Main Tidal Channel Research Articles

Mercury contamination in Marano Lagoon (Northern Adriatic sea, Italy): Source identification by analyses of Hg phases

Total Hg concentrations and Hg speciation were determined in bottom sediments of Marano lagoon to investigate the consequences of Hg phases on fish farms and shellfish cultivation areas. Mercury phases were separated into cinnabar (HgS) and non-cinnabar compounds, via a thermo-desorption technique, in surface and core sediments; both of which had been contaminated by industrial wastes and mining activity residues. The former are due to an industrial complex, which has been producing cellulose, chlor-alkali and textile artificial fibres since 1940. Processing and seepage wastewaters, which were historically discharged into the Aussa-Corno river system and therefore into the lagoon, have been significantly reduced since 1984 due to the construction of wastewater treatment facilities. The second source is the Isonzo River, which has been the largest contributor of Hg into the northern Adriatic Sea since the 16th century due to Hg mining at the Idrija mine (western Slovenia). Red cinnabar (HgS) derived from the mining area is mostly stable and insoluble under current environmental conditions. In contrast, organically bound Hg, such as Hg bound to humic acids, has the potential to be transformed into bioavailable Hg compounds (for example, methylmercury). The presence of the two Hg forms permitted each Hg source to be quantified. It also allowed the areas with the highest risk of Hg contamination from Hg-rich sediment to be identified; thus potentially avoiding the transfer of Hg from the sediment into the water column and eventually into living biota. The results show that Hg Enrichment Factors in bottom sediments exceed values of 10 and cinnabar dominates the central sector near the main tidal channel where tidal flux is more effective. Non-cinnabar compounds were found to be enriched in fine grained material and organic matter. In fact, up to 98% of total Hg at the Aussa-Corno river mouth and in the inner margin of the basin occurred in an organic form. This evidence, combined with the high contents of total Hg (4.1–6.6 μg g −1 and EF > 10) measured in surface sediments, suggest that Hg in Marano lagoon is involved in biogeochemical transformations (e.g., methylation).

Complex Sediment Transport Pattern on a Spit-Platform in the Danish Wadden Sea

Spit-platforms have not drawn much attention, despite their expected importance for understanding sediment transport mechanisms leading to siltation of tidal channels. In this article, the complex sediment transport pattern on a shallow spit-platform in the Danish Wadden Sea is described based on measurements from four current meters deployed on the spit-platform in a low- and a high-energy period. The hydrodynamic characteristics at the four stations are different in terms of impact from tidal and wave-induced currents. The sediment enters the spit-platform as sandbars oriented perpendicular to the shore migrating alongshore, because of wave activity, until the sediment reaches an ebb-dominated tidal channel. In and near this channel, the sediment is transported downdrift by the ebb-tidal currents until the sand enters a shallow dissecting ebb channel, where the sand is transported obliquely offshore and deposited on a small ebb-tidal delta. This zigzag pattern of sediment transport continues until all ebb channels (up to four) are bypassed. On the subtidal flats between the most downdrift ebb channel and the main tidal channel, the sediment is transported the remaining distance into the main tidal channel or onto the tidal flat in the lagoon landward of the spit-platform. During higher-energy conditions, the sediment transport is more affected by wave-induced currents, leading to high transport rates in the shallow parts of the spit-platform and less transport in the horizontally restricted channels.

Changes in coastal zone processes at a high sea‐level stand: a late Holocene example from Belgium

AbstractThis paper presents the results of an investigation of late Holocene deposits on the Belgian coastal plain. The upper clastic deposits overlying the uppermost intercalated peat bed were studied on the basis of lithology, radiocarbon dates and foraminiferal assemblages to determine why silting‐up phases or peat beds are lacking, and to establish the evolution of the sedimentary environments as well as the age of the deposits. Two contrasting sites, in quite different locations and showing a different stratigraphy, were studied. It appears that the coastal evolution was not gradual and continuous, but characterised by short periods with high‐energy conditions alternating with long periods where low‐energy conditions and little sedimentation prevailed. The evolution is determined mainly by tidal channel activity. At about 3400 cal. yr BP, the peat was eroded in narrow zones by the main tidal channels and this until the landward limit of the plain. The peat was only partly flooded and mudflats fringed the channels. With time, new tributaries formed and the channel network progressively enlarged. The channels were rapidly filled, followed by a period of little sedimentation, which lasted at least until 1400–1200 cal. yr BP. The following channel activity is characterised by lateral sedimentation, rather than vertical erosion, together with increased sediment reworking. The mechanisms of coastal change in the late Holocene might be similar over the entire plain, but the timing of the changes is certainly not. Copyright © 2002 John Wiley & Sons, Ltd.

Groundwater Outflow and its Linkage to Coastal Circulation in a Mangrove-fringed Creek in Kenya

Research into groundwater outflow and tidal circulation was undertaken in a 32km2mangrove-fringed tropical creek in Kenya. The study involved measurement of the groundwater level, storage and outflow, tidal currents, salinity and temperature. The study shows that groundwater outflow in the creek occurs within the main tidal channels and is not restricted along the shoreline. Groundwater outflow is evidenced by: (1) the occurrence of vertical salinity anomalies; (2) ebb–flood tide salinity differences; (3) occurrence of groundwater in shallow wells a short distance from the mangroves; and (4) enormous groundwater recharge and storage in the Mida basin. Vertical salinity anomalies are characterized by the presence of a lens of low salinity water at the bottom water column and higher salinity at the surface. It is believed that this occurs since no major density differences develop despite vertical salinity differences of up to 1·58, possibly as a result of uniform distribution of water temperature vertically. In drought conditions, seepage reduces and hypersaline conditions (salinity in the range of 36·89 to 38·57) which mask groundwater outflow, develop in the backwater region as a result of intense evaporation and restricted circulation. The strong currents reaching 2·5ms−1occur in the frontwater zone and enhance the tidal flushing while lower currents <0·5ms−1occur in the backwater zone and promote trapping of water which thereby increase the residence time to 11 days. With flood currents being more dominant than ebb currents in the main creek channel, the seaward export of material derived from groundwater is limited.

Evidence for tidally-induced vertical migration of some gelatinous zooplankton in the Wadden Sea area near Sylt

Gelatinous zooplankton is more abundant in the Wadden Sea area of North Sylt than in the adjacent North Sea. The hypothesis is tested that medusae maintain their position in the North Sylt Wadden Sea by ascending to the surface at flood tides and descending to the bottom during ebb tides, thus avoiding the strong ebb currents which could carry them out of this food-rich area. Surface sampling at a main tidal channel revealed great differences between high tide and low tide abundances of five species of medusae in the surface layer.Rathkea octopunctata, Sarsia tubulosa, Eucheilota macultata andPleurobrachia pileus were all more abundant around high tide than during low tide. Bongo net sampling in different depth layers revealed thatPleurobrachia pileus, Bougainvillia ramosa andEucheilota maculata showed a preference for the surface layers around high tide, whereas most of the individuals were found in the deepest layer around low tide. The results suggest tidally-induced vertical migration of medusae in tidal channels. This may assist maintenance of the populations in the Wadden Sea area near Sylt.

Geologic Framework and Hard Mineral Resources of Petit Bois Pass and Adjacent Inner Shelf, Mississippi-Alabama

ABSTRACT Approximately 200 line-km of high-resolution seismic profile data, 24 vibracores, and several C14 samples form the database to delineate the geologic framework and potential hard mineral resources of the Petit Bois Pass area. Petit Bois Pass is a natural tidal inlet system located between Petit Bois and Dauphin Islands at the Mississippi-Alabama state line. The inlet is 8.2 km wide with the main inlet channel located to the east adjacent to Dauphin Island. The inlet throat depth is 6.7 m below mean low water (MLW). The morphology of the main tidal channel is mixed energy, which is reflected by the absence of a flood-tidal delta and a moderately developed ebb-tidal delta. The largest percentage of sediment occurs directly between Dauphin and Petit Bois Islands. More recently, however, a secondary tidal channel and associated ebb-tidal delta are developing to the west of the main tidal inlet channel. On the adjacent inner continental shelf, several shoreface sand ridges occur and are oriented at about 40° in relation to the barrier shoreline; axes of the sand ridges have azimuths of about 120°. The origin of these ridges may be related to the evolution of Petit Bois Pass and other inlets in the area. Tidal inlets are naturally occurring, concentrated sinks of sediment captured from longshore transport. In transgressive barrier island settings, available mechanisms for concentrating sediment are restricted almost exclusively to tidal inlet systems. However, the preservation potential of the tidal inlet system during transgression is limited except for the lower part of the tidal inlet channel and the ebb-tidal delta. The remainder of the inlet system eventually is removed by shoreface erosion as the coastal system translates landward. Therefore, tidal inlets and the adjacent transgressed shelf areas are primarily locations for potential hard mineral resources. In the study area, primary hard mineral resource targets include tidal inlet channels, flood-tidal deltas, ebb-tidal deltas, shoreface sand ridges, and fluvial channels. Petit Bois Pass appears to be located in an antecedent topographic depression that is cut into underlying pre-Holocene deposits. Pre-Holocene deposits form the eastern core of Dauphin Island where they are subaerially exposed. The pre-Holocene surface dips to the west, providing a platform for the narrow, elongated Holocene split of Dauphin Island, and crops out on the shoreface at the western end of Dauphin Island to -8 to -9 m below MLW. Farther to the west, core data from the main inlet channel of Petit Bois Pass indicate only Holocene sediments occur suggesting that the pre-Holocene surface is > 11 m below MLW. This > 11 m trend continues under western Petit Bois Pass. It appears that Petit Bois Pass may occupy the eastern side of an antecedent Pascagoula River valley. End_of_Record - Last_Page 461-------

Environmental characteristics and distribution of macrobenthos in a mudflat of the west coast of Korea (Yellow Sea)

The spatio-temporal changes of environmental factors and the distribution patterns of macrobenthos were investigated on an intertidal mudflat near Panweol on the west coast of Korea. Temperatures of the surface sediments and salinities of the intertitial water were highest in summer and lowest in winter, while water content of the sediment was lowest in summer and highest in winter. Hourly variations of sediment temperature, water content and interstitial salinity during the daytime depended largely upon both the timing of ebb tide and daylight intensity. Temperature, water content and interstitial salinity showed a marked variation with sediment depth. Temperatures decreased sharply from the surface to 10 or 15 cm depth, and below this depth they were nearly constant, except in summer. Water content and interstitial salinity underwent a rapid decline in this upper layer and began to rise from this depth. From the high tide mark toward the main tidal channel, water content increased but salinity decreased. The sand fraction increased with the decrease of clay content toward the main tidal channel. This tidal flat could be divided into two zones: the “upper intertidal zone” and the “lower intertidal zone”, based on the spatial differences of the water content, interstitial salinity and grain-size composition in the surface sediment. There were three distinct zones of macrobenthos. The upper intertidal zone was dominated by burrowing deposit feeders, such as Ilyoplax dentimerosa, Helice tridens sheni, Cleistostoma dilatatum and Perinereis vancaurica tetradentata. The middle intertidal zone was characterized by other burrowing deposit feeders, such as Ilyoplax pingi, Macrophthalmus japonicus and Periserrula leucophryna. And the lower intertidal zone was dominated by suspension feeders represented by Laonome tridentata and Potamocorbula amurensis. Each zone was named after these characteristic species. Although the species composition of the upper intertidal community was different from that of the middle intertidal, the way of living and the feeding behaviour of the species occurring in these two zones were very similar. But the lower intertidal zone showed a characteristic community structure differing not only in species composition but also in ways of living and feeding of the species. Some sets of environmental factors are considered to explain the zonation. Water content of the sediment and interstitial salinity will be the main factors causing the observed zonation.

Biogenic and Physical Modes of Stratification and Shell Bed Formation in the Recent Intertidal Environments of Bahia la Choya, Northern Gulf of California

Fossil concentrations such as shell beds are preferred collection sites for paleontological and paleoecological data. Although ample attention has been paid to the roles and implications of physical processes in shell bed formation, the role of biogenic sediment reworking in generating shell beds has received little study.Subsurface shell beds in the Recent intertidal environment at Bahia la Choya form via three distinct processes: storm deposition, tidal channel migration, and biogenic reworking. Shell beds on the outer tidal flats are produced primarily by storm sedimentation. Lateral migration of the main tidal channel produces shell beds along the channel margins. Shell beds on the inner flats distant from the main tidal channel are generated by vertical “conveyorbelt” recycling of fine sediment by deposit feeders. Biogenic reworking concentrates shells at depth into subsurface shell beds, producing a distinct form of biogenic bedding, termed biogenic stratification.These three modes of shell bed formation leave recognizable taphonomic and sedimentologic signatures. Biogenic stratification in particular has important implications for sedimentation rates, sediment reworking, and faunal condensation in ancient strata.

The intertidal sediment habitats of the lower medway estuary, Kent

During an ecological survey of the benthic invertebrate macrofauna of the lower Medway estuary, Kent, sediment cores were analysed for particle size, redox potential profiles, total organic matter and free sulphide ions, to characterise the sediment habitats at ten shore sites. Visual assessments of the percentage cover of green algae, predominantly Enteromorpha intestinalis (L.) and Ulva lactuca L., over the mudflats are presented. Sediment parameters ranged from less favourable conditions at BP Horseshoe Bay to the relatively healthy conditions at the Isle of Grain. In general, those sites sheltered from the influences of the main tidal channel supported green algal blooms which, on decay, resulted in a depletion of oxygen in the sediment which was characterised by negative redox potentials, high organic matter levels and high free sulphide ion estimates. Numbers of the tubificid Peloscolex benedeni (Udekem) and Hydrobia ulvae (Pennant) increased during these periods with the appearance of the capitellid Capitella capitata (Fabricius). At sites more open to the flow of the main channel, green algae were absent from the mudflats and sediment conditions were characterised by higher redox potentials, lower organic matter levels and lower free sulphide ion estimates. Nereis diversicolor Müller was the most common polychaete at these sites. A number of polychaetes found at the Isle of Grain were either absent, or did not occur in as high numbers, at other sites.