Reflective Beaches Research Articles

Testing the Habitat Safety Hypothesis with behavioural field experiments: amphipod orientation on sandy beaches with contrasting morphodynamics

Understanding behavioural adaptations in sandy beach macrofauna has been a major step in theoretical development of sandy beach ecology. In particular, behavioural studies in supra- littoral amphipods have been useful in understanding macrofaunal responses and adaptations to environmental changes in these dynamic ecosystems. The predictions of the Habitat Safety Hypo- thesis (HSH) were tested for the first time on Atlantorchestoidea brasiliensis through orientation experiments carried out on 2 sandy beaches, one dissipative and the other reflective. Orientation pat- terns of A. brasiliensis followed the trend predicted by the HSH: even though sandhoppers oriented seawards on both beaches using a sun compass, the orientation on the reflective beach was more pre- cise than on the dissipative one. Thus, stability conditions in the supralittoral zone of the reflective beach favoured a higher precision of the sun compass. The precision of seawards orientation on the dissipative beach increased when landscape references were added to the sun compass mechanism. The response to an unexpected object was also experimentally tested by use of a visual pattern, and a higher individual variability in the orientation of the reflective beach population was found. Thus, orientation strategies in sandy beach environments seem to respond to the stability of the supralit- toral zone, widening the general pattern predicted by the HSH for the supralittoral fauna to the behavioural traits of the population.

Testing the habitat harshness hypothesis: reproductive biology of the wedge clam Donax hanleyanus (Bivalvia: Donacidae) on three Argentinean sandy beaches with contrasting morphodynamics

In order to test the habitat harshness hypothesis (HHH) the reproductive biology of Donax hanleyanus was studied histologically, comparing populations from three beaches with contrasting morphodynamics (dissipative, intermediate and reflective) over 25 months. The reproductive phase of D. hanleyanus was extended at the reflective beach compared to the other two. Males and females from the dissipative and intermediate beaches were significantly smaller and had lower biomass at maturity than those at the reflective beach. Recruits were significantly more abundant and the recruitment period was extended significantly at the dissipative beach. Spawning events took place twice each year at the dissipative (early spring and spring-summer) and the intermediate beach (winter and summer), whereas continuous gamete releases were noted at the reflective beach. Size and biomass at first maturity were lower at the dissipative beach, whereas monthly mean abundance of D. hanleyanus was higher at the reflective beach. The gametogenic cycle correlated significantly with sea-surface temperature, relative spermatozoon abundance, condition index, ash-free shell-free dry mass, and mean size and abundance of oocytes, for all three populations. At the population level, many of these reproductive responses to physical variables were opposite to those predicted by the HHH, including: greater abundance, extended reproductive cycle, extended period with spawning individuals, and larger size and higher biomass at first maturity at the reflective beach. This suggested that the hypothesis of habitat safety (HHS), originally proposed for supralittoral species, may be extended to intertidal species; a combination of narrow swashes and steep slopes makes reflective beaches a safer and more stable environment for intertidal species such as D. hanleyanus.

Effect of cold fronts on the benthic macrofauna of exposed sandy beaches with contrasting morphodynamics

The aim of this study was to determine the short-term environmental changes caused by the simultaneous passage of a high energy event on two sandy beaches with different morphodynamic states and their influence on the richness, abundance and distribution of the benthic macrofauna. Two microtidal exposed sandy beaches with contrasting morphodynamics were simultaneously sampled before, during and after the passage of two cold fronts in Santa Catarina. The reflective beach showed a higher susceptibility to the increase in wave energy produced by the passage of cold fronts and was characterized by rapid and intense erosive processes in addition to a capacity for rapid restoration of the beach profile. As regards the dissipative beach, erosive processes operated more slowly and progressively, and it was characterized further by a reduced capacity for the recovery of its sub-aerial profile. Although the intensity of the environmental changes was distinct as between the morphodynamic extremes, changes in the composition, richness and abundance of macrobenthos induced by cold fronts were not evident for either of the beaches studied. On the other hand, alterations in the distribution pattern of the macrofauna were observed on the two beaches and were related to variations in sea level, position of the swash zone and moisture gradient, suggesting that short-term accommodations in the spatial structure of the macrobenthos occur in response to changes in environmental conditions in accordance with the temporal dynamics characteristic of each morphodynamic state.

Distribution patterns of interstitial polychaetes in sandy beaches of southern Brazil

AbstractThis study describes the distribution patterns of interstitial polychaetes along morphodynamic gradients on six exposed sandy beaches in Santa Catarina and Paraná (South Brazil). Three random transects were sampled at two points on each beach, one at the swash and another at the surf zone, in winter and summer conditions. Six sediment replicates were collected at each sampling point using a corer of 4.6 cm internal diameter that removed 10 cm into the sediment. Abundance and composition of interstitial polychaete were correlated to wave height, slope, grain size, CaCO3, chlorophyll a, omega indexes, temperature and relative tide range using a canonical correspondence analysis (CCA). A factorial ANOVA showed that taxa richness, mean density and Shannon’s diversity were significantly higher at the reflective beaches, but average values differ significantly between transects and these differences change according to the beach zones on both sampling dates. PERMANOVA showed that polychaete associations differ among transects according to the beach zones. The composition of interstitial polychaete associations was significantly correlated to beach morphodynamics and features (P < 0.01). Polychaete associations of reflective beaches were more diverse than in other morphodynamic states. Intermediate beaches may also sustain diverse associations due to temporal variability of the morphodynamic patterns. Beaches presenting extreme dissipative morphodynamics and compacted sediments appear to be unfavourable for the occurrence of interstitial polychaetes.

Is the burrowing performance of a sandy beach surfing gastropod limiting for its macroscale distribution?

The size-specific burrowing capacity of Olivella semistriata, an extremely abundant surfing gastropod of exposed sandy beaches in the tropical East Pacific, was investigated in terms of sediment grain size. For all investigated sand classes, there was a significant increase in burial time with size. Burrowing was fastest in sand with grain size between 150 and 355 μm, as well as in native sediment (median grain size 209 μm) and in field conditions (median grain size 223 μm). Values of the burrowing rate index (BRI) were found to be between 3 and 7, rating the burrowing capacity of O. semistriata to be fast to very fast. Data from previous qualitative and quantitative sampling campaigns were used to identify the macroscale (i.e., between beaches) distribution of O. semistriata in terms of sediment grain size and swash conditions. Swash period was shorter than burial time, excluding this as a factor limiting the distribution. Swash standstill time (the time between uprush and backwash), however, was just long enough on the beaches, where O. semistriata was present to allow for securing firm anchorage. On reflective beaches, the swash standstill time is as short as 1 or 2 s, denying O. semistriata the time to burry itself before being swept away by the backwash. As such, swash standstill time is advocated as a valuable part of the swash exclusion hypothesis. A survey of the available literature on the burrowing of surfers shows that mole crabs are by far the fastest burrowers and the only surfers that burrow sufficiently fast to withstand the extremely short swash standstill time on reflective beaches. Burrowing ability of surfing gastropods is found in the same range as surfing bivalves, both being insufficient to cope with coarse sediment on steep beaches. Finally, we suggest that neither burial time nor BRI, yet rather the minimal burial time—the time needed to anchor securely in a certain sand at a given swash velocity—should be used to judge the limitations of burial in terms of sediment and swash conditions.

Reproductive plasticity in mole crabs, Emerita brasiliensis, in sandy beaches with contrasting morphodynamics

Ovarian macroscopical analysis, histological validation and field sampling procedures were used to evaluate the variability in reproductive traits of the mole crab Emerita brasiliensis Schmitt, 1935 (Decapoda: Hippidae) in two exposed sandy beaches of Uruguay with contrasting morphodynamics. All developmental stages involved in the complex life cycle exhibited lower abundance, individual size and temporal occurrence in a harsh reflective beach, compared with a more benign dissipative environment. In addition, this population showed more compressed events of the reproductive cycle (e.g oogenesis, encounter of potential mate and female parental care) and recruitment period. However, the beginning of the vitellogenesis, ovary maturity, male sexual differentiation and ending of spermatogenesis occurred at smaller sizes under dissipative conditions. These results disagree with the recent findings of delayed sexual maturity in dissipative beaches. We postulate that, in dissipative conditions, high food availability might allow an overlapping of reproductive and moulting processes, and thus females may reach optimum size and sexual maturity with fewer moults than in reflective beaches. Hence, reproductive responses must be considered not only in relation to environmental harshness, but also in the context of life history traits and their phylogenetic and allometric constraints.

Cross-shore Mysidacea (Crustacea) distribution in two sandy beaches with contrasting morphodynamics

The order Mysidacea comprises small and very common crustaceans that inhabit most coastal ecosystem. The present contribution describes the composition, abundance and distribution of mysids in two sandy beaches representing the extreme conditions of the morphodynamics spectrum. Six species of mysids were found at the studied beaches. Metamysidopsis neritica Bond-Buckup & Tavares, 1992 was the most abundant species with more than 97% of the total individuals collected at the two beaches. At the dissipative beach was dominant in the swash and surf zone, with the greater abundance in the inner surf zone. At the reflective beach, this species was extremely abundant in the surf zone and backward the breaker. Bowmaniella brasiliensis Bacescu, 1968 occurred in low density at the surf zone of the dissipative beach, with the greater abundance in the outer surf zone, and it was practically absent in the nearshore zone. We suggest that mysids zonation in sandy beaches may be first developed as a response to food availability patterns imposed by the nearshore circulation systems, and in the second place as a result of competitive exclusion. The suggestion of partitioning of food and spatial resources shown by the mysid species is surprising in view of the apparent physical harshness and low spatial heterogeneity that seem to characterise the beach environment.

Global Patterns in Sandy Beach Macrobenthic Communities: Biological Factors

Abstract Building on an earlier study of global trends in sandy beach macrofauna communities, based on published surveys of 161 beaches, we examine trends in biological factors, especially mean individual body size and community density. Mean individual body size decreases across the gradient from reflective to dissipative beaches, although this decrease cannot be explained by any single factor. Density increases across this gradient so that dissipative beaches support densities of macrofauna orders of magnitude above those recorded on reflective beaches, implying that the potential for competition and other biological interactions is much greater in dissipative systems. Some latitudinal variation occurs in mean individual body size, but there are minimal latitudinal effects on density. Progressing from reflective to dissipative beaches, species richness increases much faster than predicted by the usual species area relationship, indicating that changes in the nature of the beach over this gradient, rathe...

Tisular and population level responses to habitat harshness in sandy beaches: the reproductive strategy of Donax hanleyanus

Microscopic analysis and field sampling procedures were used to compare demographic and reproductive strategies of the intertidal wedge clam Donax hanleyanus (Bivalvia: Donacidae) in two exposed sandy beaches with contrasting morphodynamics (reflective vs. dissipative) during 13 consecutive months. Histological analysis showed that: (1) the reproductive cycle of D. hanleyanus was more extended in the dissipative beach, and this was true for all the three pre-active (beginning of gonadal activity), active (maturation) and spawning stages; and (2) males and females showed significantly smaller sizes at sexual maturity at the reflective beach. Even though successive increments in proportion and mature at size were observed, the sigmoid function was significantly steeper at the reflective Arachania for both sexes, suggesting an abrupt transition to maturity. Field sampling revealed a more extended recruitment period at the dissipative beach, where recruits were also significantly more abundant than at the reflective beach. These results give support for the habitat harshness hypothesis, which predicts that in intertidal species capable of sustaining populations across a wide spectrum of physical conditions, such as D. hanleyanus, abundance, recruitment, size at maturity and extent of reproductive and recruitment seasons increase from reflective to dissipative beaches. However, a recent hypothesis suggests that reflective beaches acting as sink populations were not sustained, because mature and spawning individuals of both sexes were found in the reflective beach throughout the study period. Thus, we suggest that post-settlement processes are critical in modulating population patterns for this bivalve.

Longshore drift estimation using fluorescent tracers: New insights from an experiment at Comporta Beach, Portugal

A fluorescent sand-tracer experiment was performed at Comporta Beach (Portugal) with the aim of acquiring longshore sediment transport data on a reflective beach, the optimization of field and laboratory tracer procedures and the improvement of the conceptual model used to support tracer data interpretation. The field experiment was performed on a mesotidal reflective beach face in low energetic conditions (significant wave height between 0.4 and 0.5 m). Two different colour tracers (orange and blue) were injected at low tide and sampled in the two subsequent low tides using a high resolution 3D grid extending 450 m alongshore and 30 m cross-shore. Marked sand was detected using an automatic digital image processing system developed in the scope of the present experiment. Results for the two colour tracers show a remarkable coherence, with high recovery rates attesting data validity. Sand tracer displayed a high advection velocity, but with distinct vertical distribution patterns in the two tides: in the first tide there was a clear decrease in tracer advection velocity with depth while in the second tide, the tracer exhibited an almost uniform vertical velocity distribution. This differing behaviour suggests that, in the first tide, the tracer had not reached equilibrium within the transport system, pointing to a considerable time lag between injection and complete mixing. This issue has important implications for the interpretation of tracer data, indicating that short term tracer experiments tend to overestimate transport rates. In this work, therefore, longshore estimates were based on tracer results obtained during the second tide. The estimated total longshore transport rate at Comporta Beach was 2 × 10 − 3 m 3/s, more than four times larger than predicted using standard empirical longshore formulas. This discrepancy, which results from the unusually large active moving layer observed during the experiment, confirms the idea that most common longshore transport equations under-estimate total sediment transport in plunging/surging waves.

Integral properties of the swash zone and averaging. Part 3. Longshore shoreline boundary conditions for wave-averaged nearshore circulation models

The aim of the present work, final of a three-part series, is to analyse in detail flow motions within the swash zone and define suitable shoreline boundary conditions for the longshore flow for wave-averaged circulation models. The analyses of Parts 1 and 2 are extended to cover horizontally two-dimensional flows. An analytical solution for the longshore motion representing the drift velocity of the whole swash zone water mass is found. This is seen to be approximated well by the ratio between the time integral of the longshore momentum flux crossing the swash lower boundary and the swash zone net water volume. Further, a complete set of shoreline boundary conditions, taking into account wave–wave interactions, is obtained on the basis of fully numerical solutions of the nonlinear shallow-water equations. The main focus of the work is to clarify the structure of the shoreline boundary conditions for the longshore flow, but attention has also been paid to their derivation and assessment from the numerical solutions. The latter have been obtained on the basis of a fairly broad range of input wave conditions which, though biased towards those typical of reflective beaches, are believed to cover conditions also typical of moderate dissipative beaches. Two main terms are found to contribute to the longshore drift velocity: (i) a term, proportional to the shallow-water velocity, accounting for short-wave interactions, frictional swash zone forces and continuous forcing due to non-breaking wave nonlinearities and (ii) a drift-type term representing the momentum transfer due to wave breaking.

Morphological characteristics and medium-term evolution of the beaches between Ceuta and Cabo Negro (Morocco)

This work presents the results of a combined study on the beach morphology and the evolution at short- and medium-term of the littoral between Ceuta and Cabo Negro (Morocco). It is an interesting sector showing a great increase of human occupation and coastal structures. The monitoring program allowed for the reconstruction of the beach morphological behavior and the seasonal changes. The studied beaches presented reflective profiles recording little seasonality, with the most notable morphological changes being strictly related to storms. Surf Similarity and Surf Scaling parameters highlighted the existence of intermediate and reflective beach states, characterized by plunging breakers. Aerial photographs and a satellite image have been geo-referenced and elaborated with GIS tools to reconstruct the short- and medium-term evolution of the littoral and the sediment transport pathways. The littoral showed important erosion at short- and medium-term related to a negative sedimentary budget because of offshore transport. Sand accumulation was recorded close to the main ports, i.e., Marina Smir and Marina Kabila. These port structures constituted impermeable, fixed limits, which divided the studied area into littoral cells. Other free, transit limits were also observed.

Short‐term morphological change and sediment dynamics in the intertidal zone of a macrotidal beach

AbstractThis paper describes the morphological and sedimentological evolution of a macrotidal beach over a 20 day period under varying hydrodynamic conditions (significant breaker heights of 0·3–2 m and tidal ranges of 2–5 m). During the field campaign, an intertidal bar developed around the mid‐tide level, migrated onshore, welded to the upper beach and was then flattened under energetic wave conditions. The bar had a wave breakpoint origin and its formation was triggered by a reduction in tidal range, causing more stationary water‐level conditions, rather than an increase in wave height. Most of the onshore bar migration took place while the bar was positioned in the inner to mid‐surf zone position, such that the bar moved away from the breakpoint and exhibited ‘divergent’ behaviour. The depth of disturbance over individual tidal cycles was 10–20% of the breaker height. Such values are more typical of steep reflective beaches, than gently sloping, dissipative beaches, and are considered to reflect the maximum height of wave‐generated ripples. The grain size distribution of surficial sediments did not vary consistently across the beach profile and temporal changes in the sedimentology were mostly unrelated to the morphological response. The lack of clear links between beach morphology and sedimentology may be in part due to shortcomings in the sampling methodology, which ignored the vertical variability in the sediment size characteristics across the active layer.

Population Biology and Secondary Production of Excirolana braziliensis (Isopoda: Cirolanidae) in Two Sandy Beaches of Southeastern Brazil

The population dynamics and secondary production of the cirolanid isopod Excirolana braziliensis are compared between two distinct sandy beaches. Monthly sampling from June 1998 through July 2000 was carried out at Grumari (a reflective beach with coarse sand and steeper slope) and Restinga da Marambaia (an intermediate beach with fine sand and moderate slope), both located in Rio de Janeiro State (23° S), southeastern Brazil. Sampling was conducted according to a systematic design in which two transects (fixed 500 meters apart) were established, each with 10 equally spaced sampling strata. From each stratum, five replicates (0.04 square meters) were taken. Highest population abundances were observed in spring and summer on both beaches. The beaches differed significantly in physical parameters and the characteristics of their inhabitant populations: (1) Grumari Beach had a steeper slope, coarse sands, and a short swash period; (2) the abundance of total population, fecundity at length, and egg production potential were higher at Restinga da Marambaia; and (3) individual growth of males was higher at Grumari. Mean length at sexual maturity (L50%), mortality, growth of females, secondary production, and mean annual biomass did not show significant differences between beaches. Variations in population parameters of E. braziliensis might be regulated by phenotypic adjustment to local conditions, optimizing fitness, and especially enabling species to adapt to different physical conditions.

Reproductive cycle of Mellita quinquiesperforata (Leske) (Echinodermata, Echinoidea) in two contrasting beach environments

The reproductive cycle of the irregular echinoid Mellita quinquiesperforata (Leske, 1778) was studied in populations from two sand beaches with different morphodynamic conditions in Parana coast, from February 1992 through July 1994. Gametogenesis was described by histological and gonad index methods and the differences between both populations were observed in the extension of nutrient storage and spawning period. Both populations exhibited a main reproductive period during spring/summer and a nutrient storage period during autumn/winter months. The spawning period at the intermediate-dissipative beach was shorter than at the reflective beach. Meanwhile, a nutrient storage stage was more extensive in the latter. Gonad index varied between both sexes and it was influenced by population characteristics. Different strategies in allocation of resources between maintenance and reproduction effort (oocyte size) confirm the high adaptive plasticity developed by this species to live in contrasting beach environments.

Habitat harshness and morphodynamics: life history traits of the mole crab Emerita brasiliensis in Uruguayan sandy beaches

The habitat harshness hypothesis (HHH) pos- tulates that in reflective beaches the harsh environment forces organisms to divert more energy towards main- tenance and they therefore have lower abundance, fecundity, growth and survival rates than in dissipative beaches. Recent investigations have tested this hypoth- esis through single comparisons of only two beaches, and thus the observed trends in population level vari- ables cannot be attributed incontestably to the beach state, but only to location. Here, abundance, reproduc- tion, recruitment, population structure and body size of the intertidal mole crab Emerita brasiliensis were com- pared between populations from eight microtidal ex- posed sandy beaches with contrasting morphodynamics, sampled bimonthly during 22 months throughout the 180 km Uruguayan Atlantic coast. Physical variables and compound indices of the beach state were used to categorize sandy beaches. The results of this bi-annual large-scale analysis were fully consistent with the pre- dictions of the HHH: abundance (total and population components), duration of the reproduction and recruit- ment seasons and the individual size of megalops and females of the mole crab E. brasiliensis decreased from dissipative to reflective beaches. This was reflected by linear or, mostly, nonlinear relationships between bio- logical and both physical variables and compound indices of beach state. In conclusion, this multi-beach sampling provides compelling evidence of a consistent response of demographic and life history traits of an intertidal beach species to morphodynamic characteristics.

Empirical parameterization of setup, swash, and runup

Using shoreline water-level time series collected during 10 dynamically diverse field experiments, an empirical parameterization for extreme runup, defined by the 2% exceedence value, has been developed for use on natural beaches over a wide range of conditions. Runup, the height of discrete water-level maxima, depends on two dynamically different processes; time-averaged wave setup and total swash excursion, each of which is parameterized separately. Setup at the shoreline was best parameterized using a dimensional form of the more common Iribarren-based setup expression that includes foreshore beach slope, offshore wave height, and deep-water wavelength. Significant swash can be decomposed into the incident and infragravity frequency bands. Incident swash is also best parameterized using a dimensional form of the Iribarren-based expression. Infragravity swash is best modeled dimensionally using offshore wave height and wavelength and shows no statistically significant linear dependence on either foreshore or surf-zone slope. On infragravity-dominated dissipative beaches, the magnitudes of both setup and swash, modeling both incident and infragravity frequency components together, are dependent only on offshore wave height and wavelength. Statistics of predicted runup averaged over all sites indicate a − 17 cm bias and an rms error of 38 cm: the mean observed runup elevation for all experiments was 144 cm. On intermediate and reflective beaches with complex foreshore topography, the use of an alongshore-averaged beach slope in practical applications of the runup parameterization may result in a relative runup error equal to 51% of the fractional variability between the measured and the averaged slope.

Comparing Mean High Water and High Water Line Shorelines: Should Proxy-Datum Offsets be Incorporated into Shoreline Change Analysis?

Abstract More than one type of shoreline indicator can be used in shoreline change analyses, and quantifying the effects of this practice on the resulting shoreline change rates is important. Comparison of three high water line (proxy-based) shorelines and a mean high water intercept (datum-based) shoreline collected from simultaneous aerial photographic and lidar surveys of a relatively steep reflective beach (tan β = 0.07), which experiences a moderately energetic wave climate (annual average Hs = 1.2 m), reveals an average horizontal offset of 18.8 m between the two types of shoreline indicators. Vertical offsets are also substantial and are correlated with foreshore beach slope and corresponding variations in wave runup. Incorporating the average horizontal offset into both a short-term, endpoint shoreline change analysis and a long-term, linear regression analysis causes rates to be shifted an average of −0.5 m/y and −0.1 m/y, respectively. The rate shift increases with increasing horizontal offset a...

The Application of Bagnold-Type Sediment Transport Models in the Swash Zone

BUTT, T.; RUSSELL, P.; PULEO, J., and MASSELINK, G., 2005. The application of Bagnold-type sediment transport models in the swash zone. Journal of Coastal Research, 21(5), 887‐895. West Palm Beach (Florida), ISSN 0749-0208. Measurements of cross-shore velocity (u )a tz 5 3 cm and suspended sediment concentration (SSC )a tz 5 1‐33 cm were collected from the swash zones of gently sloping (Perranporth, UK) and steep (Sennen, UK) beaches, during morphodynamic conditions ranging from highly dissipative to almost reflective and breaker heights ranging from 0.8 to 2.2 m. Nine swash-zone data collection runs were used to compare the performance of simplified versions of Bagnoldtype energetics sediment transport models on the two different beaches. Models originally designed for suspended load, where the sediment transport (I) is approximately proportional to u4, showed no apparent improvement over those intended for bedload, where I } u3. It is suggested that the lowest SSC sensors could be measuring high concentrations in the sheet-flow layer, where previous laboratory results suggest a u3 model is most appropriate. The overall performance of the model is better (average r2 5 76%) on the gently sloping beach than on the steep beach (average r2 5 49%). Modifications to account for processes, such as bore turbulence, may improve the model’s performance, particularly on steep reflective beaches.

Geomorphology induces life history changes in invertebrates of sandy beaches: the case of the mole crab Emerita analoga in Chile

The hypothesis that vital functions of one of the most successful macroinvertebrates of sandy beaches of Chile, the mole crab Emerita analoga are negatively affected by the environmental conditions of reflective beaches was tested. Twelve exposed sandy beaches distributed along 200 km of coast of central Chile, and selected to cover a wide morphodynamic spectrum were sampled every two months from April 1997 to November 1998. Significant phenotypic differences among local populations of the sand crab were detected. Important life history traits of the sand crab, linked to growth pattern, reproductive biology and burrowing capacity, were negatively affected by the environmental conditions of reflective beaches. The present results reinforce the view that environmental variations associated with beach morphodynamics, play a critical role in the life history patterns of sandy beach organisms.