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1.
Measurements of velocity profiles, bathymetry, and surface sediment characteristics across eelgrass (Zostera marina L.) meadows yielded information on community development processes and functional attributes of this ecosystem. Height/length ratios of the meadows were positively correlated with tidal current velocity. Low, medium, and high current regimes were separated by surface current velocities of approximately 50 and 90 cm s?1. Z. marina can tolerate approximately 120–150 cm/sec current velocities in the areas studied. Per cent silt-clay and organic matter content of the surface sediments are negatively associated with shear velocity, suggesting that meadows in high current areas are sources while meadows in low current areas are sinks of autochthonous detritus. Current velocity maintains seagrass meadows at different equilibrium levels (relative climaxes). We theorize these different equilibrium levels provide unequal habitat utilization potentials for the associated faunal community.  相似文献   

2.
Tethered current crosses are simple, reliable tools in making measurements of estuarine currents in the absence of surface waves. The standard error of the estimate of current speed is less than 5 cm s?1 for the particular crosses and weights used in a calibration experiment. The useful velocity range of these current crosses was 8–147 cm s?1, corresponding to a measured angle, α, from 2° to 30°. Regression of the independently measured current speed on (tan α)12 yielded coefficients of determination greater than 0·94, indicating that the drag coefficient is not a function of current speed. However, the calculated drag coefficients varied widely from 0·66 to 2·55, depending on the particular combination of cross and weight, varying drastically from the commonly assumed drag coefficient value of 1·12. Thus, in using current crosses, it is imperative to calibrate each cross against a current meter reading to determine an appropriate value for the drag coefficient for a particular current cross and weight.  相似文献   

3.
The effect of self‐shading and competition for light in the seagrass Enhalus acoroides were investigated with a density reduction experiment in Haad Chao Mai National Park, Trang Province, Thailand. The study was carried out in a monospecific meadow with a natural density of 141.0 ± 8.7 shoots·m?2. The intent was to determine the response of E. acoroides beds to loss of shoots and thinning, which often occur during typhoons and severe storm activity. Permanent quadrats were manipulated by clipping the seagrass shoots to 140, 72, 36 and 16 shoots·m?2, to yield natural, 50%, 25% and 10% densities, respectively. Reducing shoot density in E. acoroides increased underwater light intensity below the canopy, generating increased leaf surface area and shoot weight. Seagrass leaf width, growth rate, and number of leaves per shoot also increased with greater light. The extent of flowering varied among treatments with no consistent trend. Our results demonstrate that increasing the available light to E. acoroides produces an increasing leaf size response as self‐shading in the bed is reduced.  相似文献   

4.
The result of two sequential oceanographic stations of 36 hours each in the area of sand ridges are presented. One station was located in the trough between two sand ridges and the other was at the crest of a sand ridge. At these stations salinity and temperature of the sea water, currents, winds, waves, and barometric pressure were measured each hour.During the observations, a cold front passed; this generated westerly winds that grew in speed from 24 to 52 km h?1. The average height of the wind generated waves grew from 1.0 to 1.5 m and their periods increased from 7 to 10 s, and the speed of the northeast directed surface current increased from 40 to 82 cm s?1. A bottom current (also directed northeast) increased from 26 to 34 cm s?1.After the cold front had passed, the wind backed to the southeast and decreased in speed from 26 to zero km h?1. The surface current in a northwest direction decreased from 29 to 8 cm s?1. A bottom current (also directed northwest) decreased from 22 to 3 cm s?1. Later, swells from the southeast appeared and their periods increased from 5 to 9 s and their heights grew from 1.0 to 1.5 m. After 3 hours, the speeds of the surface and bottom currents increased from 8 to 72 cm s?1 and 3 to 62 cm s?1 respectively.This cold front induced strong winds and storm-wave currents able to erode sediments (assuming a threshold velocity of 20 cm s?1) and transport them in a north-northeast direction.The origin and the maintenance of these sand ridges is thought to be a function of sediments eroded from troughs and piled up at ridge crests during a storm condition. Some eroded sediments are transported north of Verga lighthouse where they are deposited on a smooth bottom.  相似文献   

5.
Volumes of seawater filtered through the intertidal zone were measured on three modally reflective microtidal beaches in Western Australia. The filtered volumes were large, 19 m3 m?1 day?1 and 73 m3 m?1 day?1 on two ‘clean’ beaches but only 0·4 m3 m?1 per tidal cycle on a beach covered in kelp and seagrass wrack. The mean residence times of this water in the interstitial system and its percolation paths were both short, 1–7 h and 2–5 m respectively. Water input was greater across a beach cusp horn than across a cusp embayment. Most input occurred in the upper swash zone where the water table was less than 20 cm deep. Tidal variations in input volumes were evident even with tide ranges of only 20 cm. The inshore zone off these beaches filters on average 0·07 m3 m?2 day?1 at an average depth of 5·5 m under 0·4 m waves of 6·5 s duration. The importance of these procedures in the mineralization of organic materials and the regeneration of nutrients for an inshore ‘lagoon ecosystem’ is estimated and discused.  相似文献   

6.
Submarine canyons may be particularly important in the transport process of drift seagrasses and seaweeds from highly productive shallow lagoon areas to deeper water. We studied the composition, export, and faunal utilization of shallow, nearshore benthic vegetation as it was transported to offshore areas via the Salt River submarine canyon on the island of St Croix, U.S. Virgin Islands. The study was conducted using a saturation diving system (NULS-1: Hydrolab) during two missions in April and August, 1980. Using bottom drifters deployed in Salt River Bay and the submarine canyon, we recorded net benthic current flow up to 2 cm s?1 moving out of the lagoon and down the canyon to deeper water. Using bottom nets set up at the canyon head and at the 29 m isobar, and from transect surveys and drift clump samples, we determined drift plant export rates and drift clump biomass and species composition. The dominant drift plants were Thalassia testudinum and Syringodium filiforme and algae in the genera Dictyota, Dictyopterus, and Diloplus. During the second mission, the seagrass Halophila decipiens became more abundant, both in the drift and in large patches along the canyon floor. In both missions, more drift was collected in the nets during high wind conditions than during calmer days. Calculated turnover times ranged from 0·01 to 4·4 days for algae in the order Dictyotales and 4·4 to 18 days for Thalassia blades. Total exported biomass of drift vegetation varied between 1·4 to 65·1 kg wet wt day?1. Samples of drift vegetation contained mostly juvenile forms of both invertebrates and fishes, but in relatively low numbers. Faunal numbers were most strongly related to rate of drift movement.  相似文献   

7.
Flower and fruit production of the abundant, tall, long-lived, dioecious, surface-pollinating seagrass species Enhalus acoroides (L.) Royle were estimated at seven sites in the reef flats off Bolinao (NW Luzon, The Philippines) featuring different fragmentation of the seagrass meadows. Fragmentation of the seagrass meadow was quantified as cover of E. acoroides and all seagrass species present in 20×20 m plots. E. acoroides and overall seagrass cover were correlated positively. The proportion of female flowers of E. acoroides that developed a fruit increased sharply as overall seagrass cover was around 50%. Apparent sex ratio bore no relationship with overall seagrass cover. This threshold-type of relationship suggests that fragmentation of seagrass meadows can have a major effect on the reproductive output of this species. A possible mechanism underlying these results would be a non-linear increase of the efficiency of trapping the surface-dispersed pollen with increasing seagrass canopy density. This provides the first evidence based on real data that fragmentation can affect the population dynamics of seagrass species.  相似文献   

8.
In July 1998, a bottom-mounted Acoustic Doppler Current Profiler was deployed at 36m depth in the centre of the Tsitsikamma National Park on the eastern Agulhas Bank, South Africa. The purpose was to investigate transport of chokka squid Loligo vulgaris reynaudii paralarvae hatched on the inshore spawning grounds (<60m) and ichthyoplankton spawned within the park. Analysis of the first 12 months of data (July 1998–June 1999) shows that surface flow was mainly eastward (alongshore), with a maximum velocity (u-component) of +115cm s?1 and an average of +24cm s?1. Generally, velocity decreased with depth, with a maximum bottom velocity (u-component) of +65cm s?1 and an average of +10cm s?1. Data from a nearby thermistor array show that the water column was usually isothermal during winter (July–September), with bottom flow in the same direction as the surface layer. In summer (December–March), vertical stratification was most intense, and surface and bottom flows differed in velocity and direction. Potential net monthly displacements calculated for three depths (5m, 23m and 31m) indicate that passive, neutrally buoyant biological material (e.g. squid paralarvae, fish eggs and larvae) would likely be transported eastwards in the surface layer for eight of the 12 months, and would generally exceed distances of 220km month–1. Displacement in the bottom layer was more evenly distributed between east and west, with net monthly (potential) transport typically 70–100km, but reaching a maximum of 200km. Wind-driven coastal upwelling, prevalent during the summer, causes the surface layer of the coastal counter-current to flow offshore for several days, resulting in potential displacement distances of 40km from the coast. These results suggest that squid paralarvae hatched on the inshore spawning grounds are not generally transported towards the 'cold ridge', a prominent semi-permanent oceanographic feature of cold, nutrient-rich upwelled water, where food is abundant, and that fish larvae, whether from the surface or bottom layer, are exported beyond the boundaries of the Tsitsikamma National Park.  相似文献   

9.
Use of the quadratic shear-stress law for estimating boundary drag requires specific knowledge of the magnitude of a drag coefficient, CD, and sectional mean velocity, u?. In previous attempts to adapt the relationship for use in studies of marine-sediment transport, the flow measurement has been standardized at a level 100 cm above the bed. The particularized value of the drag coefficient has been designated as C100.In the entrance area to Chesapeake Bay, Virginia, C100 has been found to range through unacceptably wide limits. Two-thirds of the values obtained are between 3.5 · 10?3 and 5.4 · 10?2. Mean C100 for the area is 1.3 · 10?2 as compared to 3 · 10?3 for tidal channels within Puget Sound, Washington.Present data suggest that, given a moveable bed, a size hierarchy of mobile bed forms, time-varying flow, and a lack of equilibrium between flow and bed, C100 changes continuously with boundary shear stress.Accurate evaluation of boundary shear stress in tidal entrances with high flow rates and mobile beds presently requires measurement of velocity profiles.  相似文献   

10.
Erosion rates are 15–18.5 times higher in places where there is a beach at the cliff foot compared with those where there is no beach. The zone of intense erosion associated with a beach of sand and pebbles was below a height of about 10 cm above the surface, maximum erosion being at 14.5 cm below the surface. Erosion was by corrasion and wedging, a previously unrecognised process which is the quarrying of rock aided by fine-grained particles from the beach. Wedging, perhaps with some corrasion, had a median erosion rate of 11.05 · 10?3 cm tide?1 in Upper Lias shale while the rate for corrasion was 5.79 · 10?3 cm tide?1. A smooth, notched cliff foot may be produced by these processes. Where there was no beach, erosion of the cliff foot was by the sporadic quarrying of blocks of rock and the more continuous micro-quarrying of shale fragments. The cliff foot was dominated by structural planes and had a rough surface. The horizontal erosion rate due to quarrying at one cliff foot was 2.3 cm year?1. Quarrying acts on the cliff foot above the intense zone of erosion at a beach surface. The median erosion rate for this quarrying was 0.91 · 10?3 cm tide?1.  相似文献   

11.
With full-depth LADCP velocity data collected in a wide area southeast of Kyushu, Japan, large velocity currents, occasionally exceeding 15 cm s?1, were observed in a thick, 500–1,500 m, near-homogeneous density layer below approximately 3,000 m depth around the steep topographies. The currents were found not to flow along the topographic contours, and to be strongly ageostrophic. The directions of the bottom-layer currents are rather related with phase of the semi-diurnal tides, suggesting deeply intruded internal tides generated at the steep topographies.  相似文献   

12.
The mangrove, Avicennia marina var. resinifera in a tidally-flooded explosion crater, Tuff Crater, near the southern latitudinal limit of mangroves in New Zealand adopts two distinct growth forms, taller tree-like mangroves up to 4 m tall along the banks of the tidal creek, and low stunted shrub mangroves less than 1 m tall on the mudflats. Twelve trees were felled and on the basis of a biomass/height relationship for the taller trees and a biomass/canopy width relationship for the lower, above-ground biomass (excluding pneumatophores) was estimated. Average above-ground biomass for the taller mangrove was estimated to be 104·1 t ha?1 and for the lower 6·8 t ha?1. While the value for the taller mangroves is similar to figures reported for more complex tropical mangroves, the fact that 94% of the basin is covered by low generally sparse mangroves means that total biomass for the basin is estimated to be 153 t, an average of only 7·6 t ha?1. Litter-fall beneath the taller mangroves is estimated as 7·6±2·5 t ha?1 a?1 and beneath the lower mangroves 3·3±0·5 t ha?1 a?1. The value for the taller mangroves is similar to that reported from mangroves in many other parts of the world, but because of the extensive low sparse mangroves the total for the basin is estimated as 53·7 t a?1, an average rate of 2·7 t ha?1 a?1, a very low rate of litter-fall when compared with elsewhere. Decomposition of mangrove leaves occurs relatively rapidly with leaves losing half their dry weight in 10 weeks and then continuing to degrade but at a slower rate. Substrate sediment samples contain high organic matter content, and although some organic matter appears to be exported via the tidal creek, a proportion of the detrital production is evidently recycled in situ.  相似文献   

13.
Sponges are inhabited by a wide variety of organisms and have been regarded as one of the richest biotopes in tropical seas. The aim of this study was to assess the influence of the host morphology and selected environmental conditions on macrofaunal assemblages associated with the sponge Halichondria melanadocia in an estuarine system of the southern Gulf of Mexico. This sponge exhibits different growth forms when it inhabits mangrove prop roots of Rhizophora mangle (thickly encrusting form) and adjacent seagrass beds (massive, amorphous or ramose form). From a total of 50 sponge specimens collected in each habitat, a total of eight taxa (of epi‐ and endobionts) was found associated with this sponge, with polychaetes, echinoderms and crustaceans the most abundant groups. In both morphotypes (thickly‐encrusting and massive‐ramose forms), taxon richness was positively related to sponge volume and oscular diameter. The overall mean abundance of associated fauna was also positively related to sponge volume in both morphotypes and with the oscular diameter (in the seagrass morphotype only). These findings suggest that H. melanadocia constitutes an important microhabitat for a wide variety of fauna, independent of its morphology and habitat type. However, when comparing the two morphotypes, the mangrove individuals, despite having smaller sizes, smaller oscular diameter and less structural complexity, displayed an overall mean abundance of fauna 17 times higher [0.36 ± 0.18 individuals (ind.)·ml·sponge?1] than that recorded in the seagrass individuals (0.021 ± 0.01 ind.·ml·sponge?1). There were also marked differences in the proportions of the major taxonomic groups; in fact, some of them were found exclusively in one morphotype. The variability recorded in the composition and abundance of associated fauna between the morphotypes seems to be influenced by differences in sponge morphology, environmental conditions (e.g. sedimentation rate and light intensity), substrate orientation and the fauna inhabiting the surrounding area.  相似文献   

14.
The flux of ammonia, phosphate, silica and radon-222 from Potomac tidal river and estuary sediments is controlled by processes occurring at the sediment-water interface and within surficial sediment. Calculated diffusive fluxes range between 0·6 and 6·5 mmol m?2 day?1 for ammonia, 0·020 and 0·30 mmol m?2 day?1 for phosphate, and 1·3 and 3·8 mmol m?2 day?1 for silica. Measured in situ fluxes range between 1 and 21 mmol m?2 day?1 for ammonia, 0·1 and 2·0 mmol m?2 day?1 for phosphate, and 2 and 19 mmol m?2 day?1 for silica. The ratio of in situ fluxes to diffusive fluxes (flux enhancement) varied between 1·6 and 5·2 in the tidal river, between 2·0 and 20 in the transition zone, and from 1·3 to 5·1 in the lower estuary. The large flux enhancements from transition zone sediments are attributed to macrofaunal irrigation. Nutrient flux enhancements are correlated with radon flux enhancements, suggesting that fluxes may originate from a common region and that nutrients are regenerated within the upper 10–20 cm of the sediment column.The low fluxes of phosphate from tidal viver sediments reflect the control benthic sediment exerts on phosphorus through sorption by sedimentary iron oxyhydroxides. In the tidal river, benthic fluxes of ammonia and phosphate equal one-half and one-third of the nutrient input of the Blue Plains sewage treatment plant. In the tidal Potomac River, benthic sediment regeneration supplies a significant fraction of the nutrients utilized by primary producers in the water column during the summer months.  相似文献   

15.
The vertical distributions of excess 210Pb and fall out 239, 240Pu imply a uniform sedimentation rate of 1·4–1·6 cm year?1 from 0 to 105–110 cm. This sediment accumulation rate is compatible with sulfate reduction rate data from this location. Below 70 cm only ‘aged’ refractory carbon is present (CR = 1·8% C) with an age of approximately 2400 years. This phase is present in a number of locations across Long Island Sound. Planktonic carbon (CP) is present above the 60–67 cm horizon. A value of 1·0 for AP (14C activity) at 32–37 cm was taken, AP = 1·285 was used for contemporary plankton. This was obtained by correcting the measured AP of a plankton tow sample for admixed refractory carbon. These values were then used to calculate CR, CP and CF (fossil carbon) at 32–37 cm and 6–12 cm. The only values compatible with the known sulfate reduction rate data are CR equal to pre 60–67 cm levels (1·6–1·8% C), CF being 0·3% C at both depths, and CP decreasing with depth from 0·3 to 0·4% C at 6–12 cm to close to zero at 32–37 cm.  相似文献   

16.
Variability of Northeastward Current Southeast of Northern Ryukyu Islands   总被引：8，自引：4，他引：8  
To better understand the mechanism underlying the variation of the Kuroshio south of central Japan, we have examined the variability of current structure in its upstream region, southeast of Amami-Ohshima Island in the northern Ryukyu Islands. By combined use of ship-mounted Acoustic Doppler Current Profiler (ADCP) and the TOPEX/POSEIDON satellite altimeter data on Path 214, the sea surface absolute geostrophic currents were estimated every ten days from January 1998 to July 2002. The 4.5-year mean surface current was found to flow northeastward north of 26.8°N with a maximum speed of 14 cm s−1 over the shelf slope at 3000 m depth. The moored current-meter observations at three or four mooring stations from Dec. 1998 to Oct. 2002 suggested the existence of a northeastward undercurrent with a maximum core velocity of 23 cm s−1 at 600 m depth over the shelf slope at 1600 m depth. The mean volume transport in the top 1500 m between 27.9°N and 26.7°N is estimated to be 16 × 106 m3s−1 northeastward, including the subsurface core current related component of 4 × 106 m3s−1. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

17.
Resuspension of bottom sediments accounted for 69–92% of mean deposition rates (11·6–69·3 gm?2 day?1) measured at 5 sites in the Peel-Harvey Estuarine System, Western Australia. Deposition rates at all but one of the sites were found to be correlated (P<0·05) with a function of wind combining a fetch factor with the number of hours of winds greater than 5 ms?1 weighted by the third power of wind velocity. The exception was the only site with a cover of benthic algae (Cladophora) during the study. Wind-related resuspension greatly affects the forms and amounts of productivity in this system.  相似文献   

18.
Altimeter and in situ data are used to estimate the mean surface zonal geostrophic current in the section along 115°E in the southern Indian Ocean,and the variation of strong currents in relation to the major fronts is studied.The results show that,in average,the flow in the core of Antarctic Circumpolar Current(ACC) along the section is composed of two parts,one corresponds to the jet of Subantarctic Front(SAF) and the other is the flow in the Polar Front Zone(PFZ),with a westward flow between them.The mean surface zonal geostrophic current corresponding to the SAF is up to 49 cm · s-1 at 46°S,which is the maximal velocity in the section.The eastward flow in the PFZ has a width of about 4.3 degrees in latitudes.The mean surface zonal geostrophic current corresponding to the Southern Antarctic Circumpolar Current Front(SACCF) is located at 59.7 °S with velocity less than 20 cm · s-1.The location of zonal geostrophic jet corresponding to the SAF is quite stable during the study period.In contrast,the eastward jets in the PFZ exhibit various patterns,i.e.,the primary Polar Front(PF1) shows its strong meridional shift and the secondary Polar Front(PF2) does not always coincide with jet.The surface zonal geostrophic current corresponding to SAF has the significant periods of annual,semi-annual and four-month.The geostrophic current of the PFZ also shows significant periods of semi-annual and four-month,but is out of phase with the periods of the SAF,which results in no notable semi-annual and fourmonth periods in the surface zonal geostrophic current in the core of the ACC.In terms of annual cycle,the mean surface zonal geostrophic current in the core of the ACC shows its maximal velocity in June.  相似文献   

19.
We examined the effects of the presence of larval conspecifics on larval vertical distribution of four‐arm and six‐arm plutei of Strongylocentrotus droebachiensis and bipinnaria of Asterias rubens, in laboratory experiments, by manipulating population density. Larvae were introduced to experimental columns (10 × 10 × 30 cm) at one of two or three population densities (S. droebachiensis: 0.1 and 10 larvae·ml?1; A. rubens: 0.1, 1 and 10 larvae·ml?1). Subsequent changes in vertical distribution were determined from images of the larvae in the columns illuminated by a 532‐nm laser and captured by a high‐resolution camera. Larvae of both species were found higher in the water column in the high than in the low and intermediate population densities. The relationship between vertical swimming velocity and nearest neighbour distance (NND) was measured for four‐arm plutei of S. droebachiensis, and used to determine a range in distances among individuals that may affect potential interactions. The variation in swimming velocity decreased with increasing proximity below a threshold distance among individuals of 10 mm, which was greater than the estimated distance in the high population density. We suggest that the increased larval aggregation near the water surface under high population density is the result of a behavioural response to conspecifics.  相似文献   

20.
Excess 210Pb in a core from a Mexican Coastal Lagoon, which has no connection with the sea shows a small but measurable decay over the length of the core, when different approaches were compared (excess and corrected 210Pb activity with depth, total and inorganic cumulative weights) significant differences in the values for the sedimentation rate are obtained. The best coefficient correlation was calculated when corrected 210Pb activity for the uneven distribution of organic matter and cumulative inorganic weight is considered (ω = 0·93 cm yr?1, R = ?0·86; ω = 0·51 cm yr?1 for the top 13 cm, R = ?0·90 and 1·52 cm yr?1 for the interval 14–46 with R = ?0·96).Time frames in the sedimentary column were in agreement between the 210Pb calculated time and the appearance of shells fragments probably associated with the disturbances caused by the 1961 hurricane Tara.The surface accumulation rate is equivalent to a mean deposition of 262·5 g m?2 yr?1 or organic matter which is minor but comparable to some salt marshes of United States.  相似文献   

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