Unvegetated Sand Flats Research Articles

GASTROPOD ASSOCIATIONS AS A PROXY FOR SEAGRASS VEGETATION IN A TROPICAL, CARBONATE SETTING (SAN SALVADOR, BAHAMAS)

Seagrass meadows are productive marine ecosystems that stabilize sediments and provide food and shelter for a diverse associated community. The recognition of these important habitats in the geological record is problematic, because marine angiosperms rarely fossilize. Thus, the presence of paleo-seagrass vegetation often has to be inferred from the occurrence of associated organisms with a higher potential for fossilization, such as mollusks. Because most mollusk taxa are not restricted to seagrass meadows, the species composition and feeding ecology of fossil mollusk faunas need to be considered when distinguishing paleo-seagrass meadows from other marine habitats. In this study the utility of faunal composition and feeding ecology of gastropods as an indicator of seagrass vegetation was tested using present-day ecosystems. Bulk-sediment samples containing gastropod death assemblages from shallow-water seagrass meadows and unvegetated sandflats from San Salvador Island, Bahamas were collected in July 2012. Vegetation varied across localities in terms of density and number of seagrass species. Twenty-four standardized (n = 200) samples of gastropods were compared in terms of species composition and relative abundance of feeding guilds. Multivariate analyses indicate that species composition is an effective tool for distinguishing between gastropod assemblages from vegetated versus unvegetated areas. To a lesser extent, species composition differs among vegetation zones on sandflats with seagrass cover. Feeding guild composition based on species richness also differs on seagrass-vegetated and unvegetated sandflats. The results suggest that gastropod assemblages are a useful proxy for seagrass meadows in the fossil record.

Ecological performance and possible origin of a ubiquitous but under-studied gastropod

Invasions by non-indigenous species (NIS) have been suggested to alter local, regional and global biota on unprecedented scales. To manage NIS, it is pivotal to identify whether a species is introduced or native, but even today the geographical origin of thousands of species worldwide remain uncertain. Most of these ‘cryptogenic species’ are inconspicuous and rare, but in a few instances, they can also be abundant and conspicuous species, with large impacts on community structure. The identification of cryptogenic species, and summarizing information on their most likely origin, is an important task in invasion biology, and can highlight the need for research and management. Here, we document that the gastropod Batillaria australis in the Swan River estuary (Perth, Western Australia) is a conspicuous species of uncertain origin. A literature review combined with new survey data revealed that all evidence point to a recent human-mediated transfer; for example, it is absent from the fossil record, was first collected in 1954, has a low parasite diversity, has increased its population size dramatically in recent times, is separated by >3000 km from conspecifics, has no long-distance dispersal mechanisms, and existing ocean currents run against a natural range extension. Surprisingly, despite political and scientific focus on NIS hardly any ecological data have been published on this species from Western Australia. We show that B. australis is highly abundant in both seagrass beds (424 ± 29 ind m −2) and on unvegetated sand flats (92 ± 22 ind m −2) being orders of magnitudes more abundant than any native gastropod in the Swan River. Experiments showed that high resistance to predation and environmental stress potentially explains its success. From our survey data, we calculated that >3.6 billion invasive snails today occupy the Swan River. This large snail populations support other organisms; for example, almost 1 billion macroalgae are found attached to living B. australis and >100 million hermit crabs occupy its empty shells. Given Battilaria’s high abundance, wide distribution, large size, persistent shells that support other organisms and bioturbating behavior, it seems inescapable that this potential invader has impacted the ecosystem functioning of the Swan River. We argue that the search for abundant species of uncertain origin should continue, and that these species generally should be treated with the same interest as high status invaders to mitigate impacts in already invaded systems and to avoid secondary spread into neighboring ecosystems.

Influences of biological interactions on community structure within seagrass beds and sandprawn-dominated sandflats

The faunal composition of high-shore beds of eelgrass Zostera capensis versus unvegetated sandflats dominated by the sandprawn Callianassa kraussi was investigated by experimentally transplanting plots of eelgrass into sandflats where the sandprawn was either left unmanipulated or eliminated by prior faunal suffocation. After 12 months, multidimensional scaling defined five faunal clusters that were dictated primarily by the presence or absence of eelgrass rather than sandprawns. By 18 months, eelgrass and sandprawns played equal roles in defining faunal clusters. After 30 months, sandprawns had displaced the eelgrass transplants and faunal composition converged on that of sandflat control plots. Most species were either ‘eelgrass-associated’ or ‘sandflat-associated’, but three had unique responses to the treatments: two high-shore species ( Assiminea globulus and Hydrobia sp.) that were abundant in eelgrass beds never appeared in the eelgrass transplants, whereas another eelgrass-occupant, Siphonaria compressa, became superabundant on the transplants for 18 months. Unexpectedly, the eelgrass fauna was never more rich or diverse than that of unvegetated sandflats, although abundance was greater in the eelgrass. As hypothesised, the sandflats were disproportionally populated by burrowers and eelgrass by non-burrowers, but a second hypothesis that these habitats should support mainly hard-bodied and soft-bodied animals respectively was rejected. The distinctively different faunas of eelgrass and unvegetated sandflats are seemingly respectively maintained by sediment stabilisation by eelgrass versus bioturbation by sandprawns. The transplants of eelgrass into sandflats developed an eelgrass-associated fauna provided sandprawns were excluded, but ultimately reverted to a sandflat fauna once sandprawns reinvaded.

Interactions between Zostera capensis and Callianassa kraussi: influences on community composition of eelgrass beds and sandflats

Eelgrass beds have been postulated to support different faunal communities from unvegetated sandflats, in part because of the stabilisation of sediments in eelgrass beds and their destabilisation in sandflats due to bioturbation. More specifically, eelgrass beds have been hypothesised to support disproportionately greater numbers of small, flexible and non-burrowing species, and bioturbated sandflats to support more large, hard-bodied burrowers. These concepts were tested in Langebaan Lagoon on the west coast of South Africa, where beds of the eelgrass Zostera capensis and sandflats dominated by Callianassa kraussi bioturbation are mutually exclusive. In eelgrass beds, sediment penetrability was significantly reduced compared to that in sandflats. Distinctly different faunal communities occurred in the two habitats, and the suite of species characteristic of each was consistent between years and between sites. Overall, density was higher in eelgrass beds than in sandflats. In contrast to previous studies, however, richness and diversity in sandflats were greater than, or equal to, values for eelgrass beds. There is convincing support for the hypothesis that burrowers are more prevalent in sandflats and less prevalent in eelgrass beds. Conversely, body flexibility does not differ between habitats and sizes are greater in the eelgrass beds than in the sandflats.

Linking fish and prawns to their environment: a hierarchical landscape approach

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 283:233-254 (2004) - doi:10.3354/meps283233 Linking fish and prawns to their environment: a hierarchical landscape approach S. J. Pittman1,3,*, C. A. McAlpine1, K. M. Pittman2 1School of Geography, Planning and Architecture and The Ecology Centre, The University of Queensland, Brisbane 4072, Australia 2Springfield, Mingle Lane, Cambridge, UK 3Present address: NOAA/NOS/CCMA Biogeography Program, 1305 East-West Highway, Silver Spring, Maryland 20910, USA *Email: simon.pittman@noaa.gov ABSTRACT: Little is known about the relative influence of landscape structure on the spatial distribution and abundance of marine organisms. To address this problem, we applied landscape ecology concepts and methods, together with conventional sampling techniques and path analysis, to test alternative hypotheses of linkages between marine nekton and landscape structure in Moreton Bay, Queensland (Australia). We quantified substratum structure at 3 spatial scales: (1) whole landscape mosaic (10s of hectares); (2) habitat type (benthic class) (100s m2 to hectares) and (3) within-patch scale (cm2 to m2). Substratum structure at all scales was important for assemblage density and number of species, with the landscape structure of individual habitat types explaining more of the spatial variation than either within-patch structure or the structure of the whole landscape mosaic. Density and the number of species of seagrass residents were strongly influenced by landscape composition quantified as the proportion of all seagrass habitat (r2 = 0.40 and 0.48 respectively) and the proportion of long-leaved Zostera capricorni (r2 = 0.34 and 0.30 respectively) seagrass in the landscape. An abrupt decline in assemblage density and number of species was evident at <20% seagrass cover. More species of fish used mangroves with adjacent continuous seagrass beds than mangroves with adjacent patchy seagrasses or unvegetated sandflats. Several species of fish using mangroves at high tide were more strongly influenced by the spatial configuration of mangrove patches and the composition of adjacent substratum than the internal structure of mangrove patches. The study highlights the need for a hierarchical landscape approach when investigating animal-environment relations in marine landscapes. KEY WORDS: Marine landscape ecology · Landscape structure · Spatial scale · Hierarchy · Fish · Penaeid prawns · Mangroves · Seagrass · Path analysis Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 283. Online publication date: November 30, 2004 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2004 Inter-Research.

Sex-specific intertidal habitat use in subtropically wintering Bar-tailed Godwits

Habitat use by sexually dimorphic Bar-tailed Godwits (Limosa lapponica) was examined in an Australian estuary during the midwintering (November–December) and premigratory (March) periods of a nonbreeding season. In an area with two available habitats, Zostera seagrass beds and unvegetated sand flats, males occurred almost exclusively in seagrass, whereas the females occurred at equal densities in both habitats. Estimates of intake rates seemed greater for individuals foraging in seagrass (0.114–0.128 g dry mass (DM)·min–1) than on sand (0.064–0.087 g DM·min–1). Males appeared less well adapted for foraging on sand than in seagrass, and the presence of females could interfere with males' foraging on sand. The reason for the equal distribution of the females between the habitats probably lies in the inability of one particular habitat to support the entire population of females. Little interhabitat movement by individuals was detected. Within a habitat, sexes moved synchronously at the 1-ha scale but showed a degree of segregation at the individual scale, suggesting that some avoidance existed. Three diet types were observed in the godwits in the area: (1) dominated by soldier crabs (Mictyris longicarpus) on sand, (2) dominated by sentinel crabs (Macrophthalmus spp.) in seagrass, and (3) dominated by soft-bodied prey in seagrass.

Magnitude of within‐patch variation in seagrass Halophila ovalis growth affected by adjacent Thalassia hemprichii vegetation

Seagrass beds in South‐east Asia sometimes consist of a mosaic of different species in monospecific patches. We examined whether the magnitude of within‐patch variation in the seagrass Halophila ovalis is affected by the presence or absence of surrounding vegetation consisting of another seagrass species Thalassia hemprichii in an intertidal flat in Thailand waters. We measured biomass and growth rates of H. ovalis at the edges and centers of two different types of patches: (i) H. ovalis patches adjoining T. hemprichii vegetation (HT patches), and (ii) H. ovalis patches adjoining unvegetated sand flats (HS patches). Furthermore, we examined the possible effects of interspecific interactions on the growth of H. ovalis by experimentally removing adjoining T. hemprichii at the edges of HT patches. The biomass of H. ovalis was greater at the patch centre than the patch edge in both types of patches. For the growth rate of H. ovalis, significant interactions were detected between patch types and positions in patches. The difference in growth was significant and more than 4‐fold between edges and centers of the HS patches, whereas the growth was not significantly different between edges and centers of the HT patches. The removal of T. hemprichii did not significantly affect the growth rate of H. ovalis at the edge of the HT patches. These findings demonstrate that the magnitude of within‐patch variation in H. ovalis growth is affected by the conditions of adjoining habitats. However, any effects of local competition with T. hemprichii on H. ovalis growth were not evident in this short‐term manipulative experiment.

Effects of bioturbation in controlling turtlegrass ( Thalassia testudinum Banks ex König) abundance: evidence from field enclosures and observations in the Northern Gulf of Mexico

Bioturbation by stingrays and sand dollars can limit the distribution of temperate zone seagrass (eelgrass, Zostera marina Lamarck) meadows through the disruption of the root-rhizome matrix. The effects of bioturbation by these organisms on tropical or subtropical seagrass (turtlegrass, Thalassia testudinum Banks ex König) meadows are less well known. Stingray enclosure studes in St. Joseph Bay, Florida (29°N, 85.5°W) found that rays were unable to create unvegetated patches within continuous turtlegrass and that only large (disc width ≥0.9 m) rays ( Dasyatis americana Hildebrand & Schroeder) can damage rhizomes at the turtlegrass/sand flat interface habitats. Field surveys found large rays to be abundant in turtlegrass habitats only during July–August. These data indicate that stingrays are not responsible for the widespread occurrence of unvegetated patches within St. Joseph Bay. Additional exclosure experiments found that sand dollars ( Mellitta quinquiesperforata Leske) did not affect turtlegrass colonization of unvegetated sand flats. Comparisons of sand flat perimeters enclosing sand dollars at ambient densities (5–15 individuals/m 2) and those from which sand dollars were removed showed no inhibition of turtlegrass colonization of unvegetated sand patches after a 2-yr period. However, stone crab ( Menippe spp.) burrow construction at the seaward edge of turtlegrass habitats was found to cause significant, previously unreported, losses of turtlegrass habitat (average seagrass recession > 1 m/7 months). We conclude that bioturbation has less impact on turtlegrass than eelgrass habitats due to thicker, deeper turtlegrass rhizomes, and coarser sediment grain size.

Modification of animal habitat by large plants: mechanisms by which seagrasses influence clam growth.

Field experiments withMercenaria mercenaria in a relatively high-energy environment demonstrated that clams on unvegetated sand flats failed to grow during autumn while those within seagrass beds grew substantially. Clam growth rates at the seagrass margin that first receives the faster-flowing, flood-tidal currents were about 25% less than at the opposite edge. In a second experiment, pruning, which reduced average blade length by 50-75%, was shown to enhance near-bottom current velocities and to reduce shell growth ofMercenaria during summer by about 50%. As in the first experiment, clams in the unvegetated sand flats exhibited no net growth. Clam mortality, caused mostly by predatory crabs and whelks, was much higher on sand flats than in seagrass beds and intermediate in clipped seagrass. Although consistent with some previous reports, these growth results are still surprising given that they contradict the generalization that suspension feeders grow faster under more rapid current regimes.Three types of indirect interactions might explain the observed effect of seagrass on growth of buried clams: (1) altering food supply; (2) changing the intensity of biological disturbance on feeding clams; and/or (3) affecting the physical stability of the sediments. Previous research on this question has focused almost exclusively on processes that alter food supply rates. In this study, food concentrations, as indicated by suspended chla, were 30% higher inside than outside one seagrass bed, whereas chla concentrations in two other beds were not different from those on adjacent sand flats. This result is sufficient to show that more intense food depletion was not induced by the reduction in flow velocities under the seagrass canopy. Nevertheless, the possible small difference in food concentrations between vegetated and unvegetated bottom seems insufficient to explain the absence of growth of sand-flat clams, especially given the virtual lack of food limitation among suspension feeders in this system. Two data sets demonstrated that the effects of biological disturbance agents cannot be ignored. An outdoor laboratory experiment showed that even in the absence of physical contact between predator and prey the presence of a whelk reduces the amount of time spent feeding byMercenaria. This result suggests that sand flats, where predation rates are higher, may be sites of lower clam growth than seagrass beds because of greater consumer interference with clam feeding. Furthermore, clam siphons are proportionately larger inside seagrass than on sand flats, implying that siphon nipping may not be as intense inside seagrass. This process, too, would reduce net growth of sand-flat clams. Finally, no explicit test was conducted of the hypothesis that enhanced sediment transport in the absence of flow baffling and root binding by seagrass inhibits net growth of clams on high-energy sand flats. Nevertheless, this is a reasonable explanation for the pattern of enhanced growth of seagrass clams, and could serve to explain the otherwise unexplained pattern of lower clam growth at the edge of the seagrass bed that experiences the faster flood-tidal current velocities. Each broad process, changing fluid dynamics, altering consumer access, and varying sediment stability, represents a mechanism whereby habitat structure, provided by the dominant plant, has an important indirect influence on the functional value of the habitat for resident animals.

Properties of Sea Grass and Sand Flat Sediments from the Intertidal Zone of St. Andrew Bay, Florida

Organic and carbonate carbon and textural properties of the substrates underlyingHalodule wrightii andThalassia testudinum sea grass beds in the intertidal zone of St. Andrew Bay, Florida were compared to adjacent unvegetated sand flats by physiographic divisions within the bay and to the subtidal slopes of the bay. Sea grass and sand flat sediments were principally fine-grained quartz sands. The mean particle-size of the sea grass sediments were finer-grained than those of the sand flats only in the west arm and lagoon of the bay. Size-frequency distributions of the sea grass sediments were generally slightly more negatively skewed and more leptokurtic than those of the sand flats. The sea grass sediments were less well sorted than were the sand flat sediments. The average organic and carbonate carbon contents of the sea grass beds were 1.9-fold greater than that of the sand flats but much less than that of the subtidal sea grass meadows. In the areas of pollution, sea grasses were absent; near this areaHalodule wrightii was the dominant sea grass.