Interaction of spiral and flat periodic autowaves in an active medium

Interaction between the rotating wave and a periodic external source in the model of Fitz Hugh--Nagumo type was computed. When the periods of the external source are longer than the rotation period of the spiral wave (T greater than Ts) the external source does not affect the spiral wave. At T less than Ts autowave synchronization effects are observed. The oscillation period predetermined by the external source is set in all the points of the medium except the neighbourhood of the spiral wave. The dislocation (wavebreak) persists in the medium drifting slowly at the angle to the wave vector of the flat waves. After the external source is eliminated, the spiral wave with the original period restores from this dislocation. When the dislocation reaches the interface, it disappears. In this case after the switching off of the external source the resting state is established. A theory of the drift is proposed which connects the drift velocity with the nucleus size and the rotation period of spiral wave.     

Reentry in heterogeneous cardiac tissue described by the Luo-Rudy ventricular action potential model

Heterogeneity of cardiac tissue is an important factor determining the initiation and dynamics of cardiac arrhythmias. In this paper, we studied the effects of gradients of electrophysiological heterogeneity on reentrant excitation patterns using computer simulations. We investigated the dynamics of spiral waves in a two-dimensional sheet of cardiac tissue described by the Luo-Rudy phase 1 (LR1) ventricular action potential model. A gradient of action potential duration (APD) was imposed by gradually varying the local current density of K(+) current or inward rectifying K(+) current along one axis of the tissue sheet. We show that a gradient of APD resulted in spiral wave drift. This drift consisted of two components. The longitudinal (along the gradient) component was always directed toward regions of longer spiral wave period. The transverse (perpendicular to the gradient) component had a direction dependent on the direction of rotation of the spiral wave. We estimated the velocity of the drift as a function of the magnitude of the gradient and discuss its implications.     

Interaction between spiral and paced waves in cardiac tissue

For prevention of lethal arrhythmias, patients at risk receive implantable cardioverter-defibrillators, which use high-frequency antitachycardia pacing (ATP) to convert tachycardias to a normal rhythm. One of the suggested ATP mechanisms involves paced-induced drift of rotating waves followed by their collision with the boundary of excitable tissue. This study provides direct experimental evidence of this mechanism. In monolayers of neonatal rat cardiomyocytes in which rotating waves of activity were initiated by premature stimuli, we used the Ca(2+)-sensitive indicator fluo 4 to observe propagating wave patterns. The interaction of the spiral tip with a paced wave was then monitored at a high spatial resolution. In the course of the experiments, we observed spiral wave pinning to local heterogeneities within the myocyte layer. High-frequency pacing led, in a majority of cases, to successful termination of spiral activity. Our data show that 1) stable spiral waves in cardiac monolayers tend to be pinned to local heterogeneities or areas of altered conduction, 2) overdrive pacing can shift a rotating wave from its original site, and 3) the wave break, formed as a result of interaction between the spiral tip and a paced wave front, moves by a paced-induced drift mechanism to an area where it may become unstable or collide with a boundary. The data were complemented by numerical simulations, which was used to further analyze experimentally observed behavior.     

Attraction and repulsion of spiral waves by inhomogeneity of conduction anisotropy—a model of spiral wave interaction with electrical remodeling of heart tissue

Various forms of heart disease are associated with remodeling of the heart muscle, which results in a perturbation of cell-to-cell electrical coupling. These perturbations may alter the trajectory of spiral wave drift in the heart muscle. We investigate the effect of spatially extended inhomogeneity of transverse cell coupling on the spiral wave trajectory using a simple active media model. The spiral wave was either attracted or repelled from the center of inhomogeneity as a function of cell excitability and gradient of the cell coupling. High levels of excitability resulted in an attraction of the wave to the center of inhomogeneity, whereas low levels resulted in an escape and termination of the spiral wave. The spiral wave drift velocity was related to the gradient of the coupling and the initial position of the wave. In a diseased heart, a region of altered transverse coupling corresponds with local gap junction remodeling that may be responsible for stabilization-destabilization of spiral waves and hence reflect potentially important targets in the treatment of heart arrhythmias.     

Mesh-size effects on drift sample composition as determined with a triple net sampler

Nested nets of three different mesh apertures were used to study mesh-size effects on drift collected in a small mountain stream. The innermost, middle, and outermost nets had, respectively, 425 µm, 209 µm and 106 µm openings, a design that reduced clogging while partitioning collections into three size groups. The open area of mesh in each net, from largest to smallest mesh opening, was 3.7, 5.7 and 8.0 times the area of the net mouth. Volumes of filtered water were determined with a flowmeter. The results are expressed as (1) drift retained by each net, (2) drift that would have been collected by a single net of given mesh size, and (3) the percentage of total drift (the sum of the catches from all three nets) that passed through the 425 µm and 209 µm nets. During a two day period in August 1986, Chironomidae larvae were dominant numerically in all 209 µm and 106 µm samples and midday 425 µm samples. Large drifters (Ephemerellidae) occurred only in 425 µm or 209 µm nets, but the general pattern was an increase in abundance and number of taxa with decreasing mesh size. Relatively more individuals occurred in the larger mesh nets at night than during the day. The two larger mesh sizes retained 70% of the total sediment/detritus in the drift collections, and this decreased the rate of clogging of the 106 µm net. If an objective of a sampling program is to compare drift density or drift rate between areas or sampling dates, the same mesh size should be used for all sample collection and processing. The mesh aperture used for drift collection should retain all species and life stages of significance in a study. The nested net design enables an investigator to test the adequacy of drift samples.     

Propagation of Curved Activation Fronts in Anisotropic Excitable Media

We study theoretical and numerical propagation of autowave fronts in excitable two-variable (activator-inhibitor) systems with anisotropic diffusion. A general curvature-velocity relation is derived for the case that the inhibitor diffusion is neglected. This relation predicts the break of an activation front when the front curvature exceeds a critical value, which is corroborated by computer simulations of a particular reaction-diffusion model. Some qualitative effects associated with the inhibitor diffusion are studied numerically. It is found that the critical value of curvature decreases with an increase in the inhibitor diffusion coefficient. The core of a spiral wave increases in size and turns through an angle which depends on the inhibitor diffusion coefficient. PACS Numbers: 05.50. +q, 05.70. Ln., 82.40. -g, 87.10. +e     

A model for accurate drift estimation in streams

1. This paper explores the experimental difficulties involved with the use of drift nets in small streams, and outlines a method whereby the estimation of drift density (number of specimens m⁻³ of water) can be improved.
2. Changes in the filtering efficiency of the net caused by trapping of organic debris ('clogging') has the effect of reducing net entrance velocities, causing errors in the calculation of sampled water volume, and thus drift density. A model of the reductions in net entrance velocity based on empirical measurements of trapped debris is developed.
3. Cross-sectional velocity calculations suggest that errors can also be introduced into drift density calculations by positioning sampling nets only on the bed. A method to allow this effect is demonstrated.
4. As adjustments to the calculation of sampled volume are required when sampling in rivers that undergo marked changes in discharge during the sampling period, a method whereby these effects can be accommodated to improve drift density estimations is also outlined.
5. The results of this study imply that theoretical links between flow hydraulics and short-term drift behaviour are poorly understood.     

Active entry of stream benthic macroinvertebrates into the water column

Field experiments investigated the possible active entry of stream benthos into the water column. Over a 1-year period, sediment baskets were suspended for 24 hours in a stream pool so that only swimming or floating organisms, essentially unaided by current, could colonize them. A variety of benthos, including taxa characteristic of riffles, colonized the baskets, with colonization highest in late summer and negligible in winter.A modified drift net towed through the pool was used to quantitatively sample benthic animals actually in the water column. Nighttime tows captured a diverse, abundant fauna and indicated densities substantially higher than invertebrate drift densities reported in the literature. Daytime tows yielded little. Estimated percentages of the benthos in the water column at a given time were generally < 1.0%.These findings suggest that not all invertebrate drift is the result of passive mechanical removal from the substrate by current.     

Effect of the form and anisotropy of the left ventricle on the drift of spiral waves

Three-dimensional spiral waves of electrical excitation in the myocardium are sources of dangerous cardiac arrhythmias. In this work, the dynamics of spiral waves of electrical excitation were studied in a symmetric anatomical model of the human heart left ventricle and a realistic ionic cell model of the human ventricular myocardium. Three factors that affect the drift waves in the heart were compared for the first time: the geometry of the heart wall, myocardial anisotropy, and wave chirality. Cardiac anisotropy was identified as a main factor in determining the drift of spiral waves. In the isotropic case, the dynamics were determined by the wall thickness, but did not depend on the wave chirality. In the anisotropic case, chirality was found to play a crucial role.

     

Utilization of autochthonous macroinvertebrate drift by a pool fish community in a woodland stream

Qualitative and quantitative drift data were collected simultaneously above and below a pool both before and after the pool was heavily electrofished. These data revealed no significant difference between drift organism densities (#/m³) above or below the pool before or after fish collection. Qualitative and quantitative fish gut analyses suggest that the sunfishes Lepomis megalotis, L. macrochirus and L. cyanellus may be feeding on drifting invertebrates as these organisms comprised 58%, 37% and 35% gut volume, respectively. A design for a long-term drift net apparatus is presented.     

Increased Arctic sea ice drift alters adult female polar bear movements and energetics

Recent reductions in thickness and extent have increased drift rates of Arctic sea ice. Increased ice drift could significantly affect the movements and the energy balance of polar bears (Ursus maritimus) which forage, nearly exclusively, on this substrate. We used radio‐tracking and ice drift data to quantify the influence of increased drift on bear movements, and we modeled the consequences for energy demands of adult females in the Beaufort and Chukchi seas during two periods with different sea ice characteristics. Westward and northward drift of the sea ice used by polar bears in both regions increased between 1987–1998 and 1999–2013. To remain within their home ranges, polar bears responded to the higher westward ice drift with greater eastward movements, while their movements north in the spring and south in fall were frequently aided by ice motion. To compensate for more rapid westward ice drift in recent years, polar bears covered greater daily distances either by increasing their time spent active (7.6%–9.6%) or by increasing their travel speed (8.5%–8.9%). This increased their calculated annual energy expenditure by 1.8%–3.6% (depending on region and reproductive status), a cost that could be met by capturing an additional 1–3 seals/year. Polar bears selected similar habitats in both periods, indicating that faster drift did not alter habitat preferences. Compounding reduced foraging opportunities that result from habitat loss; changes in ice drift, and associated activity increases, likely exacerbate the physiological stress experienced by polar bears in a warming Arctic.     

Drift of ostracod crustaceans in adjacent intermittent and permanent streams

Ostracods were studied in the drift of adjacent permanent and intermittent streams in Southern Ontario. More species were drifting in the permanent stream than in the intermittent one. The latter developed a pool fauna during early summer and stagnant water species were predominant. Both standing and running water species were common in the permanent stream. Eleven were recorded in the drift and all but one of them are strong swimmers. I. bradyi, the only poor swimmer usually lives among vegetation and it is suggested that truly herpobenthic ostracods are less subject to drift than the others. The pattern of ostracod drift over a period of 13 months in those streams was different. The numbers drifting at the upstream and downstream stations of the same stream in a given month was variable. Ostracods were the major component of crustacean drift in both streams. Stream drift is considered as a mechanism of ostracod dispersal.     

A study of terrestrial and aerial macroinvertebrates on river banks and their contribution to drifting fauna and salmonid diets in a Scottish catchment

Macroinvertebrates of terrestrial and aerial origin were examined on the river banks, in the drifting community and in the stomach contents of salmonids at seven stream sites within the river Nethy catchment in Scotland over a 12 month period. Differences between sites (which varied from moorland to forest), had some influence on the taxonomic composition and diversity of riparian macroinvertebrates but not on the proportion of terrestrial and aerial organisms in the drift or in salmonid guts. The abundances of terrestrial and aerial organisms collected on the river banks and in the drift were not related to the combination of riparian vegetation and altitude. The abundance on the river banks of terrestrial and aerial macroinvertebrates, particularly those possessing wings, varied seasonally and was positively correlated with temperature. Terrestrial and aerial prey contributed up to 86% of the salmonid diet. Organisms most abundant on the river banks were not always the predominant organisms in the drift or those selected by salmonids. This study concludes that it is necessary to examine the riparian and benthic macroinvertebrates (the original prey sources) and the drift (as the actual source of salmonid forays) directly to elucidate salmonid trophic pathways.     

New polyacrylamide matrices for drift-free isoelectric focusing

The major cause for pH gradient decay and cathodic drift during isoelectric focusing in polyacrylamide gels has been found to be electroendo-osmotic flow generated by fixed charges in the gel matrix. These charges have the following causes: (a) trace impurities of acrylic acid in the co-monomers; (b) covalent incorporation of catalysts (persulphate and riboflavin 5'-phosphate) as terminal groups in polyacrylamide chains; (c) hydrolysis of amide groups to acrylic acid in the gel layer underneath the cathodic filter paper strip. The result of these fixed negative charges in the matrix is a movement of counter-ions with hydration water towards the cathode (i.e. electroendo-osmosis) with concomitant drift of pH gradient and focused protein zones in the same direction. It is impossible to cure the cathodic drift by increasing the pH of the anolyte, or decreasing the pH of the catholyte, or both, as previously suggested. One way to reduce the cathodic drift efficiently is to incorporate covalently in the matrix tertiary or quaternary groups (3-dimethylaminopropylmethacrylamide) in stoichiometric amounts as compared with the negative charges.This ‘balanced’ polyacrylamide displays zero drift for at least 5000V·h, which is considered to be an ample time for equilibrium separation of protein species in isoelectric focusing.     

Village Drift and Riverine Settlement: Modeling Akimel O'odham Land Use

Regular unscheduled movements of rancherías within a confined area or settlement district result in the phenomenon described as "village drift," a process whereby a settlement may change its location gradually by several kilometers over a period of years. This article presents a model of village drift based on data acquired from recent archaeological and geomorphological field studies and archival research on the Akimel O'odham, the Gila River Pima of south-central Arizona. The model provides an excellent example of human ecodynamics---an emerging landscape perspective that emphasizes the coevolution of humans and their ecosystem---with implications for understanding prehistoric and historic settlement in desert riverine environments.     

Drift patterns in a high Andean stream

The drifting of invertebrates was sampled for six 24-hour periods from September 1976 to July 1977 in a small stream of the paramo of the Ecuadorian Andes. The composition of the drift is similar to that of the benthos, though percentages may differ markedly. Drift is relatively constant throughout the year, except in March when unusually high rates were noted; at this same period a marked reduction in the benthos was also observed. Diel periodicities in the drift are unclear, although on the whole drift is more important during the daytime; when individual data series are analyzed, weak patterns of day or night drifting can be recognized in some groups. The input of insects through drift from the small streams is thought to be an important source of food for the salmonid fish inhabiting the larger torrents.     

Residence time and drift patterns of larval June sucker Chasmistes liorus in the lower Provo River as determined by otolith microstructure

Estimates of age derived from daily ring counts from otoliths and capture rates of larval June sucker Chasmistes liorus were used to determine the relationship between discharge rates of the Provo River and residence time and patterns of larval drift. During 1997, larval drift occurred over a 22 day period when discharge rates were low (mean ±s.d. 3·2 ± 0·0 m³ s^?1). In 1998, larval drift occurred in two separate events over a 40 day period. Discharge was higher during the first larval drift period (19 days; 24·8 ± 1·3 m³ s^?1) and lower during the second larval drift period (17 days; 7·0 ± 0·9 m³ s^?1). In 1997, no larval fish were collected at the lowermost transect on the Provo River (nearest Utah Lake), and few larvae >21 days of age were found. During the first drift period of 1998, larval C. liorus were collected at all transects, and mean age of larvae collected between upstream and downstream transects increased by c. 7 days. During the second drift period of 1998, only a few were collected in the lowermost transects, and age did not increase with proximity to the lake. Patterns in catch and age distribution of larval C. liorus in the lower Provo River suggest that recruitment failure occurs during the larval drift period in years with insufficient discharge to transport larvae into the lake.     

Transport of larval fish in the Amazon

In the Amazon near Manaus, larvae of Characiformes, Clupeiformes, Tetraodontiformes, Pleuronectiformes, Gymnotiformes, Belonidae and Sciaenidae were sampled in the river during most of the year, except in June and July, when the water level was at its maximum. Characiformes, Tetraodontiformes and Siluriformes were found in the ichthyoplankton mostly during the rising waters, but Clupeiformes and Sciaenidae drifted in the river almost all year around. Egg abundance was extremely low, suggesting that they do not drift or have a very short residence time. Two types of larval drift seemed to occur: a rising water drift and a lowering water drift. Characiformes, Tetraodontiformes and some Clupeiformes drifted mostly during the rising waters and were more abundant near the banks. Their strategy seemed to be a passive dispersion towards the floodplain with the flood pulse. The high densities near the banks optimized their chances of reaching a floodplain inlet. The groups that drifted during the lowering waters showed an alternative strategy. They were flushed from the floodplain lakes and may have stayed in the main river channel for a few months before returning to the floodplain. Predation in the lakes during the period when water level decline was probably the force behind this drift.     

Diel,size-differential drift patterns of three macroinvertebrate species in the lower Mississippi River,Louisiana (USA)

We explored macroinvertebrate size-differential drift in the lower Mississippi River (a 9th order system). Because this river system is highly turbid, we hypothesized that visually-dependent vertebrate predators feeding on drifting organisms would be at a disadvantage. Thus, size-differential drift should not occur. For one 24-hour period in both January and April, six drift nets were used to sample surface drift. Nets were emptied once every four hours. Individual intra-ocular distances of three macroinvertebrate species (Hydropsyche orris: Trichoptera, Hexagenia limbata: Ephemeroptera, Macrobrachium ohione: Crustacea) were measured. Percentages of size classes in the drift were determined. In both months, large individuals of H. orris and H. limbata were prevalent in the nocturnal but scarce in the diurnal drift. In January, large M. ohione drifted regardless of time. In April, large M. ohione predominated the nocturnal drift. Our results could not be attributed solely to vertebrate predator avoidance. Other mechanisms such as diel microhabitat migration and current velocity may have accounted for the results.     

Drift dynamics of Chironomidae larvae: I. Preliminary results and discussion of importance of mesh size and level of taxonomic identification in resolving Chironomidae diel drift patterns

Diel drift samples utilizing nets with mesh size less than 200 microns were taken in Linesville Creek, Pennsylvania, an eastern deciduous forest stream, and Inlet Run, Wyoming, an alpine snow melt stream. Identification of drifting Chironomidae larvae to lowest level taxonomic categories indicated 51 species or species group categories representing 51.95% of the total insect drift in Linesville Creek and 18 species or species group categories representing 70.47% of the total insect drift in Inlet Run. Orthocladiinae were the predominant larvae in the drift in Linesville Creek, with 19 species comprising 43.84% of the Chironomidae drift. In decreasing abundance were Chironomini (12 species, 40.36% of Chironomidae drift), Tanytarsini (10 species, 8.89%), and Tanypodinae (10 species, 6.91%). By contrast, Diamesinae were the predominant larvae in the drift in Inlet Run, (5 species, 71.43%) followed by Orthocladiinae (10 species, 27.25%), Tanytarsini (2 species, 1.20%), and Podonominae (1 species, 0.12%). Comparison of drift composition with substrate samples and/ or emergence data indicated a close relationship between relative abundance in drift and relative abundance in the benthos. Behavioral drift patterns with nocturnal peaks were seen for 3 species or species groups in Linesville Creek. Four species with diurnal drift peaks were present in Inlet Run. Analysis of the size distribution of drifting larvae indicates that a mesh size as small as 200 microns is required to resolve diel drift patterns. It is postulated that random factors greatly influence the apparent diel drift pattern of Chironomidae when nets with mesh size in excess of 400 microns are employed in drift studies. Conflicting literature reports of behavioral drift for Chironomidae may be due to differing species composition of drifting larvae and net mesh size related artifacts.