Horizontal Spatial Variability Research Articles

Eddy-driven diazotroph distribution in the subtropical North Atlantic: horizontal variability prevails over particle sinking speed

Mesoscale eddies influence the distribution of diazotrophic (nitrogen-fixing) cyanobacteria, impacting marine productivity and carbon export. Non-cyanobacterial diazotrophs (NCDs) are emerging as potential contributors to marine nitrogen fixation, relying on organic matter particles for resources, impacting nitrogen and carbon cycling. However, their diversity and biogeochemical importance remain poorly understood. In the subtropical North Atlantic along a single transect, this study explored the horizontal and vertical spatial variability of NCDs associated with suspended, slow-sinking, and fast-sinking particles collected with a marine snow catcher. The investigation combined amplicon sequencing with hydrographic and biogeochemical data. Cyanobacterial diazotrophs and NCDs were equally abundant, and their diversity was explained by the structure of the eddy. The unicellular symbiotic cyanobacterium UCYN-A was widespread across the eddy, whereas Trichodesmium and Crocosphaera accumulated at outer fronts. The diversity of particle-associated NCDs varied more horizontally than vertically. NCDs constituted most reads in the fast-sinking fractions, mainly comprising Alphaproteobacteria, whose abundance significantly differed from the suspended and slow-sinking fractions. Horizontally, Gammaproteobacteria and Betaproteobacteria exhibited inverse distributions, influenced by physicochemical characteristics of water intrusions at the eddy periphery. Niche differentiations across the anticyclonic eddy underscored NCD-particle associations and mesoscale dynamics, deepening our understanding of their ecological role and impact on ocean biogeochemistry.

Airborne Remote Sensing of Upper‐Ocean and Surface Properties, Currents and Their Gradients From Meso to Submesoscales

AbstractIn this work we present a unique set of coincident and collocated high‐resolution observations of surface currents and directional properties of surface waves collected from an airborne instrument, the Modular Aerial Sensing System, collected off the coast of Southern California. High‐resolution observations of near surface current profiles and shear are obtained using a new instrument, “DoppVis”, capable of capturing horizontal spatial current variability down to 128 m resolution. This data set provides a unique opportunity to examine how currents at scales ranging from 1 to 100 km modulate bulk (e.g., significant wave height), directional and spectral properties of surface gravity waves. Such observations are a step toward developing better understanding of the underlying physics of submesoscale processes (e.g., frontogenesis and frontal arrest) and the nature of transitions between mesoscale and submesoscale dynamics.

Distributed urban drag parametrization for sub‐kilometre scale numerical weather prediction

AbstractA recently developed, height‐distributed urban drag parametrization is tested with the London Model, a sub‐kilometre resolution version of the Met Office Unified Model over Greater London. The distributed‐drag parametrization requires vertical morphology profiles in the form of height‐distributed frontal‐area functions, which capture the full extent and variability of building heights. London's morphology profiles are calculated and parametrized by an exponential distribution with the ratio of maximum to mean building height as the parameter. A case study evaluates the differences between the new distributed‐drag scheme and the current London Model setup using the MORUSES urban land‐surface model. The new drag parametrization shows increased horizontal spatial variability in total surface stress, identifying densely built‐up areas, high‐rise building clusters, parks, and the river. Effects on the wind speed in the lower levels include a lesser gradient and more heterogeneous wind profiles, extended wakes downwind of the city centre, and vertically growing perturbations that suggest the formation of internal boundary layers. The surface sensible heat fluxes are underpredicted, which is attributed to difficulties coupling the distributed momentum exchange with the surface‐based heat exchange.

Field simulation of settlement analysis for shallow foundation using cone penetration data

This paper deals with the settlement analysis of a vertically loaded strip footing by the use of two-dimensional random field finite element method. Total settlement and rotation of the footing have been calculated by elasto-plastic solution in finite element model. Deformation modulus of soil Ed was assigned conforming to 2D Gaussian random field using Karhunen–Loéve series expansion. Spatial variability of Ed was represented by the horizontal and vertical correlation lengths (θvE and θhE). CPTu database from Adapazari, Turkey has been employed to estimate the correlation lengths. Soil profile was modelled with 4 layers in accordance with random field from in-situ test results, and correlation lengths were assigned to each different layer. Random field realizations were produced by MATLAB code, and probability density functions (PDF) of maximum total settlement and rotation of the footing were constructed using Monte Carlo Simulations by iterative solutions of each realization with finite element analysis. Probabilities of failure (Pf) by settlement and rotation were calculated from PDFs. Initially, the analysis was carried out using average values of each horizontal and vertical correlation lengths that were assigned to every soil layer. Subsequently, analyses were iterated with maximum and minimum values of the correlation lengths in order that both the effect of horizontal and vertical spatial variability can be considered. 1000 calculations were performed for the 5 analysis models, with 200 random field realizations for each model. The effect of varying θvE and θhE on total settlement and rotation of the footing has also been investigated.

Environmental and climatic factors affecting winter hypoxia in a freshwater lake: evidence for a hypoxia refuge and for re-oxygenation prior to spring ice loss

Low dissolved oxygen, or hypoxia, is a common phenomenon in ice-covered lakes in winter. We measured dissolved oxygen (DO) before, during, and after ice-over to characterize the timing, severity, and spatial variability of winter hypoxia in Upper Red Rock Lake, Montana, home to one of the last remaining lacustrine populations of endemic Montana Arctic Grayling (Thymallus arcticus). Unlike most previous investigations of winterkill-prone lakes, we observed considerable horizontal spatial variability in DO, a non-linear winter oxygen depletion rate, and lake-wide re-oxygenation 2–4 weeks prior to spring ice loss. Parts of the upper 1 m of the lake and near stream mouths remained well-oxygenated even during late winter. DO levels were strongly associated with maximum daily air temperature. Our analysis of a 28-year weather record revealed large interannual variability in risk of winter hypoxia, with a slight declining trend in winter severity (number of days with maximum air temperatures ≤ 0°C) in Upper Red Rock Lake. The approach we used in our study provides a useful framework for quantifying and mapping the seasonal dynamics of the extent and severity of winter hypoxia, and for identifying critical winter habitats.

Vertical variations in the bio-optical properties of seawater in the northern South China Sea during summer 2008

Vertical variability in the bio-optical properties of seawater in the northern South China Sea (NSCS) including inherent optical properties (IOPs) and chlorophyll a concentration (Chl) were studied on the basis of in situ data collected in summer 2008 using an absorption/attenuation spectrophotometer. An empirical model was developed to estimate Chl profiles based on the absorption line height at long wavelengths, with a relative root mean square error of 37.03%. Bio-optical properties exhibited large horizontal and vertical spatial variability. As influenced by coastal upwelling and the Zhujiang River (Pearl River) discharge, both IOPs and Chl exhibited high values in the surface waters of the inner shelf, which tended to decrease with distance offshore. Subsurface maximum layers of IOPs and Chl were observed in the middle and outer shelf regions, along with significantly higher values of attenuation coefficients beneath this layer that rapidly increased towards the bottom. In the open ocean, both IOPs and Chl exhibited consistent variability, with the subsurface maximum layer typically located at 34–84 m. Phytoplankton were found to be one of the major components in determining the vertical variability of bio-optical properties, with their vertical dynamics influenced by both physical forcing and light attenuation effects. The depth of the subsurface maximum layer was found to be closely related to the fluctuation of the oceanic thermocline and the depth of the euphotic zone, which also affected the total integrated biomass of the upper ocean. Typically high values of attenuation coefficients observed in the bottom waters of the continental shelf reflected the transport of particulate matter over the bottom boundary layer. Our results reveal large spatial differences in bio-optical profiles in response to complex marine ecodynamics in the NSCS. From the perspective of marine research, high-resolution optical measurements are clearly advantageous over conventional bottle sampling.

Influence of Root Distribution on Preferential Flow in Deciduous and Coniferous Forest Soils

Root-induced channels are the primary controlling factors for rapid movement of water and solute in forest soils. To explore the effects of root distribution on preferential flow during rainfall events, deciduous (Quercus variabilis BI.) and coniferous forest (Platycladus orientalis (L.) Franco) sites were selected to conduct dual-tracer experiments (Brilliant Blue FCF and Bromide [Br−]). Each plot (1.30 × 1.30 m) was divided into two subplots (0.65 × 1.30 m), and two rainfall simulations (40 mm, large rainfall and 70 mm, extreme rainfall) were conducted in these. Vertical soil profiles (1.00 m × 0.40 m) were excavated, and preferential flow path features were quantified based on digital image analysis. Root (fine and coarse) abundance and Br− concentration were investigated for each soil profile. In deciduous forest, accumulated roots in the upper soil layer induce larger lateral preferential flow as compared to the coniferous forest soil during large rainfall events. Compared with deciduous forest, coniferous forest soil, with higher (horizontal and vertical) spatial variability of preferential flow paths, promotes higher percolation and solute leaching to deeper soil layers during extreme rainfall events. Fine roots, accounting for a larger proportion of total roots (compared to coarse roots), facilitate preferential flow in the 0–40 cm forest soil layer. Overall, our results indicate that the root distribution pattern of different tree species can exert diverse effects on preferential flow in forest soils.

Influence of material spatial variability on slope stability in soft rock

Impacts of material spatial variability usually are not considered in traditional slope stability analyses. However, most geotechnical materials are not uniform inside a slope. The objective of the present study was to evaluate the impact of material spatial variability on slopes in soft rocks. The random field model was used to obtain the material spatial variability. The numerical analyses of slopes in soft rock mass were performed using a distinct element method based program UDEC. After that, Monte Carlo simulation, UDEC, and random field models were used together to analyze the impact of material variability and the slope stability. A series of numerical experiments were performed to understand the effect of joint spacing, joint angle, and size of scale fluctuation (δ). Also, failure types and mechanisms were synthesized and evaluated. The probability of failure became higher as the slope considered horizontal spatial variability of the material. The influence of the horizontal size of δ on slopes without joints was much higher than the slopes with joints. In addition, the slopes with joints yielded fewer failures as δ increased. The results of the analyses could be used for prevention of slope failure, future support system, and geotechnical exploration.

Archaeal biogeography and interactions with microbial community across complex subtropical coastal waters.

Marine Archaea are crucial in biogeochemical cycles, but their horizontal spatial variability, assembly processes, and microbial associations across complex coastal waters still lack characterizations at high coverage. Using a dense sampling strategy, we investigated horizontal variability in total archaeal, Thaumarchaeota Marine Group (MG) I, and Euryarchaeota MGII communities and associations of MGI/MGII with other microbes in surface waters with contrasting environmental characteristics across ~200km by 16S rRNA gene amplicon sequencing. Total archaeal communities were extremely dominated by MGI and/or MGII (98.9% in average relative abundance). Niche partitioning between MGI and MGII or within each group was found across multiple environmental gradients. "Selection" was more important than "dispersal limitation" in governing biogeographic patterns of total archaeal, MGI, and MGII communities, and basic abiotic parameters (such as salinity) and inorganic/organic resources as a whole could be the main driver of "selection". While "homogenizing dispersal" also considerably governed their biogeography. MGI-Nitrospira assemblages were speculatively responsible for complete nitrification. MGI taxa commonly had negative correlations with members of Synechococcus but positive correlations with members of eukaryotic phytoplankton, suggesting that competition or synergy between MGI and phytoplankton depends on specific MGI-phytoplankton assemblages. MGII taxa showed common associations with presumed (photo)heterotrophs including members of SAR11, SAR86, SAR406, and Candidatus Actinomarina. This study sheds light on ecological processes and drivers shaping archaeal biogeography and many strong MGI/MGII-bacterial associations across complex subtropical coastal waters. Future efforts should be made on seasonality of archaeal biogeography and biological, environmental, or ecological mechanisms underlying these statistical microbial associations.

Effects of land subsidence resulted from coal mining on soil nutrient distributions in a loess area of China

Underground coal-mining results in severe land subsidence then changes soil nutrient distributions. Soil organic matter (SOM) and total nitrogen (TN) are critical indicators of soil quality and play a key role for plant growth. However, fewer scholars pay attention to the soil nutrient distributions in mining subsidence areas. In this study, a total of 64 soil sampling points, taken from 4 different plots (one unmined plot, two subsided plots and one reclaimed plot), were collected from No.3 Anjialing underground coal mine in the loess area of China. SOM and TN at the depths of 0–20, 20–40, 40–60 and 60–80 cm in these sampling plots were measured. The classical statistics and geo-statistics were used to analyze the vertical and horizontal spatial variability of SOM and TN. The mechanisms of the effects of coal mining subsidence on soil nutrient distribution were revealed based on the distribution of the surface cracks of subsided plots. All of SOM and TN at the depth of 0–20 cm in subsided plots were less than those in unmined plot. Below the depth of 20 cm, TN in subsided plots were higher than those in unmined plot and SOM were less than those in unmined plot. SOM in reclaimed plot were higher than those in subsided plots; however, TN did not exhibit a clear regularity. Land subsidence increased horizontal spatial heterogeneity of SOM and TN distributions. The crack width, crack depth and the distance between sampling point and edge of crack were the main influence factors resulting in horizontal spatial heterogeneity of SOM and TN. This study can provide a theoretical and technical reference for land rehabilitation in subsided lands in loess area.

Spatial Variability of CPT Parameters and Silty Fines in Liquefiable Beach Sands

AbstractIncreasing awareness of the role of spatial variability in the assessment of liquefaction susceptibility has driven the need to characterize the vertical and horizontal spatial variability ...

Borçka Barajı Rezervuarında Yeni Oluşmuş Bir Sediment Birikim Sahasındaki Penetrasyon Direnç Değerlerinin Uzaysal Dağılımı

This study was conducted to analyze vertical and horizontal spatial variability of penetration resistance (PR) in recently deposited sediments at Borcka Dam reservoir. Ninety-one grids with 50 m by 10 m were located and PR was measured to a depth 80 cm by 10 cm increment (8 layers) within each grid. Spatial variation of PR was assessed by geostatistical techniques in all 8 layers. Semivariograms of PR were constructed for all the layers and corresponding kriging maps were built. The PR values were greater in surface layer (0-10 cm) and gradually decreased by depth. Mean PR was greatest (0.747 MPa) in surface layer and lowest (0.413 MPa) in 61-70 cm (layer 7). Results from exploratory statistics and geostatistical analyses showed that the PR values were more variable in surface layer compared to those in deeper layers due to that the surface layer was drier at the sampling and that affected by environmental factors in a greater extent. All the studied layers exhibited differences in spatial variation of PR as sill, nugget, and range values of corresponding semivariograms and kriging-predicted surface maps showed. The shortest geostatistical range (40.8 m) occurred of surface layer while longest (173.3 m) occurred for second layer (11-20 cm). The nugget effect was greatest (44%) in third layer (21-30 cm) while lowest (13%) in the surface layer. The same study may be repeated with more variables to understand mechanisms behind development and extent of spatial variation in these newly deposited sediments.

Groundwater Simulations and Uncertainty Analysis Using MODFLOW and Geostatistical Approach with Conditioning Multi-Aquifer Spatial Covariance

This study presents an approach for obtaining limited sets of realizations of hydraulic conductivity (K) of multiple aquifers using simulated annealing (SA) simulation and spatial correlations among aquifers to simulate realizations of hydraulic heads and quantify their uncertainty in the Pingtung Plain, Taiwan. The proposed approach used the SA algorithm to generate large sets of natural logarithm hydraulic conductivity (ln(K)) realizations in each aquifer based on spatial correlations among aquifers. Moreover, small sets of ln(K) realizations were obtained from large sets of realizations by ranking the differences among cross-variograms derived from the measured ln(K) and the simulated ln(K) realizations between the aquifer pair Aquifer 1 and Aquifer 2 (hereafter referred to as Aquifers 1–2) and the aquifer pair Aquifer 2 and Aquifer 3 (hereafter referred to as Aquifers 2–3), respectively. Additionally, the small sets of realizations of the hydraulic conductivities honored the horizontal spatial variability and distributions of the hydraulic conductivities among aquifers to model groundwater precisely. The uncertainty analysis of the 100 combinations of simulated realizations of hydraulic conductivity was successfully conducted with generalized likelihood uncertainty estimation (GLUE). The GLUE results indicated that the proposed approach could minimize simulation iterations and uncertainty, successfully achieve behavioral simulations when reduced between calibration and evaluation runs, and could be effectively applied to evaluate uncertainty in hydrogeological properties and groundwater modeling, particularly in those cases which lack three-dimensional data sets yet have high heterogeneity in vertical hydraulic conductivities.

Bark beetle-induced tree mortality alters stand energy budgets due to water budget changes

Insect outbreaks are major disturbances that affect a land area similar to that of forest fires across North America. The recent mountain pine bark beetle (Dendroctonus ponderosae) outbreak and its associated blue stain fungi (Grosmannia clavigera) are impacting water partitioning processes of forests in the Rocky Mountain region as the spatially heterogeneous disturbance spreads across the landscape. Water cycling may dramatically change due to increasing spatial heterogeneity from uneven mortality. Water and energy storage within trees and soils may also decrease, due to hydraulic failure and mortality caused by blue stain fungi followed by shifts in the water budget. This forest disturbance was unique in comparison to fire or timber harvesting because water fluxes were altered before significant structural change occurred to the canopy. We investigated the impacts of bark beetles on lodgepole pine (Pinus contorta) stand and ecosystem level hydrologic processes and the resulting vertical and horizontal spatial variability in energy storage. Bark beetle-impacted stands had on average 57 % higher soil moisture, 1.5 °C higher soil temperature, and 0.8 °C higher tree bole temperature over four growing seasons compared to unimpacted stands. Seasonal latent heat flux was highly correlated with soil moisture. Thus, high mortality levels led to an increase in ecosystem level Bowen ratio as sensible heat fluxes increased yearly and latent heat fluxes varied with soil moisture levels. Decline in canopy biomass (leaf, stem, and branch) was not seen, but ground-to-atmosphere longwave radiation flux increased, as the ground surface was a larger component of the longwave radiation. Variability in soil, latent, and sensible heat flux and radiation measurements increased during the disturbance. Accounting for stand level variability in water and energy fluxes will provide a method to quantify potential drivers of ecosystem processes and services as well as lead to greater confidence in measurements for all dynamic disturbances.

Three-dimensional slope reliability and risk assessment using auxiliary random finite element method

This paper aims to propose an auxiliary random finite element method (ARFEM) for efficient three-dimensional (3-D) slope reliability analysis and risk assessment considering spatial variability of soil properties. The ARFEM mainly consists of two steps: (1) preliminary analysis using a relatively coarse finite-element model and Subset Simulation, and (2) target analysis using a detailed finite-element model and response conditioning method. The 3-D spatial variability of soil properties is explicitly modeled using the expansion optimal linear estimation approach. A 3-D soil slope example is presented to demonstrate the validity of ARFEM. Finally, a sensitivity study is carried out to explore the effect of horizontal spatial variability. The results indicate that the proposed ARFEM not only provides reasonably accurate estimates of slope failure probability and risk, but also significantly reduces the computational effort at small probability levels. 3-D slope probabilistic analysis (including both 3-D slope stability analysis and 3-D spatial variability modeling) can reflect slope failure mechanism more realistically in terms of the shape, location and length of slip surface. Horizontal spatial variability can significantly influence the failure mode, reliability and risk of 3-D slopes, especially for long slopes with relatively strong horizontal spatial variability. These effects can be properly incorporated into 3-D slope reliability analysis and risk assessment using ARFEM.

Blowing in the wind: how many roads can a phytoplanktont walk down? A synthesis on phytoplankton biogeography and spatial processes

The selected theme of the 17th Workshop of the International Association for Phytoplankton Taxonomy and Ecology (IAP), “Biogeography and Spatial Patterns of Biodiversity of Freshwater Phytoplankton,” offered the opportunity to explore one neglected aspect of phytoplankton ecology: the distribution of species in the geographic space. This paper summarizes the outcomes of 20 selected contributions among those presented at the workshop. The articles report the results from studies carried out in five continents (only Oceania is not represented) and on a wide array of aquatic ecosystems (deep and shallow natural lakes, man-made lakes, temporary and permanent ponds, rivers). The topics analyzed by the contributors are related to Island Biogeography paradigms, dispersal vectors, survival strategies, environmental filters, dispersal distances, vertical and horizontal spatial variability of phytoplankton between and within water bodies, and of course, invasive algae. The overall analysis of the results presented clearly demonstrates that, as for many others organisms, there are “rules” governing freshwater phytoplankton spatial patterns and that these organisms also have a true biogeography, as nowadays is quite evident for several other groups in the same range of size. We can definitively conclude that the statement “Everything is everywhere” is obsolete, even though human activities tend to homogenize species distribution in the biosphere.

Spatial structure of thermocline and abyssal internal waves in the Sargasso Sea

Vertical and horizontal spatial structures are analyzed for the steady and internal wave velocity contributions in one hundred full-water-depth velocity profiles collected in the Sargasso Sea in water depths between 4500 and 5500m. Temporal decompositions into subinertial, near-inertial, and high-frequency velocity contributions are obtained from multiple, but brief time series at several locations. Horizontal spatial variability is evaluated from two simultaneous velocity profiles at separations ranging from 15m to 12.5km. The total internal wave field exhibits equipartition between east and north velocity components, a decrease in energy density at the smallest vertical wavenumbers, and an overall dependence for kinetic energy (KE) on vertical wavenumber as m−2.5. Most of the internal wave energy is in near-inertial motions and, of this, most occurs at Wentzel–Kramers–Brillouin (WKB) normalized vertical wavelengths of 150–800 stretched-m (i.e., sm) with a spectral peak at 500-sm wavelength (for No=3cph) and average surrounding the peak of 3c/skm (330sm). Near-inertial contributions exhibit a power law of m−3, while higher-frequency internal waves (ω>2f) a slope of m−2. There is strong vertical polarization (clockwise>anticlockwise) (CW>ACW) of the near-inertial contribution but little or none for higher-frequency motions. There is more WKB normalized near-inertial KE in the lower than in the upper half of the WKB-scaled water column while high-frequency internal waves have comparable upper and lower halves energies. The upper half shows a deficit compared to the Garrett and Munk model spectrum at vertical wavelengths shorter than 100sm. Time-mean shear is largest in the upper half, so critical-layer processes may play a role. The internal wave KE of simultaneous but spatially separated profiles has a zero-correlation scale of 15–20km, dominantly due to near-inertial waves. Thus, deep near-inertial motions exhibit wavelengths of 60–80km in contrast to longer scales reported in the surface mixed layer and upper pycnocline. The aspect ratio k/m (330sm/70km) corresponds to a wave frequency of 1.05f. The downward group velocity is 0.6mms−1, with a vertical energy flux for the near-inertial motions of 0.6mWm−2.

Water renewal and risk assessment of water pollution in semi-enclosed domains: Application to Bilbao Harbour (Bay of Biscay)

The inclusion of water renewal behaviour in the risk assessment of water pollution for Bilbao harbour (located within the Nervión estuary, southeastern Bay of Biscay) is analysed. Several parameterisations of water renewal (integral and local perspectives), obtained from the numerical evolution analysis of a Eulerian tracer, were used. The local characterisation of water renewal confirmed a strong horizontal spatial variability. In addition, vertical variability on water renewal patterns, due to the 3-dimensional structure of the flow was also appreciated. For the period analysed (August 2008) the water renewal was less effective at the surface, than in deeper waters, because marine breezes counteracted water renewal at the surface, during the observed period. The incidence of this vertical variability, on harbour environmental management, was analysed in implementing a recently developed methodology to evaluate the risk of water pollution. Since this methodology takes into account water renewal, the values of risk of water pollution in Bilbao harbour differed considerably, as a function of the renewal pattern at the surface or within deeper waters. In conclusion, it was found that the water renewal should be parameterised properly, in order to improve environmental management strategies in semi-enclosed water bodies in coastal areas. The findings permitted knowledge to be gained on water renewal behaviour within Bilbao harbour, advancing the framework for the environmental management of harbour waters.

Spatial diversity of rocky midlittoral macro-invertebrates associated with the endangered species Patella ferruginea (Mollusca: Gastropoda) of Tunisian coastline

The present study focuses on horizontal spatial variability of benthic macrofauna associated with Patella ferruginea. Thirty-six samples collected at 12 transects belonging to 4 midlittoral sites along the rocky Tunisian coastline, were examined. A total of 44 species belonging to 5 taxa were found. Multivariate analysis applied on gathered data did not show a horizontal spatial variability at small scale (between transects), but at large scale, between sites as well as sectors. Thus, three groups of communities were identified (GI: Korbous and El Haouaria; GIIa: Zembra Island and GIIb: Kelibia). The distribution of species abundance within these groups revealed that crustaceans were the most abundant taxon, due to the overwhelming dominance of Chthamalus stellatus. This substratum appeared to create favourable micro-habitats for the installation of molluscs including gastropods. Regarding the low diversity index ( H') and evenness ( J), they seemed to reflect a disturbance and a demographic unbalance within these communities. The heterogeneity of substrate surface, created by C. stellatus specimens appeared to be caused by various complex interactions established between the key components of these communities in particular suspension feeders, predators, herbivorous molluscs and macroalgae. Thus, the dynamic status of each of these communities is the result of these complex interactions.

Impact of climate change on snowpack in the Pyrenees: Horizontal spatial variability and vertical gradients

Summary In this study, snowpack series are modeled across the Pyrenees using data derived from the HIRHAM Regional Climate Model for both the control period (1960–1990) and two emission scenarios (SRES B2 and A2) by the end of the 21st century (2070–2100). A comparison of future and control simulations enables us to quantify the expected change in snowpack for the next century. Snow simulations are performed on 20 Regional Climate Model (RCM) grid points over the Pyrenees, covering the entire north–south and east–west transects; data were downscaled for four different altitudinal levels (1500, 2000, 2500, and 3000 m a.s.l.). This procedure yields a relatively complete picture of the expected impacts of climate change in the Pyrenees, covering horizontal spatial variability as well as altitudinal gradients. According to the HIRHAM model projections following different greenhouse gas emission scenarios, the thickness and duration of snowpack in the Pyrenees will decrease dramatically over the next century, especially in the central and eastern sectors of the Spanish Pyrenees. The magnitude of these impacts will follow a marked altitudinal gradient: the maximum accumulated snow water equivalent may decrease by up to 78%, and the season with snow cover may be reduced by up to 70% at 1500 m a.s.l. The magnitude of the impacts decreases rapidly with increasing altitude; snowpack characteristics will remain largely similar in the highest sectors. The decline of the snowpack would be reduced by half if a medium–low emission scenario was considered (B2) instead of the medium–high concentrations of greenhouse gas assumed in the A2 scenario.