Seagrass Assemblages Research Articles

Observations of the association by early-juvenile western rock lobsterPanulirus cygnuswith seagrass assemblages (Decapoda: Achelata: Palinuridae)

AbstractThe fishery of the western rock lobster, Panulirus cygnusGeorge, 1962, is Australia’s most valuable wild-caught single-species fishery. Recruitment in some regions of the fishery was observed to be significantly lower than expected after the 2010/2011 West Australian marine heatwave that caused extensive disturbance of dominant coastal habitats. This event generated interest in the study of the factors influencing survival and recruitment of post-larval benthic P. cygnus after settlement. The habitat associations of the highly cryptic post-settlement early-juveniles were previously unknown, with only anecdotal observations of individuals within limestone crevices in nearshore habitats. Our study used early-juveniles derived from ongoing monitoring of puerulus settlement to examine their habitat association mechanism in mesocosm experiments. Comparison of common nearshore habitat assemblages (bare sand, limestone crevices, and seagrasses (Posidonia and Amphibolis) at varying seagrass densities) found that most early-juveniles associated strongly with Amphibolis assemblages at high stem densities (~2,100 stems m–2). A shift in association between Amphibolis fronds and stems at high stem density to Amphibolis-shaded sand and leaf debris at low stem density indicated active habitat selection by early-juveniles. Habitat choices were tested with the scents of prey items and habitat types within Amphibolis assemblages using Y-maze bioassays. No significant olfactory choices were found, suggesting that habitat associations may be driven by multiple cues. Our study provides new laboratory-based insights into the habitat association of early-juvenile P. cygnus and suggests changes in seagrass assemblage identity and density are likely to be important. Further experimentation is needed to define the cues driving these patterns. The impact of habitat change on recruitment in this important fishery remains unknown and should be an objective of future research.

Heterogeneity of Fish Taxonomic and Functional Diversity Evaluated by eDNA and Gillnet along a Mangrove-Seagrass-Coral Reef Continuum.

The effective and reliable monitoring of fish communities is important for the management and protection of marine ecosystems. Environmental DNA (eDNA) metabarcoding is a relatively new method that has been widely used in recent years, while traditional sampling via fish catching (i.e., gillnets) is one of the most common and reliable fish monitoring methods used to date. We compared the taxonomic and functional diversity of fish detected within a mangrove-seagrass-coral reef continuum using both survey methods. One liter seawater and gillnet samples were collected in August 2021 from mangrove forests, seagrass meadows and coral reef habitats (n = 3 each) in Hainan, China. Surveys using eDNA and gillnets identified 139 genera belonging to 66 families and 58 genera belonging to 42 families, respectively. Regardless of the survey method, fish detected in mangrove, seagrass and coral reef habitats were heterogeneous in their communities; however, the shared species between habitats suggest some degree of connectivity. There were no significant differences between habitats in terms of taxonomic and functional diversity, but a higher taxonomic diversity was detected using eDNA. Both methods were able to distinguish fish assemblages between different habitats; however, gillnet surveys performed better than eDNA surveys for distinguishing mangrove from seagrass assemblages. Therefore, the concurrent use of eDNA and gillnet survey methods provides a more comprehensive approach to understanding the heterogeneity of fish taxonomic and functional diversity along mangrove-seagrass-coral reef continuums.

Pecora 22: Remote Sensing for Freshwater and Marine Environments

Pecora 22: Remote Sensing for Freshwater and Marine Environments

Contrasting drivers of sponge and seagrass assemblage composition in an Indo-Pacific seagrass meadow

AbstractSponges are major components of benthic marine ecosystems across the world. However, despite seagrasses being one of the key ecosystems in tropical environments, their associated sponge assemblages have been poorly studied. In this study we investigated seagrass and sponge assemblage composition in an Indo-Pacific seagrass meadow located surrounding Hoga Island in the Wakatobi National Park, South-east Sulawesi, Indonesia. We examined a continuous seagrass meadow, spatially separated into two sites, categorized into three zones based on the distance to the shore: high-shore, middle shore and near-reef-flat zones. We investigated the species richness, composition and abundance of both seagrass and sponge species across zones and sites. We found that the sponge assemblage composition was significantly different between sites and zones, while the seagrass assemblage was not. This suggests that the seagrass and sponge assemblages have different ecological drivers. Based on our observations, we propose that sponge adaptation to the prevailing environmental conditions in different seagrass zones may be reflected by sponge morphology. Furthermore, seagrass ecosystem management strategies should consider the different drivers of seagrass and sponge distribution patterns as they might be differentially affected by specific anthropogenic stressors.

Chemotaxis is important for fine scale habitat selection of early juvenile Panulirus cygnus

The western rock lobster (Panulirus cygnus [George 1962]) is the basis for Australia's most valuable single species wild caught fishery. The ability of early juvenile western rock lobster to select and settle onto suitable habitats is critical for the recruitment of this commercially important species. Many marine organisms, including the western rock lobster, have highly complex olfactory systems that allow for various behaviours including habitat selection and the location of food. We tested the active habitat selection of early juvenile western rock lobsters using only chemotaxis in scent based trials. We used a Y-maze bio-assay with which we were able to present individuals with several different natural scent stimuli. This study used three commonly observed broad habitat complexes followed by three fine scale seagrass assemblages to provide different scent stimuli. Seagrass habitats were chosen significantly more often by the juvenile lobsters in the broad habitat scale trials over bare sand and turf habitats. At a finer scale, lobsters showed a mixed response but tended to choose the canopy forming seagrass species, Amphibolis antarctica [(Labillardière) Sonder & Ascherson ex Ascherson, 1868] and Posidonia australis [Hooker, 1858] significantly more often than the other scents provided. This study developed a Y-maze chemotaxis assay for P. cygnus and confirmed that they respond to habitat related scent cues. These findings suggest the importance of seagrass habitats to early juvenile western rock lobsters and the potential impacts that habitat change may have on the successful recruitment of this commercially important species.

Seagrass structural and elemental indicators reveal high nutrient availability within a tropical lagoon in Panama

Seagrass meadows are valued coastal habitats that provide ecological and economic benefits around the world. Despite their importance, many meadows are in decline, driven by a variety of anthropogenic impacts. While these declines have been well documented in some regions, other locations (particularly within the tropics) lack long-term monitoring programs needed to resolve seagrass trends over time. Effective and spatially-expansive monitoring within under-represented regions is critical to provide an accurate perspective on seagrass status and trends. We present a comprehensive dataset on seagrass coverage and composition across 24 sites in Bahía Almirante, a lagoon along the Caribbean coast of Panama. Using a single survey, we focus on capturing spatial variation in seagrass physical and elemental characteristics and provide data on key seagrass bio-indicators, such as leaf morphology (length and width), elemental content (% nitrogen and phosphorus) and stable isotopic signatures (δ13C and δ15N). We further explore relationships between these variables and water depth (proxy for light availability) and proximity to shore (proxy for terrestrial inputs). The seagrass assemblage was mostly monospecific (dominated by Thalassia testudinum) and restricted to shallow water (<3 m). Above-ground biomass varied widely, averaging 71.7 g dry mass m−2, yet ranging from 24.8 to 139.6 g dry mass m−2. Leaf nitrogen content averaged 2.2%, ranging from 1.76 to 2.57%, while phosphorus content averaged 0.19% and ranged from 0.15 to 0.23%. These values were high compared to other published reports for T. testudinum, indicating elevated nutrient availability within the lagoon. Seagrass stable isotopic characteristics varied slightly and were comparable with other published values. Leaf carbon signatures (δ13C) ranged from −11.74 to −6.70‰ and were positively correlated to shoreline proximity, suggesting a contribution of terrestrial carbon to seagrass biomass. Leaf nitrogen signatures (δ15N) ranged from −1.75 to 3.15‰ and showed no correlation with shoreline proximity, suggesting that N sources within the bay were not dominated by localized point-source discharge of treated sewage. Correlations between other seagrass bio-indicators and environmental metrics were mixed: seagrass cover declined with depth, while biomass was negatively correlated with N, indicating that light and nutrient availability may jointly regulate seagrass cover and biomass. Our work documents the response of seagrass in Bahía Almirante to light and nutrient availability and highlights the eutrophic status of this bay. Using the broad spatial coverage of our survey as a baseline, we suggest the future implementation of a continuous and spatially expansive seagrass monitoring program within this region to assess the health of these important systems subject to global and local stressors.

Additional new records of Caulerpa cylindracea Sonder 1845 along the West Algerian Coasts

Since considered as invasive species, Caulerpa cylindracea Sonder 1845 has been the subject of numerous prospecting works aiming its localisation and identification along the western Mediterranean Coasts. In this study, we provide new occurrence of the introduced seaweed Caulerpa cylindracea in the West Algerian coasts. C. cylindracea has been reported for the first time in Canastel and Zimba (West Algerian coasts) at different density distribution levels. The colonisation level was estimated to Level (I) for Zimba, Canastel and Marsat El Hadjaj sites, which indicated a beginning of spreading. By contrast, in Cap Carbon a Level (II) was recorded suggesting an advanced colonisation of the invasive taxon. Macroalgae and seagrass assemblage characterisation showed low cover for all species in the invasion zone. Accordingly, Caulerpa cylindracea is more present in Cap Carbon than in Zimba, Canastel and Marsat El Hadjaj. Considering the observed abundance of macroalgae and seagrasses, Caulerpa prolifera seemed to resist the invasion of C. cylindracea in Marsat El Hadjaj.

The condition and composition of seagrass and mollusca on Biak island, Papua

This research was purposed to assess coverage and species distribution of seagrass as well as mollusca in Biak Island, Biak Numfor Regency, Papua. Field sampling was conducted in February, March and September 2017 at 18 observation stations. Eight seagrass species were recorded during the sampling period namely Thalassia hemprichii, Enhalus acoroides, Halodule uninervis, Halodule pinifolia, Syiringodium isoetifolium, Cymodocea rotundata, C. serrulata, and Halophila ovalis. The highest coverage was T. hemprichii, followed by C. rotundata and the least was C. serrulata. The range of seagrass coverage from 5.11% (St.1) to 75.76% (St.17) with the average of coverage in Biak was 35.7%. Furthermore, there were 4 groups based on Bray-Curtis similarity according to the seagrass assemblage in the research area. In addition, a total of 156 molluscan species were recorded consisted of 123 species from gastropod and 33 species from bivalvia. The most distributed species from gastropod were Conomurex luhuanus and Canarium urceus, while from bivalvia was Anadara antiquata. Station 15 has the highest species number with 33 species and the lowest, with just 9 species, were found at station 1 and 7.

Patterns of distribution and drivers of change in shallow seagrass and algal assemblages of a non-estuarine Southern Caribbean mangrove lagoon

Shallow marine macrophyte communities serve key roles in the tropical coastal ecosystem but are undergoing large and rapid deterioration worldwide, as is also the case in the non-estuarine mangrove lagoon of Lac Bay, Bonaire, in the Southern Caribbean. To help better understand both the drivers of assemblage structure and potential consequences of the changes taking place in the bay, we here quantify and describe the distribution of algal and seagrass assemblages along the environmental gradient from the turbid, inner mangrove pools to the clear, open bay conditions, based on 98 randomly-chosen, 4 m2 survey plots. Seven assemblages were described along this land-to-sea gradient, five of which were dominated by marine macrophytes, one by sponges and one by a polychaete. With exception of the hypersaline backwaters which were devoid of benthic macrophyte vegetation, isolated mangrove pools showed the lowest total benthic cover, species richness and biodiversity of all habitats. Salinity and substrate particle-size composition accounted for most variation between the different assemblages and appear to be the key known determinants of assemblage composition. We developed a conceptual model to help disentangle the relationship between and the relative roles of the two principal drivers, as part of a cascade of effects which ultimately result from terrestrial run-off into the bay as mediated by mangrove encroachment into the bay. The model links spatial patterns to ongoing processes and implies that the assemblage patterns described are not only a reflection of, but also allow prediction of how the assemblages develop through time.

Spatial structure of a multi-species guild: the dominant biofilm-grazing microgastropods of seagrass

Although all-too-often ignored ecologically, gastropods < 5 mm in largest dimension may be dominant aquatic assemblage components. In Moreton Bay, Australia, for example, intertidal seagrass supports a guild of 24 such biofilm-grazing species at mean density one-third that of the whole macrobenthic seagrass assemblage. Detailed spatial patterns of the 14 guild members at one c. 1 ha locality were investigated via a 256-station lattice. Relative importance of the guild within the macrobenthic assemblage as a whole was spatially uniform but its abundance was significantly patchy at all spatial scales—a few stations supporting up to 170 times background levels of some species—and guild patchiness showed a relatively constant magnitude across those scales. Patches of individual species were independently distributed, showing no evidence of negative inter-specific interactions, the few significant spatial correlations between species being very weak and positive. Levels of syntopy (up to six species within a 0.0054 m2 area) did not differ from those expected under null models of independent assortment. Although diverse, overall guild abundance was low, appearing well below potential carrying capacity, and dominated by few species. Power–law relationships suggested temporal stability of these patterns. Possible causes of such guild structure are discussed.

Contrasting impacts of light reduction on sediment biogeochemistry in deep- and shallow-water tropical seagrass assemblages (Green Island, Great Barrier Reef)

Seagrass meadows increasingly face reduced light availability as a consequence of coastal development, eutrophication, and climate-driven increases in rainfall leading to turbidity plumes. We examined the impact of reduced light on above-ground seagrass biomass and sediment biogeochemistry in tropical shallow- (∼2 m) and deep-water (∼17 m) seagrass meadows (Green Island, Australia). Artificial shading (transmitting ∼10-25% of incident solar irradiance) was applied to the shallow- and deep-water sites for up to two weeks. While above-ground biomass was unchanged, higher diffusive O2 uptake (DOU) rates, lower O2 penetration depths, and higher volume-specific O2 consumption (R) rates were found in seagrass-vegetated sediments as compared to adjacent bare sand (control) areas at the shallow-water sites. In contrast, deep-water sediment characteristics did not differ between bare sand and vegetated sites. At the vegetated shallow-water site, shading resulted in significantly lower hydrogen sulphide (H2S) levels in the sediment. No shading effects were found on sediment biogeochemistry at the deep-water site. Overall, our results show that the sediment biogeochemistry of shallow-water (Halodule uninervis, Syringodium isoetifolium, Cymodocea rotundata and C. serrulata) and deep-water (Halophila decipiens) seagrass meadows with different species differ in response to reduced light. The light-driven dynamics of the sediment biogeochemistry at the shallow-water site could suggest the presence of a microbial consortium, which might be stimulated by photosynthetically produced exudates from the seagrass, which becomes limited due to lower seagrass photosynthesis under shaded conditions.

Determining light stress responses for a tropical multi-species seagrass assemblage

Existing mitigations to address deterioration in water clarity associated with human activities are based on responses from single seagrass species but may not be appropriate for diverse seagrass assemblages common to tropical waters. We present findings from a light experiment designed to determine the effects of magnitude and duration of low light on a mixed tropical seagrass assemblage. Mixed assemblages of three commonly co-occurring Indo-West Pacific seagrasses, Cymodocea serrulata, Halodule uninervis and Halophila ovalis were grown in climate-controlled tanks, where replicate pots were subjected to a gradient in light availability (0.9–21.6 mols PAR m−2 day−1) for 12 weeks. Increased shading resulted in declines in growth and changes in cellular and photosynthesis responses for all species, although time-scale and magnitude of response were species-specific. Applying management criteria (e.g. thresholds) relevant to one species may under- or over-estimate potential for impact on other species and the meadow as a whole.

Plant genotype identity and diversity interact with mesograzer species diversity to influence detrital consumption in eelgrass meadows

Seagrass meadows are among the world's most productive ecosystems, and as in many other systems, genetic diversity is correlated with increased production. However, only a small fraction of seagrass production is directly consumed, and instead much of the secondary production is fueled by the detrital food web. Here, we study the roles of plant genetic diversity and grazer species diversity on detrital consumption in California eelgrass Zostera marina meadows. We used three common mesograzers—an amphipod, Ampithoe lacertosa, an isopod, Idotea resecata, and a polychaete, Platynereis bicanaliculata. Each grazer consumed eelgrass detritus at rates greater than live eelgrass or macroalgae. This detrital consumption, however, was not spread evenly over leaves shed from different eelgrass clones. Palatability and consumption varied because of genotype specific differences in leaf texture, secondary metabolites (phenolics), and nutritional quality (nitrogen). Further, detritus derived from some eelgrass genotypes was palatable to all grazers, while detritus from other genotypes was preferentially consumed by only one grazer species. Under monospecific grazer assemblages, plant genetic identity but not diversity influenced detritus consumption. However, more realistic, diverse mesoconsumer communities combined with high plant‐detrital genotypic diversity resulted in greater consumption and grazer survival. These results provide a mechanism for field observations of increased mesograzer density and diversity in genetically diverse seagrass assemblages and offer a potential explanation for variation in results of resource diversity– detrital processing experiments in the literature, which often exclude macroinvertebrate taxa. More broadly, our findings support the emerging principle that biodiversity effects are strongest when diversity in both consumer and resource taxa are present.

Effects of Zostera marina on the patterns of spatial distribution of sediments and macrozoobenthos in the boreal lagoon of Furen (Hokkaido, Japan)

Understanding patterns of spatial variations in benthic seagrass assemblages is a central issue in seagrass ecology. However, how patterns of spatial variations in macrozoobenthos and associated sediments differ between vegetated and unvegetated areas remain largely unexplored. In the present study, two different habitats represented by dense Zostera marina beds (Zostera) and unvegetated sediments (Bare) were compared at three locations, 100's meters apart, located at progressive distance from the Furen river in the boreal lagoon of Furen (Hokkaido, Japan). We tested the hypothesis that Z. marina influences the patterns of spatial distribution of abiotic and biotic components along an environmental (estuarine) gradient. The results showed considerable differences between Zostera and Bare, as well as between and within locations, in the distribution of both sediment variables (mud, total organic carbon [TOC] and total nitrogen, acid volatile sulfide, chlorophyll-a and pheopigments) and macrozoobenthic assemblage metrics (total number of species [S], Shannon-Weiner diversity index [H′], total abundance and abundance of dominant species). TOC content, associated to a high mud content, was highest in Bare irrespective of differences between locations (all being above a critical TOC threshold of 3.6%), while S and H′ were higher in Zostera than in Bare at all locations. Significant location x habitat effects were found in the abundance of dominant species, represented mainly by mollusks and crustaceans. Furthermore, the proportions of spatial variance were greater at the scale of replicates (meters apart) than at the scale of stations (10's meters apart) for both sediment variables and the dominant species. Importantly, for the dominant species the spatial variance at the smaller scale was much higher in Zostera than in Bare, indicating that at the scale of meters Zostera beds increase the patchiness in the spatial distribution of individuals compared to bare sediments. Overall, our results demonstrate that Z. marina has a strong effect on the spatial heterogeneity in the intensity of the ecological processes influencing patterns of sediment and macrozoobenthos distribution along an environmental gradient. The present study provides a general framework to evaluate patterns of spatial distribution across various scales within several hundreds of meters in seagrass-dominated, eutrophic coastal lagoons.

Unexpected resilience of a seagrass system exposed to global stressors.

Despite a growing interest in identifying tipping points in response to environmental change, our understanding of the ecological mechanisms underlying nonlinear ecosystem dynamics is limited. Ecosystems governed by strong species interactions can provide important insight into how nonlinear relationships between organisms and their environment propagate through ecosystems, and the potential for environmentally mediated species interactions to drive or protect against sudden ecosystem shifts. Here, we experimentally determine the functional relationships (i.e., the shapes of the relationships between predictor and response variables) of a seagrass assemblage with well-defined species interactions to ocean acidification (enrichment of CO2 ) in isolation and in combination with nutrient loading. We demonstrate that the effect of ocean acidification on grazer biomass (Phyllaplysia taylori and Idotea resecata) was quadratic, with the peak of grazer biomass at mid-pH levels. Algal grazing was negatively affected by nutrients, potentially due to low grazer affinity for macroalgae (Ulva intestinalis), as recruitment of both macroalgae and diatoms were favored in elevated nutrient conditions. This led to an exponential increase in macroalgal and epiphyte biomass with ocean acidification, regardless of nutrient concentration. When left unchecked, algae can cause declines in seagrass productivity and persistence through shading and competition. Despite quadratic and exponential functional relationships to stressors that could cause a nonlinear decrease in seagrass biomass, productivity of our model seagrass-the eelgrass (Zostera marina)- remained highly resilient to increasing acidification. These results suggest that important species interactions governing ecosystem dynamics may shift with environmental change, and ecosystem state may be decoupled from ecological responses at lower levels of organization.

Constancy despite variability: Local and regional macrofaunal diversity in intertidal seagrass beds

The importance of seagrass habitat for the diversity of benthic fauna has been extensively studied worldwide. Most of the information available is, however, about α diversity while little consideration has been given to β diversity. To fill the knowledge gaps regarding the variability of epifaunal and infaunal seagrass assemblages at large spatial and temporal scales, we scrutinized an extensive dataset covering five years of monitoring of eight intertidal Zostera marina meadows around Brittany (France). High species richness arose at the regional scale from the combination of high local diversity of the meadows and substantial among-meadows β diversity. Epifauna and infauna appeared as distinct self-communities as they displayed different spatial and temporal patterns and varied in their responses to local hydrological conditions. Infauna had higher total β diversity than epifauna due to a tighter link to the great variability of local environmental conditions in the region. Both exhibited substantial variations in species composition and community structure with variations of dominant species that were accompanied by extensive change in numerous rare species. The dominant epifaunal species were all grazers. Changes in species composition were induced mostly by species replacement and rarely by richness differences between meadows. Indeed, species richness remained within a narrow range for all seagrass beds, suggesting a potential carrying capacity for species richness of the meadows. Overall, all meadows contributed equally to the regional turnover of seagrass macrofauna, emphasizing high variability and complementarity among beds at the regional scale. The implications of this substantial within-seagrass variability for the functioning of benthic ecosystems at broad scale and for conservation purposes in habitat mosaics warrant further investigations but our results clearly advocate taking into account within-habitat variation when evaluating the diversity of benthic habitats and the potential effect of habitat loss.

Biodiversity differentials between the numerically-dominant macrobenthos of seagrass and adjacent unvegetated sediment in the absence of sandflat bioturbation

Species composition, abundance and biodiversity of the South African estuarine invertebrate fauna are known to show marked differentials between seagrass beds and adjacent unvegetated sands in enclosed estuarine/marine bays. Such differentials were investigated at four disparate localities in a bay lacking the callianassid bioturbation of other local sites. Here there were no such marked or consistent differences: <50% of differentials were statistically significant, with seagrass showing the lower, not higher, level in half of those. Overall, faunal abundance was lower in seagrass in the ratio of 0.64 : 1, whilst species density was higher but only by 1.13 to 1. Seagrass assemblages at a given locality were more similar to those of the adjacent bare sand than they were to seagrass assemblages at other localities, and likewise in respect of those in the bare sand. This suggests that marked differentials, where they occur, may result not from any supposed favourability of seagrass as a habitat but from the operation of processes within the unvegetated-sediment compartment of the system.

Watershed discharge patterns, secondary consumer abundances, and seagrass habitat condition in Yap, Micronesia

Watershed discharge, water quality, and seagrass assemblages were examined along the western coast of Yap Proper, Micronesia. Submarine groundwater discharge (SGD) during low tides associated with new and full moons contributed disproportionally to freshwater delivery where compromised Thalassia habitats existed. Despite SGD influence, nutrient sampling indicated that one characteristic regime may be a net import of new nitrogen and phosphorous (NO3 and PO4) from offshore to inshore waters, agreeing with sparse watershed development. Biologically recycled nitrogen (NH4), however, was highest where SGD contribution was largest. Time-and-tide-limited sampling likely precluded generalized relationships between SGD and NH4 across the entire study area, however, spatial profiling of SGD during low-tide events (i.e., a proxy to nutrient input) was strongly associated with seagrass habitat condition (defined within). Concomitantly, sea cucumber densities were over a magnitude of order lower than in regionally comparable Thalassia habitats, and negatively correlated with seagrass condition. Both top-down and bottom-up considerations are discussed.

Shore height and differentials between macrobenthic assemblages in vegetated and unvegetated areas of an intertidal sandflat

Intertidal macrobenthic faunal assemblages of a dual seagrass/callianassid-structured sandflat system were investigated in subtropical Moreton Bay, Queensland. Consistently across all 20 stations, the gastropod-dominated seagrass supported greater abundance (2.5×) and species richness (2×) than the amphipod-dominated sandflat. There was no evidence of along-shore or up-shore variation in the overall assemblage properties such as total abundance, species richness or diversity within either habitat type, except for variation in sandflat abundance between sites. But seagrass and sandflat assemblages both varied significantly in composition from site to site, and seagrass assemblage composition also varied with shore height. Shore height and site, however, only accounted for ≤41% of total variation. The two faunal assemblages showed a Bray–Curtis dissimilarity of 97.7% and within-habitat similarities of <20%. There was no consistency in distribution of greater diversity, dominance or evenness. No differential between any assemblage features in adjacent sandflat and seagrass samples changed with shore height, supporting hypotheses that such differentials are not maintained by predation. Macrofaunal species richness and diversity were closely coupled within sandflat stations but were uncoupled within seagrass ones, questioning the value of diversity as a comparative measure.

The critical scale of small-scale spatial variation in ecological patterns and processes in intertidal macrobenthic seagrass assemblages

Earlier work on benthic faunal assemblages in intertidal seagrass beds has shown maximal structural variation at spatial scales <150 m. To examine more closely spatial variation of species assemblages within this range, numbers of target taxa in the beds on North Stradbroke Island, Moreton Bay, Australia, were sampled at nested spatial scales of 0.5, 5, 30 and 90 m at two localities 900 m apart. Variance components were largest at the smallest scales, that at 0.5 m comprising 55% of the total and that at 5 m 13%. Individual species were patchily dispersed and their assemblages were randomly structured at all studied spatial scales. Although assemblage composition differed significantly between the two localities, overall faunal abundance and species richness were relatively uniform across the whole study area. These seagrass beds show little turn-over of dominant species and little evidence of niche partitioning through space over scales of <1 km. In this they contrast markedly with the equivalent intertidal dwarf-eelgrass beds of Knysna in South Africa that form a high turn-over, strongly structured, deterministic system.