Unvegetated Sand Research Articles

Aeolian-climatic thresholds and sand dunes at the Hanford site, south-central Washington, U.S.A.

Aeolian-climatic thresholds are quantified using time sequence analysis of 1948-87 aeolian and climatic data from the Hanford Site, south-central Washington. Calculations indicate that 30 unvegetated sand dunes will maintain their total 1987 volume if they are exposed to a climate having an average annual precipitation, wind speed, and temperature of approximately 15·5 cm, 3·5 ms −1 , and 11·8°C, respectively. Calculations also show that if average 1948-87 climate conditions continue for the next approximately 10–15 years, the volume of unvegetated dune sand in the dunes studied will decrease by approximately 18%. Aeolian-climatic projections are based on the assumptions that there will be no major changes in human, animal, or botanical impact on the surface and no major variations from historical seasonal climatic patterns.

The role of seagrass as preferred habitat for juvenile sillaginodes punctata (cuv. & val.) (sillaginidae, pisces): habitat selection or feeding?

Previous experiments have shown that the relationship between abundance of juvenile fishes and seagrass density is the result of habitat selection rather than direct predation, but have not distinguished between habitat selection and simple feeding. The simple feeding model, in which fish swim until they find food, eat it, and swim again, predicts that fish will be found where the most food is available regardless of the presence or density of seagrass. Food availability was manipulated in tanks in which juvenile Sillaginodes punctata (Cuv. & Val.) (King George whiting) were offered the choice of two habitats, live eelgrass ( Zostera muelleri Irmisch ex Aschers.) or unvegetated sand. When food was available, fish were initially distributed in a way that strongly supported the feeding model, but once they were satiated, the distribution of fish fitted the predictions of neither model. In the absence of food, fish selected eelgrass habitat. At night, the distribution of fish showed that they took little account of food availability or habitat.

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.

Effects of an Introduced Aquatic Plant, Hydrilla verticillata, on Benthic Communities in the Upper Chesapeake Bay

The introduced submersed aquatic plant, Hydrilla verticillata, is currently spreading in the Chesapeake Bay, and is colonizing previously unvegetated areas and displacing native flora. To examine the effects of this invasion, we compared faunal densities between Hydrilla beds and unvegetated sand areas. Transplants of Hydrilla and sampling from natural Hydrilla and unvegetated patches indicate that this plant enhances the abundances of a variety of benthic fauna. Among two clam species transplanted into Hydrilla patches and adjacent unvegetated areas, one demonstrated enhanced survival within Hydrilla patches while the other exhibited reduced tissue growth. Similarly, the growth form of Hydrilla may have been affected by clam presence. Dissolved oxygen, sediment characteristics, and sediment resuspension also varied between Hydrilla and unvegetated areas. The spread of Hydrilla in the Chesapeake Bay clearly has significant, though complex, effects on the bottom community. Such effects emphasize the potential importance of introduced plants in altering community characteristics.

Observations of aeolian transport and niveo‐aeolian deposition at three lowland sites, Canadian arctic archipelago

AbstractObservations of wind action were made in the late winter, spring and summer at three lowland sites located on Banks Island (1977‐79), Melville Island (1986‐87) and Ellesmere Island (1989‐90). Erosion pin measurements in north‐central Banks Island showed minimal activity on interfluves and the highest rates at a blowout site in an unvegetated sand plain. The timing of aeolian activity during the year depended on the site characteristics. Where the snow cover was incomplete in winter, either because a rugged microtopography resulted in terrain elements protruding above the surface of the snow or because sublimation of snow exposed the ground surface, aeolian transport could occur year‐round. In areas with a flatter microtopography, summer transport took place but deflation was negligible over the winter and niveo‐aeolian deposition was confined to the autumn and spring when the snow cover was patchy. Windblown sediment became spatially concentrated by denivation at some sites and was deposited on slopes as fans or layers up to 8 cm thick. In the years this occurred, the rate of deposition was greater than sediment removal by wash or other processes and, at least in the short term, the form of the slope was changed.

Intertidal habitats used by juvenile chinook salmon ( Oncorhynchus tshawytscha) rearing in the North Arm of the Fraser River estuary

Results from surveys of juvenile chinook ( Oncorhynchus tshawytscha) and their invertebrate food items in the North Arm of the Fraser River estuary are presented and related to habitat zones on narrow beaches. Fish catches were highest on unvegetated habitat in the low tide area. The weight of food items in stomachs was highest from fish in the mid intertidal area. The distribution and abundance of ten common prey used by chinook were also examined. Emergent vegetation (sedges ( Carex lyngbyei), rushes ( Scirpus spp., Typha spp.)) and riparian shrubs and trees in the middle and upper intertidal zones, respectively, were identified as vital components providing detritus and habitat for the chinook food organisms. Water volume over the unvegetated sand and mud flats in the low intertidal zone needs to carefully managed in this part of the estuary where extensive dyking and filling has been conducted in the past. For the purposes of managing fish habitat to achieve a goal of no net loss of habitat in the Fraser estuary, biologists are assigning highest values to the sedges and rushes. Our data support this as an interim measure, but further research is needed to investigate the importance of riparian vegetation as its significance may be underestimated.

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.

Sediment stabilization by Halophila decipiens in comparison to other seagrasses

While seagrasses are known for their sediment stabilization qualities, some genera, such as the diminutive Halophila, are not. An underwater flume was used to evaluate the threshold of sediment motion in a 20 m deep Halophila decipiens bed. Results were compared to unvegetated sand and seagrasses elsewhere. Despite having substantially smaller leaves and leaf biomass than other seagrass species, H. decipiens increases the threshold velocity for sediment motion significantly as compared to sand, and was similar to threshold velocity effects by larger seagrasses. Allocation of leaf biomass and rhizomes closer to the sediment-water interface than observed for other seagrasses was hypothesized as the primary physical basis for the sediment stabilization effects provided by this species.

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.

Orientation of Empty Pelecypod Shells and Shell Fragments in Quiet Water

ABSTRACT Participation in man-in-the-sea experiments Tektite I and II permitted an examination of the orientation of pelecypod shells and shell fragments on sediment unaffected by waves or currents. Whether on vegetated or unvegetated unrippled sand or on solid coral, the particles lie predominantly with concave sides up. This orientation occurs both for open articulated empty valves and for isolated disarticulated valves. The degrees of orientation differs with shell size; the smaller shells are less preferentially oriented concave-up than are the larger shells. The concave-up orientation results from the activity of predators and scavengers, from bioturbation, or from a combination of the two mechanisms. An experiment that exposed nearly 1200 shells to 40 days of bioturbation demonstrated that disturbance by organisms is an effective agent for producing a dominantly concave-up orientation. Shell size and shell shape are significant factors in bioturbational rotation. In general, the tendency to be rotated to a concave-up position increases with increasing shell size to the point where shells are too large to be readily overturned by most organisms. The percent of concave-up shells also depends on the angle of balance of the shell; shells rotated by bioturbation tend to behave like loaded dice. Shells that became buried during the experiment howed a more random orientation than did those exposed on the surface after 40 days. The orientation of shells in quiet water differs markedly from those orientations produced under most conditions of waves or currents. Although concave-up assemblages can be produced by deposition from turbidity currents or from the transport of shells across small ripples, analysis of orientation as it relates to shell size and shape may provide a means of identifying specific depositional mechanisms or conditions.