Reflective Beaches Research Articles

Flow Separation over a Prograding Beach Step at Pensacola Beach, Florida

ABSTRACT Houser, C., 2013. Flow separation over a prograding beach step at Pensacola Beach, Florida. Beach steps are a common feature of poorly sorted intermediate and reflective beaches that can develop as an accretionary wedge from foreshore sediment moved seaward by the backwash. Although there have been several studies describing the development and evolution of steps in response to wave and tidal forcing, characterization of the separated flow and development of the backwash vortex has been limited to a small number of laboratory experiments. This study presents the first field data in which flow separation and the backwash vortex are quantified using a pulse-coherent acoustic Doppler profiler over a 0.10-m prograding beach step. As the step migrated beneath the instrument station under a quasi-steady wave forcing it maintained its height and length, allowing for a space-for-time substitution. Flow separation developed under subcritical flows, when the backwash exceeded ∼0.50 m s−1 and extended over ...

Major food web properties of two sandy beaches with contrasting morphodynamics, and effects on the stability

We determined major structural properties influencing the food webs of two sandy beaches with contrasting morphodynamics in the Atlantic coast of Uruguay: reflective (narrow and steep) and dissipative beaches (wide and flat). Furthermore, we evaluated how these characteristics could influence the stability of the local food webs. To this end, we examined the correlation of several food web properties with different ecosystem types (including freshwater habitats, estuary, marine, and terrestrial environments) using a principal components analysis. Sandy beach food web components included detritus, phytoplankton, zooplankton, benthic invertebrates, fishes, and seabirds. Our results revealed that the dissipative beach presented higher trophic levels, a higher number of trophic species, more links per species, as well as a higher proportion of intermediate trophic species, but lower connectance and proportion of omnivorous species than the reflective beach. The variation in the food web properties was explained by two principal components. Sandy beach food webs contribute mainly to one dimension of the principal components analysis that was determined by the number of trophic species, links per species, the trophic similarity, and the characteristic path length. We suggest that species and link characteristics, such as predominance of scavengers and detritivorous, the relatively high connectance and the short path length are drivers in the food web structure and may play a role in the community dynamic.

Predicting beach profile evolution with group method data handling-type neural networks on beaches with seawalls

A major goal of coastal engineering is to develop models for the reliable prediction of short- and long-term near shore evolution. The most successful coastal models are numerical models, which allow flexibility in the choice of initial and boundary conditions. In the present study, evolutionary algorithms (EAs) are employed for multi-objective Pareto optimum design of group method data handling (GMDH)-type neural networks that have been used for bed evolution modeling in the surf zone for reflective beaches, based on the irregular wave experiments performed at the Hydraulic Laboratory of Imperial College (London, UK). The input parameters used for such modeling are significant wave height, wave period, wave action duration, reflection coefficient, distance from shoreline and sand size. In this way, EAs with an encoding scheme are presented for evolutionary design of the generalized GMDH-type neural networks, in which the connectivity configurations in such networks are not limited to adjacent layers. Also, multi-objective EAs with a diversity preserving mechanism are used for Pareto optimization of such GMDH-type neural networks. The most important objectives of GMDH-type neural networks that are considered in this study are training error (TE), prediction error (PE), and number of neurons (N). Different pairs of these objective functions are selected for two-objective optimization processes. Therefore, optimal Pareto fronts of such models are obtained in each case, which exhibit the trade-offs between the corresponding pair of the objectives and, thus, provide different non-dominated optimal choices of GMDH-type neural network model for beach profile evolution. The results showed that the present model has been successfully used to optimally prediction of beach profile evolution on beaches with seawalls.

Global patterns in sandy beach macrofauna: Species richness, abundance, biomass and body size

Global patterns in species richness in sandy beach ecosystems have been poorly understood until comparatively recently, because of the difficulty of compiling high-resolution databases at continental scales. We analyze information from more than 200 sandy beaches around the world, which harbor hundreds of macrofauna species, and explore latitudinal trends in species richness, abundance and biomass. Species richness increases from temperate to tropical sites. Abundance follows contrasting trends depending on the slope of the beach: in gentle slope beaches, it is higher at temperate sites, whereas in steep-slope beaches it is higher at the tropics. Biomass follows identical negative trends for both climatic regions at the whole range of beach slopes, suggesting decreasing rates in carrying capacity of the environment towards reflective beaches. Various morphodynamic variables determine global trends in beach macrofauna. Species richness, abundance and biomass are higher at dissipative than at reflective beaches, whereas a body size follows the reverse pattern. A generalized linear model showed that large tidal range (which determines the vertical dimension of the intertidal habitat), small size of sand particles and flat beach slope (a product of the interaction among wave energy, tidal range and grain size) are correlated with high species richness, suggesting that these parameters represent the most parsimonious variables for modelling patterns in sandy beach macrofauna. Large-scale patterns indicate a scaling of abundance to a body size, suggesting that dissipative beaches harbor communities with highest abundance and species with the smallest body sizes. Additional information for tropical and northern hemisphere sandy beaches (underrepresented in our compilation) is required to decipher more conclusive trends, particularly in abundance, biomass and body size. Further research should integrate meaningful oceanographic variables, such as temperature and primary production, in deciphering latitudinal trends.

Patterns of sandy-beach macrofauna production

Using data available from the literature, patterns of biomass, production and productivity of sandy-beach macrofauna populations were examined, considering environmental (temperature, exposure, grain size and beach slope) and biological variables (life span and mean body mass) and feeding and taxonomic groups. A total of 102 estimates of both production and biomass and 105 estimates of P/B ratios were collected from 52 studies carried out between 42°46′S and 54°05′N, for 83 sandy-beach macrofauna populations. The negative relationship between P/B ratio and beach slope for the supralittoral amphipods agrees with the Habitat Safety Hypothesis, according to which these forms would show higher mortality in dissipative than in reflective beaches. The observed higher production of filter-feeders in exposed than in sheltered beaches suggests that more food is available for filter-feeders in exposed beaches. The higher production of filter-feeders (represented by bivalves and decapods), than of scavengers/predators (peracarids and gastropods) showed the importance of filter-feeders in the food web of sandy beaches. The P/B ratios were strongly related to life span, but weakly or not related to the mean body mass. The high amphipod P/B ratio was attributed to the short life span of these crustaceans; conversely, gastropods showed the lowest P/B ratio, in accordance with their longer life span. The observed differences in biomass, production and P/B ratios within crustaceans and molluscs were attributed to differences in life-history traits and feeding mode.

Responses of fish assemblage structures to sandy beach types in Kyushu Island, southern Japan

To determine differences in fish assemblage structures between beach morphodynamic types on an exposed sandy shore, three beach types (reflective, intermediate, and dissipative) were sampled at Fukiagehama Beach, Kyushu Island, southern Japan, in May, August, and November 2006 and 2007. Distinct differences in physical conditions and the amount of major prey invertebrates, such as copepods, mysids, and amphipods, were recognized between the three beach types. The reflective beach was characterized mainly by turbulent swashes, coarse sediment, and a low abundance of major prey invertebrates. In contrast, the dissipative beach was characterized mostly by benign swashes, fine sediment, and a high abundance of prey invertebrates. The intermediate beach tended toward an intermediate condition between the reflective and dissipative beaches. The fish assemblage structures also differed clearly between the three beach types, with species and individual numbers being greatest on the dissipative beach and lowest on the reflective beach. A similarity index indicated differences in species composition of the assemblages between the beach types. Such differences arose primarily from the differential distributions of two dominant trophic groups, zooplankton (mostly copepods) and epibenthic crustacean (mainly mysids and amphipods) feeders, between the beach types, both groups tending to increase in species and individual numbers from the reflective beach to the dissipative beach. In addition, the mean standard length of all fish collected in each month tended to decrease toward the dissipative beach. These results suggested that beach types strongly influence sandy beach fish assemblage structures.

Direct estimation wave setup as a medium level in swash

Díaz-Sánchez, R., López-Gutiérrez, J.S., Lechuga, A., Negro, V., Esteban, M.D., 2013. Direct estimation wave setup as a medium level in swash.The extreme runup is a key parameter for a shore risk analysis in which the accurate and quantitative estimation of the upper limit reached by waves is essential. Runup can be better approximated by splitting the setup and swash semi-amplitude contributions. In an experimental study recording setup becomes difficult due to infragravity motions within the surf zone, hence, it would be desirable to measure the setup with available methodologies and devices. In this research, an analysis is made of evaluated the convenience of direct estimation setup as the medium level in the swash zone for experimental runup analysis through a physical model. A physical mobile bed model was setup in a wave flume at the Laboratory for Maritime Experimentation of CEDEX. The wave flume is 36 metres long, 6.5 metres wide and 1.3 metres high. The physical model was designed to cover a reasonable range of parameters, three different slopes (1/50, 1/30 and 1/20), two sand grain sizes (D50 = 0.12 mm and 0.70 mm) and a range for the Iribarren number in deep water (ξ0) from 0.1 to 0.6. Best formulations were chosen for estimating a theoretical setup in the physical model application. Once theoretical setup had been obtained, a comparison was made with an estimation of the setup directly as a medium level of the oscillation in swash usually considered in extreme runup analyses. A good correlation was noted between both theoretical and time-averaging setup and a relation is proposed. Extreme runup is analysed through the sum of setup and semi-amplitude of swash. An equation is proposed that could be applied in strong foreshore slope-dependent reflective beaches.

WAVE RUNUP PREDICTION FOR FLOOD MAPPING

Wave runup determines the extent over which waves act. Wave runup is therefore an important parameter to determine flood inundation extents from coastal storms. Cross-shore and longshore sediment transport are a function of the hydrodynamics on the beach and are therefore related to wave runup. Several benchmark wave runup data sets are summarized and used to evaluate the available tools for predicting wave runup for flood hazard assessment. Benchmark data span a range of shoreline conditions including sandy beaches on the Pacific and Atlantic coasts, dissipative to reflective beaches, as well as structures ranging from impermeable smooth levees to rough permeable rubble mounds. Data include laboratory and prototype measurements. Tools for predicting wave runup are analyzed including empirical equations, computer programs based on empirical equations, and the CSHORE time-averaged cross-shore model. Most of the tools show fairly high degrees of skill but some do not. The study recommends using CSHORE to model runup for most beach and structure conditions. However, CSHORE is not likely to predict wave runup on infragravity-dominated dissipative beaches well. For these cases, it is recommended that one of the recommended empirical equations for beaches be used.

Sediment transport processes and morphodynamics on a reflective beach under storm and non-storm conditions

New field experiments of sediment transport on the shoreface of a reflective beach (R=0.4–0.6) during pre-storm, storm and post-storm conditions are reported. Data were collected outside the breakpoint and include water surface elevations, cross-shore and long-shore flow velocity, suspended sediment concentrations, bedform dimensions and the morphological response. Instruments included two pulse-coherent ADPs, an ADV, multiple OBS and a pencil-beam sonar. The depth-averaged mean velocity was always offshore and increased by a factor five during the storm, and this increase is attributed to undertow being enhanced by strong backwash plumes from cusp bays. Estimates of cross-shore suspended sediment transport are landward during the pre- and post-storm phases, consistent with the morphological response. During the storm, the shoreface quickly appeared to reach equilibrium, despite energy levels being maintained, with sediment transport rates reducing rapidly after the initial phase of the storm. Streaming in the wave boundary layer was observed and strongest during the pre-storm phase, consistent with the observed shoreward transport. During the storm the near-bed streaming was too small to reverse the increased offshore current, consistent with the observed net offshore transport. The generally small orbital velocity skewness observed on this steep reflective beach suggests that wave reflection may reduce the overall skewness of the velocity, in turn contributing to the stability of reflective beach types.

An integrated approach to characterize the interaction between coastal morphodynamics, geomorphological setting and human interventions on the Mediterranean beaches of northwestern Morocco

The purpose of this study is to determine the geographic distribution of types of beaches according to their different degrees of exposure to natural and anthropogenic forcings, an approach that is important for the implementation of appropriate coastal management strategies. Accordingly, the morphodynamic behavior of five Mediterranean beaches of Tetouan (in northwestern Morocco) was monitored over the course of a two-year project. The results show that seasonal beach changes are not very significant; the most important variations were recorded after storms. The type and mobility of beaches are a function of their curvature and distance from headlands, exposure to waves, grain size and sediment supply. Morphological changes are faster and more excessive in reflective beaches located north of the Cabo Negro promontory; moreover, these beaches have a greater tendency for erosion. Erosion vulnerability hazards have also increased in the northern part of the study area, due to coastal constructions. The timescale of the study and the datasets obtained are, however, too short to identify the imprint of chronic erosion. Continuous dune destruction under development pressures and a sand supply deficit from the nearby wadis will indeed be detrimental, especially at the more exposed beaches, leading to the destruction of the very resource that underpins tourism development in the area. Mitigation of coastal erosion requires not only a nexus between science and management but also the rapid implementation of measures aimed at restoring the coastal sediment budget taking into account the morphodynamics of the beach–dune units.

Coupling between macrofauna community structure and beach type: a deconstructive meta-analysis

Species richness and abundance of macrofauna on sandy shores increase from micro - tidal reflective to macrotidal dissipative beach conditions. However, no attempt has been made to deconstruct these patterns. Using information on the macrofauna from 63 microtidal sandy beach sur- veys in South America, we deconstructed the community to discriminate among taxonomic groups, supralittoral and intertidal forms, and groups with different feeding habits and development modes. We also separated the effects of development mode and beach zone (intertidal vs. supralittoral) at the species level and scaled body size to density estimates. Total species richness decreased towards reflective beaches, but this trend was less marked in crustaceans than in molluscs or polychaetes. Supralittoral air-breathing crustaceans increased in richness and abundance from dissipative to reflective conditions, a reverse trend to that seen in intertidal crustaceans. Development modes (spe- cies with direct development and with planktonic larvae) showed the same response to beach type as the community as a whole. Filter feeders, scavengers and deposit feeders showed the same trend, but the latter were scarce or absent on reflective beaches. Zone was more important than either develop- ment or feeding modes in determining individual species responses to beach type. Body size decreased and density increased from reflective to dissipative beaches, suggesting that smaller forms are more sensitive to harsh conditions where density is lower, and hence there is less scope for bio- logical interactions in reflective beaches. The deconstruction approach provided insights into the relationships between life cycle characteristics and richness and abundance patterns and the relative importance of environmental variables in beach ecology.

Performance of intertidal topography video monitoring of a meso-tidal reflective beach in South Portugal

This study discusses site-specific system optimization efforts related to the capability of a coastal video station to monitor intertidal topography. The system consists of two video cameras connected to a PC, and is operating at the meso-tidal, reflective Faro Beach (Algarve coast, S. Portugal). Measurements from the period February 4, 2009 to May 30, 2010 are discussed in this study. Shoreline detection was based on the processing of variance images, considering pixel intensity thresholds for feature extraction, provided by a specially trained artificial neural network (ANN). The obtained shoreline data return rate was 83%, with an average horizontal cross-shore root mean square error (RMSE) of 1.06 m. Several empirical parameterizations and ANN models were tested to estimate the elevations of shoreline contours, using wave and tidal data. Using a manually validated shoreline set, the lowest RMSE (0.18 m) for the vertical elevation was obtained using an ANN while empirical parameterizations based on the tidal elevation and wave run-up height resulted in an RMSE of 0.26 m. These errors were reduced to 0.22 m after applying 3-D data filtering and interpolation of the topographic information generated for each tidal cycle. Average beach-face slope tan(β) RMSE were around 0.02. Tests for a 5-month period of fully automated operation applying the ANN model resulted in an optimal, average, vertical elevation RMSE of 0.22 m, obtained using a one tidal cycle time window and a time-varying beach-face slope. The findings indicate that the use of an ANN in such systems has considerable potential, especially for sites where long-term field data allow efficient training.

A HYBRID MODEL OF SWASH-ZONE LONGSHORE SEDIMENT TRANSPORT ON REFLECTIVE BEACHES

The hydrodynamics and sediment transport in the swash zone is currently outside the domain of coastal-area models, which is a significant limitation in obtaining littoral sediment-transport estimates, especially on steep reflective beaches where the waves practically break on the beachface. In this study, an existing process-based coastal model (MIKE 21) is combined with a theoretical derivation of swash processes, resulting in an innovative hybrid modelling approach that is capable of estimating longshore sediment transport in the swash zone. The method relies upon estimation of swash hydrodynamics from an extended ballistic swash model with friction included. The terminal bore and other incident wave properties were computed from the output of a spectral-wave model (MIKE 21 SW). The Bagnold-type equation was applied to estimate gross transport volumes and the longshore component was computed for the sand volume displaced during the up-rush. The newly developed hybrid modelling approach was applied to Jimmys beach, a steep reflective beach (D50 = 0.3 mm, gradient=0.1) along the northern shoreline of Port Stephens, Australia. The model results yield the alongshore swash transport pathways and the indicative transport volumes. A point of divergence is identified at the beach erosion area, which is of critical importance in terms of shoreline erosion and management. The preliminary results suggest that swash-zone transport can account for a large percentage of the total littoral drift for such beaches. However, further field or laboratory data are required to test model utility, as well as to tune calibration parameters based on the site-specific conditions.

Mapping beach morphodynamics remotely: A novel application tested on South African sandy shores

Sandy beaches have been identified as threatened ecosystems but despite the need to conserve them, they have been generally overlooked. Systematic conservation planning (SCP) has emerged as an efficient method of selecting areas for conservation priority. However, SCP analyses require digital shapefiles of habitat and species diversity. Mapping these attributes for beaches from field data can take years and requires exhaustive resources. This study thus sought to derive a methodology to classify and map beach morphodynamic types from satellite imagery. Since beach morphodynamics is a strong predictor of macrofauna diversity, they could be considered a good surrogate for mapping beach biodiversity. A dataset was generated for 45 microtidal beaches (of known morphodynamic type) by measuring or coding for several physical characteristics from imagery acquired from Google Earth. Conditional inference trees revealed beach width to be the only factor that significantly predicted beach morphodynamic type, giving four categories: dissipative, dissipative-intermediate, intermediate and reflective. The derived model was tested by using it to predict the morphodynamic type of 28 other beaches of known classification. Model performance was good (75% prediction accuracy) but misclassifications occurred at the three breaks between the four categories. For beaches around these breaks, consideration of surf zone characteristics in addition to beach width ameliorated the misclassifications. The final methodology yielded a 93% prediction accuracy of beach morphodynamic type. Overlaying other considerations on this classification scheme could provide additional value to the layer, such that it also describes species’ spatial patterns. These could include: biogeographic regions, estuarine versus sandy beaches and short versus long beaches. The classification scheme was applied to the South African shoreline as a case study. The distribution of the beach morphodynamic types was partly influenced by geography. Most of the long, dissipative beaches are found along the west coast of the country, the south coast beaches are mostly dissipative-intermediate, and the east coast beaches range from short, estuarine pocket and embayed beaches in the former Transkei (south east), to longer intermediate and reflective beaches in KwaZulu-Natal (in the north east). Once combined with the three biogeographic regions, and distinguishing between estuarine and sandy shores, the South African coast comprised 24 different beach types. Representing shorelines in this form opens up potential for numerous spatial analyses that can not only further our understanding of sandy beach ecology at large spatial scales but also aid in deriving conservation strategies for this threatened ecosystem.

Food web structure of sandy beaches: Temporal and spatial variation using stable isotope analysis

The food web structure of two sandy beach ecosystems with contrasting morphodynamics (dissipative vs. reflective) was examined using stable carbon (δ 13C) and nitrogen (δ 15N) isotope analysis. Organic matter sources (POM: particulate organic matter; SOM: sediment organic matter) and consumers (zooplankton, benthic invertebrates and fishes) were sampled seasonally in both sandy beaches. Food webs significantly differed between beaches: even though both webs were mainly supported by POM, depleted δ 13C and δ 15N values for food sources and consumers were found in the dissipative system (following the reverse pattern in δ 13C values for consumers) for all the four seasons. Primary consumers (zooplankton and benthic invertebrates) use different organic matter sources on each beach and these differences are propagated up in the food web. The higher productivity found in the dissipative beach provided a significant amount of food for primary consumers, notably suspension feeders. Thus, the dissipative beach supported a more complex food web with more trophic links and a higher number of prey and top predators than the reflective beach. Morphodynamic factors could explain the contrasting differences in food web structure. The high degree of retention (nutrients and phytoplankton) recorded for the surf zone of the dissipative beach would result in the renewed accumulation of POM that sustains a more diverse and richer fauna than the reflective beach. Further studies directed to assess connections between the macroscopic food web, the surf-zone microbial loop and the interstitial compartment will provide a deeper understanding on the functioning of sandy beach ecosystems.

Trophic models in sandy beaches with contrasting morphodynamics: Comparing ecosystem structure and biomass flow

We model and compare for the first time the ecosystem structure and trophic networks of two sandy beaches with contrasting morphodynamics (i.e. dissipative and reflective). To this end, an Ecopath model was implemented to represent the macroscopic food web on each sandy beach ecosystem. The dissipative beach model comprised 20 compartments and the reflective nine, including detritus, phytoplankton, zooplankton, benthic invertebrates, fishes and seabirds. Input data mainly came from direct surveys, whereas additional information was gathered from published and unpublished sources. Results revealed a major number of top predators and higher trophic levels (TLs) in the dissipative beach (seabirds, fishes, gastropods and the polychaete Hemipodus olivieri) than in the reflective one (fishes and Hemipodus olivieri). Detritivorous and filter feeding benthic invertebrates constituted intermediate trophic levels on both beaches. Exportation of most primary production and detritus was a common feature, with a detritivory:herbivory ratio of 0.42 in the dissipative beach and 0.51 in the reflective, indicating a higher utilization of the primary production in the former. The aggregation analysis showed five TLs in the dissipative beach and four in the reflective. Lower transfer efficiencies in the reflective beach could be attributed to a lower diversity of predators when compared to the dissipative system. Comparison of global ecosystem properties showed that the dissipative system had higher values for total system throughput, total biomass, net production, Ascendency and total number of pathways. We concluded that the greater diversity and biomass in the dissipative beach are reflected in a higher dimension and ecosystem organization than in the reflective beach, thus providing new evidences at the ecosystem level about well-established differences in biological descriptors between beach types. Finally, we stressed the need for acquiring experimentally-based information on benthic invertebrate's consumption rates and the inclusion of the surf zone microbial loop and the interstitial compartment to test for differences in structure and functioning of these coastal ecosystems.

The response of three main beach types to tropical storms in South China

Based on the observational data of 6 storms and 8 different beaches on the coasts of southern China, the paper is concerned with the characteristics of storm-effects on beaches with different morphodynamic types in South China. A dissipative beach is very stable and has a weak response to tropical storms. The storm-induced morphological changes for this type of beach are characterized by erosion at the dune toe and minor changes on the beach face. A low-tide terrace beach has a strong response to storms, with marked changes in the upper profile. A barred beach usually experiences berm erosion, swash bar smoothening, and seaward bar migration during storms. Overwash may occur for the beach without backshore dune or protection. In this paper, backshore changes and the alteration of beach gradient in response to tropical storms are discussed. With taking into account tidal influences, a new morphodynamic parameter ( ε T ) is proposed in order to highlight the beach response intensity to storm action. Results show that the inherent morphodynamic properties have significant effects on the beach response to storm. The more dissipative beach has a smaller response intensity and shows a moderate change in beach profile during a storm, while a more reflective beach has a greater response intensity and changes markedly under storm conditions.

Surf zone physical and morphological regime as determinants of temporal and spatial variation in larval recruitment

Larvae of intertidal species develop in the coastal ocean, and the last body of water they must cross while migrating back to shore is the surf zone. We hypothesized that the surf zone is a semipermeable barrier to this shoreward migration and that differences in water exchange across the surf zone result in temporal and spatial variation in larval delivery to the shore. We tested the hypotheses that larval delivery 1) should increase with larger waves and 2) should be higher on more dissipative beaches than on more reflective beaches. We found a significant positive correlation between the daily averaged ratio of wave height to wave period ( H/ T) and daily cyprid settlement at Dike Rock, California and Bastendorff Beach, Oregon, USA. We tested the second hypothesis by comparing populations of barnacles, limpets, and benthic algae on rocks on four more dissipative and six more reflective sandy beaches in northern California and southern Oregon. Newly recruited barnacles and limpets were significantly more abundant at more dissipative than reflective beaches, and the higher abundance was most likely due to differences in settlement rather than post-settlement mortality. The density and percent cover of barnacles and the density of limpets were significantly higher at more dissipative beaches. In contrast, the density and percent cover of algae were significantly higher at more reflective beaches. The results are consistent with the hypothesis that the surf zone is a semipermeable barrier to the shoreward migration of larvae and that differences in water exchange across the surf zone as function of the beach hydrodynamics result in temporal and spatial variation in larval delivery to the shore.

Physical determinants of intertidal communities on dissipative beaches: Implications of sea-level rise

Sea-level rise is likely to cause significant changes in the morphodynamic state of beaches in the higher latitudes, resulting in steeper beaches with larger particle sizes. These physical changes have implications for beach invertebrate communities, which are determined largely by sediment particle size, and hence for ecosystem function. Previous studies have explored the relationships between invertebrate communities and environmental variables such as particle size, beach slope and exposure to wave action, and often these physical variables can be integrated in various indices of morphodynamic state. Most of these studies incorporated a full range of beach types that included wave-dominated surf beaches, where the wave action is harsh enough to enable reliable estimates of breaker height, a parameter included in several of the indices, and concluded that more dissipative beaches with gentler slopes and finer particle sizes often support a higher number of species and greater abundance than more reflective beaches. Whether these predictions remain valid for less wave-dominated beaches, where breaker height is more difficult to determine, is uncertain. In the present study, the abundance of meio- and macrofauna was quantified across a range of beaches in the UK, which are generally towards the lower energy end of the morphodynamic gradient, and their relationships with beach physical properties explored. No significant relationships were found between abundance and the standard morphodynamic indices, but significant relationships were found for both macro- and meiofaunal abundance when these indices were combined with an exposure index (derived from velocity, direction, duration and the effective fetch). All the relationships identified between abundance and combined morphodynamic indices indicated a higher abundance of both macro- and meiofauna on the more dissipative beaches. The reverse was however found for species richness. If predictions that accelerated sea-level rise will move beaches towards a more reflective morphodynamic state are correct, this could lead to declines in the abundance of meio- and macrofauna, with potential adverse consequences for ecosystem functioning.

Effects of morphodynamic and estuarine gradients on the demography and distribution of a sandy beach mole crab: implications for source–sink habitat dynamics

Several conceptual models relating biological and morphodynamic variables have been developed in sandy beach ecology. On estuarine sandy beaches, a major environmental factor influ- encing macrofaunal distribution is salinity. In this setting, the concurrent role of large-scale estuarine and morphodynamic gradients in shaping demographic patterns is still uncertain. To explain large- scale variations in demographic aspects of the mole crab Emerita brasiliensis, 16 Uruguayan sandy beaches distributed along the estuarine gradient generated by the Rio de la Plata were characterized in terms of beach morphology, sediment traits and salinity over a 2 yr period. Abundance (total and discriminated by reproductive and recruitment components), biomass and body size of E. brasiliensis significantly increased from estuarine reflective to oceanic dissipative beaches. Further, generalized models (both additive and linear) showed that salinity, mean grain size and swash width were the most important explanatory factors of spatio-temporal variations in abundance. Sexually mature indi- viduals and megalopae were almost absent in estuarine and oceanic reflective beaches, whereas oceanic dissipative beaches showed polymodal size structures with fully represented population components, together with extended reproductive and recruitment periods. We conclude that (1) morphodynamic and estuarine gradients are inextricably linked, affecting mole crab demography; and (2) estuarine and oceanic reflective beaches operate as sink habitats where populations do not prosper, whereas oceanic dissipative beaches act as source habitats. We thus provide empirical sup- port to the source-sink hypothesis recently developed for sandy beaches. These long-term variations in demographic processes should be complemented by connectivity studies and genetic analyses in order to gain a deeper understanding of metapopulation dynamics in sandy beaches.