Exposed Sandy Beaches Research Articles

Community structure and intertidal zonation of the macroinfauna in intermediate sandy beaches in temperate latitudes: North coast of Spain

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 ( Ω), 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.

The Impacts of Recreational Four-Wheel Driving on the Abundance of the Ghost Crab (Ocypode cordimanus) on a Subtropical Sandy Beach in SE Queensland

There is increasing concern regarding the impacts of recreational four-wheel driving (4WDing) on sandy beach environments. The ghost crab Ocypode cordimanus is a widely distributed Australian species that utilizes beaches and dunes for constructing burrows and for foraging. Comparisons of ghost crab abundances (using burrow counts) in areas “open” and “closed” to recreational 4WDing were conducted on exposed sandy beaches on North Stradbroke Island, off the coast of southeast Queensland. Beaches where recreational 4WD activity is present had significantly lower ghost crab abundances than beaches where it is absent. The most plausible reason for this difference in abundance is that ghost crabs are highly vulnerable to being crushed by beach traffic when feeding on the beach at night. To mitigate the impacts of recreational 4WDing on ghost crab population management intervention is needed. Possible changes to the management of recreational 4WD activity include the setting aside of areas free of recreational 4WD activity for the conservation of biodiversity, or a prohibition on driving on the beaches between dusk and dawn. There is also a need for a consistent and transparent approach in Queensland for quantifying and monitoring 4WD activity in sandy beach environments.

Effects of a freshwater canal discharge on an ovoviviparous isopod inhabiting an exposed sandy beach

The present study evaluates the effects of an artificial freshwater discharge (Canal Andreoni) on the ecology of the ovoviviparous isopod Excirolana armata. Bimonthly, 17 environmental variables plus isopod abundance, biomass, fecundity, growth and mortality were compared between three sites: ‘Barra del Chuy’ (undisturbed), at 13 km from the canal, ‘Coronilla’ (moderately disturbed), at 1 km, and ‘Andreoni’ (grossly disturbed), at the canal mouth. Environmental (salinity, slope, beach width, and swash width) and some biological (isopod abundance, biomass and growth rates) variables significantly decreased towards Canal Andreoni. Salinity was the most important explanatory variable of spatial trends in isopod biomass. However, the reproductive output, fecundity, survival and individual weight were not affected, suggesting that E. armata is regulated by density-dependent and abiotic factors operating together: the former were more intense on undisturbed conditions, whereas the latter prevailed in impacted ones. Internal brooding counteracts the effect of fresh water, which explains the lack of effect of environmental harshness on reproductive traits.

Ecology of exposed sandy beaches in northern Spain: Environmental factors controlling macrofauna communities

Thirty-four exposed sandy beaches on the northern coast of Spain (from 42°11′ to 43°44′N, and from 2°04′ to 8°52′ W; ca. 1000 km) were sampled over a range of beach sizes, beach morphodynamics and exposure rates. Ten equally spaced intertidal shore levels along six replicated transects were sampled at each beach. Sediment and macrofauna samples were collected using corers to a depth of 15 cm. Morphodynamic characteristics such as the beach face slope, wave environment, exposure rates, Dean's parameter and Beach State Index were estimated. Biotic results indicated that in all the beaches the community was dominated by isopods, amphipods and polychaetes, mostly belonging to the detritivorous-opportunistic trophic group. The number of intertidal species ranged from 9 to 31, their density being between 31 and 618 individuals m − 2 , while individuals per linear metre (m − 1 ) ranged from 4962 to 17 2215. The biomass, calculated as total ash-free dry weight (AFDW) varied from 0.027 to 2.412 g m − 2 , and from 3.6 to 266.6 g m − 1 . Multiple regression analysis indicated that number of species significantly increased with proximity to the wind-driven upwelling zone located to the west, i.e., west-coast beaches hosted more species than east-coast beaches. The number of species increased with decreasing mean grain size and increasing beach length. The density of individuals m − 2 increased with decreasing mean grain size, while biomass m − 2 increased with increasing food availability estimated as chlorophyll-a concentration in the water column of the swash zone. Multiple-regression analysis indicated that chlorophyll-a in the water column increased with increasing western longitude. Additional insights provided by single-regression analysis showed a positive relationship between the number of species and chlorophyll-a, while increasing biomass occurred with increasing mean grain size of the beach. The results indicate that community characteristics in the exposed sandy beaches studied are affected by physical characteristics such as sediment size and beach length, but also by other factors dependent on coastal processes, such as food availability in the water column.

Biological impacts of oil pollution and cleaning in the intertidal zone of exposed sandy beaches: Preliminary study of the “Prestige” oil spill

On 13th September 2002, the oil tanker “Prestige” sunk off the Galician coast, causing one of the most important oil spills in history, which affected the entire coastline, particularly the exposed rocky shores and sandy beaches. Seventeen exposed sandy beaches were analysed along the Galician coast, in May 2003, and results were compared with previous data for September 1995 and 1996. The intertidal areas of the beaches were sampled in four tidal zones along the beach profile: swash, resurgence, retention and dry sand. Six cores of 0.05 m 2 were taken at each level and washed through a 1 mm mesh. Sediment samples were collected at each level for sediment analysis. The species were grouped into six main taxonomic groups: polychaetes, molluscs, marine crustaceans, semi-terrestrial crustaceans, insects and others. The total number of species was calculated in each group before and after the oil spill. The disturbance effect on each tidal level was determined. A decrease in the species richness was generally observed in all the studied beaches, although this decrease was not homogeneous in all the taxonomic groups. Polychaetes, insects, semi-terrestrial crustaceans and others lost species in all cases, while marine crustaceans did not show this tendency, losing species in some cases and gaining in others. The most affected beaches lost up to 66.7% of the total species richness after the oil spill. The most disturbed levels were swash, losing most of the polychaetes, and dry sand, with decrease in insects and semi-terrestrial crustaceans in many cases. Dry sand level received a high amount of oil and was more affected by grooming and cleaning activities where fuel and polluted material were removed, including algal wrack that is used by the supratidal macrofauna as food and shelter.

Horizontal and vertical distribution of meiofauna on sandy beaches of the North Sea (The Netherlands, Belgium, France)

Sandy intertidal zones were analysed for the presence of meiofauna. The material was collected on six macro-tidal sandy beaches along the North Sea (The Netherlands, France, Belgium), in order to analyse the vertical and horizontal meiofaunal distribution patterns. Eleven higher meiofauna taxa (one represented by larval stage—Copepoda nauplii) were recorded. The maximum total meiofauna abundance was observed on the Dutch beach (4,295±911 ind. 10 cm−2) in the Westerschelde estuary, while the lowest values (361±128 ind. 10 cm−2) were recorded in France at the Audresselles beach. Meiofauna of the different localities consisted mainly of nematodes, harpacticoids and turbellarians. Nematodes numerically dominated all sampled stations, comprising more than 45% of the total meiofauna density. Meiofauna was mainly concentrated at the sand surface with about 70% present in the uppermost 5 cm. Meiofauna occurred across the entire intertidal zone. A clear zonation pattern in the distribution of meiofauna taxa across the beaches was observed. The present work suggests that designation of exposed sandy beaches as physically controlled (McLachlan 1988) does not explain their biological variability.

Oxygen isotopic evidence for greater seasonality in Holocene shells of Donax variabilis from Florida

Donax variabilis, the variable coquina clam, has been a common inhabitant of exposed sandy beach intertidal and shallow subtidal zones in the southeastern United States throughout the Pleistocene and Holocene. It is ideally suited for paleotemperature studies because it is restricted to environments of well-mixed, normal-marine seawater with a fairly uniform isotopic composition. As a result, oxygen isotopic variability in D. variabilis shells is largely explained by temperature variation. Although D. variabilis is small and short-lived, its shell represents an important paleoclimate archive because of its unique habitat preference. High-resolution sampling of individual D. variabilis shells and comparison of oxygen isotopic temperature profiles with historical seawater temperatures from the northeastern Florida coast indicate rapid shell growth over a brief life span of three to five or six months. Analysis of two modern shells reveals a close correspondence between isotopically determined water temperatures and historical water temperatures during the spring–summer growing season. Paleotemperature profiles from four archaeological shells, however, suggest a longer growth interval spanning summer–autumn. Two Preceramic Archaic Period shells (ca. 4240 and 5570 14C yr BP) and two Orange Period Archaic shells (ca. 3600 and 3760 14C yr BP), from four different archaeological sites, yield paleotemperatures that average 3.5 °C higher than present summer–autumn water temperatures. These warm paleotemperatures highlight seasonality differences associated with the mid-Holocene Hypsithermal climatic interval in this region.

Biodiversity of marine nematodes in Australian sandy beaches from tropical and temperate regions

The universal occurrence and abundance of nematodes provides opportunities to investigate ecological factors that may influence biodiversity. Clarke and Warwick (2001) have proposed diversity indices average taxonomic distance(AvTD), variation in taxonomic distinctness (VarTD) for computing marine nematode biodiversity based on classification trees. Faith [Biological Conservation 61 (1992) 1] had previously proposed a diversity index based on phylogenetic trees, though not applied to nematodes. Clarke and Warwick (2001) also considered an index AvPD analogous to AvTD. These indices have been applied to five very large collections of free-living nematodes from three exposed sandy beaches in Australia. Two were from a beach close to Darwin in northern Australia, two from the temperate southeast coast of Australia and one from the south of the Australian mainland. The collections extend over a considerable range of latitude, from 12°26′S to 38°33′S and the controversial hypothesis that latitudinal gradients per se influence the biodiversity of marine nematode assemblages is examined. AvTD did not vary among collections and its value for any collection was indistinguishable from that of random samples of the same size drawn from the total species pool. VarTD showed no variation for three of the collections, but was slightly higher than expected for the mid-latitude beach, attributed to unevenness in the classification trees. AvTD and VarTD were not greatly affected by differences in sampling intensity. PD varied directly with the number of species found but observed PD did not differ from the PD of random samples of the same number of species taken from the total species pool. Thus, the observed variation in PD is wholly accounted for by variations in species richness. The number of species found increased with decreasing latitude. It appears that species richness by itself is an adequate index of biodiversity for the free-living nematodes of these sandy beaches, and more complex indices such as AvTD, VarTD and PD are unnecessary.

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.

THE ASSOCIATION BETWEEN THE COQUINA CLAM LOVENELLA GRACILIS CLARKE

Abstract The northern coquina clam, Donax fossor, is often found in association with the hydroid Lovenella gracilis in the intertidal zone of exposed sandy beaches of southern New Jersey. Although previous workers have suggested that the Donax-Lovenella relationship is one of commensalism, this assumption rarely has been quantified or experimentally investigated. Our objectives are: (1) to eliminate the possibility that the association is a recent occurrence, (2) to quantify whether the hydroid has any affect on predation rates of the clam, (3) to quantify the effect of the hydroid on the ability of the clam to swash-ride, and (4) to document seasonal variation in the association. Monthly field samples collected at two sites over a 2-y period revealed that D. fossor and L. gracilis are seasonal in occurrence. The proportion of the D. fossor population with an epibiotic hydroid varied among season, clam size, and field site. Hydroid abundance also varied between years, among seasons, and between field site...

Temporal variability of intertidal benthic metabolism under emersed conditions in an exposed sandy beach (Wimereux, eastern English Channel, France)

Benthic community metabolism during emersion was measured in a three-year survey by monitoring CO 2 fluxes in benthic chambers on an exposed sandy beach of the eastern English Channel (Wimereux, France). The three-year chronology of variations in benthic metabolism was characterised by a high variability around a low value for gross community primary production (GCP: 17.47 ± 40.85 mgC m −2 h −1, mean ± SD) and community respiration (CR: 1.66 ± 1.97 mgC m −2 h −1, mean ± SD). Although benthic metabolism remained low most of the time, some high values of primary production and respiration were occasionally detected. High primary production rates (up to 213.94 mgC m −2 h −1 measured at the end of summer) matched with the development of Euglena sp., together with the occurrence of phytoplanktonic species on the sediment, whereas high community respiration rates were detected at the end of spring on Phaeocystis sp. foam deposits. Community respiration was positively correlated with bacterial abundance, suggesting that CR was mainly supported by microfauna.

Projection and local interneurons in the sixth abdominal ganglion of the sand crab Emerita analoga (Hippidae)

Hippid crabs are adapted for life in the surf zone of exposed sandy beaches, and their tailfan differs from the tailfans of other crustaceans with respect to morphology and motor control and in having nonspiking stretch receptors (NSR). To investigate how these crabs' mechanosensory systems are adapted to this turbulent environment, I used axonal back-filling and intracellular recording with dye-filled microelectrodes to describe afferent projections from the telson and morphologies and physiological responses of intersegmental and local neurons in abdominal ganglion 6 (A6) in Emerita analoga, then compared them with descriptions of corresponding neurons in A6 of crayfish. The density of afferent terminals and the proportion of projection neuron somata is lower in anterior A6 of E. analoga than in crayfish, perhaps correlated with a reduction in hydrodynamic setae. Many interneurons responded to afferent nerve stimulation and displayed activity correlated with levels of A6 motor output. NSR stretch depolarizes unilateral local neurons and terminals of axons entering A6 from the connective and hyperpolarizes bilateral local and projection neurons. The timing and duration of this inhibition would suppress mechanosensory input from the telson during uropod beating (homologue of crayfish's nongiant tail-flipping). Suppression of reafference during uropod beating may have been pivotal for evolution of hippids' ability to move rapidly across the water-sand interface in the slosh zone of sandy beaches. Homologies between A6 neurons in E. analoga and crayfish, suggested by morphological and physiological similarities, indicate that the NSRs connect to a neuronal network regulating exteroceptive input that was inherited from their tail-flipping ancestors.

Abundance – occupancy relationships in macrofauna on exposed sandy beaches: patterns and mechanisms

We studied the relationship between abundance and extent of occupancy of 158 species of macrofauna inhabiting 66 sandy beaches around the coast of Great Britain. We also used these data to test the predictions of two hypotheses proposed to explain positive abundance‐occupancy relationships. We found a strong positive relationship between abundance and extent of occupancy; this pattern was apparent in taxonomic subsets of organisms which have contrasting reproductive and dispersal traits such as planktotrophic/lecithotrophic development in the plankton vs brood development under parental care. Moreover, the abundance‐occupancy relationships in these taxonomic subsets had statistically indistinguishable slopes, and elevation. We propose that this lends support to the notion that differences in population structure such as the tendency to form metapopulations may not be primary determinants of the abundance‐occupancy pattern in these taxa as proposed by the rescue/metapopulation hypothesis. To test the predictions of the niche‐breadth hypothesis we derived values describing the range of sediment grain‐sizes exploited by members of two taxonomic subgroups: amphipods and bivalves. We found a weak, statistically non‐significant relationship between this niche‐breadth measure and occupancy in bivalves which have been shown to respond to grain‐size in previous studies, however this was negated after correction for possible artefacts of sampling effort. All other relationships between abundance or occupancy and grain‐size range were non‐significant. The consistency of the demonstrated abundance‐occupancy relationship with those demonstrated in other studies of primarily terrestrial fauna indicates some shared mechanistic explanation, but our data fail to provide support for the two mechanistic hypotheses investigated.

Musculature of the primitive gastrotrich Neodasys (Chaetonotida): functional adaptations to the interstitial environment and phylogenetic significance

Gastrotrichs are acoelomate micrometazoans common to marine interstitial environments of sublittoral sediments and exposed sandy beaches. The genus Neodasys (Chaetonotida) contains three marine species known from oceans and seas worldwide, and figures prominently in discussions of gastrotrich origins. To gain insight into the phylogenetic position of Neodasys and to understand the adaptive significance of muscle anatomy in marine interstitial gastrotrichs, a fluorescent phalloidin-linked marker was used to view the organization of muscles in two species from North America and Australia. Muscular topography of Neodasys cirritus from Florida, USA, and Neodasys cf. uchidai from Queensland, Australia, was found to be similar between species and to basal species of Macrodasyida; muscles were present in circular, longitudinal and helicoidal patterns. Musculature of the midgut region was partially reduced relative to basal macrodasyidan gastrotrichs, but well developed relative to most other chaetonotidan gastrotrichs. In general, muscle patterns in species of Neodasys closely correspond with those of other gastrotrichs of similar size and body type, and may therefore reflect a common adaptive solution to the physical demands of the interstitial environment. Results also suggest that reductions in midgut musculature may be functionally related to oviposition and, as such, are probably not homologous with similar reductions of circular muscles in other species of Chaetonotida.

Community structure of intertidal meiofauna along a gradient of morphodynamic states on an exposed North Sea beach

An exposed sandy beach located at Sylt was studied to analyse the patterns in the number of major taxa, abundance, biomass, and zonation of meiofauna. Three sites with different morphodynamic states were sampled. The results of a canonical correspondence analysis showed that water content, porosity and grain size were the best predictor variables of meiofaunal density. The highest meiofaunal densities and biomass occurred at the mid‐shore levels of each site: up to 9592 individuals 10 cm and 7.5 g m (ash‐free dry weight) at the reflective site, 11,640 individuals 10 cm and 9.4 g m at the low intermediate site, and 2993 individuals 10 cm and 1.5 g m at the mid‐intermediate site. The total abundance and biomass of the meiofauna per running metre of beach were higher at the low intermediate site (141 × 10 individuals m and 98 g m ), compared with the reflective site (74 × 10 individuals m and 55 g m ) and the mid‐intermediate site (69 × 10 individuals m and 33 g m ). The number of major taxa was nine, ten, and eight at the reflective, low intermediate and mid‐intermediate sites, respectively. The across‐shore variability in densities and community assemblage structure of the meiofauna decreased through the dissipative state.

Composition, structure and zonation of intertidal macrofauna in relation to physical factors in microtidal sandy beaches at Río de Janeiro State, Brazil

SUMMARY: The intertidal macroinfauna of fiftheen exposed sandy beaches located in Rio de Janeiro State was studied to determine species richness, density, biomass and zonation. Morphodynamic influence on biological parameters is also evaluated in this paper. Dean’s morphodynamic index (Ω) ranged from reflective to intermediate values and mean grain size ranged from fine to coarse sands. A total of 17 species were identified, with the crustaceans Emerita brasiliensis, Excirolana braziliensis and Pseudorchestoidea brasiliensis being the most abundant species. Species richness had a significant positive linear relationship with Ω values, swash period and slope, and a negative relationship with grain size. Significant positive relationships were also observed for total density with the swash period and slope, total biomass with slope, density of Emerita brasiliensis with swash period and slope and biomass of Emerita brasiliensis with slope. Zonation analysis showed that two or three biological zones could be recognised, with the two-zone pattern being observed in most of the beaches. This pattern consisted of a lower zone characterised by Emerita brasiliensis, Donax haleyanus and Hemipodus oliveri; and an upper zone characterised by Excirolana braziliensis, Pseudorchestoidea brasiliensis and Phaleria testacea. The threezone pattern as a disruption of the general two-zone pattern and its formation was attributable to the variation in the distributions of P. brasilensis and E. braziliensis. Our results suggest that beach morphodynamics is the best predictor for the variability of macroinfaunal community composition.

The response of macrofauna communities and shorebirds to macrophyte wrack subsidies on exposed sandy beaches of southern California

To investigate the influence of marine macrophyte wrack subsidies on community structure, relationships between community attributes, including species richness, abundance, and biomass of macrofauna and abundance of shorebirds, and a variety of factors, including the standing crop of wrack and beach morphodynamics, were examined on 15 exposed sandy beaches on the southern California coast. The beaches sampled were primarily modally intermediate morphodynamic types, and three were groomed regularly. Species richness, abundance, and biomass of the macrofauna were high compared to values reported for similar beach types in other regions and were not predicted by morphodynamics or other physical factors. Overall species richness and abundance, and the species richness, abundance, and biomass of wrack-associated fauna and selected taxa were significantly correlated with the standing crop of macrophyte wrack. Wrack-associated macrofauna, such as amphipods, isopods, and insects, made up an average of >37% of the species on ungroomed beaches and comprised 25% or more of the total abundance on half of those beaches. The abundance of two shorebird species, plovers that forage using visual cues, was positively correlated with the standing crop of wrack and with the abundance of wrack-associated invertebrates. Significant differences in community structure, including depressed species richness, abundance, and biomass of macrofauna, especially for wrack-associated taxa, were associated with beach grooming and provided strong evidence for the bottom-up effects of wrack subsidies. Grooming also reduced the prey available to vertebrate predators, such as shorebirds. Substantial ecological effects of the large-scale disturbance and removal of organic material, food resources, and habitat are associated with beach grooming. These results suggest that macrophyte wrack subsidies strongly influence macrofaunal community structure, higher trophic levels, and ecological processes on exposed sandy beaches. The supply of macrophyte wrack should be considered as a factor in ecological studies of exposed sandy beaches, particularly where macrophyte production is high.

Population structure, growth and production of the surf clam Donax serra (Bivalvia, Donacidae) on two Namibian sandy beaches

Population structure, growth and production of the surf clam Donax serra (Bivalvia, Donacidae), inhabiting highly exposed sandy beaches of Namibia, were investigated between November 1997 and December 1999. From length–frequency distribution and tagging–recapture data, a von Bertalanffy growth function with an asymptotic length ( L ∞ ) of 82 mm and a growth constant ( K) of 0.274 yr −1 was established. Regarding growth performance of Donacidae, D. serra fits in a group of species inhabiting cold temperate and upwelling regions. The intertidal biomass of the studied population ranged between 141 and 546 g ash-free dry mass (AFDM) m −2 yr −1. Individual production was maximal at 56.5 mm shell length (0.83 g AFDM ind. −1 yr −1), and annual production ranged between 167 and 637 g AFDM m −2 yr −1, resulting in productivity values (P/ B ̄ ) between 1.167 and 1.589 yr −1. These data underline the importance of D. serra for the beach/surf ecosystem. Further, the findings of this study are crucial to support future aquaculture or exploitation activities and management.

Shorebird use of an exposed sandy beach in southern California

Frequent morning surveys of birds were conducted on 1 km of beach in southern California to investigate shorebird use of an exposed sandy beach. The overall mean abundance (98.6 individuals km −1), estimated biomass (9.6 kg km −1), and species richness (5.5 species km −1) of shorebirds observed were very high for a sandy beach in the temperate zone. Eight species, sanderling ( Calidris alba), semipalmated plover ( Charadrius semipalmatus), marbled godwit ( Limosa fedoa), black-bellied plover ( Pluvialis squatarola), western sandpiper ( Calidris mauri), willet ( Catoptrophorus semipalmatus), surfbird ( Aphriza virgata), and whimbrel ( Numenius phaeopus), occurred in overall mean abundances >1 bird km −1 and accounted for 97% of the abundance and biomass of shorebirds. Sanderlings were the most abundant shorebird every year (64% of individuals and 35% of the biomass). Different species of abundant shorebirds exhibited distinct patterns of use of beach habitat, including fall, spring, and winter peaks in abundance. Temporal variation in shorebird use on seasonal and interannual scales was associated with migration patterns, and also with habitat availability and condition. Seasonal variation in monthly mean abundance and estimated biomass of shorebirds varied over more than an order of magnitude and followed a similar pattern in each year, reaching maxima in the fall or winter (161–280 individuals km −1 and 15.4–23.9 kg km −1) and minima in May or June (3–11 individuals km −1 and 0.8–2.2 kg km −1). A minor peak in shorebird abundance and biomass coinciding with spring migration was observed in April of most years. The number of species of shorebirds observed in individual surveys ranged from 0 to 11 species km −1 and was positively and significantly correlated with abundance. Monthly mean species richness and the total species observed monthly followed similar seasonal patterns, ranging from annual maxima of 7.4–9.1 and 12–17 species km −1 between August and October to minima of 0.8–2.1 and 2–8 species km −1, respectively, during June. In contrast, species turnover was lowest (1.1–1.7) in October and November, and generally highest (2–4) during early summer (June). The amount of sandy intertidal habitat available to shorebirds on the transect was estimated using sand elevations and predicted tide heights. In the fall and winter, the abundance of shorebirds was significantly and positively correlated with tide height, possibly reflecting feeding opportunities and high tide refuge effects during the highest tides. In the spring when sand levels were low, the abundance of shorebirds was negatively correlated with tide height. Prey availability, beach condition and the local availability, and condition of alternative foraging habitats may influence those relationships. Interannual variations in shorebird use and beach condition were observed in the course of the study. During an El Nino Southern Oscillation (ENSO) event (1997–1998), the extent of sandy habitat was greatly reduced and intertidal habitat was mostly converted to rocky substrate. The overall abundance of shorebirds and the mean abundance of some common species (e.g. sanderling) were depressed, and an uncommon species (surfbird, A. virgata) was unusually abundant during the ENSO event. In summary, the results suggest that sandy beaches are important habitat for many species of shorebirds, particularly in areas where alternative coastal foraging habitats, such as coastal wetlands, have become scarce. Understanding the dynamics of and threats to exposed sandy beaches may be increasingly important for shorebird conservation in many coastal regions.

The relative accuracy of standard estimators for macrofaunal abundance and species richness derived from selected intertidal transect designs used to sample exposed sandy beaches

In order to ensure that patterns detected in field samples reflect real ecological processes rather than methodological idiosyncrasies, it is important that researchers attempt to understand the consequences of the sampling and analytical designs that they select. This is especially true for sandy beach ecology, which has lagged somewhat behind ecological studies of other intertidal habitats. This paper investigates the performance of routine estimators of macrofaunal abundance and species richness, which are variables that have been widely used to infer predictable patterns of biodiversity across a gradient of beach types. To do this, a total of six shore-normal strip transects were sampled on three exposed, oceanic sandy beaches in the Eastern Cape, South Africa. These transects comprised contiguous quadrats arranged linearly between the spring high and low water marks. Using simple Monte Carlo simulation techniques, data collected from the strip transects were used to assess the accuracy of parameter estimates from different sampling strategies relative to their true values (macrofaunal abundance ranged 595–1369 individuals transect −1; species richness ranged 12–21 species transect −1). Results indicated that estimates from the various transect methods performed in a similar manner both within beaches and among beaches. Estimates for macrofaunal abundance tended to be negatively biased, especially at levels of sampling effort most commonly reported in the literature, and accuracy decreased with decreasing sampling effort. By the same token, estimates for species richness were always negatively biased and were also characterised by low precision. Furthermore, triplicate transects comprising a sampled area in the region of 4 m 2 (as has been previously recommended) are expected to miss more than 30% of the species that occur on the transect. Surprisingly, for both macrofaunal abundance and species richness, estimates based on data from transects sampling quadrats at 3 m intervals across the intertidal outperformed those based on data from transects sampled at either 2 or 4 m intervals, instead being more similar to those from transects sampled at 1 m intervals (the highest level of sampling effort considered). This suggests that across-shore patches of organisms might be distributed in response to some process acting at spatial scales close to 3 m. Together these results provide a compelling argument against combining data from surveys employing different sampling regimes. They also indicate a need for increased sampling effort, improved estimation techniques or both. Considering the limitations imposed on sandy beach ecology, improved estimation seems most reasonable, with emerging techniques such as geostatistics, extrapolative estimates of species richness and general additive models representing areas of potential progress.