Wave Exposure

The relationships between fish assemblages, their associated habitat, and degree of protection from fishing were evaluated over a broad spatial scale throughout the main Hawaiian islands. Most fish assemblage characteristics showed positive responses to protection whether it was physical (e.g. habitat complexity), biological (e.g. coral cover growth forms), or human-induced (e.g. marine reserves). Fish biomass was lowest in areas of direct wave exposure and highest in areas partially sheltered from swells. Higher values for fish species richness, number of individuals, biomass, and diversity were observed in locations with higher substrate complexity. Areas completely protected from fishing had distinct fish assemblages with higher standing stock and diversity than areas where fishing was permitted or areas that were partially protected from fishing. Locations influenced by customary stewardship harbored fish biomass that was equal to or greater than that of no-take protected areas. Marine protected areas in the main Hawaiian islands with high habitat complexity, moderate wave disturbance, a high percentage of branching and/or lobate coral coupled with legal protection from fishing pressure had higher values for most fish assemblage characteristics.

We review how intertidal biodiversity is responding to globally driven climate change, focusing on long-term data from rocky shores in the British Isles. Physical evidence of warming around the British Isles is presented and, whilst there has been considerable fluctuation, sea surface temperatures are at the highest levels recorded, surpassing previous warm periods (i.e. late 1950s). Examples are given of species that have been advancing or retreating polewards over the last 50 to 100 yr. On rocky shores, the extent of poleward movement is idiosyncratic and dependent upon life history characteristics, dispersal capabilities and habitat requirements. More southern, warm water species have been recorded advancing than northern, cold water species retreating. Models have been developed to predict likely assemblage composition based on future environmental scenarios. We present qualitative and quantitative forecasts to explore the functional consequences of changes in the identity, abundance and species richness of gastropod grazers and foundation species such as barnacles and canopy-forming algae. We forecast that the balance of primary producers and secondary consumers is likely to change along wave exposure gradients matching changes occurring with latitude, thereby shifting the balance between export and import of primary production. Increases in grazer and sessile invertebrate diversity are likely to be accompanied by decreasing primary production by large canopy-forming fucoids. The reasons for such changes are discussed in the context of emerging theory on the relationship between biodiversity and ecosystem functioning.

Patterns of community structure in the marine environment are strongly influenced by population relationships to biotic and physical gradients. The aim of this work is to explore the relationships of tropical rocky reef fish assemblages to wave exposure and benthic coverage in a gradient of distance from the coast. The study was conducted on the Guarapari Islands, southeastern Brazilian coast. Fish were sampled by underwater visual census (166 transects) and benthic cover was estimated with quadrats (223 replicates). Two main kinds of habitats were found to be derived from the close interrelation between exposure and benthic coverage: (1) exposed areas subjected to major hydrodynamic forcing, and (2) sheltered or moderately exposed areas. The first group is associated with mid-water schooling species like planktivorous labrids and Chromis, piscivorous Caranx, as well as gregarious omnivores like Abudefduf and Diplodus. In terms of benthic composition, macroalgae and encrusting calcareous algae prevail in this high-energy habitat. The second group is characterized by site-attached and reef associated species like territorial pomacentrids, invertebrate feeders such as Halichoeres poeyi and Chaetodon striatus, and small cryptobenthic fishes (e.g. blenniids and labrisomids). Turf algae, zoanthids and massive corals dominate this environment. Environmental plasticity is also common with some genera showing high abundances in all habitats (e.g. Holocentrus, Haemulon, Acanthurus). Examples of the coupling of food availability and fish abundance were found. Planktivores, territorial herbivores, macroalgae browsers and spongivores were positively related with the abundance of their preferred food items along the exposure gradient. Within-family analyses of Pomacentridae and Labridae showed that niche partitioning is likely occurring and seems to be mediated by swimming ‘ability’ and associated feeding performance.

Shallow eutrophic lakes tend to be either in a turbid state dominated by phytoplankton or in a clearwater state dominated by submerged macrovegetation. Recent studies suggest that the low water turbidity in the clear-water state is maintained through direct and indirect effects of the submerged vegetation. This study examined what mechanisms may cause a recession of the submerged vegetation in the clear-water state, and thereby a switch to the turbid state. The spatial distribution of submerged vegetation biomass was investigated in two shallow eutrophic lakes in the clearwater state in southern Sweden. Biomass of submerged vegetation was positively correlated with water depth and wave exposure, which also were mutually correlated, suggesting that mechanisms hampering submerged vegetation were strongest at shallow and/or sheltered locations. The growth of Myriophyllum spicatum, planted in the same substrate and at the same water depth, was compared between sheltered and wave exposed sites in two lakes. After 6 weeks the plants were significantly smaller at the sheltered sites, where periphyton production was about 5 times higher than at the exposed sites. Exclosure experiments were conducted to evaluate the effects of waterfowl grazing on macrophyte biomass. Potamogeton pectinatus growth was decreased by grazing, whereas M. spicatum was not affected. The effects were greater at a sheltered than at a wave-exposed site, and also negatively related to distance from the reed belt. These results suggest that competition from epiphytes and waterfowl grazing hamper the development of submerged vegetation at sheltered and/or shallow locations. An increased strength of these mechanisms may cause a recession of submerged vegetation in shallow eutrophic lakes in the clear-water state and thereby a switch to the turbid state.

The effects of single-season tropospheric ozone (0,) exposures on growth, leaf abscission, and biomass of trembling aspen (Popu1u.s tremuloides Michx.) rooted cuttings and seedlings were studied. Plants were grown in the Upper Peninsula of Michigan in open-top chambers with 0, exposures that ranged from 7 to 92 ppm-h. Depending on the genotype, total seasonal 0, exposure in the range of 50-92 ppm-h had negative impacts on stem, retained leaf, and root biomass accumulation and on diameter growth. Leaf abscission generally increased with increasing 0, exposure and was the principal cause of the decrease in leaf biomass of the OX-treated plants. Considerable genetic variation in 0, responses occurred, as shown by differences in sensitivities among clones and among seedlings. However, the responses to 0, of rooted cuttings and seedlings were similar when seedling means were compared with clonal means for leaf abscission, diameter growth, retained leaf biomass, and root biomass. Comparison of a single square-wave treatment (52 ppm-h) with 70 and 92 ppm-h episodic exposures suggested that the plant response to the square-wave exposure was similar to the response to the highest episodic exposure even though the 92 ppm-h episodic exposure was almost twice the square-wave exposure. Our results are consistent with previous studies that show that I? tremuloides is highly responsive to 0, exposure and this response has a strong genetic component.

Effects of sea temperature regime, substratum type and degree of wave exposure on the composition of rocky intertidal communities in the Cape of Good Hope were examined using cluster analyses of data for biomass of 310 species on 12 rocky shores. The Cape of Good Hope forms an overlap area between the south and west coast intertidal biota of South Africa (Stephenson. 1939) and species composition was found to be largely influenced by temperature regime, species richness being greatest within the area of overlap. However, biomass is dominated by very few species, the 10 species with highest biomass on each beach accounting for an average of 92 % of total biornass. If the 10 species exhibiting greatest biornass on each of the 12 beaches considered are pooled, a total of only 45 species is involved; 25 of these reveal biomass trends in response to gradients of wave exposure. Thus, while temperature regime establishes, by exclusion, a framework of species composition on each beach, the actual biomass of the species present is largely influenced by degree of exposure. Cluster analyses based on biomass data reveal groupings based primarily on the degree of exposure and secondarily on temperature. While specific influences of substratum (especially of substratum instability) were apparent, any overall effects on the biota were masked by those of exposure and temperature.

A paradigm is a set of mutually supportive hypotheses that provides a frame of reference within a field. In 1962, Kuhn proposed that paradigms form within the dual contexts of empirical evidence and intellectual history. Facts potentially contradictory to a paradigm may not be recognized until they are observed repeatedly and incorporated as supportive evidence into a new theory. Support for this interpretation can be found in the history of a paradigm of predation in rocky intertidal communities. Hypotheses were developed in the contexts of innovative field experiments and historical arguments of competition theory. The resulting paradigm proposed that predators restrict populations of competitively dominant prey to refuges. Different types of refuge, or no refuge, prevail in different areas of the intertidal zone, accounting for patterns of prey distribution, the coexistence of natural enemies, and the local maintenance of diversity. An ensuing period of criticism made reference to potential contradictions. Rather than by predation alone, prey abundances are determined largely by an interplay of varying rates of predation and prey production. Furthermore, prey refuges are neither necessary nor sufficient to explain all observed instances of local coexistence of predators and prey.

1 We investigated morphological responses of the submerged macrophyte Myriophyllum spicatum L. to water depth and wave exposure when grown in the same substrate at two sites in two eutrophic lakes. Periphyton production was 4 -8 times higher at sheltered than at wave-exposed sites and its influence was further investigated in a glasshouse experiment. Morphological responses in both experiments were compared by allometric analyses, with shoot weight as covariate. 2 In the field study, plants shoots exhibited similar responses (increased plant height and branch length, and decreased branch number) to sheltered conditions as to deep water. The partitioning between above-and below-ground biomass however, differed, with below-ground decreasing with an increasing water depth, but increasing or remaining unaffected at sheltered compared with exposed conditions. 3 In the glasshouse experiment, plant responses to water depth were similar to those in the field study. Furthermore, plant height increased when plants were overgrown with periphyton. 4 High production of periphytic algae at sheltered sites appears to cause light limitation of macrophytes. However, other factors such as nutrient uptake also appears to determine morphological responses. At sheltered sites, where leaf nutrient uptake is reduced by abundant periphyton and thick boundary layers, plants allocate more biomass to roots. At deep and wave-exposed sites, the absence of periphyton allows plants to take up nutrients through their leaves and allocation of biomass to shoots increases photosynthesis. 5 Overall, relative allocation to shoot and root biomass appears to be primarily controlled by nutrient availability, whereas allocation of available shoot biomass to particular structures is controlled by light availability.

The spatial arrangement of seagrass beds varies from scales of centimeters to meters (rhizomes, shoot groups), meters to tens of meters (patches), to tens of meters to kilometers (seagrass landscapes). In this study we examine the role of patch scale (patch size, seagrass % cover, seagrass biomass), landscape scale (fractal geometry, patch isolation) and wave exposure (mean wind velocity and exceedance) variables in influencing benthic community composition in seagrass beds at three intertidal sites in northern New Zealand (two sites in Manukau Harbour and one site in Whangapoua Harbour). Analysis of univariate community measures (numbers of individuals and species, species richness, diversity and evenness) and multivariate analyses indicated that there were significant differences in community composition inside and outside of seagrass patches at each of the three sites. Partialling out the spatial and temporal components of the ecological variation indicated that seagrass patch variables explained only 3-4% of the patch scale variation in benthic community composition at each of the sites. The temporal component was more important, explaining 12-14% of the variation. The unexplained variation was high (about 75%) at all three sites, indicating that other factors were influencing variation in community composition at the scale of the patches, or that there was a large amount of stochastic variation. Landscape and wave exposure variables explained 62.5% of the variation in the species abundance data, and the unexplained variation at the landscape level was correspondingly low (12%). Canonical correspondence analysis produced an ordination that suggests that, while mean wind velocity and exceedance were important in explaining the differences between the communities in the two harbours, spatial patterning of the habitat, primarily fractal dimension, and secondarily patch isolation (or some factors that were similarly correlated), were important in contributing to variability in community composition at the two sites in Manukau Harbour. This study suggests that spatial patterning of seagrass habitat at landscape scales, independent of the patch scale characteristics of the seagrass beds, can affect benthic community composition. Community composition inside and outside seagrass habitats involves responses to seagrass bed structure at a series of hierarchical levels, and we need to consider more than one spatial scale if we are to understand community dynamics in seagrass habitats.

A simple food chain model of community structure was used to evaluate the roles of bottom-up and top-down factors in a rocky intertidal community. Predictions of the model were modified to incorporate known variation in the strength of species interactions and nutrient delivery rates along a wave-exposure gradient. To test the predictions of the model, I manipulated nutrients and consumers in tide pools chiseled into mudstone benches at two sites that varied in degree of wave exposure. The pools were located in the mid-intertidal zone between ϳ1 and 1.5 m above mean lower low water (MLLW), at Boiler Bay, Oregon, USA. The focal organisms were the benthic macroalgae and mobile invertebrate herbivores that dominate naturally occurring tide pools at this site.

The distribution of Mytilus edulis Linnaeus, M. galloprovincialis Lamarck, and their hybrids was examined in mussel populations in southwest England in 1996 and. This is a region where both parental taxa and populations containing large numbers of hybrids cooccur yet a fine-scale mapping of the hybrid populations has not been conducted. In this study the geographic distribution of hybrid populations was determined in southwest England over 360 km of coast from Tintagel Castle in north Cornwall to Beer in south Devon. Sample localities were spaced from 5 to 20 km apart. Genotypes for individual mussels were determined using PCR to amplify a size polymorphism at the Glu-5¢ locus that is completely differentiated between M. edulis and M. galloprovincialis. Hybrid populations, characterized by high frequencies of individuals with heterozygous genotypes and a pattern of decreasing frequency of M. edulis alleles with increasing shell length, were continuously distributed along 180 km of open coast in southwest England. This ''hybrid patch'' was bordered at one end by geographically extensive populations of pure M. edulis and at the other end by nearly pure populations of M. galloprovincialis. Strong natural selection in hybrid populations results in a decline in the frequency of M. edulis alleles with increasing size. Waveexposure has previously been implicated as the agent producing this pattern of selection, but in the present study the relationship between allele frequency and body size was not correlated with variation in wave shock intensity among localities within the hybrid zone. The transition between hybrid populations and those con-taining pure populations of M. edulis or M. galloprovincialis is abrupt which suggests that coastal circulation patterns may provide strong barriers to larval dispersal which accounts for the position and maintenance of the hybrid zone.

The North Sea Benthos Project 2000 was initiated as a follow-up to the 1986 ICES North Sea Benthos Survey with the major aim to identify changes in the macrofauna species distribution and community structure in the North Sea and their likely causes.

Historical oil spills have shown that environmental damage on the seashore can be measured by acute mortality of single species and destabilisation of the communities. The biota, however, has the potential to recover over some period of time. Applied to the understanding of the fate of oil and population and community dynamics, the impact can be described by the function of the following two factors: the immediate extent and the duration of damage. A simple and robust mathematical model is developed to describe this process in the Svalbard intertidal. Based on the integral of key biological and physical factors, i.e., community speci®c sensitivity, oil accumulation and retention capacity of the substrate, ice-cover and wave exposure, the model is implemented by a Geographical Information System (GIS) for characterisation of the habitatÕs sensitivity and vulnerability. Geomorphologic maps and georeferenced biological data are used as input. Digital maps of intertidal zone are compiled, indicating the shoreline sensitivity and vulnerability in terms of coastal segments and grid aggregations. Selected results have been used in the national assessment programme of oil development in the Barents Sea for priorities in environmental impact assessments and risk analyses as well as oil spill contingency planning.

The rocky intertidal mussel Mytilus californianus is exposed to potentially damaging thermal conditions during low tide. However, because the temperatures of ectothermic organisms are driven by multiple climatic factors, we do not fully understand what the body temperatures of intertidal invertebrates are under field conditions, or how thermal stress varies between intertidal sites. We designed a temperature logger that thermally matches (similar size, color, morphology and thermal inertia) living mussels of the species M. californianus, and tested its ability to provide realistic measurements of body temperature in the field. As part of these tests, we examined the propensity of M. californianus to ''gape,'' a behavior in which the mussel opens its shell valves, and which may permit evaporative cooling. Because our instruments were unable to mimic this behavior, we tested the degree to which gaping contributes to animal cooling by exposing M. californianus to a range of climatic conditions while recording body temperatures, gaping behavior and water loss. Results indicated no significant influence of gaping on body temperature. Tests comparing temperatures of loggers to those of real mussels under laboratory and field conditions showed that thermally matched loggers recorded temperatures within a few degrees of living animals and that unmodified loggers regularly incurred errors of up to 14°C. We then deployed a series of thermally matched loggers at two sites in central Oregon (Boiler Bay and Strawberry Hill) previously hypothesized to display site-specific differences in aerial temperature, and at various wave-exposure regimes within each site. Significant differences were demonstrated between sites using a subset of temperature metrics in a multivariate analysis. Yearly peaks in maximum temperature, average daily maximum temperature, and degree hours were useful in discriminating between sites. In 2001, wave-exposed sites at Strawberry Hill displayed fewer degree-hours than wave-protected sites, but an equivalent or greater maximum temperature. In 2002 both of these metrics were significantly lower in waveexposed sites. Boiler Bay and Strawberry Hill differed in thermal regime, but not in a way that would indicate one was ''hotter'' than the other.

In this study the potential of predicting critical erosion shear stresses from microphytobenthos chlorophyll a surface concentrations in the upper mm of the sediment surface was investigated. The results should provide a better basis for estimating sediment surface stability on large scales by means of optical remote sensing. To achieve a comprehensive picture, tidal #ats in the German and Danish Wadden Sea, which di!ered in tidal range, wave exposure, sediment type and microphytobenthos composition, were studied. The selection of sampling sites was biased towards water-free and visually undisturbed sediment surfaces. The relationship between critical erosion shear stresses and chlorophyll a concentration was found to be highly site-speci"c. Most of the variation within the data set cannot be explained by the chlorophyll a concentration and the surface sediment types present. Instead, the data suggest distinct dependencies on other site speci"c features, such as the intensity of sediment surface reworking, microphytobenthos assemblage composition and the endobenthic macrofauna present. It is concluded that to estimate critical erosion shear stresses from chlorophyll a concentration by means of optical remote sensing techniques requires extensive and repeated site speci"c calibrations since their validity is limited by the patchy and sporadic occurrence of some of the determinant parameters.

Marine reserves provide a large-scale experimental framework to investigate the effects of fishing on food web dynamics and how they vary with environmental context. Because marine reserves promote the recovery of previously fished predators, spatial comparisons between reserve and fished sites are often made to infer such effects; however, alternative explanations for differences between reserve and fished sites are seldom tested (e.g., environmental variation among sites). We investigated the context dependency of the predator-urchin-kelp trophic cascade reported in northeastern New Zealand by comparing the abundance of herbivorous sea urchins (Evechinus chloroticus), the extent of urchin barrens habitat, and macroalgal biomass between reserve and fished sites within six locations that span an environmental gradient in wave exposure, sedimentation, and water clarity. At depths where differences in urchin abundance or macroalgal biomass were found between reserve and fished sites we used a model selection approach to identify which variables (fishing or environmental factors) best explained the variation among sites. Differences between reserve and fished sites were not ubiquitous across the locations examined and were highly depth specific. At sheltered locations, urchins were rare and barrens absent at both reserve and fished sites. At moderately exposed coastal locations, actively grazing urchins were most abundant at 4-6 m depth, and significant differences in macroalgal biomass between reserve and fished sites were observed. In contrast, at offshore island locations, urchins extended into deeper water, and differences between reserve and fished sites were found at 4-9 m depth. These differences could only be attributed to trophic cascades associated with protection from fishing in two of the six locations examined. In other cases, variation between reserve and fished sites was equally well explained by differences in sediment or wave exposure among sites. These results suggest that trophic cascades are not ubiquitous to northeastern New Zealand's subtidal reefs and the importance of sea urchins, and indirectly predators, in controlling macroalgal biomass will vary at local and regional scales in relation to abiotic factors. A better mechanistic understanding of how environmental variation affects the strength of species interactions across multiple spatial scales is needed to predict the ecosystem-level effects of fishing.

Patterns of selection on gastropod shell morphology are generally believed to be different on wave-exposed and wave-sheltered shores. The heavy surf on wave-exposed shores is thought to select for small size whereas the high risk of shell-breaking predation on wave-sheltered shores is thought to select for increased shell size and thickness. We compared the risk of shell-breaking predation to littorinid gastropods of different sizes and shell-thicknesses by tethering them on wave-exposed and wave-sheltered shores of the Northeastern Pacific. Over 2 years we found that the predation rate on the direct-developing gastropod Littorina sitkana was consistently much lower at two moderately wave-exposed sites (less than 0.01% d−1) than on the two wave-sheltered sites (8% d−1 and 2% d−1 respectively). At least 30% of the shell-breaking predation resulted in diagnostic “peeled” shell breakage patterns that could be directly attributed to predatory crabs. Observations with SCUBA at high tide suggested that most of the remainder of the shell-breaking predation was from the red rock crab, Cancer productus, and that only a small amount was from pile perch, Rhacochilus vacca. In contrast to our expectations, the smallest size-class of L. sitkana suffered significantly lower rates of predation than the largest size-class at one of the wave-sheltered sites. The effect of shell thickness on predation mortality was as predicted from previous laboratory experiments. The thin-shelled littorinid species, Littorina subrotundata, suffered significantly higher rates of predation than two thicker-shelled species, L. sitkana and L. scutulata s.l., at three of our four sites. We conclude that the higher rates of shell-breaking predation on wave-sheltered shores of the Northeastern Pacific selects for L. sitkana with thicker but not necessarily larger shells than those on wave-exposed shores.

Two ecotypes of Nucella lapillus can occur allopatrically or sympatrically, according to different degrees of wave exposure and microhabitats, on rocky shores of NW Spain. We studied differences in shell size and shape in adults and families of shelled embryos captured in the wild, and molecular variation at 9 microsatellite loci and 4 mtDNA regions. Both adults and shelled embryos of the two ecotypes showed significant differences in size and shell-shape components (the most important being size of the shell aperture). Strong among-family variation suggests that shell-shape variation has an important genetic component, and the pattern of genetic differentiation was in agreement with traits being affected by divergent natural selection. No significant overall molecular differentiation was observed between the two ecotypes for mtDNA, although we found significant genetic structure at two microsatellite loci. Microsatellite differentiation between ecotypes at these two loci was larger in sympatry than in allopatry. These results and the comparison of tenacity suggest that the two forms are distinctly adapted to differences in shore level and degree of exposure to wave action.

Mosaics of habitat dominated by canopy-forming macroalgae and canopy-free (open-gap) habitat are prominent features of temperate subtidal reefs. However, the persistence and mechanisms underlying the arrangement of these patterns are not well understood. We described patterns in the proportion of reef covered by each of these habitats, and the length of patches of each habitat, at 20 sites encompassing a gradient in wave exposure in south-western Australia. Our aims were to characterise patterns in habitat distribution, and the strength of associations with potential influences, in order to develop models of habitat mosaic generation and maintenance. Modelled seabed orbital velocities explained approximately 35% of the variation in the length of open-gap patches, with longer patch lengths at sites that experienced highest water velocities. This observation is consistent with the hypothesis that waves create open-gaps by dislodging canopy algae. Herbivorous damselfish (Parma spp.) were 5.6 times more likely, and the sea urchin Heliocidaris erythrogramma was 20 times more likely, to be encountered at sheltered inshore sites than at exposed sites further from shore. Parma were 8.2 times more likely to be found in open-gap habitat. However, there was no relationship between the occurrence of either herbivore and the proportion of open-gap habitat among sites. These observations do not support a hypothesis that grazing by herbivores creates open-gaps, but grazing might still be a process that influences the persistence of some open-gaps. Massive sponges were three times as likely to be found in open-gap habitat, and hard corals were 91 times more likely to be encountered in open-gap habitat. The strength of these associations suggests that canopy algae negatively influence sessile invertebrates. Further, the large size and likely old age of sessile invertebrates, particularly hard corals, indicates that patches of open-gap habitat can persist for decades. The patterns observed suggest that wave-induced disturbances create open-gaps and that these gaps are persistent features of temperate reefs.

1. The effects of physical disturbance in terms of wave exposure, shore slope and substrate mobility on the presence, species richness, cover and depth limits of the low-growing, shallow water macrophyte community (called the low mixed community) were examined at 41 shore sites in Lake Wanaka, South Island, New Zealand. 2. Wave exposure at a site was described by the previous year's maximum depth of sediment motion and maximum vertical extension of waves, determined from a computer wave model. Shore slope at each site was recorded from )1 to 0 m depth, and sediment stability was assessed as the cover of small gravel. 3. The low mixed community was only present on sites where the previous year's maximum depth of sediment motion was <8 m, maximum wave run-up was <0.3 m, shore slope <0.12 m m )1 , and small gravel cover <78%. 4. Species richness, cover and depth limits of the low mixed community decreased with increasing disturbance on the sites. Sixty-two percentage of the variation in species richness could be explained by physical disturbance variables when all sites were included (N ¼ 41). When only sites with a low mixed community were included (N ¼ 22), only 18% of the variation was explained. Species richness within sites supporting a low mixed community is thus poorly explained by physical disturbance, whereas presence or absence is better explained. There was no evidence to support the intermediate disturbance hypothesis in this study. 5. Disturbance as a result of waves explained 86% of the variation in cover among sites with a low mixed community and accounted for 68 and 58% of the variation in upper and lower depth limit of the low mixed community, respectively. 6. The models obtained in this study can be used as predictive models for the low mixed community in New Zealand lakes in relation to natural physical disturbance on the shore. By integrating these results with previous studies on the effect of water level fluctuation, we describe a generalised optimum physical habitat for the low mixed community in New Zealand lakes.

The Point Conception, California, USA region (hereafter PC) is one of the most important biogeographic and oceanographic discontinuities on the US west coast. Here we address how mesoscale oceanographic and environmental variability in the region around PC, CA may influence the distribution, abundance and size of the mussel Mytilus californianus, a competitively dominant species in rocky intertidal assemblages along the northeast Pacific. Strong upwelling and high wave exposure dominate the California coast north of PC, and weak, seasonal upwelling and warmer water temperatures are characteristic of the region south/ east of PC. We hypothesized that the gradient in temperature, upwelling, and wave exposure around PC would greatly influence patterns of recruitment and abundance of mussels, potentially underlying large-scale differences in community structure. We evaluated these predictions by surveying intertidal community structure, mussel distribution, size, abundance and recruitment at a range of intertidal sites around PC. We found that intertidal communities north of PC were dominated mainly by macrophytes, while mussels and barnacles were relatively scarce. Intertidal communities south of PC were dominated by mussels and barnacles, with a low abundance of macrophytes. Mussels were larger and mussel beds were more expansive and extended lower in elevation at sites ranging from north to south around PC. At northern sites, high abundances of sea star predators and elevated wave exposure effectively displaced the entire mussel zone upwards. We found no differences in the numbers of mussel recruits to sites around PC, suggesting that spatial patterns of mussel abundance were not driven by differential recruitment. These results suggest that unlike other well-studied systems, supply of benthic larvae does not underly the large-scale gradient in community structure around PC. We suggest that environmental conditions favorable to macroalgal growth north of PC, and conditions favorable to filter-feeder growth south of PC may underly mesoscale patterns of intertidal community structure in this region.

A large data set~ collected under the national Danish monitoring program, was used to evaluate the importance of photon flux density (PFD), relative wave exposure (REI), littoral slope, and salinity in regulating eelgrass cover at different depth intervals in Danish coastal waters. Average eelgrass cover exhibited a bell-shaped pattern with depth, reflecting that different factors regulate eelgrass cover at shallow-and deep-water sites. The multiple logistic regresMon analysis was used to identify regulatiug factors and determine their role in relation to eelgrass cover at different depth intervals. PFD, REI, and salinity were main factors affecting eelgrass cover while littoral Mope had no significant effect. Eelgrass cover increased with increasing PFD at water depths of more than 2 m~ while cover was inversely related to REI in shallow water. This pattern favored eelgrass cover at intermediate depths where levels of PFD and REI were moderate. Salinity had a minor, but siguificant~ effect on eelgrass cover that is most likely related to the varying costs of osmoregulation with changing salinity. The analysis provided a useful conceptual framework for understanding the factors that regulate eelgrass abundance with depth. Although the regression model was statistically significant and included the factors generally considered most important in regulating eelgrass cover, its explanatory power was low, especially in shallow water. The largest discrepancies between predicted and observed values of cover appeared in cases where no eelgrass occurred despite sufficient light and moderate levels of exposure (ahnost 50% of all observations). These discrepancies suggest that population losses due to stochastic phenolnena, such as extreme wind events, played an important regulating role that is not adequately described by average exposure levels. A more thorough knowledge of the importance of such loss processes and the time scales involved in recovery of seagrass populations after a severe disturbance are necessary if we are to understand the regulation of seagrass distribution in shallow coastal areas more fully.

Geospatial and field data were merged to characterize the invasion of an exotic macrophyte into a large (∼6192 ha), reservoir, the Chippewa Flowage, Wisconsin, USA. Multi-scale analyses provided data for quantitative models of human, abiotic and biotic influences that would explain the presence and abundance of Myriophyllum spicatum L., a widespread, aquatic species. Human and abiotic factors were analysed in a geographic information system and additional abiotic and biotic factors were sampled at 2778 sample points. Using multivariate logistic and multiple linear regressions the relationships between 29 factors and the species' presence and abundance were quantified. M. spicatum occurred at 181 of 992 vegetated points. M. spicatum abundance was significantly influenced by two geospatial variables (wave action and water-level drawdown) and two site variables (water depth and native species presence). Separate models for shallow and deep water explained 32% and 30% of M. spicatum abundance, respectively (compared to 22.8% using all 181 points). The species' vertical distribution (range = 0.30-4.27 m) was related to depth of winter drawdown (∼1.3 m on average) and to water depth (an indicator of decreasing light), as well as to shading by floating-leaved macrophytes. Spatial extrapolation of model predictions for M. spicatum presence indicated that a total 940 hectares are at risk for M. spicatum invasion (∼15% of the Chippewa Flowage; about 2.5 times larger than the current distribution).

In northwest Europe, sheltered rocky shores are dominated by fucoid canopy algae and barnacles are rare, although the latter are extremely abundant on exposed shores. The supply of the intertidal barnacle Semibalanus balanoides (L.) to sheltered, fucoid dominated rocky shores was investigated to determine the importance of larval supply in limiting the abundance of adults in shelter. Larval supply was measured at two spatial scales, at the scale of shore (100s of metres), by comparing larval concentrations at exposed and sheltered sites, and at a smaller spatial scale (m), by examining the role of fucoid canopies in limiting supply to the substratum. Replicate plankton trawls were carried out above the intertidal zone at high water at two sheltered sites and nearby exposed headlands. The concentration of S. balanoides cyprid larvae was significantly higher at the sheltered sites on two out of three sampling occasions with up to 14 times greater larvae on one occasion than the nearby exposed site. The effect of the macroalgal canopy on supply to the substratum was assessed in two ways: directly, by pumping water from the substratum in areas with and without Ascophyllum nodosum (L.) Le Jolis, and indirectly by measuring cyprid settlement in a canopy-manipulation experiment. Pumped plankton samples from mid tide level showed that the A. nodosum canopy did not form a barrier to larval supply and may have had a positive effect on larval concentrations at the substratum. Cyprid settlement was assessed in the mid shore A. nodosum and low shore Fucus serratus L. zones to areas with canopy algae (but protected from the sweeping effects of macroalgal fronds) and without canopy. Settlement over three consecutive 24-h periods showed a consistent pattern; settlement was consistently lower beneath the F. serratus canopy than in cleared areas, suggesting that this algal species forms a barrier, limiting supply of cyprid larvae to the substratum.

Chondrus crispus Stackhouse alternates between two isomorphic life history phases that differ in cellwall phycocolloid composition. It has been long hypothesized that the gametophyte, with strong-gelling kappa-type carrageenans, is mechanically superior to the tetrasporophyte, with nongelling lambda-type carrageenans, which could contribute to the observed gametophytic dominance in many wave-swept environments. Standard mechanical tests were performed on distal tissues of C. crispus sampled from a range of environments in Narragansett Bay, Rhode Island, using a tensometer equipped with a video extensometer. Life history phase was by far the most important determinant of mechanical properties, whereas environmental factors had only modest influence (vertical distribution) or no effect (exposure); gametophytic distal tissues were 43% stronger, 21% more extensible, and 21% stiffer than tetrasporophytic distal tissues. However, the superior strength of gametophytic tissues was not evident at the stipe/holdfast junction (where breakage typically occurs), and the two phases were equally susceptible to dislodgment by a given force. The primary ecophysiological role of carrageenans in C. crispus may not be the provision of a structure to resist wave action.

Wave action is known to inXuence the abundance and distribution of intertidal organisms. Wave action will also determine the duration and suitability of various foraging windows (high-tide and low-tide, day and night) for predation and can also aVect predator behaviour, both directly by impeding prey handling and indirectly by inXuencing prey abundance. It remains uncertain whether semiterrestrial mobile predators such as crabs which can access intertidal prey during emersion when the eVects of wave action are minimal, are inXuenced by exposure. Here, we assessed the eVect of wave action on the abundance and population structure (size and gender) of the semi-terrestrial intertidal crab Pachygrapsus marmoratus on rocky

We studied the role of hydrodynamics in the establishment of seagrass seedlings for two Mediterranean sea grass species, Posidonia oceanica and Cymodocea nodosa, by combining flume and field studies. Flume measurements under both unidirectional and oscillatory flow showed that P. oceanica seedlings experienced higher drag forces than C. nodosa, which could be related to the larger total leaf area. Drag coefficients were between 0.01 and 0.1 for Reynolds numbers of 103 and 105. As a result, P. oceanica seedlings required 40-50% of root length anchored to the sediment before being dislodged, whereas C. nodosa required ≈ 20%. To validate the flume results, seedling survival in sandy beds was evaluated for two depths (12 and 18 m) at two field locations. To calculate near-bottom orbital velocities at the planting sites, deep-water waves were propagated to shallow water using a numerical model. Our results showed that P. oceanica seedlings experienced high losses after the first autumn storms when near-bottom orbital velocities exceeded 18 cm s-1. The loss of C. nodosa seedlings was much lower and some seedlings survived velocities as high as 39 cm s-1. Thus, flume and field results are consistent in explaining relative higher losses of P. oceanica seedlings than for C. nodosa.

The structure of cryptic reef Wsh assemblages was assessed on sheltered and exposed aspects of coastal breakwaters at two locations in the northwestern Adriatic Sea. There were distinct diVerences between the two levels of exposure, which were consistent between locations. Habitat characteristics, measured on scales of tens of centimetres, explained 50% of the variability in assemblage structure between exposures, whereas 'exposure' alone (implying direct eVects of wave energy on the Wsh) explained <5% of the variation. The most important explanatory variables were the presence of macroalgae, sandy habitat and oyster shell, the last of which increased the degree of small-scale complexity and provided nesting sites for blennies. We found little evidence to suggest that wave action had large direct eVects on the Wsh assemblages, although this may be in part due to the relatively small degree of diVerence between 'exposed' and 'sheltered' samples under the calm conditions of a sea with a relatively short fetch. These results suggest that wave action acts mainly indirectly as a structuring force on cryptic reef Wsh communities, by altering the composition and/or the relative density of epibiota that inXuence the distribution of Wsh. Thus, relative wave energy may provide a useful means of predicting Wsh assemblage structure only at large spatial scales. Microhabitat, composed of a combination of physical complexity and biological elements, always explained the greater part of variability at small (<1 m) spatial scales.

Distribution of submerged macrophytes was assessed at 15 littoral sites in large, shallow Lake Balaton using the echo sounding technique. Sites represented the conspicuous longitudinal and transverse environmental gradients. Absence/presence of plants, water depth, bottom consistency and distance from the shore have been derived from echo sounding data. Turbidity and meteorological variables were recorded at a single site from 2004 to 2006. Site-specific variability in underwater light conditions (an indicator of stress) and exposure to breaking waves (an indicator of disturbance) were estimated by simple models.

Intertidal macroalgae often experience greater risk of dislodgment with increasing size because of underscaling of breaking force of their stipes relative to drag on their thalli. This ratio (breaking force/drag) indicates safety from breakage at a given flow speed, with values greater than one indicating safety from breakage and values lower than one indicating danger of breakage. We examined this

The in¯uence of wave exposure and of tidal height on mussel (Perna perna Linnaeus) population structure (size, density, biomass and adult/juvenile correlations) was examined at 18 sites along the south coast of South Africa. Sites were classi®ed as exposed or sheltered prior to sampling, without reference to the biota, on the basis of aspect, topography and wave regime. A single set of samples was collected from each site during three spring tide cycles. Adult mussels on these shores almost always attach directly to the rocks, and layering of mussels is virtually absent. Shore height always had a strong in¯uence on population structure, but exposure had signi®cant eects only lower on the shore, and almost exclusively on mussel sizes. Principal component analysis (PCA), based on size distribution data for each population, revealed a general upshore decrease in the modal size of the adult cohort. The eects of exposure on size distribution, however, varied with tidal height. PCA separated exposed zones, with larger mussels, from sheltered zones on the low-shore. Farther upshore the two shore types were increasingly confounded. The maximum size of mussels showed a similar pattern, with signi®cant dierences (ANOVA, p < 0.05) between exposed and sheltered sites only on the low-and mid-shores. Density was calculated from randomly placed quadrats (i.e. not necessarily from areas of 100% cover) and showed a dierent pattern. Adult (>15 mm) densities decreased up the shore, with low-, mid-and high-shore zones being signi®cantly dierent from one another (ANOVA, p < 0.0001; followed by multiple range tests). However, exposure had no signi®cant eect on density, nor was there a signi®cant interaction with zone. Recruit (<15 mm) densities were positively correlated with adult (>15 mm) densities in all zones and for both exposure regimes (p < 0.05 in all cases), but there was considerable variability and extremely low predictability in these relationships (r 2 generally <0.2). Predictability tended to be greater towards the high-shore, where adults were more clumped. As with density, biomass was not aected by exposure, but decreased upshore as mean size and density decreased. A reduction in the in¯uence of exposure farther upshore may be caused by greater emersion overriding the eects of exposure. The presence of free space within mussel beds and sig-ni®cant correlations between recruit and adult densities suggest that these mussel populations are recruit limited.

Nineteen intermediate exposed sandy beaches, located along the northern coast of Spain, were sampled during the summer 1999. Data from 10 of the beaches, located at the eastern part of this coast, were previously reported to evaluate environmental factors affecting benthic macrofauna. Data from nine of the beaches, located at the western part of this coast, were included to compare community structure and intertidal zonation of the macroinfauna on intermediate sandy beaches in temperate latitudes. Morphodynamic parameters such as Dean's parameter (U), Beach State Index (BSI) and relative tide range (RTR) were estimated at each beach. Beach length, width, intertidal slope, medium grain size, sorting, swash amplitude and wave characteristics were also analyzed. The highest macroinfaunal densities and biomass occurred at the mid and lower shore levels of each beach. Crustaceans, mainly cirolanid isopods, were the dominant group found on these beaches, whereas molluscs were the least representative. In general, the relationship between community structure and beach morphodynamics was similar to that found for the macroinfauna worldwide suggesting that macroinfauna in intermediate sandy beaches is affected, in the same way, by the physical processes associated with different beach types. Histograms and kite diagrams representing the intertidal distribution of the macroinfauna and multivariate analysis were used to show the zonation pattern on these exposed beaches. Intertidal slope values and beach profile pattern were found similar in all the beaches sampled. We hypothesized that this particular beach profile could influence the pattern of macroinfauna zonation. All the 19 beaches have two zones in common: the supralittoral zone of air breathers present on all shores at and above the drift line and the littoral zone extending from the drift line down the midshore to just above the water table outcrop. Ordination analyses identified two possible zones within the lower beach levels on seven of the beaches, but this cannot be clearly established. The Monte Carlo permutation test was used to select beach slope, length and wave height as the best predictor variables of macroinfaunal characteristics and it seems that the species most affected by the main variables showed the clearest zonation on the beaches.

Elevated rates of sediment run-off, as a result of changes in land-use and climate, are a significant threat to marine coastal communities, with a potential to cause broad-scale, long-term alteration of habitats. Individual sedimentation events can smother estuarine flats with terrigenous sediments, creating a significant disturbance to local benthic communities. Variations in the degree to which a habitat is altered, the rate at which mixing occurs, and species-specific dispersal and responses to the altered habitat, suggest that colonisation of terrestrial sediment depositions will vary with location, both between and within estuaries. This study was designed to explore the effect that variations in wave-induced hydrodynamics would have on long-term colonisation of terrestrial sediment depositions on intertidal flats. Sites for the experimental deposition of terrestrial sediment were located along a gradient in wave exposure, with only limited variation in immersion times (30 min) and ambient sediment particle sizes (predominantly fine sand). Over 20 months, periodic measurements were made of factors predicted to affect colonisation: the sediment characteristics of the deposited sediment; local-scale wave climate; bioturbation of the deposited sediment; and local populations of benthic invertebrates. Neither opportunistic use of the new resource, progressive recovery or facilitation by colonising macrofauna was observed. Little vertical mixing of the deposited and existing sediment by either waves or bioturbators occurred; instead bedload transport was the dominant process. Local differences in hydrodynamic conditions and macrobenthic communities resulted in site-specific colonisation of the experimental plots. The strength and duration of the macrofaunal response to deposited sediment observed in this study suggest that chronic small-scale (m's) patchy 0022-0981/03/$ -see front matter D

Morphological variation and vertical distribution of Fucus vesiculosus were quantified at several sites in the Finnish archipelago (Baltic Sea). F. vesiculosus samples were obtained from skerries at geographical distances of 1 km or more (large scale) and at intervals of ca 100 m around a single island (small scale). The results were examined in relation to wave exposure, calculated by Baardseth and effective fetch cartographic methods. Despite the fact that the exposure indices were calculated differently they correlated strongly. Vegetative morphological characteristics of F. vesiculosus illustrate the morphological differences both within and between exposure gradients. The tallest and widest F. vesiculosus plants were found at the sheltered end of the large-scale exposure gradient. Those from equally sheltered sites of the island were smaller in all respects. Thus, the trend from small narrow plants to large wide sheltered plants was expressed differently over the different geographical scales. Consequently localities with similar exposure indices may have morphologically different F. vesiculosus populations. Shores with similar cartographic exposure indices can be different in nature. Underwater topography and shore locations, either close to the mainland or at the outermost sites of the archipelago, affect the exposure. Although a sheltered shore is indicated, the sublittoral zone may be quite exposed to the movements of water. In contrast, in an open shore environment underwater rocks, boulders and shallow water areas can provide sheltered habitats. The depth range of the F. vesiculosus belt exhibited two distinctive patterns. At sheltered sites, around islands in the outermost reaches of the archipelago F. vesiculosus can grow to a maximum depth of 5 m. In exposed habitats the belt becomes narrower, reaching a maximum depth of 3 m. Closer to the mainland F. vesiculosus is found at exposed sites to a maximum depth of 5 m; the depth range at sheltered sites is narrower, only reaching depths of 2 m or less. In conclusion, the changes in plant morphology and in the vertical belt distribution are similar to each other along both gradients at the exposed ends of the wave action spectrum; however, the two gradients diverge at the sheltered ends of the spectrum.

Morphological development of Laminaria hyperborea was examined at localities with different degrees of wave exposure. Biomass data were converted to estimates of annual growth of lamina, stipe and hapteron. Allocation of annual growth to the hapteron constituted a small amount of the total production, but increased with increasing wave exposure. At the most sheltered locality, lamina growth amounted to between 80 and 95% of total growth in all age groups, and lamina to stipe ratios (fresh weight) were about 10. A higher portion of the annual growth was allocated to the stipe at the more wave-exposed localities. This resulted in lamina to stipe ratios of between 1 and 2 in old plants, but within these localities no gradual decrease in lamina to stipe ratio with increasing wave exposure was observed. At some localities a maximum of growth allocation to the stipe was found in the intermediate age groups. This was most pronounced at the locality with the highest density of canopy plants and intermediate wave exposure. Also, at this locallty the stipes were longer in relation to their weight than at the other localities. These results indicate that light environment, in additlon to wave exposure, Inay influence the stipe morphology and the relationship between stipe and lamina growth. At one station stipe growth was also calculated in individual tagged plants using the lengthweight relationship. The results agreed with those obtained by the biomass method. The highest allocation of growth to the stipes was found in 3 and 4 yr old plants. Maximum annual stipe growth (weight increase) occurred in 4 and 5 yr old plants and decreased in older plants, while maximum lamina growth was found in the oldest plants. KEY WORDS: Laminaria hyperborea Kelp. Growth allocation Age Morphology. Plasticity Wave exposure O lnter-Research 1995 Resale of full art~cle not permitted

The bleaching event of 1998 caused widespread mortality on coral reefs in the Maldives. Nearly 10 years after the coral mass mortality, the state of Maldivian reefs was evaluated paying specific attention to three ecological stages, linked to the 3D structure of the coral community: young (high living hard coral cover), mature (a balance between living coral cover and loose sediment), and regressive (high amount of rubble and sand). The relative importance of three biogeomorphological descriptors (living hard corals; rubble and sand; coralline algae on rock) in the reef flat (2-7 m depth) and in the slope (7-18 m) of three reefscapes related to wave exposure was assessed. The role of wave energy in shaping ecological stages is different in the reef flat (early stages are found in low energy conditions) and in the slope (early stages are associated with high energy sites).

The mapping of seabed environments is fundamental to successful fisheries management and environmental monitoring, however, there is an emerging need to better characterise habitats based upon appropriate physical parameters. In this study, relationships between seabed geomorphology and the distribution of benthic habitats were examined using multibeam sonar, underwater video, predicted wave energy, and sediment data for Esperance Bay, part of the Recherche Archipelago. This shallow (<50 m), high energy, biogenic sediment dominated environment is located in temperate southwestern Australia. Exposure to wave energy appears to determine the distribution of unconsolidated substrate, and is the most useful regional scale predictor of rhodolith and seagrass habitats. Although they are intermittently smothered by mobile sediments, limestone reefs provide habitat for a wide range of sessile organisms, even in very high wave exposure environments. The distribution of rhodolith beds is related to poorly sorted sediments that contain high gravel, mud, and CaCO3 percentages. Our results reveal that in the Recherche Archipelago, wave abrasion coupled with localised sediment transport and accumulation play a major role in increasing the diversity of inner shelf benthic habitats. This highlights the value of assessing geomorphic processes in order to better understand the distribution and structure of benthic habitats.

During 2005 monthly samples of the solitary ascidian Herdmania momus were collected along the Mediterranean and Red Sea coasts of Israel, in order to investigate possible differences in life history strategies of the two populations. The samples were preserved, dissected, and measurements were made of length, total weight, gonad weight, oocyte diameter and the occurrence of symbionts was recorded. Additionally, field surveys showed that in the Mediterranean H. momus exclusively inhabit artificial substrates, and are common at greater depths than in Eilat (Red Sea). Individuals of H. momus in Eilat reproduced year round. Although individuals from the Mediterranean were significantly larger than individuals collected in Eilat their gonad indices and oocyte diameter measurements indicate that they have a short reproductive season. Copepods were found in 50% of the samples from both sites, while a pontoniine shrimp was found in 14% of the samples from Eilat only. The marked differences between the native vs. non-indigenous populations of H. momus are attributed to differential food availability, water temperature, currents and wave exposure. The increasing evidence of negative effects of non-indigenous ascidians on natural fauna from other regions in the world emphasizes the need for additional research regarding the ecology of ascidians along the coasts of Israel.

This paper describes the morphology of near-horizontal basalt and calcarenite shore platforms around Lord Howe Island as well as the lithological and process environment in which they occur. The morphology of platforms around the island is highly varied. For instance, they occur at a wide range of elevations between low-tide level and several metres above the highest tides, and their width ranges between just a few metres to over 100 m. However, common to most platforms is that they have near-horizontal surfaces that terminate abruptly in a steep scarp at their seaward edge. Correlations indicate that the variability in platform elevation and width is attributable to variability in key parameters of erosion, such as rock resistance and shoreline water depth. Whereas wider platforms occur in rocks of lesser resistance, platform elevation is shown to increase both as rock resistance and the depth of water at the shoreline increases. Wave exposure has a significant control on platform width in some instances, but an over-arching relationship was not detected. Most platforms around the island appear to have developed over the Holocene and a conceptual model is proposed to account for the varied morphology of platforms that have developed over this period. Resistant plunging cliffs occur along about 18% of the rocky coastline of Lord Howe Island and represent the starting point for the model. Where cliffs have yielded to wave erosion platforms have been initiated at an elevation controlled predominantly by rock resistance and shoreline water depth. At Lord Howe Island narrow platforms up to about 20 m wide in exposed locations generally occur about 4 to 6 m above high tide level, and have apparently been subject to very infrequent erosion. Shore platforms about 20 to 40 m wide have generally formed in less resistant rocks, are somewhat lower at about 2 to 3 m above high tide level, and have been subject to more frequent erosion over the Holocene. Most basalt platforms around the island fall within these first two categories, but there are some instances of platforms about 50 m wide. These platforms have formed in rocks of comparatively low resistance and are relatively low in elevation occurring close to the level of high tide. An interesting feature of some of these wide platforms is that, unlike the morphology of narrower platforms, raised ramparts sometimes occur on their outer edge.

The Northwestern Mediterranean Sea is a microtidal system, where barometric pressure and local wave exposure play the paramount role in sea water level variation. Herein, Cystoseira amentacea var. stricta and Cystoseira compressa are common species, generally co-occurring on exposed shores where they form a fucoid belt that characterise a narrow area called the infralittoral fringe. To better understand the eventual zonation patterns and the interspecific relationships of Cystoseira species, a multi-response manipulative experiment based on cross-transplantation, was set up. The fitness of the two species was assessed in terms of survival, growth and reproductive conditions; the role of a given Cystoseira and of the presence of a hard to measure environmental gradient was assessed on the recruitment of the two species and on the composition of assemblages re-colonising the bare substrate. Our results highlight a different response to the manipulation of the two species: while C. compressa fitness does not seem to be affected by the vertical position on the shore, C. amentacea var. stricta survival, growth and reproduction change in function of the gradient. The height on the shore should therefore be taken into account for a successful restoration of lost habitats. The recruitment of the two species and the early phases of succession in re-colonisation of the substratum are also affected by the gradient, and the dispersal of both species seems to be more limited than it was assumed until now (approximately 20 cm). C. amentacea, able to spread in continuous belts in the infralittoral fringe, may be a better competitor than C. compressa, thriving at a higher level on the shore, with its lower limit potentially confined by competitive exclusion of C. amentacea var. stricta. The two Cystoseira species cannot be considered as ecological analogues and determinism seems to drive their distribution in the infralittoral fringe. Nevertheless, stochastic factors, and in particular the very limited dispersal, can be assumed as major factor in regulating large scale distribution of these species.