Artificial Seagrass Research Articles

Flow in seagrass canopies: The influence of patch width

Seagrass beds and the communities they form are well known for their ability to alter their local hydrodynamic environment, reducing current velocities and altering turbulent structure in and around the canopy. Much of the quantitative information that has been published on the interaction of seagrass canopies with flowing water has been derived from laboratory flume studies. The few studies that have been conducted all point to similar patterns of flow alteration around the seagrass canopy. Differences among the results of the study are likely primarily due to different experimental configurations. Some studies have used seagrass beds much narrower than the width of the flume while others have used seagrass beds extending the full width of the flume. The validity of the latter design has often been called into question because of scaling issues. In this study, artificial seagrass was used to examine the effects of bed width in a laboratory flume on the spatial pattern of water velocity and turbulence intensity within the bed. As seagrass bed width was increased, blocking more of the cross-sectional area of the flume, the seagrass became less effective at reducing within-canopy current velocities while over-canopy flow was increased. Narrow patches (0.3 m in a flume, 1.0 m wide) were significantly more effective at reducing current velocity within the canopy than were wider patches, but experienced higher turbulence intensity. Using laboratory findings from experiments to predict field flow conditions when patch geometry differs substantially from that of a flume may either over- or under-estimate flow reduction and turbulence intensity. This is particularly the case within the first meter of horizontal distance as flow enters the canopy. Therefore, flume conditions where the bed width equals the flume width may be more appropriate for mimicking flow interaction with broad and shallow seagrass beds. Use of bed widths narrower than the flume width are likely more accurate for modeling small, developmentally arrested patches, or recently established patches such as those arising from restoration projects.

Do Habitat Corridors Influence Animal Dispersal and Colonization in Estuarine Systems?

Studies investigating animal response to habitat in marine systems have mainly focused on habitat preference and complexity. This study is one of the first to investigate the affect of benthic habitat corridors and their characteristics on dispersal and colonization by estuarine macrofuana. In this study, mark-recapture field experiments using artificial seagrass units (ASUs) assessed the effects of seagrass corridors, interpatch distance (5 m vs. 10 m), and the ratio of corridor width to patch width (0.5 m:1 m vs. 0.25 m:1 m) on dispersal of two benthic organisms: the highly mobile grass shrimp, Palaemonetes sp., and the less mobile bay scallop, Argopecten irradians, in two estuarine systems in southeastern North Carolina (NC). The presence of a seagrass corridor, interpatch distance, and corridor width to patch width ratios did not significantly affect shrimp or scallop dispersal to receiver patches. Bay scallop dispersal to receiver patches was significantly higher at one site (Drum Shoals) with relatively high flow, compared to a second site (Middle Marsh) with lower flow. We then examined colonization of estuarine macrofauna to seagrass patches with and without corridors to determine which, if any, taxonomic groups respond positively to corridors at scales of 10 m and over 1 month. Colonization of estuarine macrofauna to seagrass patches was enhanced in the presence of corridors at a relatively large interpatch distance (10 m), which was statistically significant for relatively slow moving polychaete worms. Thus, although benthic habitat corridors may facilitate dispersal of relatively slow moving estuarine animals between otherwise isolated seagrass patches, several common seagrass fauna such as grass shrimp and bay scallops apparently use water currents to rapidly disperse across the seagrass/sand landscape.

Periphyton light transmission relationships in Florida Bay and the Florida Keys, USA

Light transmission was measured through intact, submerged periphyton communities on artificial seagrass leaves. The periphyton communities were representative of the communities on Thalassia testudinum in subtropical seagrass meadows. The periphyton communities sampled were adhered carbonate sediment, coralline algae, and mixed algal assemblages. Crustose or film-forming periphyton assemblages were best prepared for light transmission measurements using artificial leaves fouled on both sides, while measurements through three-dimensional filamentous algae required the periphyton to be removed from one side. For one-sided samples, light transmission could be measured as the difference between fouled and reference artificial leaf samples. For two-sided samples, the percent periphyton light transmission to the leaf surface was calculated as the square root of the fraction of incident light. Linear, exponential, and hyperbolic equations were evaluated as descriptors of the periphyton dry weight versus light transmission relationship. Hyperbolic and exponential decay models were superior to linear models and exhibited the best fits for the observed relationships. Differences between the coefficients of determination ( r 2) of hyperbolic and exponential decay models were statistically insignificant. Constraining these models for 100% light transmission at zero periphyton load did not result in any statistically significant loss in the explanatory capability of the models. In most all cases, increasing model complexity using three-parameter models rather than two-parameter models did not significantly increase the amount of variation explained. Constrained two-parameter hyperbolic or exponential decay models were judged best for describing the periphyton dry weight versus light transmission relationship. On T. testudinum in Florida Bay and the Florida Keys, significant differences were not observed in the light transmission characteristics of the varying periphyton communities at different study sites. Using pooled data from the study sites, the hyperbolic decay coefficient for periphyton light transmission was estimated to be 4.36 mg dry wt. cm −2. For exponential models, the exponential decay coefficient was estimated to be 0.16 cm 2 mg dry wt. −1.

Influence of seagrass landscape structure on juvenile blue crab habitat-survival function

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 300:179-191 (2005) - doi:10.3354/meps300179 Influence of seagrass landscape structure on the juvenile blue crab habitat-survival function Kevin A. Hovel1,2,*, Mark S. Fonseca1 1NOAA/NOS Center for Coastal Fisheries and Habitat Research, 101 Pivers Island Road, Beaufort,North Carolina 28516, USA2Present address: Department of Biology, San Diego State University, 5500 Campanile Drive, San Diego, California 92182-4614, USA *Email: hovel@sciences.sdsu.edu ABSTRACT: Organismal survival in marine habitats is often positively correlated with habitat structural complexity at local (within-patch) spatial scales. Far less is known, however, about how marine habitat structure at the landscape scale influences predation and other ecological processes, and in particular, how these processes are dictated by the interactive effect of habitat structure at local and landscape scales. The relationship between survival and habitat structure can be modeled with the habitat-survival function (HSF), which often takes on linear, hyperbolic, or sigmoid forms. We used tethering experiments to determine how seagrass landscape structure influenced the HSF for juvenile blue crabs Callinectes sapidus Rathbun in Back Sound, North Carolina, USA. Crabs were tethered in artificial seagrass plots of 7 different shoot densities embedded within small (1–3 m2) or large (>100 m2) seagrass patches (October 1999), and within 10 × 10 m landscapes containing patchy (<50% cover) or continuous (>90% cover) seagrass (July 2000). Overall, crab survival was higher in small than in large patches, and was higher in patchy than in continuous seagrass. The HSF was hyperbolic in large patches and in continuous seagrass, indicating that at low levels of habitat structure, relatively small increases in structure resulted in substantial increases in juvenile blue crab survival. However, the HSF was linear in small seagrass patches in 1999 and was parabolic in patchy seagrass in 2000. A sigmoid HSF, in which a threshold level of seagrass structure is required for crab survival, was never observed. Patchy seagrass landscapes are valuable refuges for juvenile blue crabs, and the effects of seagrass structural complexity on crab survival can only be fully understood when habitat structure at larger scales is considered. KEY WORDS: Habitat structure · Blue crab · Habitat fragmentation · Seagrass · Habitat-survival function Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 300. Online publication date: September 16, 2005 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2005 Inter-Research.

Seagrass bed patchiness: effects on epifaunal communities in San Diego Bay, USA

Seagrass habitat structure influences epifaunal density, diversity, community composition and survival, but covariation of structural elements at multiple scales (e.g., shoot density or biomass per unit area, patch size, and patch configuration) can confound studies attempting to correlate habitat structure with ecological patterns and processes. In this study, we standardized simulated seagrass shoot density and bed area among artificial seagrass beds in San Diego Bay, California, USA to evaluate the singular effect of seagrass bed configuration (“patchiness”) on the density and diversity of seagrass epifauna. Artificial seagrass beds all were 1 m 2, but were composed of a single large patch (“continuous” treatment), four smaller patches (“patchy” treatment), or 16 very small patches (“very patchy” treatment). We allowed epifauna to colonize beds for 1 month, and then sampled beds monthly over the next 3 months. Effects of seagrass bed patchiness on total epifaunal density and species-specific densities were highly variable among sampling dates, and there was no general trend for the effects of fragmentation on epifaunal densities to be positive or negative. Epifaunal diversity (measured as Simpson's index of diversity) was highest in very patchy or patchy beds on two out of the three sampling dates. Very patchy beds exhibited the highest dissimilarity in community composition in the first two sampling periods (August and September), but patchy beds exhibited the highest dissimilarity in the third sampling period (October). Our results indicate that seagrass patch configuration affects patterns of epifaunal density, diversity, and community composition in the absence of covarying bed area or structural complexity, and that patchy seagrass beds may be no less valuable as a habitat than are continuous seagrass beds. The spatial pattern employed when harvesting or planting seagrass may influence epifaunal habitat use and should be a key consideration in restoration plans.

Reefs as contributors to diversity of epiphytic macroalgae assemblages in seagrass meadows

The links between macroalgae-covered reefs and diversity and biomass of epiphytic algae assemblages in adjacent seagrass meadows were investigated. Algae assemblages were sam- pled in 3 habitats: on-reef, seagrass meadow near-reef ( 3000 m), off the coast of Fremantle, Western Australia. We interpreted differences among these habitats as evidence that proximity to reefs influences epiphytic algae assemblages. Propagules in the water column, epiphytes recruiting onto artificial seagrass units (ASU) and assem- blages on natural Posidonia sinuosa were sampled in each habitat. For propagules, ASU and natural P. sinuosa, there were significant differences in the structure of algae assemblages near and distant from reefs, while assemblages near reefs tended to resemble those found on reefs. Multivariate analysis (ANOSIM) confirmed that propagule assemblages near reefs and on reefs were similar, but those distant from reefs differed from those on reefs. Near-reef habitat recorded the highest total number of algal taxa growing on ASU (53), followed by on-reef (44) and distant-from-reef (37) habi- tats. ANOSIM again confirmed that assemblages at on- and near-reef sites were not significantly dif- ferent, but both differed from those at sites distant from reef. We identified 59 species of algal epi- phytes growing on mature P. sinuosa leaves at near-reef and distant-from-reef sites, with only 19 species being common to both habitats, and multivariate analyses were suggestive of differences in assemblage structure. Biomass was only recorded for the ASU epiphyte assemblages and was great- est in the near-reef habitat (0.63 ± 0.11 g shoot -1 ) followed by the on-reef habitat and the distant- from-reef habitat (0.18 ± 0.04 and 0.11 ± 0.01 g shoot -1 , respectively). Our results demonstrate that diversity and biomass of macroalgae epiphytes in seagrass ecosystems differ between habitats of varying proximity to reefs.

Evaluating vegetative transplant success in Posidonia australis: a field trial with habitat enhancement

Seagrasses in Western Australia grow on open coastlines that are wave and surge exposed, which are known to reduce transplant success. This study investigated the use of habitat enhancement procedures to improve the survival and expansion of Posidonia australis transplants. Limited success transplanting P. australis vegetative propagules (plugs) was achieved after habitat enhancement was implemented, using artificial seagrass (ASG) mats to emulate a “surrogate” seagrass meadow. The ASG mats enhanced one aspect of the habitat (stabilised sediment grain size) but did not suitably stabilise erosion and accretion. Transplant survival was significantly increased (up to 50% survival) in 60% of the sites where ASG mats were present. However, only 8% of the total transplant population exhibited rhizome extension (3.2 ± 0.11 mm d −1).

Patch shape and orientation influences on seagrass epifauna are mediated by dispersal abilities

While there have been theoretical arguments supporting the importance of the shape and orientation of habitat patches for determining species abundances, there have been few empirical demonstrations that these processes actually operate. Instead, most field studies have focussed on the importance of patch area, isolation and edge effects. I demonstrate that passively dispersed seagrass epifauna respond to the shape and orientation of artificial seagrass patches when currents, the dispersal mechanism, are strong, but not when they are weak. Orientation is important because animals dispersing via tidally induced water currents move predominantly in a single direction, and thus patches oriented across the current intercept more potential colonists than do those patches oriented perpendicular to the current. Currents less affect taxa that actively disperse, or that are relatively sedentary. Fish species that tend to use intertidal areas at high tide, however, were more abundant in patches perpendicular to shore (and parallel to the current), presumably because these patches offer the greatest amount of edge to animals undergoing tidal migrations.

Habitat selection of two gobies ( Microgobius gulosus, Gobiosoma robustum): influence of structural complexity, competitive interactions, and presence of a predator

Herein I compare the relative importance of preference for structurally complex habitat against avoidance of competitors and predators in two benthic fishes common in the Gulf of Mexico. The code goby Gobiosoma robustum Ginsburg and clown goby Microgobius gulosus (Girard) are common, ecologically similar fishes found throughout the Gulf of Mexico and in the southeastern Atlantic Ocean. In Florida Bay, these fishes exhibit habitat partitioning: G. robustum is most abundant in seagrass-dominated areas while M. gulosus is most abundant in sparsely vegetated habitats. In a small-scale field survey, I documented the microhabitat use of these species where their distributions overlap. In a series of laboratory experiments, I presented each species with structured (artificial seagrass) versus nonstructured (bare sand) habitats and measured their frequency of choosing either habitat type. I then examined the use of structured versus nonstructured habitats when the two species were placed together in a mixed group. Finally, I placed a predator ( Opsanus beta) in the experimental aquaria to determine how its presence influenced habitat selection. In the field, G. robustum was more abundant in seagrass and M. gulosus was more abundant in bare mud. In the laboratory, both species selected grass over sand in allopatry. However, in sympatry, M. gulosus occupied sand more often when paired with G. robustum than when alone. G. robustum appears to directly influence the habitat choice of M. gulosus: It seems that M. gulosus is pushed out of the structured habitat that is the preferred habitat of G. robustum. Thus, competition appears to modify the habitat selection of these species when they occur in sympatry. Additionally, the presence of the toadfish was a sufficient stimulus to provoke both M. gulosus and G. robustum to increase their selection for sand (compared to single-species treatments). Distribution patterns of M. gulosus and G. robustum likely result from a synthesis of various biotic and abiotic filters, including physiological tolerances to environmental factors, dispersal ability of larvae, and availability of food. Selection for structural complexity, competition, and presence of predators may further define the resulting pattern of distribution observed in the field.

Settlement and Density of Juvenile Fish Assemblages in Natural, Zostera Capricorni (Zosteraceae) and Artificial Seagrass Beds

Few studies have validated the use of artificial seagrass to study processes structuring faunal assemblages by comparison with natural seagrass. One metric (fish recruitment) for evaluating the use of artificial seagrass was used in the present study. Settlement and recruitment of juvenile fish was estimated in natural, Zostera capricorni Aschers, and artificial seagrass in Botany Bay, NSW, over 6 consecutive days. Tarwhine, Rhabdosargus sarba, dominated the catch from both habitats, and there was no significant difference in abundance of recruits among the habitats. This was at least partly caused by large spatial and temporal variation in abundance. Daily abundances of R. sarba recruits suggested movement between seagrass beds, but could not be confirmed without tagging individual fish. Rhabdosargus sarba settlers were less abundant than recruits, but were also patchily distributed amongst natural and artificial seagrass beds. Most other species were also found in similar abundance in the two habitats, except stripey, Microcanthus strigatus, which was more abundant in artificial seagrass. Overall, fish assemblages in natural and artificial seagrass were similar. Artificial seagrass may therefore be useful for monitoring settlement and recruitment of juvenile fishes to disturbed habitats, to predict the success of habitat remediation. However, if artificial seagrass is used to model processes occurring in natural seagrass, it is necessary to consider species-specific responses to the artificial habitat.

Substrate selection by blue crab Callinectes sapidus megalopae and first juvenile instars

Various marine and estuarine species utilize chemical cues during settlement. We inves- tigated responses by megalopae and first juvenile (J1) blue crabs to common Chesapeake Bay sub- strates in mesocosm and field experiments. Mesocosm trials examined responses of megalopae or J1 crabs to sand, marsh mud, live oysters Crassostrea virginica, sun-bleached oyster shell, eel grass Zos- tera marina and artificial seagrass in replicate 160 l tanks. Either 10 megalopae or J1 crabs isolated in each of 6 substrates were allowed total access after acclimation to test the null hypothesis of equal distribution among substrates after 13 h. Thirty-five percent of megalopae were recovered from Z. marina, with the remaining substrates containing fewer than half that many. In contrast, 30 % of J1 crabs (with only 17% recovered from Z. marina) were found in live C. virginica. A field experiment quantified responses of ingressing megalopae to Z. marina, marsh mud, and C. virginica. Overnight settlement was significantly higher in Z. marina (x = 3.3 ind.; 60% of total) when compared to mud (x = 0.9; 16 %) or C. virginica (x = 1.3; 24 %). Likewise, J1 crabs were significantly more numerous in Z. marina (x = 3.7 ind.; 55% of total) than in C. virginica (x = 1.8; 27 %) and mud (x = 1.2; 18 %). J1 crab distribution in field plots likely reflected habitat selection by megalopae; laboratory results were equivocal and probably due to artifacts associated with density-dependent agonism. The initial non-random distribution of blue crabs in Chesapeake Bay may be deterministic and due to habitat- selection behavior by megalopae. Selection for seagrass assures the greatest likelihood of maximal survival and accelerated growth. Similar relationships may also exist in estuarine-dependent species with comparable habitat requirements and life-history characteristics.

Faunal colonization of artificial seagrass plots: The importance of surface area versus space size relative to body size

An index of structural habitat complexity was devised: the average inter-structural space size within a habitat/the width of the prey organism of concern (Sp/Py). Prey survivorship should be low at Sp/Py<1 as the prey will be effectively excluded from using the habitat as refuge (they cannot maneuver through the spaces). At Sp/Py near to 1, survivorship should be high, as the spaces within the habitat are ideal for the prey and their predators are excluded (assuming they are larger than the prey). As Sp/Py increases, prey survivorship should drop rapidly until reaching a lower plateau where no predators are excluded by the structure. Sp/Py is dimensionless, and is potentially applicable across different scales and habitat types. Some of the predictions of this model were tested using artificial seagrass plots deployed in a seagrass bed in the York River, Virginia. The plots had 5 different structural treatments: control (a base with no ribbon), low, medium and high densities, as well as a heterogeneous treatment (composed of 1/3 low, medium and high density in a single treatment). The abundance of 2 mobile fauna size classes (<3.5 mm width and 3.5 to 9.5 mm width) and total species richness were compared among the different density treatments. The abundance of the smaller fauna increased with increasing density, and this response was proportional to the total surface area of the plots. The small fauna apparently did not respond to the smaller, ideal space sizes associated with the higher density plots. The larger fauna responded to the treatments as well, with the highest abundances occurring in the heterogeneous and high density treatments. The larger fauna did not respond to the structure proportional to the surface area within the plots, and its is possible that they responded to the inter-structural space sizes appropriate to their body sizes, although the results do not clearly support this conclusion. The different treatments did not affect species richness when the effect of total abundance on richness was controlled.

Elements of habitat complexity that influence harpacticoid copepods associated with seagrass beds in a temperate bay.

The influence of habitat structure on abundance and taxonomic richness of epibenthic harpacticoid copepods in seagrass beds of Port Phillip Bay, Australia was investigated using artificial seagrass plants. The density and length of artificial seagrass plants was manipulated at three sites over two sampling times. Results for artificial plants were also compared with controls without plants. The presence of habitat structure in the form of artificial seagrass resulted in a significant increase in harpacticoid abundance at all sites and taxonomic richness at one site. In terms of artificial seagrass treatments, higher blade density resulted in higher harpacticoid abundance, but blade length and surface area had no significant effect. Taxonomic richness did not vary amongst artificial seagrass treatments. At the site where taxonomic richness was increased in the presence of artificial seagrass, rarefaction showed that the result was consistent with a passive increase related to increased sample size. In contrast, although abundances in artificial seagrass were significantly higher than in controls at the other two sites, the taxonomic richness was similar to controls, suggesting that the full range of taxa available was represented in control samples. This study shows that structural aspects of complexity can have importance beyond the simple provision of complexity in the form of increased surface area of habitat, and may depend on the scale examined. Further, the study emphasises the importance of spatial and temporal replication of experiments to give generality to results.

HABITAT FRAGMENTATION IN A SEAGRASS LANDSCAPE: PATCH SIZE AND COMPLEXITY CONTROL BLUE CRAB SURVIVAL

Habitat fragmentation is increasingly common on land and in the sea, leading to small, isolated habitat patches in which ecological processes may differ substantially from those in larger, continuous habitats. Seagrass is a productive but fragmented subtidal habitat that serves as a refuge from predation for many animals because its structural complexity limits the detection and capture of resident prey. The singular influence of seagrass habitat fragmentation (e.g., patch size) on faunal survival is largely unknown and has been difficult to quantify because seagrass habitat complexity (e.g., shoot density) and patch size are often confounded and vary seasonally. In early summer 1998 we quantified the effect of seagrass habitat fragmentation on juvenile blue crab (Callinectes sapidus) survival in the absence of covarying complexity by exposing tethered crabs to predators in density-controlled, artificial eelgrass (Zostera marina) plots embedded within natural seagrass patches of four broad size classes (<1 m2 to >30 000 m2). We repeated this experiment in late summer 1998 with three different shoot densities, after predictable environmental events (defoliation and bioturbation) had increased seagrass habitat fragmentation and decreased shoot density. In early summer, crab survival was inversely correlated with seagrass patch area; survival of juvenile blue crabs increased as patch size decreased, in contrast to patterns typically observed in terrestrial and marine systems. This pattern appears to have been due to low abundance of adult blue crabs, the chief predator of juvenile conspecifics, in small patches. In late summer, blue crab survival was greater than in early summer, and survival increased with artificial seagrass shoot density but did not vary with patch size. The breakdown of the relationship between crab survival and patch size in late summer may have resulted from influx of cownose rays, which fragmented large, continuous patches of seagrass into smaller patches in midsummer, potentially equalizing fragmentation across the seagrass meadow. These results show that (1) fragmented seagrass landscapes hold significant refuge value for juvenile blue crabs, (2) fragmentation and crab survival vary temporally, and (3) crab survival increases with habitat complexity (shoot density) regardless of patch size. The findings indicate that habitat patch size and complexity jointly drive organismal survival, and that their influence differs temporally in this dynamic landscape. Thus, ecological processes are sensitive to landscape structure, and studies of habitat structure should incorporate multiple scales of space and time, as well as potentially confounding structural variables.

Active habitat selection for sand by juvenile western king prawns, Melicertus latisulcatus (Kishinouye).

This paper presents the results of a series of habitat selection experiments aimed at determining if juvenile Melicertus latisulcatus generally occur on intertidal sand- and mud-flats as a result of active selection of unvegetated areas, or due to extrinsic factors (e.g. differential predation). In the laboratory, juvenile M. latisulcatus showed a clear preference for habitats containing sand irrespective of the presence or absence of predators. If sand was not available, artificial seagrass was chosen as a secondary preference but was avoided when sand alone was also present. Importantly, the combinations of habitats chosen for testing allowed us to determine that artificial seagrass provided a good surrogate for real seagrass, and that the presence of potential food (epiphytes) did not appear to influence habitat selection. There was also no difference in the habitat selected between day and night, and only minor differences with prawn size. Thus, juvenile M. latisulcatus appear to have a hierarchy of mechanisms for avoiding predators, with burying in sand being the preferred option. If burying is not possible, then seagrass is used for shelter. Active habitat selection to avoid predation appears likely to play a substantial role in determining the distribution of these animals on unvegetated sand- and mud-flats.

The effects of seagrass (Zostera japonica) canopy structure on associated fauna: a study using artificial seagrass units and sampling of natural beds.

The importance of seagrass canopy to associated fauna was assessed by comparing the species richness, abundance and diversity of the epi- and infaunal macroinvertebrate assemblages in a seagrass (Zostera japonica Ascherson and Graebner) bed and the adjacent unvegetated area in Hong Kong. Seagrass cover had significant effects on the composition and abundance of the associated fauna and the amount of detritus accumulated on the sediment surface. Detritus abundance was significantly higher in the seagrass bed, and was positively correlated with both the above- and belowground biomass of Z. japonica. Both the abundance and species richness of the epi- and infauna were significantly positively correlated with the belowground biomass of the seagrass and detritus standing crop. Macrofaunal species richness was higher (118) in the seagrass bed than the adjacent unvegetated areas (70), with a higher degree of similarity between the infauna than the epifauna of the two habitats. While all species recorded from the unvegetated areas were found in the seagrass bed, 48 species occurred only in the seagrass-covered areas. Species richness of epifauna was significantly higher in the seagrass bed, but there was no difference between infaunal species of the two habitats. On the contrary, faunal (epi- and infauna) abundance was significantly higher in seagrass areas. The seagrass bed also supported species of small tellinid bivalves previously not recorded from Hong Kong. Artificial seagrass units (ASUs, 0.2 m(2)) with four combinations of leaf density and leaf length and a control (bare sand) were placed at short distances from natural patches of Z. japonica. The composition, abundance and biomass of the epibenthos associated with the ASUs and the control were recorded after 3 months in the field. While species richness did not differ among the treatments, total abundance of epibenthos was significantly higher in the high density-long leaves (HL) treatment than in the control. Results of a discriminant analysis using log-transformed abundance data suggest that the gastropod Clithon oualaniensis, the mussel Musculista senhousia and the crab Thalamita sp. were important species distinguishing the assemblages in the various treatments. All the three species were significantly more abundant in the HL treatment than in the low density-short leaves (LS) treatment and the control. By contrast, there was no significant difference in the biomass of the epifauna, but discriminant analysis again separated the five treatments based on the composition of the biomass, with the same three species identified as the most important discriminating species. The species richness and abundance of the epifauna associated with the ASUs were similar to the adjacent unvegetated areas, but significantly lower than in the Zostera patches. The physical canopy structure of Z. japonica beds increased the abundance of the epibenthos, potentially through provision of canopy and indirectly through trapping of detritus.

Impacts of Seagrass Habitat Architecture on Bivalve Settlement

We investigated the effects of differing spatial scales of seagrass habitat architecture on the composition and abundance of settling bivalves in a sub-tropical seagrass community. The density of newly settled bivalves was generally greater atThalassia testudinum grass bed edge ( 10 m). Deviation from this generalized pattern occurred when high densities of newly settled tulip mussels (Modiolus americanus) were recorded from the interior of the meadow, associated with aggregations of adult mussels. Bivalve settling densities appear to reflect settlement shadows of passively delivered larvae, bedload transport of newly settled individuals from unvegetated regions, as well as gregarious settlement among adult conspecifics. We also investigated the impact of seagrass patch shape and size on settlement by using artificial seagrass units (ASU) in separate short-term and long-term experiments. We found a positive relationship between ASU perimeter and bivalve abundance, suggesting that larval encounter rates with seagrass habitat may determine initial settlement patterns. Using ASUs we also investigated the relative role seagrass epiphytes play in determining the density of settling bivalves. Results showed greater settling densities where epiphytic secondary structure was elevated compared to controls, and bivalve density was significantly greater when ASUs were fouled with a natural community of epiphytes, suggesting that both microstructure and biofilms positively influenced bivalve settlement. We conclude that structural components of seagrass habitats increase bivalve settlement at multiple spatial scales, including epiphytic micro-structure, small-scale patch shape and size, and large-scale within habitat differences.

Settlement preferences and early migration of the tropical sea cucumber Holothuria scabra.

Settlement and post-settlement processes of the sea cucumber Holothuria scabra Jaeger were studied in the laboratory. Independent and paired choice experiments revealed that several substrates could induce metamorphosis into pentactulae, but that specific substrates favoured settlement. Leaves of seagrass Thalassia hemprichii, with or without their natural bio-film, yielded the highest settlement rates (4.8-10.5%). T. hemprichii was preferred as a settlement substrate over sand, crushed coral, several other plant species and artificial seagrass leaves with or without a bio-film. Only settlement on the seagrass, Enhalus acoroides, was similar to that recorded for T. hemprichii. In the absence of a substrate, the larvae delayed settlement for nearly 96 h and survival was less than 0.5%. Sand and crushed coral, either alone or together, induced settlement from <1.5% of the available larvae. The pentactulae found on sand, coral and in bare containers were 10-35% smaller than those on T. hemprichii leaves. Soluble extracts from T. hemprichii and E. acoroides successfully induced metamorphosis and settlement on clean plastic surfaces. Newly settled juveniles remained on the seagrass leaves for 4-5 weeks before migrating to sand at around 6 mm in length. Prior to this, the juveniles spent 4-5 days moving on and off the leaves. Once on the sand, the juveniles became deposit-feeders, but did not show the typical burrowing behaviour of older specimens until they reached around 11 mm in length. The larvae of H. scabra appear to actively select seagrass leaves, possibly through chemical detection. We hypothesise that larvae settling on seagrass have an increased chance of growth and survival because they are provided with a suitable substrate on which to grow, and a bridge to sand substrates as they become deposit-feeders.

Zoobenthic community establishment and habitat complexity-the importance of seagrass shoot-density, morphology and physical disturbance for faunal recruitment

Seagrass meadows are among the most diverse coastal ecosystems in the Baltic Sea. There is, however, an apparent lack of quantitative data on functional aspects of these systems, such as plant-animal interactions. We tested the importance of seagrass density and morphology for ben- thic infaunal recruitment in a 2 mo (June/July 1997) field-experiment with both bare and vegetated (3 densities of artificial Ruppia maritima and Zostera marina) colonization trays with azoic sediment. These artificial seagrass patches were placed at 3 m depth in an unvegetated area of a sandy bottom seagrass site on the Aland Islands, northern Baltic Sea. Faunal succession was followed by SCUBA- diving and core sampling with 10 to 20 d intervals. The data showed strong effects of seagrass com- plexity and wind disturbance on (1) physical processes such as accumulation of drifting algae, parti- cle trapping and sediment binding, (2) development of community parameters (abundance, species richness, diversity) and (3) species-specific colonization patterns. Our data further demonstrated the importance of post-settlement events for distribution of juvenile macrofauna (e.g. resuspension or transport by means of drifting algae), and showed negative and positive effects of wind-mediated dis- turbance in low- and high-complexity habitats, respectively. It is concluded that wind disturbance may act as a mechanism creating and maintaining high animal diversity in seagrass meadows.

Macrofaunal associations with seagrass epiphytes

Abstract Attached epiphytes often make important contributions to total primary production in seagrass meadows. Additionally, they may increase the spatial complexity of seagrass habitats. Experiments conducted using artificial seagrass units (ASU) manipulated both epiphytic structure and epiphytic food resources. Previous work suggested that the increase in faunal density associated with epiphytes was related to increases in structure, but our results indicate that the primary impact of epiphytes lies in their trophic role. Data showed that epifaunal density was significantly greater in conditioned ASUs fouled with a live community of epiphytes (12 285 individuals m −2 ) compared to ASUs with artificially created epiphytic structure (5099 inds. m −2 ) and to control ASUs (5955 inds. m −2 ). This response to epiphytic trophic resources was significant for most herbivore/omnivore taxa, but not necessarily for filter feeding or predatory epifauna. However, densities of two predatory taxa (fish and mud crabs) were significantly greater where epiphytic biomass was higher, which may reflect their response to increased prey abundance. Additionally, ASUs conditioned with live epiphytes had greater taxa richness than other ASUs. Epiphytic structure appeared to play only a limited role in determining the density of most mobile epifauna, but epiphytic structure appeared to be important in augmenting the settlement of bivalves. By using ASUs we were able to control aspects of blade length and shoot density, but the pre-experiment conditioning of treatments fouled with live epiphytes may have played a role in determining absolute differences in macrofaunal density among ASU treatments. Overall, our work suggests that the trophic role of epiphytes can have a dramatic impact on associated epifaunal communities, although future investigations are needed to assess this relationship more fully.