Small-scale Disturbances Research Articles

Human Influence on Banj Oak (Quercus leucotrichophora, A. Camus) Forests of Central Himalaya

The present study suggests that the impact of human-induced small-scale disturbances (lopping of branches and leaf litter removal) adversely impacts the functioning of banj oak (Quercus leucotrichophora, A. Camus) forests of Central Himalaya. Significantly higher (p < .001) biomass stocks, carbon sequestration rates, soil carbon, leaf area index (LAI), litter fall, and faster litter decomposition rates were observed in least human influenced (LHI) forests as compared to moderately human influenced (MHI) forests and highly human influenced (HHI) forests. Three replicate forest stands of each category were selected for the observation. The study is used as a background to suggest alternative strategies to conserve the forests, taking into account the social and economic concerns of the village community.

Description of ionospheric disturbances observed by Vertical Ionospheric Sounding at 3 MHz

&lt;p&gt;High Frequency radio waves reflected by the ionosphere can provide a relevant amount of information within the composite received signal. The ionosphere is indeed a frequency dispersive, bi-refractive, absorbing medium, in which multipath propagation occurs due to disturbance on a varied time-spatial scale. On the time-spatial level of Small Scale Disturbances (SSD) the ionosphere dynamics, detectable by Vertical Ionospheric Sounding (VIS), is mainly dependent on wrinkled layers acting as multi-reflectors. The present paper discusses different aspects of the effects of multipath fading suffered by the wave along the propagation path and potentially associated with SSD. To achieve these objectives, a VIS campaign at a fixed frequency of 3.0 MHz was conducted at the ionospheric observatory in Rome (Latitude 41.8 N; Longitude 12.5 E), by collecting a series of measurements of the power variations in received echo signals recorded between two consecutive ionograms whose sounding repetition rate was set to 15 min. The obtained results show that: 1) the fading suffered by the wave follows either a Rayleigh trend or a Nakagami-Rice trend, or a mix of them, the mixed case being the most frequent (about 65 % of the analysed cases); 2) the predominant periodicities characterizing the power variation are less than 25 s; such values are compatible with the small scale ionospheric disturbances; 3) for all the 24 hours of the day the ionospheric reflector is pretty stable and for time intervals of 10-30 s the periods of stability occur with a percentage of occurrence ranging between 55% and 95 %; for time intervals of 190- 210 s the periods of stability occur instead with a percentage of occurrence ranging between 5% and 54 %.&lt;/p&gt;

Intraspecific Variation in Physiological Condition of Reef-Building Corals Associated with Differential Levels of Chronic Disturbance

Even in the absence of major disturbances (e.g., cyclones, bleaching), corals are subject to high levels of partial or whole-colony mortality, often caused by chronic and small-scale disturbances. Depending on levels of background mortality, these chronic disturbances may undermine individual fitness and have significant consequences on the ability of colonies to withstand subsequent acute disturbances or environmental change. This study quantified intraspecific variations in physiological condition (measured based on total lipid content and zooxanthellae density) through time in adult colonies of two common and widespread coral species (Acropora spathulata and Pocillopora damicornis), subject to different levels of biological and physical disturbances along the most disturbed reef habitat, the crest. Marked intraspecific variation in the physiological condition of A. spathulata was clearly linked to differences in local disturbance regimes and habitat. Specifically, zooxanthellae density decreased (r2 = 26, df = 5,42, p<0.02, B = −121255, p = 0.03) and total lipid content increased (r2 = 14, df = 5,42, p = 0.01, B = 0.9, p = 0.01) with increasing distance from exposed crests. Moreover, zooxanthellae density was strongly and negatively correlated with the individual level of partial mortality (r2 = 26, df = 5,42, p<0.02, B = −7386077, p = 0.01). Conversely, P. damicornis exhibited very limited intraspecific variation in physiological condition, despite marked differences in levels of partial mortality. This is the first study to relate intraspecific variation in the condition of corals to localized differences in chronic disturbance regimes. The next step is to ascertain whether these differences have further ramifications for susceptibility to periodic acute disturbances, such as climate-induced coral bleaching.

Resilience of Zostera muelleri seagrass to small‐scale disturbances: the relative importance of asexual versus sexual recovery

Resilience is the ability of an ecosystem to recover from disturbance without loss of essential function. Seagrass ecosystems are key marine and estuarine habitats that are under threat from a variety of natural and anthropogenic disturbances. The ability of these ecosystems to recovery from disturbance will to a large extent depend on the internsity and scale of the disturbance, and the relative importance of sexual versus asexual reproduction within populations. Here, we investigated the resilience of Zostera muelleri seagrass (Syn. Zostera capricorni) to small-scale disturbances at four locations in Lake Macquarie – Australia's largest coastal lake – and monitored recovery over a 65-week period. Resilience of Z. muelleri varied significantly with disturbance intensity; Z. muelleri recovered rapidly (within 2 weeks) from low-intensity disturbance (shoot loss), and rates of recovery appeared related to initial shoot length. Recovery via rhizome encroachment (asexual regeneration) from high-intensity disturbance (loss of entire plant) varied among locations, ranging from 18-35 weeks, whereas the ability to recover was apparently lost (at least within the time frame of this study) when recovery depended on sexual regeneration, suggesting that seeds do not provide a mechanism of recovery against intense small-scale disturbances. The lack of sexual recruits into disturbed sites is surprising as our initial surveys of genotypic diversity (using nine polymorphic microsatellite loci) at these location indicate that populations are maintained by a mix of sexual and asexual reproduction (genotypic diversity [R] varied from 0.24 to 0.44), and populations consisted of a mosaic of genotypes with on average 3.6 unique multilocus genotypes per 300 mm diameter plot. We therefore conclude that Z. muelleri populations within Lake Macquarie rely on clonal growth to recover from small-scale disturbances and that ongoing sexual recruitment by seeds into established seagrass beds (as opposed to bare areas arising from disturbance) must be the mechanism responsible for maintaining the observed mixed genetic composition of Z. muelleri seagrass meadows.

Flow dynamics of Byrd Glacier, East Antarctica

AbstractForce-balance calculations on Byrd Glacier, East Antarctica, reveal large spatial variations in the along-flow component of driving stress with corresponding sticky spots that are stationary over time. On the large scale, flow resistance is partitioned between basal (~80%) and lateral (~20%) drag. Ice flow is due mostly to basal sliding and concentrated vertical shear in the basal ice layers, indicating the bed is at or close to the pressure-melting temperature. There is a significant component of driving stress in the across-flow direction resulting in nonzero basal drag in that direction. This is an unrealistic result and we propose that there are spatial variations of bed features resulting in small-scale flow disturbances. The grounding line of Byrd Glacier is located in a region where the bed slopes upward. Nevertheless, despite a 10% increase in ice discharge between December 2005 and February 2007, following drainage of two subglacial lakes in the catchment area, the position of the grounding line has not retreated significantly and the glacier has decelerated since then. During the speed-up event, partitioning of flow resistance did not change, suggesting the increase in velocity was caused by a temporary decrease in basal effective pressure.

Does mollusc shell debris determine patterns of macrofaunal recolonisation on a tidal flat? Experimental evidence from reciprocal transplantations

Mollusc shells introduce complexity and heterogeneity into soft-sediment habitats and might regulate the structure and functioning of benthic assemblages. Two areas of an unvegetated tidal flat located at the mouth of Paranaguá Bay (southern Brazil) largely differ in substrata characteristics and macrofaunal assemblage structure. Sediments of the inner part of the studied site are structurally more complex than those of the peripheral portion, due to high densities of empty shells of the bivalve Anomalocardia brasiliana. The effects of this biogenic matrix on macrofaunal recolonisation during a three-month period were assessed through a reciprocal transplantation experiment of defaunated sediment between shelled and bare habitats. Hypotheses were tested to distinguish between three possible causes of variation: (i) habitat complexity, (ii) intrinsic factors of each area (regardless of the presence/absence of shells), and (iii) their interaction. Habitat structural heterogeneity provided by empty shells does not explain the large differences between macrofauna in shelled and bare habitats. Assemblages that colonised transplanted sediments became similar to those in controls at the area to which they were moved, but differed from those in controls at the area where they originated. The lack of a consistent response of assemblages to the presence of shells can be explained not only by the extremely fast recovery following small-scale experimental disturbances, but also by the dispersal mechanisms related to reproductive traits of the dominant taxa. Changes in assemblages were greatly influenced by the tanaid Monokalliapseudes schubarti, the numerically-dominant species that showed consistent differences among treatments. The reduced dispersal capacity due to the lack of planktonic larval stages associated with high fecundity, continuous reproduction and fast growth were determinant features for the rapid colonisation of defaunated sediments, regardless of habitat complexity. Despite the well-recognised impact of mollusc shell production on soft-sediment habitats, our study showed that its effects are scale-dependent, both spatially and temporally, and might be regulated by the structure of local assemblages and species-specific responses related to dispersal mechanisms.

The resilience of the forest field layer to anthropogenic disturbances depends on site productivity

The resilience of the boreal forest field layer (herbs plus dwarf shrubs) to anthropogenic disturbances is insufficiently understood because of the multitude of direct or indirect driver pathways and environmental conditions involved. We hypothesized that the impact of the forest-management-induced disturbances on field layer varies along the gradient of site productivity. To explore that we proposed a method for estimating the proportional effect of each driver on the field-layer composition, in a survey data of 273 mature or overgrown boreal forests, by combining variogram analysis with multifactorial general linear modelling. In forest types of very low and high productivity, field-layer composition was sensitive to the management disturbances in general and, particularly, to the management-controlled variations in the structure of the stand and its understory, i.e., in environmentally stressful conditions the main limiting factors were light availability and its spatiotemporal variability. At intermediate productivity, instead, the natural heterogeneity of ground layer conditions was the dominant driver, pointing to the limitation of regeneration microsites. Accordingly, on soils with low and high productivity, biodiversity-oriented sustainable forestry should diversify silvicultural approaches among stands and (or) enhance the within-stand mosaic, whereas small-scale natural disturbances of the ground-level “organic blanket” should be promoted in forests of intermediate productivity.

Effects of Canopy Gap Disturbance on Forest Birds in Boreal Forests

We studied the effects of small-scale disturbance on breeding, forest passerine birds in an old-growth and managed boreal forests in northern Finland. Small-scale disturbance (< 2 ha) in an old-growth and managed forests originated from wind falls and small clear cuts. Continuous forest without gaps was used as a control for both management types (old-growth and managed forests). Passerines' response to disturbance was examined by estimating species richness and abundance of different ecological groups. Species richness and the total abundance of birds did not differ between gap and non-gap plots, neither did the abundance of most ecological groups. Management type or study year were the most important factors explaining abundances. Our results differ from studies conducted in temperate forests, mainly in North America, where small-scale disturbance have been found to increase avian diversity and abundance. Differences between boreal and temperate forests or in avian assemblages between continents may exp...

Gap disturbance patterns in an old-growth sessile oak (Quercus petraea L.)–European beech (Fagus sylvatica L.) forest remnant in the Carpathian Mountains, Romania

In recent decades, natural forest remnants have become increasingly important as reference objects for maintaining or restoring old-growth characteristics in managed forests. Canopy gaps play an important role in forest regeneration, particularly for the establishment and development of tree species with different ecological recruitment patterns. Yet quantitative descriptions of such patterns are still scarce, particularly for oak-dominated forests. The old-growth sessile oak–European beech forest remnant in the Runcu-Grosi Natural Reserve provided a unique opportunity to study natural disturbance regimes with minimal human influence in an ecosystem type rarely investigated. The study site comprised the best preserved part of the Reserve. Its 32.3 ha are dominated by sessile oak. A complete gap survey was carried out. The size, shape, spatial pattern and traits of the gapmakers of all 321 gaps were recorded. Additionally, the gap age as well as the structure and composition of gapfillers were investigated in 70 randomly sampled expanded gaps.The canopy gaps and the expanded gaps covered 12.8% and 28.5% of the study site, respectively. The frequency distribution of the canopy gap sizes corresponded to the negative exponential distribution, with most of the gaps (60%) smaller than 100 m2, 34% between 100 and 300 m2 and only 2% larger than 500 m2. Canopy gaps smaller than 300 m2 were responsible for 71% of the total gap area, suggesting a dominance of small and intermediate gaps in this forest. The pattern of the canopy gaps is characterized by a pronounced soft-core effect, and in one part of the study area a tendency towards regularity. Most of the gaps (84%) were caused by more than one gapmaker and seemed to be created in more than one disturbance event since, in 72% of the gaps, gapmakers of at least two different decay classes were found. The disturbance regime was driven by the mortality of sessile oaks, the main gapmaker species, caused mostly by uprooting. The other main canopy tree species was European beech, which died often by snapping. It was less common as gapmaker (20%), but was the main gapfiller (91%). In contrast sessile oak was almost absent among the gapfillers.These results suggest that the current small-scale disturbance pattern dominating this old-growth forest is more suitable for shade–tolerant species such as European beech, accentuating the already steady decline of oaks in mixed sessile oak–European beech stands.

Tree regeneration in the threatened forest of Robinson Crusoe Island, Chile: The role of small-scale disturbances on microsite conditions and invasive species

Biological invasions are a major driver of biodiversity loss on islands. After disturbances, invasive plant species can establish affecting forest regeneration microsites. On Robinson Crusoe Island (33°S, Juan Fernández Archipelago, Chile) small-scale treefall canopy gap microsites are most frequently used by “endemic montane forest” species for regeneration. Regeneration can be hampered when invasive species establish and alter gap conditions. We evaluated the role of small-scale disturbances on regeneration and identified tree regeneration microsites in gaps, gap borders, and closed forest. We collected information on the effects caused by invasive species by sampling 30 gaps with a range of invasive species cover, including gaps where invasive species were removed. We analyzed the impact of native ferns and invasive species, regeneration substrates and light availability on native tree species regeneration and juvenile tree performance traits. Our aim was to analyze small-scale disturbances and identify threshold values for the variables related with tree regeneration presence–absence, density and performance, particularly considering invasive species competition. We used classification and regression trees to identify variables and their threshold values influencing native tree species regeneration. Gap borders and small gaps (<200m2) were preferred microsites for regeneration. Native tree species seemed able to compete as long as invasive species cover did not exceed 10%. Fern cover >10% facilitated tree regeneration and performance. Competition from invasive species for space, water and nutrients was likely more important than for light. Restoration should attempt to recreate intermediate disturbance conditions considering the threshold values identified. Thresholds for variables important for restoration processes can help in the control of invasive species.

Contributions of gopher mound and casting disturbances to plant community structure in a Cascade Range meadow complex

Pocket gophers (Geomyidae) are major agents of disturbance in North American grasslands. Gopher mounds bury existing plants and influence community structure through various mechanisms. However, in mountain meadows that experience winter snowpack, gophers also create winter castings, smaller tube-shaped deposits, previously ignored in studies of plant–gopher disturbance relationships. We studied the influences of the Mazama pocket gopher (Thomomys mazama Merriam, 1897) in montane meadows of the Oregon Cascades, quantifying community patterns at larger spatial scales than previously studied in this system and considering, for the first time, effects of both mounds and castings. We measured cover of disturbance and individual plant species along twenty 5 m transects in each of four plots with differing species composition. Total plant cover was negatively correlated with mounds and castings. However, only mounds influenced growth-form dominance, reducing graminoid cover and increasing the forb–graminoid ratio — effects attributable to the greater volume and longevity of mounds. Forb-disturbance relationships were variable among plots, likely due to differences in species’ tolerance of burial. Transect-scale richness and heterogeneity (variation in composition within transects) increased with disturbance. We conclude that frequent, small-scale disturbances create a shifting mosaic of vegetation states, reducing graminoid dominance and enhancing species richness and heterogeneity at larger spatial scales.

New insights into vegetation dynamics and settlement history in Hümmling, north-western Germany, with particular reference to the Neolithic

Palynological investigations on two well-dated peat profiles provide insights into Neolithic vegetation and settlement history from Hummling in north-western Germany. The site selections allow comparisons between local and regional vegetation changes and are used to estimate the extent of Neolithic influence on the vegetation. The interpretation of the fossil spectra relied on radiocarbon dating, evaluation of pollen indicator taxa, non-pollen palynomorphs and multivariate techniques. During the late Mesolithic the vegetation was dominated by mixed oak forests while openings in forest cover were detected, with a decline in elm reflected in the regional pollen record around 4250 cal. b.c. The presence of humans is shown by settlement indicators that are first recorded at ca. 3800 cal. b.c. Vegetation changes were small between 4300 and 3600 cal. b.c. This suggests that regional vegetation was relatively resilient to small-scale disturbances. Possible indications of grazing were recorded in the spectra of the local pollen profile but there is no clear-cut evidence for Neolithic activity. Between 3520 and 2260 cal. b.c. decreases in forest cover were inferred from both profiles and increases in settlement indicators reflect farming activity. These changes coincide with the emergence in the area of the Funnel Beaker Culture and the subsequent Single Grave Culture. Both profiles suggest that settlement probably ceased between ca. 3230 and 3050 cal. b.c. This lull or cessation in activity was probably regional in character. After 2260 cal. b.c. human impact on the vegetation decreases and woodlands regenerate. The longevity of the regeneration phase—ca. 690 years—was probably connected with the low resilient capability of the vegetation on the poor soils.

Global stability of the ballooning mode in a cylindrical model

Ballooning disturbances in a finite-pressure plasma in a curvilinear magnetic field are described by the system of coupled equations for the Alfven and slow magnetosonic modes. In contrast to most previous works that locally analyzed the stability of small-scale disturbances using the dispersion relationship, a global analysis outside a WKB approximation but within a simple cylindrical geometry, when magnetic field lines are circles with constant curvature, is performed in the present work. This model is relatively simple; nevertheless, it has the singularities necessary for the formation of the ballooning mode: field curvature and non-uniform thermal plasma pressure. If the disturbance finite radial extent is taken into account, the instability threshold increases as compared to a WKB approximation. The simplified model used in this work made it possible to consider the pattern of unstable disturbances at arbitrary values of the azimuthal wavenumber (ky). Azimuthally large-scale disturbances can also be unstable, although the increment increases with decreasing azimuthal scale and reaches saturation when the scales are of the order of the pressure nonuniformity dimension.

Silvicultural disturbance has little impact on tree species diversity in a Central African moist forest

Timber production is an important economic sector in most forested countries of Central Africa, where about 14 million hectares of lowland moist forests are now planned for management. This production is expected to be sustainable, but the actual impact of logging on biodiversity is still questioned.To answer this question, we used a unique long-term controlled experiment implemented more than 20years ago in an old-growth semi-deciduous moist forest of the Central African Republic (CAR). We tested whether (i) anthropogenic disturbances associated with silvicultural operations had an effect on the composition and diversity of tree communities, and (ii) there is a relationship between diversity and disturbance intensity in those forests.For this, we botanically identified all trees⩾10cm DBH in 28 1-ha plots where no treatment (controls), logging and logging+thinning operations were implemented 24years ago and created a strong gradient of disturbance. We investigated the relationships between five diversity indices and a disturbance index calculated for each 1-ha plot, for all species and separately for three regeneration guilds.We found a strong positive monotonic relationship between the intensity of disturbance and the percentage of pioneer species in the tree communities, which proved to be equally detrimental, in terms of relative abundance, to the non-pioneer light-demanding and the shade-bearing species.Overall, disturbance appeared to have a weak monotonous negative effect on diversity, irrespective to the index considered. The diversity of shade-bearers slightly decreased along the disturbance gradient without significant decrease in species density; disturbance had no effect on non-pioneer light demanders, but a clear significant negative effect on the diversity of pioneers, with a significant decrease in species density. This negative effect was associated with the massive recruitment of the early-successional, fast-growing Musanga cecropioides R. Br. (Urticaceae), which rapidly preempted space and resources in the most disturbed plots. Despite this effect, disturbance did not significantly affect the local heterogeneity in species distribution.These results suggest that the semi-deciduous moist forests of CAR are locally resilient to small-scale disturbances associated with silvicultural operations. This may be a consequence of the past anthropogenic and/or climatic disturbances, which have been stronger and more long-lasting than elsewhere within the tropical forest biome, and would have removed the most vulnerable species. Because logging intensity in these forests is usually low, we do not expect any direct major impact on tree species diversity, at least after the first felling cycle.

Second-mode attenuation and cancellation by porous coatings in a high-speed boundary layer

AbstractNumerical simulations of the linear and nonlinear two-dimensional Navier–Stokes equations, and linear stability theory are used to parametrically investigate hypersonic boundary layers over ultrasonic absorptive coatings. The porous coatings consist of a uniform array of rectangular pores (slots) with a range of porosities and pore aspect ratios. For the numerical simulations, temporally (rather than spatially) evolving boundary layers are considered and we provide evidence that this approximation is appropriate for slowly growing second-mode instabilities. We consider coatings operating in the typical regime where the pores are relatively deep and acoustic waves and second-mode instabilities are attenuated by viscous effects inside the pores, as well as regimes with phase cancellation or reinforcement associated with reflection of acoustic waves from the bottom of the pores. These conditions are defined as attenuative and cancellation/reinforcement regimes, respectively. The focus of the present study is on the cases which have not been systematically studied in the past, namely the reinforcement regime (which represents a worst-case scenario, i.e. minimal second-mode damping) and the cancellation regime (which corresponds to the configuration with the most potential improvement). For all but one of the cases considered, the linear simulations show good agreement with the results of linear instability theory that employs an approximate porous-wall boundary condition, and confirm that the porous coating stabilizing performance is directly related to their acoustic scattering performance. A particular case with relatively shallow pores and very high porosity showed the existence of a shorter-wavelength instability that was not initially predicted by theory. Our analysis shows that this new mode is associated with acoustic resonances in the pores and can be more unstable than the second mode. Modifications to the theoretical model are suggested to account for the new mode and to provide estimates of the porous coating parameters that avoid this detrimental instability. Finally, nonlinear simulations confirm the conclusions of the linear analysis; in particular, we did not observe any tripping of the boundary layer by small-scale disturbances associated with individual pores.

Short-term effects of dam removal on macroinvertebrates in a Taiwan stream

Dam removal is an approach for restoring rivers. However, there are increasing concerns about the impact of removal on downstream biota. We examined the short-term responses of benthic macroinvertebrates and their avian predator (Brown Dipper, Cinclus pallasii Temminck) in reaches downstream of a check dam after it was removed from a mountain stream in central Taiwan. The density and taxonomic richness of downstream macroinvertebrates decreased immediately after dam removal. The decreases were associated with scouring or burial by sediments from the upstream impoundment. Ten weeks post-removal, downstream macroinvertebrate densities, although marginally recovering, remained lower than both pre-removal and upstream densities. Substantial changes in community structure were not significantly associated with an increase in the proportion of taxa with short life spans. However, this small-scale disturbance had no strong effect on the abundance of their very mobile, avian predator. This study and other studies of dam removal have found that downstream sedimentation following dam removal can reduce macroinvertebrate densities and that they may recover over time. Thus, timescale must be considered when interpreting the consequences of dam removal, especially when the long-term goal is stream restoration.

Assess ecosystem resilience: Linking response and effect traits to environmental variability

Disturbances, nature as well as human, are putting constant pressure on ecosystems. These include small scale disturbances like a falling tree, but also large scale disturbances like eutrophication and climate change. Resilience is a useful indicator to assess whether an ecosystem has the capacity to maintain functioning with environmental variability. In this study we tested whether plant functional traits can be distinguished to develop a response-and-effect framework for general predictions concerning resilience. We defined response traits to assess the system's resistance to disturbance, and effect traits to assess its recovery after disturbance. We used a dataset with 932 vegetation plots containing 104 species from a selected wetland area in The Netherlands. The environmental variability was related to response traits and the response traits to effect traits with RLQ analysis, fourth-corner analysis and Spearman's rank correlation. As a result, combinations of traits that specify effects of environmental change on ecosystem resilience were found. A strong resistance to environmental variability was shown, and consequently, a positive effect on resilience. Due to correlations between response and effect traits, combinations of traits were identified having a variable effect on the resilience of the system. In this way this study argues to further develop a response-and-effect framework to understand and assess ecosystem resilience. The selection of traits is system-specific, and therefore, one should only select those response and effect traits that differentiate between response to environmental variability and effects on ecosystem functioning.

A model of bubble-induced turbulence based on large-scale wake interactions

AbstractNavier–Stokes simulations of the agitation generated by a homogeneous swarm of high-Reynolds-number rising bubbles are performed. The bubbles are modelled by fixed momentum sources of finite size randomly distributed in a uniform flow. The mesh grid is regular with a spacing close to the bubble size. This allows us to simulate a swarm of a few thousand bubbles in a computational domain of a hundred bubble diameters, which corresponds to a gas volume fraction $\alpha $ from 0.6 % to 4 %. The small-scale disturbances close to the bubbles are not resolved but the wakes are correctly described from a distance of a few diameters. This simple model reproduces well all the statistical properties of the vertical velocity fluctuations measured in previous experiments: scaling as ${\alpha }^{0. 4} $, self-similar probability density functions and power spectral density including a subrange evolving as the power $- 3$ of the wavenumber $k$. It can therefore be concluded that bubble-induced agitation mainly results from wake interactions. Considering the flow in a frame that is fixed relative to the bubbles, the combined use of both time and spatial averaging makes it possible to distinguish two contributions to the liquid fluctuations. The first is the spatial fluctuations that are the consequence of the bubble mean wakes. The second corresponds to the temporal fluctuations that are the result of the development of a flow instability. Note that the latter is not due to the destabilization of individual bubble wakes, since a computation with a single bubble leads to a steady flow. It is a collective instability of the randomly distributed bubble wakes. The spectrum of the time fluctuations shows a peak around a frequency ${f}_{cwi} $, which is independent of $\alpha $. From the present results it is possible to determine the origin of the overall properties of the total fluctuations observed in the experiments. The scaling of the velocity fluctuation as ${\alpha }^{0. 4} $ is a combination of the scalings of the spatial and temporal fluctuations, which are different from each other. As the time fluctuations are symmetric in the vertical direction, the asymmetry of the probability density function of the vertical velocity comes from that of the spatial fluctuations. Both contributions exhibit a ${k}^{- 3} $ spectral behaviour around the same range of wavenumbers, which explains why it is observed regardless of the nature of the dominant contribution.

Integrating connectivity science and spatial conservation management of coral reefs in north-west Australia

The geographically isolated, environmentally unique and biologically diverse coral reefs of north-west Australia are under threat from climate change and other localised human impacts. Similar to many other regions around the world, effective mitigation of these threats through ecosystem-based marine spatial planning is currently constrained by a lack of knowledge of the extent and direction of dispersal of eggs, larvae, recruits, juveniles or adults among the coral reefs (termed population connectivity). Here, we present the outcomes of a series of consultations between Marine Protected Area planners, managers and scientists aimed at facilitating the integration of this connectivity knowledge into effective management guidelines and policy in north-west Australia. Through this process, we first synthesised current knowledge of population connectivity in the fields of oceanography, larval biology and population genetics, and then identified and answered to the best of our ability the most useful questions for the spatial planning of Marine Protected Area networks. Key findings indicate that these systems are likely ecologically independent for many coral reef organisms, with hard corals exhibiting the most localised dispersal of species studied so far. Thus, given that hard corals are also the habitat forming species, and that more widely dispersing species such as fish are likely less vulnerable to small scale disturbance, we propose that no-take areas that facilitate resilience of hard coral populations should maximise biodiversity more generally. This means no-take areas that are large enough to encompass routine dispersal distances of corals (10–20km), and are spaced at similar distances, will not only maintain self-replenishment, but also aid recovery after disturbance through connectivity between no-take areas. The results can be applied in a regional and wider context, and provide a valuable template for transfer of scientific knowledge into effective policy.

Characteristics of nonlinear evolution of wavepackets in boundary layers

The nonlinear evolution of a finite-amplitude disturbance in a 3-D supersonic boundary layer over a cone was investigated recently by Liu et al. using direct numerical simulation (DNS). It was found that certain small-scale 3-D disturbances amplified rapidly. These disturbances exhibit the characteristics of second modes, and the most amplified components have a well-defined spanwise wavelength, indicating a clear selectivity of the amplification. In the case of a cone, the three-dimensionality of the base flow and the disturbances themselves may be responsible for the rapid amplification. In order to ascertain which of these two effects are essential, in this study we carried out DNS of the nonlinear evolution of a spanwise localized disturbance (wavepacket) in a flat-plate boundary layer. A similar amplification of small-scale disturbances was observed, suggesting that the direct reason for the rapid amplification is the three-dimensionality of the disturbances rather than the three-dimensional nature of the base flow, even though the latter does alter the spanwise distribution of the disturbance. The rapid growth of 3-D waves may be attributed to the secondary instability mechanism. Further simulations were performed for a wavepacket of first modes in a supersonic boundary layer and of Tollmien-Schlichting (T-S) waves in an incompressible boundary layer. The results show that the amplifying components are in the band centered at zero spanwise wavenumber rather than at a finite spanwise wavenumber. It is therefore concluded that the rapid growth of 3-D disturbances in a band centered at a preferred large spanwise wavenumber is the main characteristic of nonlinear evolution of second mode disturbances in supersonic boundary layers.