Intertidal Landscape Research Articles

Modeling of Inundation Characteristics of a Microtidal Saltmarsh, Grand Bay National Estuarine Research Reserve, Mississippi

Ennis, B.; Peterson, M.S., and Strange, T.P., 2014. Modeling of inundation characteristics of a microtidal saltmarsh, Grand Bay National Estuarine Research Reserve, Mississippi. Intertidal saltmarsh habitats are unique productive estuarine systems in that their relative productivity is strongly influenced by the morphology of the landscape and the degree of water inundation. Despite the recognized effect that flooding in saltmarsh intertidal zones has on the natural history of resident fauna and the conservation of the habitat itself, limited data are available with regard to hydrological conditions to conduct proper assessment of these habitats. In this paper we describe a geographic information system–based model that was used to quantify the inundation characteristics of intertidal habitats within the microtidal Crooked Bayou, Grand Bay National Estuarine Research Reserves (GNDNERR), Mississippi, system. Using water levels from a nearby National Oceanic and Atmospheric Administration tide station, inundation characteristics were generated by the model for multiple sample sites and for plots within each site along a common marsh elevation gradient. The derived characteristics generated by the model were used in a principal components analysis to create factor scores depicting flooding conditions. These factor scores were then used as the means of comparison of cumulative inundation characteristics between sites and the plots within sites. Overall, inundation characteristics were found to vary significantly between sampling sites (Kruskal-Wallis, χ2 = 26.13, p < 0.001) and among the plots (Friedman's, χ2 = 19.27, p < 0.001) across the elevation gradient. In general, although the inundation potential of the sampling plots within each site significantly decreased as the marsh elevation gradient increased, there were marked differences among and within sites because of the considerable microtopography patterns we identified that exemplify the microtidal, low-lying intertidal landscape of GNDNERR.

Kite aerial photography for low-cost, ultra-high spatial resolution multi-spectral mapping of intertidal landscapes.

Intertidal ecosystems have primarily been studied using field-based sampling; remote sensing offers the ability to collect data over large areas in a snapshot of time that could complement field-based sampling methods by extrapolating them into the wider spatial and temporal context. Conventional remote sensing tools (such as satellite and aircraft imaging) provide data at limited spatial and temporal resolutions and relatively high costs for small-scale environmental science and ecologically-focussed studies. In this paper, we describe a low-cost, kite-based imaging system and photogrammetric/mapping procedure that was developed for constructing high-resolution, three-dimensional, multi-spectral terrain models of intertidal rocky shores. The processing procedure uses automatic image feature detection and matching, structure-from-motion and photo-textured terrain surface reconstruction algorithms that require minimal human input and only a small number of ground control points and allow the use of cheap, consumer-grade digital cameras. The resulting maps combine imagery at visible and near-infrared wavelengths and topographic information at sub-centimeter resolutions over an intertidal shoreline 200 m long, thus enabling spatial properties of the intertidal environment to be determined across a hierarchy of spatial scales. Results of the system are presented for an intertidal rocky shore at Jervis Bay, New South Wales, Australia. Potential uses of this technique include mapping of plant (micro- and macro-algae) and animal (e.g. gastropods) assemblages at multiple spatial and temporal scales.

Ecosystem engineering by seagrasses interacts with grazing to shape an intertidal landscape.

Self-facilitation through ecosystem engineering (i.e., organism modification of the abiotic environment) and consumer-resource interactions are both major determinants of spatial patchiness in ecosystems. However, interactive effects of these two mechanisms on spatial complexity have not been extensively studied. We investigated the mechanisms underlying a spatial mosaic of low-tide exposed hummocks and waterlogged hollows on an intertidal mudflat in the Wadden Sea dominated by the seagrass Zostera noltii. A combination of field measurements, an experiment and a spatially explicit model indicated that the mosaic resulted from localized sediment accretion by seagrass followed by selective waterfowl grazing. Hollows were bare in winter, but were rapidly colonized by seagrass during the growth season. Colonized hollows were heavily grazed by brent geese and widgeon in autumn, converting these patches to a bare state again and disrupting sediment accretion by seagrass. In contrast, hummocks were covered by seagrass throughout the year and were rarely grazed, most likely because the waterfowl were not able to employ their preferred but water requiring feeding strategy (‘dabbling’) here. Our study exemplifies that interactions between ecosystem engineering by a foundation species (seagrass) and consumption (waterfowl grazing) can increase spatial complexity at the landscape level.

A simplified model for frictionally dominated tidal flows

The response of vegetated tidal wetlands to environmental changes is governed by vegetation‐flow‐landscape feedbacks occurring over large spatial and temporal scales, and thus typically studied using simplified models. Here, we present a new simplified approach to describe frictionally dominated tidal flows, characteristic of tidal wetlands. The derivation of the model follows from an expansion of the momentum and mass conservation equations in terms of small dimensionless parameters, which arise once the relevant variables are suitably scaled. A comparison with a full‐fledged finite element model indicates that the proposed simplified approach ensures a reliable description of the flow field; suggesting that the model provides a key advancement to study the bio‐morphodynamic feedbacks which shape intertidal landscapes.

A comparison of GPS and lidar salt marsh DEMs

ABSTRACTDigital elevation models (DEMs) were compared to characterize how well airborne lidar (light detection and ranging) data depict the microtopography of a salt marsh. 72,000 GPS points and 700,000 lidar points from a 1 km2 salt marsh island were linearly interpolated using identical DEM configurations. Overall, 78% of lidar elevations were within ±0.15 m of the high precision GPS elevations. Spatial arrangement of difference values reveal that lidar performed best on the marsh platform, and poorly along tidal creeks and creek heads. Also, the overall shape of the salt marsh was poorly defined, even where lidar data were within the reported range of accuracy. These observations indicate that lidar appears to be a robust tool for mapping intertidal landscapes. However, lidar DEMs may not adequately resolve the microtopographic variations of a salt marsh, and for research questions that require accurate depiction of small‐scale tidal creek networks and subtle terrain features lidar data should be augmented with other information. Copyright © 2011 John Wiley &amp; Sons, Ltd.

Landscape patterns in boundary intensity: a case study of mussel beds

This work examines the proposition that positive interactions among neighboring individuals within a population may produce landscape patterns in boundary intensity. The large scale patterns emerge because the interactions favor an aggregated distribution in the face of a potential limiting factor, and the strength of that factor varies over the landscape. The consequences of spatially varying neighborhood processes were explored using cellular automata simulating the structure of mussel beds in 2-dimensional intertidal landscapes, each characterized by a vertical gradient of tidal immersion and a horizontal gradient of wave energy. Running the model with and without the neighborhood processes demonstrated that the facilitating neighborhood processes elevate intensity above that caused by the gradients, and consequently abrupt (high intensity) boundaries emerged in the midst of gradual environmental variation. Trends generated on the 2-D landscape by the model were compared with those in photo-mosaics of intertidal mussel beds, Mytilus californianus on rocky shores of the British Columbia. The analysis involved interpolation of boundary locations using a spatially-constrained cluster algorithm, and then estimation of the corresponding boundary intensities using a landscape index aggregation (CLUMPY). The general similarity between predicted and real trends in intensity over the wave energy gradients suggests that spatially varying neighborhood processes determine much of the landscape scale variation in boundary intensity, while certain discrepancies (e.g. a more rapid rise of observed intensities with increasing wave exposure) suggest modifications of the theory and new empirical work.

Salt-marsh characterization, zonation assessment and mapping through a dual-wavelength LiDAR

Linking intertidal processes to their natural patterns within a framework of coastal erosion requires monitoring techniques providing high-resolution spatio-temporal data from the scale of processes to this of patterns. The Scanning Hydrographic Operational Airborne LiDAR Survey (SHOALS) consists of a ubiquitous topographic and bathymetric LiDAR (Light Detection And Ranging) system that has become an important technology for generating high-resolution Digital Terrain Models (DTM) and Digital Surface Models (DSM) over intertidal landscapes. The objectives of this project are i) to highlight the capacity of SHOALS Topography and intensity data (Red and Near-InfraRed) to detect intertidal vegetation, ii) to assess the salt-marsh zonation, and iii) to map intertidal habitats and its adjacent coastal areas (Gulf of St. Lawrence, Canada). The study area was selected based on the spectrum of land cover types, encompassing beach, salt-marsh, arable farm and urban coastal environments. Surfaces constructed from the LiDAR survey included DSM, DTM, Normalized Surface Model (NSM), Digital Intensity Model for InfraRed (DIMI), Digital Intensity Model for Red (DIMR), and Normalized Difference LiDAR Vegetation Index Model (NDLVIM), derived from the two previous models. The correlation between the so-called NDLVI and the amount of salt-marsh vegetation, measured in situ, was 0.87 ( p < 0.01). Then, LiDAR-assessed salt-marsh ecological zonation allowed finding out intermediate and strong relationships between NDLVI and Topography ( r 2 = 0.89, p < 0.038) and Topographic heterogeneity ( r 2 = 0.54, p < 0.1394), respectively. Finally, NDLVI and Topography surfaces were classified using maximum likelihood algorithm into 17 classes, whose overall accuracy and kappa coefficient were 91.89% and 0.9088, respectively. These results support that (1) intertidal vegetation can be discriminated by NDLVI, (2) salt-marsh ecological zonation pattern, and (3) accurate coastal land cover maps can be satisfactorily generated from a single LiDAR survey using the NDLVIM and DTM approach.

Effects of De-Embankment on Pore Water Geochemistry of Salt Marsh Sediments

Salt marshes form part of the widely distributed intertidal landscape. The salt marshes of the East Frisian barrier island Langeoog (NW Germany) belong to the barrier-connected salt marsh type and were protected by a summer dike that was removed in 2004. In this study, pore water and sediment data were combined to investigate the effects of the de-embankment along a transect including sites on seawater-influenced grassland, high and low salt marsh, and transition zone tidal flat–low salt marsh. Pore waters were sampled with in situ pore water samplers for 13 mo and analysed for trace metals (Fe, Mn), nutrients (NH4 , PO43−), dissolved organic carbon, and sulphate. Additionally, on site measurements of pH and salinity were carried out. Pore water ultrafiltration experiments with 5000 Da MWCO (molecular weight cut off) complemented the water analyses. Sediment samples were taken from hammer corings and were analysed for bulk parameters (total carbon [TC], total inorganic carbon [TIC], total organic carbon [TOC], total sulphur [TS]) and selected major elements (Si, Al, Fe, Mn). Additionally, reactive iron and manganese were analysed.Sediments along the study transect are characterised by quartz dominance and very low TC (TOC and TIC) and TS contents. The iron content is comparable to other salt marsh sediments. The high percentage of reactive iron (up to 40%) indicates that salt marsh sediments form an important iron source for pore waters, as confirmed by high pore water concentrations of dissolved iron (up to 583 μM). Dissolved iron in pore waters most likely results from reduction and dissolution of oxidised iron minerals by organic ligands or Fe(II) organic complexes. Iron complexation by humic substances and siderophores in combination with circum-neutral pH values keep iron in solution. Therefore, the studied salt marshes presumably form an important iron reservoir, which may account for elevated pyrite contents frequently observed in Holocene coastal peats.Flooding of the salt marsh during a storm surge resulted in a considerable increase in pore water iron (8-fold), manganese (21-fold), phosphate (5-fold), and ammonia (7-fold) concentrations. These results show that seawater restoration (de-embankment) should be handled very carefully, especially with regard to nutrient release and subsequent changes in pore water quality.

Mapping and monitoring intertidal benthic habitats: a review of techniques and a proposal for a new visual methodology for the European coasts

Mapping seafloors is a fundamental step for managing and preserving coastal zones. Moreover, in a context of current global environmental changes, new methods allowing long-term monitoring are increasingly required. Various methods have been used to map seafloors, primarily benthic macrofauna and sediment sampling along regular grids or transects, and remote sensing methods. These methods map very different things, do not have the same accuracy levels, and have different costs in time and money. Furthermore, such methods often require the competencies of highly skilled scientists and exclude non-specialists otherwise best placed to perform them. In this paper, we test a method based on Direct Field Observations (‘DFO method’), which can be used by non-specialists, and assess if it is sufficient for mapping and monitoring intertidal habitats. We further compare this method with other conventional ones. The results show that such a simple method is relatively rapid and inexpensive given the results obtained. Moreover, it is particularly suitable for highly fragmented intertidal landscapes where other methods are often very limited. In consequence, in areas such as the European coasts, it can be used by non-specialists, such as protected-area managers, and because it is an inexpensive and quick method long-term monitoring is also possible.

Diversity in fragments of artificial and natural marine habitats

In fragmented landscapes the matrix, i.e. the surrounding but different habitat, can have a strong influence on the biodiversity in fragments. The amount of natural habitat converted for urban use is increasing on every continent, so fragments of natural habitat are becoming more likely to be embedded in a matrix of human-made habitat, rather than other natural habitats. This is partic- ularly prevalent on urbanised coastlines. Yet there has been little research to help us understand the dynamics and consequences of fragmented coastal landscapes. In Sydney, Australia, most of the nat- ural intertidal landscape of Sydney Harbour (a mosaic of rocky shore, mudflats and sandflats) has been replaced by artificial habitat (such as seawalls). I tested the hypothesis that biota on rocky shores in Sydney Harbour would differ depending on whether shores were embedded in a matrix of artificial habitat (here called 'complete fragments'), natural habitat (here called 'natural patches'), or a mixture of the 2 types of matrix (called mixed fragments). Assemblages at mid-shore levels were significantly different among these types of fragments. At mid- and low-shore levels, assemblages were more variable in mixed fragments than in complete fragments and natural patches. The kinds of mobile and sessile taxa that contributed most to differences among fragments differed, depending on which types of fragments were compared. Many taxa were found in greater abundance in natural patches than in mixed or complete fragments, although not all comparisons were statistically signifi- cant. Artificial habitat seems to interrupt natural connectivity among local populations of intertidal marine organisms in this urbanised estuary.

Interplay between physical and predator landscapes affects transferability of shorebird distribution models

Coastal landscapes with extensive intertidal mudflats provide non-breeding habitat for Arctic shorebirds. Few attempts have been made to develop and test landscape-level models predicting the intertidal distribution of these birds. We modelled the distribution of a Holarctic species, Dunlin (Calidris alpina), at a hemispherically important non-breeding site, the Fraser River Delta, British Columbia, Canada, in seasons with different predator landscapes. We trained the models during a season when nocturnal predators were common and tested temporal transferability of the models on independent datasets when nocturnal predators were absent. Snowy Owls (Nyctea scandiaca) influenced Dunlin distribution and thus model transferability. After accounting for their presence, models displayed good to excellent discrimination, i.e. prediction of the instantaneous and cumulative (over low tide period) probability of mudflat use by Dunlin, in fore- and backcasting applications. Model calibration was good or else, where over-prediction was observed, the reason for the bias was identified. The distribution models may predict mudflat use by Dunlin and possibly related species given relevant data describing the intertidal landscape. The models are amenable to GIS application, describe the amount of use per hectare of the intertidal zone and can be used to determine and visualise relative and absolute suitability of intertidal areas.

The invasion of the acorn barnacle Balanus glandula in the south-western Atlantic 40 years later

The acorn barnacle Balanus glandula (Darwin 1854) is native to the Pacific coast of North America and was accidentally introduced in Argentina in the early 1970s. Here the invasion status of this species is reviewed in the south-western Atlantic focusing on geographical patterns of density, cover, biomass, size and recruitment, and also estimating its rate of spread. Field surveys along rocky shores and a literature review show that B. glandula invaded most of the rocky shores of Argentina at a high rate of spread. Density and cover of barnacles do not follow a latitudinal trend; instead both variables show a bimodal pattern with the highest values in two distant locations (Puerto Lobos and Bahía Bustamante). However, the size of the barnacles increases with latitude, and is positively related to biomass. Recruitment of this species varies between wave exposed and protected areas, and over time. At lower latitudes barnacles recruit in winter, while at higher latitudes they recruit in spring and summer. The differences observed in density and recruitment suggest that along the Argentinean coast, oceanographic processes have a stronger influence in the distribution and success of the barnacles than the gradient in wave exposure. Balanus glandula is a successful invader which has completely re-shaped the native intertidal landscape. Moreover, considering the wide temperature range that this barnacle tolerates in native and invaded regions, the entire coastline of South America could be colonized by this species in the future.

Fractal characterization of Mytilus edulis L. spatial structure in intertidal landscapes using GIS methods

The blue mussel, Mytilus edulis L., forms dense and variable patch mosaics composed of aggregates of mussel individuals. Knowledge of mussel bed spatial pattern at multiple scales is important for understanding the form and function of intertidal systems where mussels are prominent features. This study extends prior work demonstrating fractal patterns of mussel boundaries in soft-bottom systems at the quadrat-scale by investigating fractal structure using GIS methods at both the quadrat- and bed-scales. The study pursues three goals for mussel beds in eastern Maine: (1) to compare quadrat-scale fractal dimensions obtained using manual methods with those obtained using digital imagery and techniques, (2) to determine if fractal patterns identified at the quadrat-scale are also present at the bed-scale, (3) and to evaluate the effectiveness of aerial photography and image analysis techniques. Photographs of randomly located quadrats (2500 cm2 each) were scan digitized and classified into mussel presence/absence classes. Fractal dimensions of mussel/non-mussel boundaries were calculated using the box-counting method and compared with results obtained using analog photographs and methods. Digital aerial photographs at low tide were acquired for beds at two sites and classified using image processing techniques, and bed-scale fractal dimensions were calculated. At the quadrat-scale, fractal dimensions and their relationship with percent cover differed consistently in absolute value from results using manual methods but agreed in demonstrating fractal patterns for all quadrats and a parabolic trend with percent cover very similar to the one revealed manually. At the bed-scale, both sites were shown to be fractal, with higher dimension value for the bed that subjectively appeared more fragmented and highly dissected. Because mussels are important soft-bottom ecosystem engineers, i.e., foundation species that regulate species composition and abundances, the fractal spatial distribution identified in this study suggests that the species affected by them also exhibit fractal patterns. These results indicate the effectiveness of archive imagery and GIS methods for characterizing intertidal systems and point to the feasibility of future image acquisition.

Critical bifurcation of shallow microtidal landforms in tidal flats and salt marshes

Shallow tidal basins are characterized by extensive tidal flats and salt marshes that lie within specific ranges of elevation, whereas intermediate elevations are less frequent in intertidal landscapes. Here we show that this bimodal distribution of elevations stems from the characteristics of wave-induced sediment resuspension and, in particular, from the reduction of maximum wave height caused by dissipative processes in shallow waters. The conceptual model presented herein is applied to the Venice Lagoon, Italy, and demonstrates that areas at intermediate elevations are inherently unstable and tend to become either tidal flats or salt marshes.

Geomorphic analysis of tidal creek networks

The purpose of this study is to determine if concepts in terrestrial channel network analysis provide insight on intertidal creek network development and to present new metrics for their analysis. We delineated creek network geometry using high‐resolution digital images of intertidal marsh near Georgetown, South Carolina. Analyses reveal that intertidal creek networks may be topologically random. Length‐area relationships suggest that salt marsh and terrestrial networks have similar scaling properties, although the marsh networks are more elongate than terrestrial networks. To account for recurrent water exchange between creek basins at high tide, we propose that the landscape unit of geomorphic analyses should be the salt marsh island as opposed to salt marsh creek drainage basin area. Using this approach, the relationship between maximum channel length per island and island area is well described by a power function. A similar power relationship exists for cumulative channel length versus island area, giving a nearly unit slope; this implies that marsh islands have a spatially uniform drainage density. Since the island boundaries are easily identified in remote sensing, taking the island as the unit of geomorphic analysis will eliminate discrepancies in delineating basin boundaries and preclude the need for defining basin area in intertidal landscapes. Analyses and results presented here may be used to quantify salt marsh reference condition and provide indicator variables to assess salt marsh disturbance.

Mating system and gene flow in the red seaweed Gracilaria gracilis: effect of haploid-diploid life history and intertidal rocky shore landscape on fine-scale genetic structure.

The impact of haploid-diploidy and the intertidal landscape on a fine-scale genetic structure was explored in a red seaweed Gracilaria gracilis. The pattern of genetic structure was compared in haploid and diploid stages at a microgeographic scale (< 5 km): a total of 280 haploid and 296 diploid individuals located in six discrete, scattered rock pools were genotyped using seven microsatellite loci. Contrary to the theoretical expectation of predominantly endogamous mating systems in haploid-diploid organisms, G. gracilis showed a clearly allogamous mating system. Although within-population allele frequencies were similar between haploids and diploids, genetic differentiation among haploids was more than twice that of diploids, suggesting that there may be a lag between migration and (local) breeding due to the long generation times in G. gracilis. Weak, but significant, population differentiation was detected in both haploids and diploids and varied with landscape features, and not with geographic distance. Using an assignment test, we establish that effective migration rates varied according to height on the shore. In this intertidal species, biased spore dispersal may occur during the transport of spores and gametes at low tide when small streams flow from high- to lower-shore pools. The longevity of both haploid and diploid free-living stages and the long generation times typical of G. gracilis populations may promote the observed pattern of high genetic diversity within populations relative to that among populations.

Investigating biotic and abiotic factors affecting the recruitment of an intertidal clam on an exposed sandy beach using a generalized additive model

The existence of biotic interactions among macroinfauna resident on exposed sandy beaches and the potential roles that such interactions might play with respect to structuring these assemblages is currently a controversial topic. To elucidate the relative importance of biotic versus abiotic factors in determining the spatial distribution of beach fauna, we use a generalized additive model, which can account for nonlinear relationships between beach clam recruitment and four explanatory variables: two abiotic (beach gradient and distance from a major river mouth); and two biotic (the abundance of adult clams and the abundance of their juveniles). The study organism, Donax serra Röding, is a fairly typical intertidal filter feeder that dominates the macrofaunal communities on many southern African sandy beaches. Results indicate that a mixture of biotic and abiotic factors mediates recruit abundance, with beach gradient being the most influential, followed by adult and juvenile abundances. Of particular importance was strong evidence of intraspecific interactions between D. serra adults and recruits. Furthermore, despite the existence of a published hypothesis stating that the highest recruitment of this species is found beside river mouths and that the subsequent alongshore distribution of older age classes is determined primarily by distance from a major river mouth, we found the opposite. Lowest recruit abundance was in the vicinity of a river mouth and maximum recruit abundance was near the center of the beach. Distance from the river mouth was also less important in structuring the distribution of recruits than the biotic factors. This emphasizes the complex ecology of organisms inhabiting apparently simple intertidal landscapes, and illustrates how the generalized additive modelling approach can be fruitfully applied to problems in beach ecology.

Response of an Estuarine Benthic Community to Application of the Pesticide Carbaryl and Cultivation of Pacific Oysters ( Crassostrea gigas) in Willapa Bay, Washington

Oyster culture operations on the West coast of North America have developed into complete farming operations for the introduced Japanese oyster, Crassostrea gigas, which now covers vast areas of the intertidal landscape, particularly in Washington State where the pesticide carbaryl has also been used to control burrowing thalassinid shrimp for more than 30 years. Field experiments were conducted to examine the effects of these habitat modifications on the benthic community in Willapa, Bay Washington (124°06 ′W,46°24 ′N) where 50% of the state's oyster production occurs. Results indicated that the primary long-term effect of carbaryl application was removal of the two species of thalassinid shrimp ( Neotrypaea californiensis and Upogebia pugettensis), which dominated the community at the start of the experiment and clearly influenced community composition themselves. Small peracarid crustaceans like the amphipods Corophium acherusicum and Eohaustorius estuarius experienced the most significant short-term mortalities, but generally recruited back to treated sites within 3 months, and were often more abundant on treated than untreated sites 1 year after carbaryl application. Results for molluscs were mixed, with no significant effect on Macoma spp, but a significant effect on the commensal clam Crytomya californica and mixed results for the cockle Clinocardium nutalli. Polychaetes were the least susceptible to carbaryl and with the exception of a short-term effect on oligochaetes, no significant negative effects were observed. The addition of oysters did not affect the infaunal community in this study, however greater abundance of epifaunal organisms like mussels, scaleworms, and the amphipod Amphithoe valida, which builds tubes in algae attached to shells, was observed. Carbaryl, which is currently applied to roughly 242 ha (<6% of the intertidal) in Willapa Bay on an annual basis, has a variable but relatively short-term effect on the benthic community, which should be viewed in the context of other oyster culture operations like the addition of oysters themselves to a community often dominated by burrowing thalassinid shrimp which clearly control its dynamics.

High‐resolution remote sensing of intertidal ecosystems: A low‐cost technique to link scale‐dependent patterns and processes

Linking experimentally tested local processes to natural patterns in intertidal ecosystems requires data‐acquisition techniques that provide spatiotemporal data from the scale of local processes to the scale of patterns. We developed a low‐cost, high‐resolution remote‐sensing technique based on the use of a 6‐m helium‐inflated blimp, a standard 35‐mm camera, and photogrammetric numerical tools in order to acquire high‐resolution data of environmental (i.e.,\ topographic) and biological (i.e., algal biomass) variables over intertidal landscapes. The camera was calibrated for photogrammetric analysis, and overlapping color aerial photographs were taken at an altitude of 80 and 50 m. We performed stereo analysis of digitized images and numeric topographic restitution over an 18 3 18 m area with an error of 0.02 m along the Z axis. A normalized vegetation index (NDVI) from color‐infrared images at 0.02‐m resolution over the same area was computed. Algal biomass sampled within the photographed area allowed us to calibrate NDVI with algal biomass (R2 = 0.73, p &lt; 0.01). Aggregation analysis performed on a height above zero level, local topographic heterogeneity, and algal biomass confirmed, at the landscape level, previous local experi‐mental evidence of a relationship between topographic heterogeneity and algal biomass increasing from scales of 0.5 to 2 m. Our method permits multiscale testing of local scale‐dependent processes over a natural landscape.

Environmental heterogeneity and balancing selection in the acorn barnacle Semibalanus balanoides.

The northern acorn barnacle Semibalans banlanoides occupies several intertidal microhabitats which vary greatly in their degree of physical stress. This environmental heterogeneity creates distinct selection regimes which can maintain genetic variation in natural populations. Despite considerable attention placed on the link between spatial variation in fitness and balancing selection at specific loci, experimental manipulations and fitness estimates for molecular polymorphisms have rarely been conducted in the wild. The aim of this transplant experiment was to manipulate the level of physical stress experienced by a cohort of barnacles in the field and then investigate the spatial variation in fitness for genotypes at three loci: two candidate allozymes and the mitochondrial DNA control region. The viability of mannose-6-phosphate isomerase (Mpi) genotypes was dependent on the level of physical stress experienced in the various treatments; alternative homozygotes were favoured in alternative high stress-low stress environments. In contrast, the fitness of genotypes at other loci was equivalent among treatments and unaffected by the manipulation. Evaluated in the light of balancing selection models, these data indicate that the presence of multiple environmental niches is sufficient to promote a stable Mpi polymorphism in barnacle populations and that allelic variation at this locus reflects the process of adaptation to the heterogeneous intertidal landscape.