Transport of Crustacean Larvae Between a Low-Inflow Estuary and Coastal Waters

The effectiveness of larval behavior in regulating transport between well-mixed, low-inflow estuaries and coastal waters in seasonally arid climates is poorly known. We determined the flux of an assemblage of benthic crustacean larvae relative to physical conditions between a shallow estuary and coastal waters on the upwelling coast of northern California (38°18′N, 123°03′W) from 29 to 31 March 2006. We detected larval behaviors that regulate transport in adjacent coastal waters and other estuaries for only two taxa in the low-inflow estuary, but they were apparent for taxa outside the estuary. Vertical mixing in the shallow estuary may have overwhelmed larvae of some species, or salinity fluctuations may have been too slight to cue tidal vertical migrations. Nevertheless, all larval stages of species that complete development in nearshore coastal waters were present in the estuary, because they remained low in the water column reducing seaward advection or they were readily exchanged between the estuary and open coast by tidal flows. Weak tidal flows and gravitational circulation at the head of the estuary reduced seaward transport during development for species that completed development nearshore, whereas larval release during nocturnal ebb tides enhanced seaward transport for species that develop offshore. Thus, nonselective tidal processes dominated larval transport for most species back and forth between the low-inflow estuary and open coastal waters, whereas in adjacent open coastal waters, larval behavior in the presence of wind-induced shear was more important in regulating migrations between adult and larval habitats along this upwelling coast.     

Persistence of tidally-oriented vertical migration by zooplankton in a temperate estuary

Tidal vertical migration by zooplankton is a common phenomenon in estuaries, usually associated with landward movement of meroplankton or position maintenance of holoplankton. Little is known about the persistence of this behavior, its spatial variability, or its response to changing environmental conditions. We extended a previous study of tidal movements of zooplankton in the low-salinity zone (LSZ) of the San Francisco estuary in 1994 to include data from two additional years with very different hydrology. Freshwater flow during sampling in 1995 was about 7-fold greater than in 1994; the LSZ was about 28 km further seaward, and gravitational circulation in the LSZ was strong. In 1996 freshwater flow and LSZ position were intermediate but, because the LSZ was in shallower water in 1996 than in 1995, gravitational circulation was uncommon. Behavior of copepods in both years was similar to that reported in 1994 with some tidal migration observed during most cruises. An exception was the introduced carnivorous copepodTortanus dextrilobatus, which did not migrate and maintained a position deep in the water column (1995 only). In 1996, mysids mainly stayed near the bottom with evidence for vertical migration from only 1 of 6 data sets, whereas amphipods migrated slightly on a diel schedule; these behaviors contrasted with the tidal migration observed in 1994. The bay shrimpCrangon franciscorum did not appear to migrate, but was more abundant in the water column during both ebb and flood, suggesting passive vertical dispersal. Zooplankton did not appear to maintain position by interactions with lateral circulation cells. The results for copepods suggest rigidity in behavior with little or no relaxation of the vertical movement in 1995 when strong gravitational circulation would have made upstream movement relatively easy. Mysids and amphipods altered their behavior depending on local conditions related to freshwater flow.     

Colonisation of Beach-Cast Macrophyte Wrack Patches by Talitrid Amphipods: A Primer

Sandy beaches are characterised by heterogeneously distributed food sources both in time and space. The major energy supply is derived from marine subsidies in the form of beach-cast macrophyte wrack. Wrack patches are short-lived, and their position on the beaches varies with tidal and seasonal cycles as well as weather conditions. Little is known about how sandy beach inhabitants orient themselves towards, and colonise, wrack patches. In a series of field studies on islands off Vancouver Island (British Columbia, Canada), colonisation patterns of wrack patches by beach fleas and sand hoppers (Amphipoda: Talitridae), the most abundant macrofaunal detritivores, were studied. As indicated by colonisation of experimental patches of wrack and wrack surrogates that were either visible or buried in the sand, beach fleas (Traskorchestia traskiana) rely on olfactory cues for locating freshly deposited wrack patches in their patchy and dynamic habitat. Dense colonisation of freshly deposited algal wrack generally occurred within less than 1 h but depended upon the tidal height of wrack patches. Beach flea density in freshly deposited wrack patches increased with increasing tidal height. By contrast, sand hoppers (Megalorchestia californiana) colonised freshly deposited wrack patches in densities that decreased with tidal height. Discussing these interspecific differences, we provide a primer for future detailed studies on transport of matter along the marine–terrestrial gradient of sand beaches.     

Of sand and mud: Sedimentological criteria for identifying the turbidity maximum zone in a tidally influenced river

The thickness and lateral distribution of sand and mud beds and bedsets on channel bars from the tidally influenced Fraser River, British Columbia, Canada, are quantitatively assessed. Fifty‐six vibracores totalling ca 114 m of vertical section are used to tabulate bed thicknesses. Statistical calculations are undertaken for nine channel bars ranging from the freshwater and tidal zone, to the sustained brackish water and tidal zone. The data reveal that thickness trends can be organized into three groups that broadly correspond to time‐averaged hydrodynamic and salinity conditions in the various distributary channels. Thick sand beds (up to 30 cm) and thin mud beds (up to 5 cm) characterize the freshwater tidal zone. The tidal and freshwater to brackish‐water transition zone comprises thin sands (up to 10 cm) and thicker muds (up to 19 cm), and the sustained brackish water tidal zone consists of thin muds (up to 6 cm) with relatively thicker sands (up to 25 cm). The results suggest that the locus of mud deposition occurs in the tidal freshwater to brackish‐water zone, probably reflecting mud flocculation and deposition at the turbidity maximum. Landward of the turbidity maximum, mud deposition is linked to tidal influence (tidal backwater effect and reverse eddy currents on channel margins) as mud beds thin in the landward direction. These results support the hypothesis that mud deposition is greatest at the turbidity maximum and decreases in both the seaward and landward direction. This study also showcases that mud‐bed thicknesses are greatest towards the turbidity maximum and thin in both the landward and seaward direction. In the rock record, the apex of mud deposition probably marks the position of the palaeo‐turbidity maximum.     

Recruitment Ecology of Burrowing Shrimps in US Pacific Coast Estuaries

Recruitment is a strong determinant of year class strength and adult population density especially for sessile benthic invertebrates where post-settlement mortality and competition are low or relatively stable over time. A series of surveys were undertaken to characterize recruitment and post-settlement processes for two species of burrowing shrimps, Neotrypaea californiensis and Upogebia pugettensis in order to determine how they influenced broader adult populations in US west coast estuaries. On average, U. pugettensis decapodids settled earlier (April–July), recruited almost exclusively to areas with conspecific adults, and grew more rapidly during their first summer than N. californiensis. Neotrypaea californiensis decapodids settled and recruited over a longer period (June–November) and were distributed across the tidal flat. While initially more abundant in areas with conspecific adults, they also either survived better or redistributed as small juvenile shrimp to areas where adults were absent. Linear relationships were found between abundance of newly recruited (0+ age class) shrimp and that of older 1+ shrimp a year later. Positive slopes were close to one for N. californiensis but less than one for U. pugettensis, suggesting lower survival. Annual recruitment varied dramatically but was more consistent for both species in Yaquina Bay. Patterns in strong recruitment years amongst estuaries, particularly for U. pugettensis, suggest the presence of multi-estuary metapopulations linked via larval dispersal. These results have important implications for shrimp population management including control for shellfish aquaculture, but also conservation of estuarine habitats due to the strong influence of these ecosystem engineers on the benthic community.     

The morphological dynamics of intertidal megaripples in the Mawddach Estuary, North Wales, and the implications for palaeoflow reconstructions

The morphology and migration rate of tidal bedforms are important because of their use in interpretation of modern and ancient sediment transport regimes. Tidal flow, megaripple morphology and migration were studied in the mesotidal Mawddach Estuary, North Wales, to examine the veracity of published flow-bedform relationships, quantify spatial variations in migration and assess consequences for palaeoflow reconstruction. Two transects were surveyed along a megarippled intertidal shoal (mean grain size 280 μm) for a period of 22 semi-diurnal tidal cycles. A vertical array of current meters recorded tidal current profiles at the centre of one of the transects. Flood tidal currents dominate at Fegla Fach shoal, with peak velocities over 1 m s^?1 at spring tides, and 0.5 m s ^?1 at neaps, and bed sediment transport was also flood-dominated. Over the lunar cycle, the morphology of the megaripples on the survey lines was divisible into three phases: 1 the neap mode-consisting of near-moribund two-dimensional (2-D) flood-orientated megaripples of wavelength c. 6 m and height c. 0.2 m; 2 a transitional mode-where, on rising tidal ranges, scour pits formed and developed into 3-D megaripples which underwent net migration with the flood tide; 3 the spring mode-consisting of 3-D megaripples of wavelength c. 4 m and height c. 0.2 m. Despite complete re-orientation by the ebb tide, these were recognizable from one low water survey to the next, and net migration was c. 1 m per tide with the flood tide. We infer the presence of the equilibrium ‘spring tidal form’ occurring as flood-orientated megaripples during the flood tide. The data support previously reported separation of 2-D and 3-D megaripples at a depth to grain size ratio of 8000, and at a depth-mean velocity of the dominant tide (U_maxdom) of 0.75-0.8 m s^?1. A migration threshold exists at U_maxdom of c. 0.53-0.57 m s^?1. Measures of migration which might be used on preserved sections have been applied to the data. These measures systematically overestimated bedform migration at most stages of the lunar cycle (by <25% at spring tides and <140% in the post-spring transition period), but were accurate when the megaripples had developed into their 'spring tidal form’. There is significant variation of migration rates within the survey populations. We conclude that whilst the occurrence of megaripple cross-sets may be used as a palaeoflow indicator, and sedimentary structures associated with 2-D to 3-D transitions may also be indicative of palaeoflows, there are likely to be significant uncertainties involved in using tidal bundles as an indication of sediment transport rates.     

Nekton use of salt marsh creeks in the upper Tejo estuary

The use of the Tejo estuary, Portugal, salt marsh creeks by nekton was examined based on sampling surveys with a fyke net from September 1998 until August 2001. From the 20 taxa (14 fish species, 5 decapod crustacean species, and 1 cephalopod species) identified in the studied creeks, 16 were regularly caught throughout the sampling period. The shrimpPalaemonetes varians was the most numerically abundant species in the creeks, while the biomass was dominated by the mulletLiza ramada. The nekton assemblage was mainly represented by marine-estuarine opportunist species, comprising 85% of the total. A high seasonality was detected on the species abundance patterns: the most abundant species (P. varians, Crangon crangon, L. ramada, Pomatoschistus microps, Syngnathus sp., andAnguilla anguilla) occurred throughout the sampling period,Sardina pilchardus, Dicentrarchus, labrax, andAtherina boyeri were particularly abundant in spring and summer, andEngraulis encrasicholus, Liza aurata, Gambusia holbrooki, Palaemon longirostris, andPalaemon serratus were most abundant in autumn and winter.L. ramada occurred in the tidal creeks in high numbers during neap tides, while the majority of the remaining taxa were most abundant during spring tides, suggesting a differential pattern of habitat use occording to species.     

Temporal and vertical distribution of crab larvae in a tidal pass

We investigated the vertical and temporal distributions of portunid zoea andCallinectes megalopae in a tidal pass of the Texas coast. Zoea were equally abundant on ebb and flood tide but were more abundant during the day than at night. Megalopae were more abundant during flood tide and at night than during ebb tide or during the day. We examined the evidence for selective tidal stream transport in both life-history stages. Depth of the centers of mass of the larval distributions and the dispersions around those centers were unrelated to temperature, salinity, current velocity, and time of day. Scaling arguments suggest that the absence of pattern in the vertical distributions was not due to turbulent mixing. There was little evidence that either larval stage used these environmental characteristics as cues for changes in behaviour. However, the presence of megalopae in the water column primarily during flood tide does support the tidal transport hypothesis. Megalopae may have difficulty sensing and reacting to environmental cues in wellmixed estuaries with semidiurnal tides.     

Phytoplankton biomass, cell diameter, and species composition in the low salinity zone of Northern San Francisco Bay estuary

Phytoplankton chlorophyll a concentration, biovolume, cell diameter, and species composition differed across the narrow, low salinity zone between 0.6‰ to 4‰ and may influence copepod food availability in the northern San Francisco Bay Estuary. The highest chlorophyll a concentrations (range 3.2–12.3 μg 1^?1), widest cell diameters (>5 μm diam), highest diatom densities and highest production rates of >10 μm diam cells occurred at the landward edge of the salinity zone in April during a strong spring tide and May during a strong neap tide. Near optimum predator/prey ratios, large prey estimated spherical diameters, and high chlorophyll a concentrations suggest these phytoplankton communities provided good food quantity and quality for the most abundant copepods, Eurytemora affinis, Sinocalanus doerrii, and Pseudodiaptomus forbesi. At the center of the zone, chlorophyll a concentrations, diatom densities, and production rates of >10 μm diam cells were lower and cell diameters were smaller than upstream. Downstream transport was accompanied by accumulation of phytoplankton with depth and tide; maximum biomass occurred on spring tide. The lowest chlorophyll a concentrations (1.4–3.6 μg 1^?) and consistently high densities (3,000–4,000 cells ml^?1) of <5 μm diam cells occurred at the seaward edge of the zone, where the green alga Nannochloris spp. and the bluegreen alga Synechococcus spp. were the most abundant phytoplankton. Low chlorophyll a concentrations and production rates of >10 μm diam cells, small prey estimated spherical diameters, and high predator/prey ratios suggested the seaward edge of the zone had poor phytoplankton food for copepodids and adult copepods. The seaward decrease in phytoplankton chlorophyll a concentration and cell diameter and shift in species composition in the low salinity zone were probably a function of an estuary-wide decrease in chlorophyll a concentration, cell diameter, and diatom density since the early 1980s that was enhanced in the low salinity zone by clam herbivory after 1987. *** DIRECT SUPPORT *** A01BY090 00008     

Structure of gravel beaches and their relationship to tidal range

The height of accretionary gravel beaches, determined by the vertical distance between the large sphere zone at the seaward margin and the large disc zone at the landward margin, is determined mainly by tidal range. As the beach builds seaward, this height translates into thickness and so the complete thickness of gravel beach accumulations is a good indicator, in the geological record, of tidal range. Beaches with a small tidal range have the zone between the large discs and the large spheres compressed and sometimes steepened. These beaches are characterized mainly by cusps and berms, both of which yield steeply dipping, seaward building, cross‐strata. Beaches built by large, persistent waves generally have a steep profile, whereas those with small wave height appear to have a shallow profile. Cusps generally have extremely well‐sorted gravels, often in an openwork fabric and with a landward crown of coarse gravel that may grade seaward into shape‐sorted, gently dipping gravels. Berms, which may extend for hundreds of metres along the coast, comprise, in section, very well‐sorted cross‐stratified gravels that may intermittently build seaward for a distance >15 m. Extensive selection pavements typify beaches with a high tidal range and comprise sheets of gravel which are well‐sorted and dip at comparatively low angles towards the sea. These selection pavements form beds that, in section, can be >50 m in seaward length; they grow from clasts, supplied to them from swash or backwash, and which are selected by the fabric of the existing sheet. In some cases, the selection pavements can be very mature having only a specific clast shape and size remaining. Total preservation of the beach sequence in the geological record is more likely to take place in areas of rapid deposition of sediments, such as the marine fringes to a fan delta.     

Suspended-Sediment Flux and Retention in a Backwater Tidal Slough Complex near the Landward Boundary of an Estuary

Backwater tidal sloughs are commonly found at the landward boundary of estuaries. The Cache Slough complex is a backwater tidal region within the Upper Sacramento–San Joaquin Delta that includes two features that are relevant for resource managers: (1) relatively high abundance of the endangered fish, delta smelt (Hypomesus transpacificus), which prefers turbid water and (2) a recently flooded shallow island, Liberty Island, that is a prototype for habitat restoration. We characterized the turbidity around Liberty Island by measuring suspended-sediment flux at four locations from July 2008 through December 2010. An estuarine turbidity maximum in the backwater Cache Slough complex is created by tidal asymmetry, a limited tidal excursion, and wind-wave resuspension. During the study, there was a net export of sediment, though sediment accumulates within the region from landward tidal transport during the dry season. Sediment is continually resuspended by both wind waves and flood tide currents. The suspended-sediment mass oscillates within the region until winter freshwater flow pulses flush it seaward. The hydrodynamic characteristics within the backwater region such as low freshwater flow during the dry season, flood tide dominance, and a limited tidal excursion favor sediment retention.     

Effects of common reed (Phragmites australis) invasion on marsh surface macrofauna: Response of fishes and decapod crustaceans

The tidally inundated marsh surface is an importnat site for energy exchanges for many resident and transient species. In many areas along the East Coast of the U.S. the dominant vegetation,Spartina alterniflora, has been replaced by the common reed (Phragmites australis). This shift has caused concern about the impact ofPhragmites on marsh fauna but research in this area has been limited. During 1997 and 1998, we examined the effect ofPhragmites on fish and decapod crustacean use of the marsh surface in the brackish water reaches of the Mullica River, in southern New Jersey, U.S. Fish and decapod crustaceans were sampled with an array of shallow pit traps (rectangular glass dishes, 27.5×17.5×3.7 cm) and with flumes (1.3 m wide×10 m long of 3.2-mm mesh). Fish (2–60 mm TL) dominated pit trap collections withFundulus heteroclitus andFundulus luciae significantly more abundant atSpartina sites.Fundulus heteroclitus was also the dominant fish (15–275 mm TL) collected in flumes but collections with this gear, including a number of species not collected in pit traps, showed no distinct preferences for different marsh vegetation types. Decapod crustaceans (1–48 mm CW) collected in pit traps were generally less abundant than fishes withCallinectes sapidus andPalaemonetes spp. most abundant inSpartina, whileRhithropanopeus harrisii was most abundant inPhragmites. The same decapod crustacean species (2–186 mm CW) dominanted the flume collections and, similar to the pattern of fish collected by the flumes, there were no distinct habitat preferences for different marsh vegetation types. As a result of these observations, with different sampling techniques, it appears there is an overall negative effect ofPhragmites on larval and small juvenile fish but less or no effect on larger fish and decapods crustaceans.     

The influence of phytoplankton community composition on primary productivity along the riverine to freshwater tidal continuum in the San Joaquin River,California

Differences in phytoplankton community composition along a riverine to, freshwater tidal continuum was an important factor affecting the primary productivity and quantity of phytoplankton biomass available to the San Francisco Estuary food web downstream. The relative contribution of riverine and freshwater tidal phytoplankton was determined using measurements of primary productivity, respiration, and phytoplankton species composition along a riverine to freshwater tidal gradient in the San Joaquin River, one of two major rivers that flow into, the San Francisco Estuary. Chla-specific net primary productivity was greater in the freshwater tidal habitat and was correlated with both a higher growth efficiency and maximum growth potential compared with the river upstream. Cluster analysis indicated these differences in growth parameters were associated with differences in species composition, with greater percent diatom and green algal species biomass upstream and flagellate biomass downstream. Correlation between the chla specific net productivity and phytoplankton species composition suggested the downstream shift from riverine diatom and green algal species to flagellate species contributed to the seaward increase in net primary productivity. Environmental conditions, such as specific conductance and water transparency, may have influenced primary productivity along the riverine to freshwater tidal continuum through their effect on both species composition and growth rate. Data suggest light was not the sole controlling factor for primary productivity in this highly turbid estuary; phytoplankton growth rate did not increase when riverine plankton communities from low light conditions upstream were exposed to higher light conditions downstream. This study suggests that the availability of phytoplankton biomass to the estuarine food web may be influenced by management of both phytoplankton growth and community composition along the riverine to freshwater tidal continuum.     

Biosedimentological zonation of Boundary Bay tidal flats, Fraser River Delta, British Columbia

Boundary Bay tidal flats on the inactive southern flank of the Fraser Delta have surface sediments consisting almost entirely of very fine to fine, well to very well sorted sands which show a gradual fining-shorewards trend. Five floral/sedimentological zones form distinct biofacies. These are, from the shoreline seaward, the saltmarsh, algal mat, upper sand wave, eelgrass and lower sand wave zones. The lower limit of the saltmarsh lies at a constant level above which the maximum duration of continuous exposure rises abruptly from ～ 12 to 40 days. Similarly, at the lower limit of the algal mat zone the maximum duration of continuous exposure jumps from 1 to ～ 2 days, and at the upper limit of the eelgrass zone from ～ 0·5 to ～ 0·8 days. These correlations between exposure and zonation are suggested to be causal. In the algal mat and eelgrass zones microtopography of biogenic origin, only a few centimetres high, creates lateral heterogeneity within the zonal biofacies. In the upper sand wave zone, very low amplitude (a～ 0·1 m) symmetrical sand waves (λ～ 30 m) of probable storm-wave origin have a similar effect. In the lower sand wave zone, sand waves (a～ 0·5 m, λ～ 60 m) are formed by tidal currents or wave action and physical sedimentary structures dominate over biogenic ones. The densities of the following macrofaunal organisms which produce distinctive biogenic sedimentary structures were determined on two surveyed transects: Callianassa californiensis and Upogebia pugettensis, both thalassinidean shrimps; three polychaete worms, Abarenicola sp., Spio sp. and Clymenella sp.; the bivalve Mya arenaria and the gastropods Batillaria attramentaria and Nassarius mendicus. Callianassa excavate unlined temporary feeding burrows, whereas Upogebia build mud-lined permanent dwelling burrows. Upogebia are restricted to below mean sea-level where continuous exposure is < 0·5 days, whereas Callianassa extend up to a level, just below mean higher high water, where maximum continuous exposure rises abruptly from 4 to 9 days. This difference in range is probably due to the latter's greater anoxia tolerance—a necessary adaptation for life in an unlined feeding burrow.     

Comparative patterns of occupancy by decapod crustaceans in seagrass,oyster, and marsh-edge habitats in a Northeast Gulf of Mexico estuary

Decapod crustaceans occupying seagrass, salt marsh edge, and oyster habitats within the St. Martins Aquatic Preserve along the central Gulf coast of Florida were quantitatively sampled using a 1-m² throw trap during July–August 1999 and March–April 2000. Relative abundance and biomass were used as the primary measures to compare patterns of occupancy among the three habitat types. Representative assemblages of abundant and common species from each habitat were compared using Schoener's Percent Similarity Index (PSI). In all, 17,985 decapods were sampled, representing 14 families and 28 species. In the summer sampling period, mean decapod density did not differ between oyster and seagrass habitats, which both held greater densities of decapods than marsh-edge. In the spring sampling period oyster reef habitat supported greater mean decapod density than both seagrass and marsh-edge, which had similar densities of decapods. Habitat-specific comparisons of decapod density between the two sampling periods indicated no clear seasonal effect. In summer 1999, when seagrasses were well established, decapod biomass among the three habitats was not significantly different. During spring 2000, decapod biomass in oyster (41.40 gm⁻²) was greater than in marshedge (4.20 gm⁻²), but did not differ from that of seagrass (9.73 g m⁻²). There was no significant difference in decapod biomas between seagrass and marsh-edge habitats during the spring 2000 sampling period. The assemblage analysis using Schoener's PSI indicated that decapod assemblages associated with oyster were distinct from seagrass and marshedge habitats (which were similar). The results of this study suggest that in comparison to seagrass and marsh-edge habitats, oyster reef habitats and the distinct assemblage of decapod crustaceans that they support represent an ecologically important component of this estuarine system.     

Influence of vegetation on aeolian sand transport rate from a backshore to a foredune at Hasaki, Japan

The influence of vegetation on aeolian sediment transport rate in the region from a backshore to a foredune was investigated at the Hasaki Coast in Japan, where an onshore wind was predominant and the creeping beach grasses Carex kobomugi and Calystegia soldanella were major species. The comparison of cross-shore distributions of the cross-shore component of aeolian sand transport rate with and without vegetation, which were estimated on the basis of the beach profile changes and a mass conservation equation, showed that the creeping grasses influenced the aeolian sand transport rate. The landward aeolian sand transport rate rapidly decreased landward from the seaward limit of vegetation when the grasses grew. The aeolian sand transport rate reduced by 95% with a vegetation cover of 28%. On the other hand, when the grasses were absent, the landward aeolian sand transport rate did not decrease near the seaward vegetation limit, but near the foot of the foredune.     

Nekton use of submerged aquatic vegetation, marsh, and shallow unvegetated bottom in the Atchafalaya River delta, a Louisiana tidal freshwater ecosystem

We sampled nekton (fishes and decapod crustaceans) in submerged aquatic vegetation (SAV) (Potanogeton nodosus, Najas guadalupensis), in emergent marsh vegetation (Sagittaria spp. andScirpus americanus), and over unvegetated bottom associated with three islands in the Atchafalaya River Delta, Louisiana. The purpose of our study was to quantify nekton densities in these major aquatic habitat types and to document the relative importance of these areas to numerically dominant aquatic organisms. We collected a total of 33 species of fishes and 7 species of crustaceans in 298 1-m² throw trap samples taken over three seasons: summer (July and August 1994), fall (September and October 1994), and spring (May and June 1995). Fishes numerically accounted for >65% of the total organisms collected. Vegetated areas generally supported much higher nekton densities than unvegetated sites, although bay anchoviesAnchoa mitchilli were more abundant over unvegetated bottom than in most vegetated habitat types. Among vegetation types, most species showed no apparent preference between SAV and marsh. However, inland silversidesMenidia beryllina and freshwater gobiesGobionellus shufeldti were most abundant inScirpus marsh in summer, and blue crabsCallinectes sapidus were most abundant in SAV (Potamogeton) in spring. Several species (sheepshead minnowCyprinodon variegatus, rainwater killifishLucania parva, and blue crab) apparently selected the vegetated backmarsh of islands (opposite of riverside) over stream-sideScirpus marsh. Freshwater gobies, in contrast, were most abundant in streamsideScirpus marsh. Densities of juvenile blue crabs were high (up to 17 m⁻²) in vegetated delta habitat types and comparable to values reported from more saline regions of Gulf Coast estuaries. Shallow vegetated habitat types of the Atchafalaya River Delta and other tidal freshwater systems of the Gulf Coast may be important nursery areas for blue crabs and other estuarine species.     

Rapidly entrainable tidal vertical migrations in the salt marsh snailLittoraria irrorata

The salt marsh periwinkleLittoraria irrorata (Say) remains on the substratum during low tide but climbs above the water on stalks ofSpartina alterniflora Loisel during high tide. Rhythmic tidal migrations may allowL. irrorata to avoid predators such as blue crabsCallinectes sapidus Rathbun that forage when the marsh is inundated. These tidal rhythms may be driven by endogenous clocks or they may be easily entrained. Snails with flexible and entrainable climbing rhythms may be able to avoid predators in unpredictable environments (e.g., when water unexpectedly covers the substratum as in storm surges). We tested the behavioral response ofL. irrorata to different simulated tidal regimes in the laboratory, and the effect of remaining above mean high water (MHW) on snail survivorship in a smallS. alterniflora salt marsh. In laboratory mesocosms, vertical snail position was measured under constant water levels, simulated tidal cycles, and simulated tidal cycles 180° out of phase (reversed). Under constant water levels, snails ceased to migrate vertically after 1 d. When exposed to tidal and reversed tidal cycles, snails migrated in synchrony with the appropriate simulated rhythm.L. irrorata entrained quickly to differing tidal cycles and maintained their position above the water surfce when water levels were high. In a field experiment, snails were tethered toS. alteriflora plants near the substratum and above MHW in the marsh for 1 wk to assess survival. Survival of snails tethered above MHW was sigificantly greater than for snails tethered at the base of plants; no snails in control cages died. Rapid alteration of tidal vertical migrations may allowL. irrorata to avoid predators that forage when water inundates the marsh predictably or unexpectedly.     

Hydrodynamics and sediment dynamics of The Wash embayment, eastern England

Water and sediment movement in The Wash embayment has been determined from an extensive set of data, consisting of tidal current readings and suspended sediment concentration measurements. Instantaneous and residual currents in the embayment show a distinct lateral inhomogeneity, whereas vertically the water column is almost homogenous. The central deep water area (30–40 m) of the embayment is dominated by a residual landward water movement, whilst on the margins, the residual movement is seaward. Sediment is supplied predominantly in suspension from the north, through the northern extremity of Boston Deep. Suspended sediment pathways are coincident with the spring tide water movements and the subtidal channels act as the main conduits. Approximately 6·8 × 10⁶ tonnes yr^?1 of suspended sediments are supplied to the embayment from offshore areas. Bedload sediment supply is of lesser importance, ～ 1·4 × 10⁴ tonnes yr^?1. Whereas suspended sediment movement appears to be the dominant mode of transport throughout the embayment, bedload transport is important in reforming the sea bed into a variety of bedforms which are particularly well developed on the margins of channels and shoals.     

Facies architecture in a tectonically influenced estuarine incised valley fill of Miocene age, northern Brazil

The Miocene Barreiras Formation in the Middle Rio Capim area records an incised valley system for which facies analysis and ichnology (Skolithos, Ophiomorpha, Planolites, Gyrolithes, Taenidium) suggest an estuarine character. Three stratigraphic units are recognized (from bottom to top): Unit 1 includes an inner estuarine tidal channel complex and tidal flat/salt marsh deposits; Unit 2 consists of estuarine bay/lagoon and flood tidal delta deposits related to the estuary mouth; and Unit 3 includes a tidal channel with a tidal point bar, as well as tidal flat/salt marsh deposits similar to those from Unit 1. These units and their bounding surfaces record the history of relative sea level changes in the estuary. After a sea level drop, the valley was inundated and formed an amalgamated sequence boundary and transgressive surface. Transgression (Unit 1) promoted the landward shift of flood tidal deltas and lagoon settings (Unit 2). The system then moved seaward, with the superposition of inner estuarine deposits (Unit 3) over Unit 2. Facies architecture seems to have been controlled by tectonics, as shown by: the paleovalley orientation according to the main tectonic structures of the basin; the presence of faults and fractures that displace the basal unconformity; and the abundance of soft sediment deformation.