Low Tidal Range Research Articles

Of Manatees, Mangroves, and the Mississippi River: Is There an Estuarine Signature for the Gulf of Mexico?

Important parameters of estuarine variability include morphology, flushing times, nutrient loading rates, and wetland: water ratios. This variability both reflects and disguises underlying relationships between the physics and biology of estuaries, which this comparative analysis seeks to reveal, using the Gulf of Mexico (GOM) estuaries as a starting point. A question used to focus this analysis is: are the GOM estuaries unique? The GOM receives the Mississippi River, a uniquely large, world-class river, which dominates the freshwater and nutrient inflows to the GOM continental shelf, whose margins include 35 major estuarine systems. These GOM estuaries have 28% and 41% of the U.S. estuarine wetlands and open water, respectively. Within the GOM, estuarine nitrogen, phosphorus, and suspended matter loading varies over 2 orders of magnitude. Anoxic estuarine events tend to occur in estuaries with relatively slow freshwater turnover and high nitrogen loading. Compared to estuaries from other regions in the U.S., the average GOM estuary is distinguished by shallower depths, faster freshwater flushing time, a higher wetland area:open water area ratio, greater fisheries yield per area wetland, lower tidal range, and higher sediment accumulation rates. The average GOM estuary often, but not always, has a flora and fauna not usually found in most other U.S. estuaries (e.g., manatees and mangroves). Coastal wetland loss in the GOM is extraordinarily high compared to other regions and is causally linked to cultural influences. Variations in nutrient loading and population density are very large among and within estuarine regions. This variation is large enough to demonstrate that there are insufficient systematic differences among these estuarine regions that precludes cross-system analyses. There are no abrupt discontinuities among regions in the fisheries yields per wetland area, tidal amplitude and vegetation range, salt marsh vertical accretion rates and organic accumulations, nitrogen retention, or wetland restoration rates. These results suggest that a comparative analysis emphasizing forcing functions, rather than geographic uniqueness, will lead to significant progress in understanding how all estuaries function, are perturbed, and even how they can be restored.

Subsurface hydrology and nutrient export from barrier island marshes at different tidal ranges.

The vertical flux of water, ammonium, and phosphatewas measured from two developing barrier islandmarshes; Hog Island, VA (1.3 m tidal range) andPritchards Island, SC (2.5 m tidal range). Verticalflow and nutrient concentrations from the deposit weremeasured using seepage meters placed on the surface ofthe marsh. Measured water flow was not significantlydifferent between the sites (average = 60.1 and 57.4l m-2h-1for the VA and SC marsh). Verticalflow, calculated from piezometric head gradients andhydraulic conductivity, only accounted for 15 and 4%of measured flow at the VA and SC marshes,respectively. Horizontal flow, calculated as thedifference between measured total flow and calculatedvertical flow, was larger at the SC marsh. Therefore,tidal range seems to influence the direction(horizontal vs. vertical), but not the magnitude oftotal water flow from these developing marshes.Despite a lower tidal range, phosphate flux wassignificantly higher at the VA marsh (69 mmol m-2yr-1) relative to the SC marsh (11 mmol m-2yr-1) probably because of higher porewaterphosphate concentrations. Measured ammonium flux wasnot significantly different between the sites (194 and174 mmol m-2yr-1for the VA and SC marsh).Nutrient concentration must be considered togetherwith tidal range when assessing nutrient export. Thisstudy represents one of few that quantify nutrient andwater export from naturally-developing marshes.Although young marshes are thought to export smallquantities of nutrients and other materials, rates ofnutrient export from these developing marshes weresimilar to rates measured in older marshes. Therelatively high export rates from these developingmarshes were attributed to high rates of subsurfaceflow and high nutrient input from a buried marsh ormudflat located one meter below the present marshsurface. These results have implications fordevelopment of created or restored marshes especiallywhere the goals of those activities are to maximizewetland functions such as nutrient exchange.

Recent, arctic deltaic sedimentation: Olivier Islands, Mackenzie Delta, North‐west Territories, Canada

Despite a low tidal range and relatively low wave conditions, the Mackenzie Delta is not prograding seaward but rather is undergoing transgressive shoreface erosion and drowning of distributary channel mouths. In the Olivier Islands region of the Mackenzie Delta the resultant morphology consists of a network of primary and secondary channels separated by vegetated islands. New ground is formed through channel infilling and landward‐directed bar accretion. This sedimentation is characterized by seven sedimentary facies: (1) hard, cohesive silty clay at the base of primary channels which may be related to earlier, offshore deposition; (2) ripple laminated sand beds, believed to be channel‐fill deposits; (3) ripple laminated sand and silt, interpreted as flood‐stage subaqueous bar deposits; (4) ripple laminated or wavy bedded sand, silt and clay, representing the abandonment phase of channel‐fill deposits and lateral subaqueous bar deposition from suspension settling; (5) a well sorted very fine sand bed, presumed to result from a single storm event; (6) parallel or wavy beds of rooted silt, sand and clay, interpreted as lower energy emergent bar deposits; and (7) parallel or wavy beds of rooted silt and clay, believed to represent present‐day subaerial bar aggradation. The distribution of sedimentary facies can be interpreted in terms of the morphological evolution of the study area. Initial bar deposition of facies 3 and channel deposition of facies 2 was followed by lateral and upstream bar sedimentation of facies 3 and 4 which culminated with the deposition of the storm bed of facies 5. Facies 6 and 7 signify bar stabilization and abandonment.Patterned ground formed by thermal contraction and preserved in sediments as small, v‐shaped sand wedges provides the most direct sedimentological indicator of the arctic climate. However, winter ice and permafrost also govern the stratigraphic development of interchannel and channel‐mouth deposits. Ice cover confines flow at primary channel mouths, promoting the bypassing of sediments across the delta front during peak discharge in the spring. Permafrost minimizes consolidation subsidence and accommodation in the nearshore, further enhancing sediment bypass. Storms limit the seaward extent of bar development and promote a distinctive pattern of upstream and lateral island growth. The effects of these controls are reflected in the vertical distribution of facies in the Olivier Islands. The sedimentary succession differs markedly from that of a low‐latitude delta.

Characterization of the Ebre and Rhone estuaries: A basis for defining and classifying salt‐wedge estuaries

River discharge is the main factor controlling the hydrologic dynamics of the Ebre and Rhone estuaries. The topography of the estuary bed also influences the extent, advance, and retreat of the salt wedge. Tides have little influence because of the low tidal range. Mean annual river discharge is close to the critical value determining the formation and breakup of the salt wedge. This is related to the fact that river flow controls the sedimentary dynamics of the estuary, including the sandbar at the mouth. When discharge is lower than the mean annual river flow, the salt wedge is established and the estuary becomes a depositional environment. With higher flows the salt wedge is washed away and erosive conditions prevail. On the basis of the conditions in which the salt wedge is formed, two types of highly stratified estuaries can be identified. In the first one, a salt‐wedge regime is established during low river flows, whereas during high flows the wedge is washed away and the estuary becomes a river. This circulation pattern corresponds to the “salt‐wedge estuary” or type 4 of the Hansen‐Rattray classification, typical of river‐dominated estuaries in microtidal seas. In the second one, a salt wedge is established during high river flows, whereas a partially mixed estuary occurs during low flows. This pattern corresponds to types 3b and 2b of the Hansen‐Rattray classification.

A 7 year record of above-ground net primary production in a southeastern Mexican mangrove forest

Spatial and temporal variations in net above-ground primary production (NPP) and litter turnover rate were studied, from 1987 to 1993, in a mangrove forest bordering Laguna de Terminos, Mexico. NPP, the sum of total litter fall and wood production, was measured over the entire study period in three zones in a basin forest: zone I, where Rhizophora mangle (red mangrove) occurs but Avicennia germinans (black mangrove) is the dominant species; zone II, a scrub forest of A. germinans; zone III, where larger A. germinans trees occur. In 1991, a fringe zone dominated by A. germinans and R. mangle was added to the study. Three distinctive climatic seasons occur in the region: rainy, dry, and cold front (locally named ‘nortes’). Average total litter fall in the fringe zone (793 g m −2 year −1) was significantly higher than in the basin forest (496, 307, and 410 g m −2 year −1 for basin zones I, II, and III, respectively). All zones showed significant differences among seasons with the norte season having significantly lower litter fall. Litter turnover rates were about 7 months in zones I and II and 10 months in zone III, reflecting the low tidal range that occurs in the basin forest. Low litter turnover rates in the basin forest were reflected in a high organic matter standing crop. Annual average stem growth was significantly higher in zones I and III (1.27 and 1.36 kg per tree year −1, respectively) than in zone II (0.62 kg per tree year −1). Above-ground NPP rates in the basin forest (399–695 g m −2 year −1) were lower than in fringe and riverine forests, reflecting patterns of litter fall and wood production. There was no seasonal variation in soil salinity but the basin forest had significantly higher soil salinity than the fringe forest. Spatially, mean soil salinity was inversely related to litter fall. Long-term patterns in soil salinity, precipitation and air temperature explained 74% of the inter-annual litter fall variability. Over the 7 year study, productivity in zone II was more variable than in zones I and III, and productivity (litter fall and wood growth) were less variable than litter standing crop and turnover.

Holocene evolution of coastal wetlands in wet-tropical northeastern Australia

Pollen in sediments drilled from the Innisfail coastal plain, northeast Queensland, Australia, was examined to reconstruct the evolution of Holocene wetlands in a wet-tropical environment. In contrast to monsoonal Australia, stable environmental conditions created by year-round rainfall and low tidal range caused abrupt, unidirectional transitions in wetland zonation as marine influence changed. This has enabled a detailed reconstruction of the marine transgression and subsequent progradation. Mangroves colonized in response to marine transgression at c. 7400 BP, as riverine mangroves of low salt tolerance migrated up the Mulgrave River. These were replaced around 7000 BP by extensive Rhizophora-dominated mangroves, coinciding with the development of a Rhizophora-dominated community in Wyvuri embayment. Mangroves reached their greatest extent as sea-level rise slowed towards 6000 BP, with a stillstand indicated by a brief return to terrestrial conditions at one site. High freshwater input depressed salt intrusion in the upper reaches of the Mulgrave estuary and prevented the development of hypersalinity in the upper tidal zone. Drainage conditions then controlled whether mangroves were succeeded by freshwater swamp or swamp-forest.

Impacts of Sea-Level Rise on Deltas in the Gulf of Mexico and the Mediterranean: The Importance of Pulsing Events to Sustainability

In deltas, subsidence leads to a relative sea-level rise (RSLR) that is often much greater than eustatic rise alone. Because of high RSLR, deltaic wetlands will be affected early by an acceleration of eustatic sea-level rise. If there is sufficient vertical accretion, wetlands can continue to exist with RSLR; however, lack of sediment input eventually leads to excessive water logging and plant death. Areas with low tidal range, such as the Mediterranean and Gulf of Mexico, are especially vulnerable to rising water levels because the elevational growth range of coastal vegetation is related to tide range. Reduction of suspended sediments in rivers and prevention of wetland flooding by river dikes and impoundments have reduced sediment input to Mediterranean and Gulf of Mexico deltaic wetlands. This sediment deficit will become more important with an acceleration in sea-level rise from global warming. Most sediment input occurs during strong pulsing events such as river floods and storms, and management policies and decisions are especially designed to protect against such events. Management approaches must be reoriented to take advantage of pulsing events to nourish marsh surfaces with sediments. We hypothesize that deltas can be managed to withstand significant rates of sea-level rise by taking advantage of pulsing events leading to high sediment input, and that this type of management approach will enhance ecosystem functioning.

The Silty Beds: a tidal flat sequence at the junction of the Lower Greensand and Gault (Albian, Lower Cretaceous) of Bedfordshire, England

The Silty Beds are a Lower Albian tidal flat deposit with sediments of the intertidal and subtidal zone. They are interpreted as the deposit of a sheltered shore, near an estuary mouth, with low wave energy and low tidal range. The Silty Beds are a well-bedded series of sands, silts and clays with minor ironstone and intermittent carbonate lenses occurring towards the top of the unit. These beds include carbonaceous material, pyritized, glauconitic and other trace fossils as well as glauconite-rich sands. Some beds are highly bioturbated, while others show only moderate burrowing. Where there is no burrowing activity, detailed sedimentary structure has been preserved, such as fine parallel-lamination, cross-stratification, fine ripple lamination and abundant flaser bedding. The environment of deposition is interpreted as tidal sand and mud flats with intermittent freshwater and brackish lagoons. Ironstone development was probably early diagenetic. The top of the succession culminates in emergence and erosion of the topmost ironstone. Cavities, thus formed, were subsequently infilled with a bioclastic limestone (the Shenley Limestone) during the transgression leading into the deeper water marine clays of the Gault.

SEQUENCE STRATIGRAPHY AND THE DEPOSITIONAL HISTORY OF THE MURTA MEMBER (UPPER HOORAY SANDSTONE), SOUTHEASTERN EROMANGA BASIN, AUSTRALIA: IMPLICATIONS FOR THE DEVELOPMENT OF SOURCE AND RESERVOIR FACIES

The Hooray Sandstone (Namur Sandstone Member and overlying Murta Member) was deposited during the Early Cretaceous in southwestern Queensland and northeastern South Australia. A sequence stratigraphie model for the Hooray is proposed. The basal beds of the Hooray Sandstone are the braided fluvial deposits of the Namur Sandstone Member, deposited during the late Berriasian and earliest Valanginian on a Type 1 sequence boundary developed during the 131.5 Ma sea level lowstand. Two sequences are recognised within the Namur (Namur Nl and N2) with a Type 1 sequence boundary interpreted within the member (128.5 Ma sea level fall). The upper Namur is gradational with the lower Murta Member M6 which forms a transgressive systems tract. The M5 and M4 are highstand systems tract sediments deposited over the maximum flooding surface at the base of the M5 (127.5 Ma condensed section). Low sea levels led to incision of channels basinwards of exposed basement rocks at the end of M4 time (about 126 Ma). Sea level rise led to the gradual Willing of these channels until the southeastern highlands were drowned but islands in the southeast contributed vitrinitic material during mid M3 time, which formed the source for the oil in the Nockatunga area. Reservoir sandstones are developed within the basal M3 incised valley fill sands and in the transgressive microtidal barrier facies sands at the top of the M3. The former can produce stratigraphic traps, whereas the latter form structural traps. Reservoir quality is probably best in the thicker mid 'estuarine' sands due to tidal reworking, and generally poorer in the barrier facies due to little winnowing of fines because of the low tidal range, weak wave activity, and gene-contemporaneous carbonate cementation. During later M3 time, transgression led to the landward movement of barrier bars, forming a generally sheet-like sand body. The top M2 and M3 barriers probably formed in a microtidal environment under a seasonal climate with periodic storm conditions. Following inundation of the islands during M2 and Ml times, organic matter supplied to the Nockatunga area was dominantly finely comminuted, bacterially-altered plant matter and algal material that forms a potentially rich but immature oil source. The 'hot gamma' shales of the Ml are interpreted as the 123.5 Ma condensed section.

River-beach interaction on mixed sand and gravel coasts: a geomorphic model for water resource planning

A distinctive sequence of river mouth offsetting and freshwater lagoon development is described from mixed sand and gravel coasts on the east coast of the South Island, New Zealand, particularly the Rakaia river. Though total annual sediment load exceeds 4 × 10 6t, the Rakaia is a ‘small’ river in that the coarse fraction nourishing beaches and the regional longshore drift is insufficient to maintain either the river mouth or the adjacent coasts against long-term erosion by the sea. Dominance of marine over fluvial processes is an important overall control of the lagoon and spit development sequence. Long, narrow spits grow generally, but not exclusively northward for up to 3 km at the Rakaia under the influence of longshore drift in low to moderate stages of river flow. Under higher river flows the spits are breached, the lagoon circulation alters and a ‘slug’ of the regional drift sediments temporarily stored in the spits is bypassed to the downdrift coast. River-injected materials contained in flood formed subtidal deltas also contribute to the drift and to marked modulation of the regional net longshore sediment transport which displays a ‘pulsational’ character at downdrift sites. The river mouth lagoons are non-tidal and non-saline owing to generally steep bed gradients in the braided rivers and to low tidal range, though backwater effects are observable and some salt enters by storm overwash and spray. Permeability variations within enclosing spits are important in both the process of mouth offsetting and in flood breaching. A range of potential and actual environmental impacts on such river mouths relating to use of freshwater resources for activities such as hydroelectric power generation and irrigation is discussed in the light of the fact that the coastal lagoons present important concentrations of recreational, wildlife and fisheries values. Irrigation drawoff and hydroelectric power generation can affect the magnitudes of river flow and its temporal pattern at the coast, as well as delivery of coarse sediments to the spits enclosing the mouth, thus markedly altering the interaction with marine processes in the river mouth region. Water resource development on the river has the potential to increase freshwater flooding adjacent to the mouth, to cause protracted closure of the mouths and to accentuate coastal erosion.

A tempestite-stromatolite-evaporite association (late Vendian, East Greenland): a shoreface-lagoon model

Sections of the Canyon Formation (310 m) and overlying Spiral Creek Formation (25 m) have been studied on Ella Ø (Ella Island), central East Greenland. These strata cap Vendian glacial deposits and are overlain disconformably by probable basal Cambrian sandstones. The Canyon Formation is informally divided from bottom to top into mudrock (250 m), siltstone (15 m), flakestone (25 m) and stromatolitic (27 m) members. Overlying a thin basal sandstone interpreted as littoral facies, the main bulk of the mudrock member consists of variably dolomitic silt-shales (offshore deposits below wave base). Bituminous films in the upper half of the member are thought to represent microbial mats. The overlying units represent a major shallowing-upwards sequence. The siltstone and flakestone members consist largely of sharp-based coarse dolomitic siltstones (0.05-0.6 m thick) containing combinations of: gutter casts, parallel and undulatory lamination, wave-ripple lamination and flexed dolostone intraclasts. A tempestite origin is indicated. Dolostone clasts in the flakestone member are analogous to shell fragments in Phanerozoic tempestites and are derived by reworking of thin dolomite crusts within the background sediments of the flakestone member. There is a transitional boundary to the overlying stromatolitic member which consists dominantly of finely crystalline dolostones with thin (<0.05 m) storm layers and stromatolite biostromes and bioherms. The lower half of the member lacks evidence of desiccation, but does contain subaqueous shrinkage cracks. The upper half contains a number of scoured and desiccated dolostone surfaces, abundant rigid intraclasts and several thin oolitic beds. Stromatolite morphology is characteristically broad (metre-scale), fairly smooth domes, whose synoptic relief tends to decrease upwards in individual beds and overall in the member. Scattered anhydrite crystal and nodule pseudomorphs are common, varying in abundance at different horizons. The Spiral Creek Formation contains a basal cross-stratified sandstone, with desiccated dolostone layers, interpreted as a tidal sandflat deposit. The central part of the Spiral Creek Formation consists of centimetre-scale alternations of dolomitic sandstones with short-wavelength wave ripples, and mudrocks with desiccation cracks and halite pseudomorphs interpreted as playa lake deposits. The overall facies assemblage above the mudrock member is interpreted as representing the progradation of a mudrock-dominated evaporative lagoon-sandflat-mudflat-playa assemblage over a shoreface dominated by storm action. The lagoon may have been partly barred by stromatolites, but is seen to be transitional with the shoreface. This requires a new facies model in which muddy coastal and lagoonal carbonate facies are not bounded seaward by a continuous reef or sandbody. This is indicative of low wave energy, except during infrequent storms, and a low tidal range, so that strong tidal currents are only locally developed.

Mechanisms of sediment transport and dispersion in a tectonically active submarine valley/canyon system: Zakynthos Straits, NW Hellenic Trench

Physical oceanographic and geophysical data sets are used to examine sediment transport processes. Currents were directed generally along the axis of the valley/canyon system and, in an area of low tidal range, are attributed to regional weather patterns. Near-bed and near-surface currents are opposed, both in terms of instantaneous observations and net (Lagrangian) drift. Near-bed down-canyon water movement provides a mechanism for the offshore movement of fine-grained sediment. Slope failures have been identified in the geophysical records and are considered to be controlled by earthquakes and salt diapirism, in a tectonically active area. These failures, as mass displacement or slow down-slope creep, result in debris infill and the generation of turbidity currents. Fine-grained sediments, resuspended by the turbidity currents, would be transported down-canyon by the slowly moving near-bed residual water movement. Hence, the dominant transport mechanisms in the valley/canyon system are: (1) frequent mass sediment failure; and (2) nearly continuous down-channel fine-grained suspended sediment movement.

Shallow-marine carbonate facies and facies models

Summary Shallow-marine carbonate sediments occur in three settings: platforms, shelves and ramps. The facies patterns and sequences in these settings are distinctive. However, one type of setting can develop into another through sedimentational or tectonic processes and, in the geologic record, intermediate cases are common. Five major depositional mechanisms affect carbonate sediments, giving predictable facies sequences: (1) tidal flat progradation, (2) shelf-marginal reef progradation, (3) vertical accretion of subtidal carbonates, (4) migration of carbonate sand bodies and (5) resedimentation processes, especially shoreface sands to deeper subtidal environments by storms and off-shelf transport by slumps, debris flows and turbidity currents. Carbonate platforms are regionally extensive environments of shallow subtidal and intertidal sedimentation. Storms are the most important source of energy, moving sediment on to shoreline tidal flats, reworking shoreface sands and transporting them into areas of deeper water. Progradation of tidal flats, producing shallowing upward sequences is the dominant depositional process on platforms. Two basic types of tidal flat are distinguished: an active type, typical of shorelines of low sediment production rates and high meteorologic tidal range, characterized by tidal channels which rework the flats producing grainstone lenses and beds and shell lags, and prominent storm layers; and a passive type in areas of lower meteorologic tidal range and higher sediment production rates, characterized by an absence of channel deposits, much fenestral and cryptalgal peloidal micrite, few storm layers and possibly extensive mixing-zone dolomite. Fluctuations in sea-level strongly affect platform sedimentation. Shelves are relatively narrow depositional environments, characterized by a distinct break of slope at the shelf margin. Reefs and carbonate sand bodies typify the turbulent shelf margin and give way to a shelf lagoon, bordered by tidal flats and/or a beach-barrier system along the shoreline. Marginal reef complexes show a fore-reef—reef core—back reef facies arrangement, where there were organisms capable of producing a solid framework. There have been seven such phases through the Phanerozoic. Reef mounds, equivalent to modern patch reefs, are very variable in faunal composition, size and shape. They occur at shelf margins, but also within shelf lagoons and on platforms and ramps. Four stages of development can be distinguished, from little-solid reef with much skeletal debris through to an evolved reef-lagoon-debris halo system. Shelf-marginal carbonate sand bodies consist of skeletal and oolite grainstones. Windward, leeward and tide-dominated shelf margins have different types of carbonate sand body, giving distinctive facies models. Ramps slope gently from intertidal to basinal depths, with no major change in gradient. Nearshore, inner ramp carbonate sands of beach-barrier-tidal delta complexes and subtidal shoals give way to muddy sands and sandy muds of the outer ramp. The major depositional processes are seaward progradation of the inner sand belt and storm transport of shoreface sand out to the deep ramp. Most shallow-marine carbonate facies are represented throughout the geologic record. However, variations do occur and these are most clearly seen in shelf-margin facies, through the evolutionary pattern of frame-building organisms causing the erratic development of barrier reef complexes. There have been significant variations in the mineralogy of carbonate skeletons, ooids and syn-sedimentary cements through time, reflecting fluctuations in seawater chemistry, but the effect of these is largely in terms of diagenesis rather than facies.

Some observations on Seabirds breeding in the Tsitsikamma Coastal National Park.

In 1980 and 1981 more than 50 pairs of kelp gulls Lams dominicanus, 70 of Cape cormorants Phalacrocorax capensis and 20 of whitebreasted cormorants P. carbo nested in the Tsitsikamma Coastal National Park. Kelp gulls were breeding in the Park in the mid 1960's but no records could be found of breeding by Cape cormorants prior to 1980. The earliest record for nesting by whitebreasted cormorants was 1971 and the population apparently increased in the late 1970's. Small numbers of African black oystercatchers Haematopus moquini nested in the park in 1980 and 1981. Brown mussels Perna perna and limpets Patella spp. dominated their hardshelled diet. Whereas oystercatchers at St Croix Island fed mainly on organisms from the mid intertidal region, those at Tsitsikamma appear to have favoured molluscs from the lower tidal range.

Aspects of growth, recruitment and conservation of the Brown Mussel &lt;i&gt;Perna perna&lt;/i&gt; along the Tsitsikamma Coast

Beds of brown mussels Perna perna are a dominant feature of the lower tidal range of rocky shores in the Tsitsikamma Coastal National Park. Mussels in this region attain a length of 30-40 mm by the end of their first year, and the majority probably exceed 50 mm at two years of age. Secondary settlement of plantigrades occurs from midspring through early winter, especially in summer and early autumn, and growth of adult forms is slowest prior to this settlement. Settlement occurs serially and powerful cohorts may be formed. At Nature's Valley density and biomass of mussels inside the park's boundary are significantly higher than in non-protected areas. Older age groups are also more abundant within the park.

A Regressive Barrier and Barrier--Protected Deposit: Depositional Environments and Geographic Setting of the Late Tertiary Cohansey Sand

ABSTRACT Two facies sequences. which are interpreted as regressive barrier and barrier-protected deposits, make up the Upper Tertiary Cohansey Sand of southern New Jersey. The facies and environments inferred from them, in the 6 m thick barrier sequence are from the base up: interbedded sand and grit with multidirectional trough sets (surf zone); gently dipping laminated sand (foreshore); burrowed laminated sand with heavy mineral concentrations (backshore-dune); peat (freshwater marsh); and laminated clay (salt water marsh). The facies and inferred environments of the 6 m thick barrier-protected sequence, which conformably caps the barrier sequence, are from the base up: cross-bedded sand characterized by tabular sets (subtidal channel); burrowed cross-bedded sand characterized by trough sets and Ophiomorpha (shallow subtidal channel or channel margin); and burrowed, massive sand (sand flat). Lenticular bodies of interbedded sand and clay (restricted or abandoned tidal channels) are scattered throughout the sequence. The Cohansey sediment was deposited along a submergent coastline receiving abundant sediment and characterized by moderate wave energy and a low tidal range. This 30 m thick quartz arenite deposit was built up during at least two regressive-transgressive cycles.

The Wave Pump: Conversion of Wave Energy to Current Energy

The Wave Pump converts wave energy to current energy utilizing the momentum of waves which are close to or at the breaking stage. To improve the efficiency of the Pump so that even smaller waves become effective, waves are concentrated in a funnel. Funnel geometry therefore is an important parameter. So is the wave height over depth ratio. The effectiveness of the pump is not reduced even if the direction of wave propagation deviates —approx. 10? from the centerline of the pump. The efficiency of the pump may be further improved by the installation of a ramp at the entrance to the discharge channel. The pump has been thoroughly tested by hydraulic model tests. The first prototype was built at the Palmas Del Mar marina on Puerto Rico as a flushing device in a low tidal range area. It has been in operation for approx. 2 yr. The pump is also useful for flushing of ice in arctic waters. It may be used as an energy producing device on a modest scale.

An ecological study of the Thecamoebina of Christchurch Harbour, England

Summary Christchurch Harbour is the lagoonal estuary of the Rivers Avon and Stour. Due to the low tidal range much of it remains submerged at low water. The microfauna living on the bottom sediment includes Foraminiferida and Thecamoebina. The latter are normally restricted to the lower reaches of rivers but when low salinity conditions prevail in the lagoonal part of the estuary they have a wider distribution. It is shown that a number of species will tolerate brackish water if the calcium content is less than 1 mg-atom/l. Most species show seasonal changes in abundance related partly to temperature changes. It is suggested that fossil Thecamoebina may be more common than has previously been recognized, especially in Pleistocene and Holocene deposits, and that they would be useful paleoecological indicators.