures and salinity of Selayar Island waters during the southeast monsoon season were 2°C lower and 0.5 psu higher than during the first transitional season. Differences in mean current velocity values tended to be more affected by local tidal conditions. Different salinity was thought to be influenced by upwelling phenomena and local climatic factors such as rainfall, wind, and river flow discharge.

Evaluation of XBeach performance for the erosion of a laboratory sand dune

A new set of laboratory data is used to investigate the bathymetry change of a steep sand dune exposed to waves and high water levels, and subsequently compared to the results of numerical simulations using XBeach. Bichromatic wave boundary conditions are used to simulate a combined short-wave and long-wave field for two water level elevations corresponding to collision with the dune face, and overwashing of the dune crest. In the collision regime case, episodic slumping due to the undercutting of the dune results in sudden erosional events followed by long periods of wave-driven reshaping at the dune toe. In the overwash regime case, morphological changes are faster and sediment transport rates are higher.The XBeach model was used to simulate wave-driven erosion of the dune at the two water levels observed in the laboratory. The model was not able to precisely recreate the cross-shore spatial variability of significant wave height observed in the experiments, however near-bed wave-orbital and mean current velocities were in good agreement with observations. Following rapid initial adjustment, the model results were in agreement with measured dune morphology at successive times. XBeach was sensitive to several parameters that control the rate of erosion including the critical avalanching slope under water, the threshold water depth and the sediment transport formulation, and performed well after careful selection of the best combination of these parameters. Overall, the model predictions were in better agreement with laboratory observations for dune erosion in the overwash regime case than the collision regime case.

Contourite identification along Italian margins: The case of the Portofino drift (Ligurian Sea)

A brief review of the published evidence of current deposits around Italy is the occasion to test the robustness of matching bottom current velocity models and seafloor morphologies to identify contourite drifts not yet documented. We present the result of the regional hydrodynamic model MARS3D in the Northern Tyrrhenian and Ligurian Sea with horizontal resolution of 1.2 km and 60 levels with focus on bottom current: data are integrated over summer and winter 2013 as representative of low and high intensity current conditions.The Eastern Ligurian margin is impacted by the Levantine Intermediate Water (LIW) with modeled mean velocity of bottom current up to 20 cm s−1 in winter 2013 and calculated bottom shear stress exceeding 0.2 N m−2 in water depth of 400–800 m. By crossing this information with seafloor morphology and geometry of seismic reflections, we identify a sediment drift formerly overlooked at ca 1000 m water depth. The Portofino separated mounded drift has a maximum thickness of at least 150 m and occurs in an area of mean current velocity minimum. Independent evidence to support the interpretation include bottom current modelling, seafloor morphology, seismic reflection geometry and sediment core facies. The adjacent areas impacted by stronger bottom currents present features likely resulted from bottom current erosion such as a marine terrace and elongated pockmarks.Compared to former interpretation of seafloor morphology in the study area, our results have an impact on the assessment of marine geohazards: submarine landslides offshore Portofino are small in size and coexist with sediment erosion and preferential accumulation features (sediment drifts) originated by current-dominated sedimentary processes. Furthermore, our results propel a more general discussion about contourite identification in the Italian seas and possible implications.

Current and wave effects around windfarm monopile foundations

Laboratory measurements were undertaken to investigate wave and current velocities in the vicinity of a wind turbine monopile foundation, in order to inform environmental impact assessments and to quantify flow variability in the region of the power take off cable. Flow measurements were made up to 15.5 pile diameters (D) downstream of the pile. Measurements were also taken around the perimeter of the pile (~0.75 D from the pile centre) at the approximate representative height of the power cable.In current-only tests, the mean flow was reduced immediately downstream of the pile, but returned to within 5% of background levels by 8.3 D downstream of the pile centre in representative conditions. A new parameterisation of the velocity recovery is given. The turbulent eddy shedding frequency was well predicted by the Strouhal number. Turbulence peaked at 1.5 D from the pile centre, and the subsequent decay was parameterised. Velocity magnitudes at the side of the pile were up to 1.35 times greater than background flow rates, in line with potential flow theory. Velocities in the wake region were much less than predicted by potential flow theory, corresponding with increased turbulence.Tests with waves indicated that oscillatory velocities reduced immediately down-wave of the pile, but returned quickly to background levels (by 1.65 to 3.5 D of the pile centre). The general near-pile distribution of the orbital velocity maximum was well represented by potential flow theory. Orbital velocities were reduced immediately up-wave and down-wave of the pile. At the side of the pile in wind sea conditions, the velocity increased up to 1.66 times the background level. This increased to 1.85 times in swell conditions.For orthogonal currents and waves, a velocity parameter was calculated as the mean current plus wave orbital velocity, resolved. With the mean current direction as a reference, the maximum flow was observed at the side of the pile. At 0.75 D from the pile centre, the flow was enhanced by up to 1.2 times the no–pile case. Spectral peaks in the velocity were evident at both wave frequency and at the Strouhal frequency, immediately down current from the pile.

OCEANOGRAPHY AND WATER QUALITY CONDITION IN SEVERAL WATERS OF THOUSAND ISLANDS AND ITS SUITABILITY FOR WHITE SHRIMP Litopenaeus vannamei CULTURE

The purposes of this study were to determine oceanographic and water quality parameters and their suitability for white shrimp Litopenaeus vannamei culture. The measurements were carried out on dry season in Semak Daun Island, Karya Island, and Panggang Island waters of Thousand Islands, with areas of 315.0, 12.0, and 102.8 ha, water depth average of 4.6 m (0.5-28.1 m), 14.6 m (0.5-26.7 m), and 5.3 m (0.8-13.6 m), mean current water velocity of 12.9, 12.7, and 13.5 cm/second, respectively. In the study areas, we found a diurnal tidal pattern with high wave in January and July-August. Based on temperature, salinity, and water density in Semak Daun Island waters, there seemingly occurred a turn over indicating a good water circulation, while in Panggang Island and Karya Island waters tended to have a stratification. Generaly, water qualities in the study areas were in the op-timum range for white shrimp culture, i.e., temperature of 29.6-30.8oC, turbidity of 0.10-1.05 NTU, transparency of 5.8-9.7 m, total suspended solid of <8 mg/L, total dissolved solid of 20-164 mg/L, pH of 6.89-7.22, salinity of 32.2-32.3, dissolved oxygen of 5.8-10.8 mg/L, ammonia of 0.068-0.145 mg/L, nitrate 1.247-2.589 mg/L, and phosphate of 1.021-2.352 mg/L. Moreover, in Semak Daun Island wa-ters, we found the highest suitability for white shrimp culture due to its better water circulation.Keywords: mariculture, coral reef waters, strait, water current, turnover, stratification.

The suppression of surfzone cross‐shore mixing by alongshore currents

AbstractMixing by horizontal surfzone eddies is important to the cross‐shore exchange of material through the surfzone. Surfzone cross‐shore mixing for normally and obliquely incident waves, with very similar incident significant wave heights and directional spreads, is quantified using Boussinesq modeled drifter trajectories. For t < 100 s, surfzone cross‐shore diffusivities K(t) are independent of incident wave angle owing to similar eddy velocities m s−1. For 600 < t < 10,000 s, K(t) is maximum for normally incident waves and decreases with incident wave angle. Thus, the mixing parameterization is not completely applicable to the surfzone, because and LE do not change for these experiments. Reduced mixing for obliquely incident waves results from large eddies (wavelengths >200 m) propagating at a velocity C different than the mean current velocity V(x)—the same mechanism that reduces mixing for mesoscale eddies.

The implementation of rare events logistic regression to predict the distribution of mesophotic hard corals across the main Hawaiian Islands.

Predictive habitat suitability models are powerful tools for cost-effective, statistically robust assessment of the environmental drivers of species distributions. The aim of this study was to develop predictive habitat suitability models for two genera of scleractinian corals (Leptoserisand Montipora) found within the mesophotic zone across the main Hawaiian Islands. The mesophotic zone (30–180 m) is challenging to reach, and therefore historically understudied, because it falls between the maximum limit of SCUBA divers and the minimum typical working depth of submersible vehicles. Here, we implement a logistic regression with rare events corrections to account for the scarcity of presence observations within the dataset. These corrections reduced the coefficient error and improved overall prediction success (73.6% and 74.3%) for both original regression models. The final models included depth, rugosity, slope, mean current velocity, and wave height as the best environmental covariates for predicting the occurrence of the two genera in the mesophotic zone. Using an objectively selected theta (“presence”) threshold, the predicted presence probability values (average of 0.051 for Leptoseris and 0.040 for Montipora) were translated to spatially-explicit habitat suitability maps of the main Hawaiian Islands at 25 m grid cell resolution. Our maps are the first of their kind to use extant presence and absence data to examine the habitat preferences of these two dominant mesophotic coral genera across Hawai‘i.

Experimental study of wave-induced mass transport

ABSTRACTAn original method is proposed to extract drift velocity induced by waves propagating in a wave flume. The particle image velocimetry technique is employed to measure velocity fields under regular gravity waves propagating at a constant depth. Based on the measurements, the Eulerian mean current velocity is determined. The instantaneous velocity fields are integrated to calculate particle trajectories and the Lagrangian time-averaged vertical distribution of mass-transport velocity. The Stokes drift velocity is determined by subtracting the Eulerian mean current from the Lagrangian mass-transport velocity. The results are in good agreement with theoretical second-order irrotational solution and confirm applicability of the weakly nonlinear solution for calculating transport processes associated with propagating ocean waves.

Transient tracer distributions in the Fram Strait in 2012 and inferred anthropogenic carbon content and transport

Abstract. The storage of anthropogenic carbon in the ocean's interior is an important process which modulates the increasing carbon dioxide concentrations in the atmosphere. The polar regions are expected to be net sinks for anthropogenic carbon. Transport estimates of dissolved inorganic carbon and the anthropogenic offset can thus provide information about the magnitude of the corresponding storage processes. Here we present a transient tracer, dissolved inorganic carbon (DIC) and total alkalinity (TA) data set along 78°50′ N sampled in the Fram Strait in 2012. A theory on tracer relationships is introduced, which allows for an application of the inverse-Gaussian–transit-time distribution (IG-TTD) at high latitudes and the estimation of anthropogenic carbon concentrations. Mean current velocity measurements along the same section from 2002–2010 were used to estimate the net flux of DIC and anthropogenic carbon by the boundary currents above 840 m through the Fram Strait. The new theory explains the differences between the theoretical (IG-TTD-based) tracer age relationship and the specific tracer age relationship of the field data, by saturation effects during water mass formation and/or the deliberate release experiment of SF6 in the Greenland Sea in 1996, rather than by different mixing or ventilation processes. Based on this assumption, a maximum SF6 excess of 0.5–0.8 fmol kg−1 was determined in the Fram Strait at intermediate depths (500–1600 m). The anthropogenic carbon concentrations are 50–55 µmol kg−1 in the Atlantic Water/Recirculating Atlantic Water, 40–45 µmol kg−1 in the Polar Surface Water/warm Polar Surface Water and between 10 and 35 µmol kg−1 in the deeper water layers, with lowest concentrations in the bottom layer. The net fluxes through the Fram Strait indicate a net outflow of ∼ 0.4 DIC and ∼ 0.01 PgC yr−1 anthropogenic carbon from the Arctic Ocean into the North Atlantic, albeit with high uncertainties.

The altitudinal limit of Leptohyphes Eaton, 1882 and Lachlania Hagen, 1868 (Ephemeroptera: Leptohyphidae, Oligoneuriidae) in Ecuadorian Andes streams: searching for mechanisms

ABSTRACTWe explored mechanisms determining the upper altitudinal limit of ephemeropterans from two different genera: Leptohyphes Eaton, 1882 (Leptohyphidae) and Lachlania Hagen, 1868 (Oligoneuriidae). For this, we (1) surveyed the two taxa in 165 stream sites along a wide altitudinal gradient; (2) sampled benthic fauna at short altitudinal intervals along a stream, from 2780 to 3150 m above sea level; (3) collected adults at the lowest and highest sites; and (4) transplanted nymphs from the lowest to the highest study site in our stream to determine survival over time. Densities of the two taxa declined gradually with altitude and both disappeared between 2950 and 3080 m a.s.l. The upper altitudinal limit in the stream seemed to be most closely related to mean oxygen saturation, temperature, and current velocity. Adults were collected where the nymphs were found, but not at the upstream site where the nymphs were absent, implying limited upstream dispersal of adults and some of the altitudinal constraint lying at the adult stage. Short-term survival of transplanted nymphs was lower than that of controls, suggesting that the distribution was limited at the juvenile stage, and that at least some of the altitudinal constraint is related to the abiotic stream environment.

自律制御ボート型ロボットの開発と河川流速計測の試み

The present study developed autonomous-mobile floating-robot using one-board microcomputer Arduino which could measure automatically mean velocity in an open-channel flow. Camera tracking system and PID control method could make the robot remain the position against main stream, and then time-averaged streamwise velocity was evaluated reasonably by a duty-ratio of screw propeller motor. Reliable laboratory experiments with electromagnetic velocimetry provided the calibration curve that connects the duty-ratio and mean current velocity. Furthermore, the present floating could be found to move successfully in not only the laboratory flume but also in natural creek.