Muddy Tidal Flats Research Articles

Biogeochemical sulfur transformations in the cohesive and permeable tidal flat sediments of Jade Bay (North Sea)

Intertidal flats are highly productive coastal marine ecosystems which are affected by fast changes in environmental conditions and host dynamic biogeochemical cycles in their sediments. Bioturbation by burrowing organisms and roots of plants strongly affects speciation and cycling of redox-sensitive elements in intertidal sediments. In this work, we have studied the impact of sediment type and vegetation on the cycling of redox-sensitive elements including sulfur, iron, and manganese in sandy and muddy tidal flats sediments in the Jade Bay (North Sea) and adjacent area. The redox speciation of these elements was analyzed in the pore-waters and the total sediment. The isotopic compositions of sulfur species were measured in non-vegetated sediments and in sediments which are inhabited by various plants. In the cohesive sediments, which are not affected by vegetation, a decrease in sulfate concentration, coupled with the presence of relatively high concentrations of hydrogen sulfide in the pore-waters and the presence of sulfide minerals as well the isotopic compositions of sulfur species are consistent with fast rates of sulfate reduction in the sediments. In the cohesive sediments affected by roots of Salicornia stricta and sediments desiccation, a cryptic sulfur cycle, which is characterized by microbial sulfate reduction coupled to fast reoxidation of hydrogen sulfide by Fe(III) (hydr)oxides and, possibly, by oxygen is present. Below the roots penetration depth, speciation of redox-sensitive elements is similar to those in the baren sediments. In the cohesive sediments affected by the roots of Spartina anglica and Triglochin maritima, which have longer roots, a cryptic sulfur cycle was detected in the upper 30 cm of sediments. At the sites that are characterized by permeable surface sediments and alternating permeable and cohesive layers in the deeper sediments, the composition of the sediment has a similar or even more significant impact on the speciation of the redox-sensitive elements than penetration of relatively weak roots of Spartina anglica. These sediments are characterized by the formation of hydrogen sulfide and sulfide oxidation intermediates in the cohesive layers and their diffusion to (and oxidation at) the boundaries between cohesive and permeable sediments. We conclude that in the cohesive sediments, the penetration of roots and desiccation leads to the formation of overall oxidized conditions, while in the sediments with alternating layers, permeability may provide a more significant control for speciation of redox-sensitive elements.

Climate change impacts on a sedimentary coast—a regional synthesis from genes to ecosystems

Climate change effects on coastal ecosystems vary on large spatial scales, but can also be highly site dependent at the regional level. The Wadden Sea in the south-eastern North Sea is warming faster than many other temperate coastal areas, with surface seawater temperature increasing by almost 2 °C over the last 60 years, nearly double the global ocean mean increase. Climate warming is accompanied by rising sea levels, which have increased by approximately 2 mm yr−1 over the last 120 years. For this sedimentary coast, the predicted acceleration of sea-level rise will have profound effects on tidal dynamics and bathymetry in the area. This paper synthesises studies of the effects of ocean warming and sea level rise in the northern Wadden Sea, largely based on research conducted at the Wadden Sea Station Sylt of the Alfred Wegener Institute Helmholtz Centre for Polar and Marine Research. An increasing rate of sea level rise above a critical threshold will lead to coastal erosion and changes in sediment composition, and may cause the transition from a tidal to lagoon-like environment as tidal flats submerge. This involves changes to coastal morphology, and the decline of important habitats such as muddy tidal flats, salt marshes and seagrass meadows, as well as their ecological services (e.g. carbon sequestration). Ocean warming affects plankton dynamics and phenology, as well as benthic community structure by hampering cold-adapted but facilitating warm-adapted species. The latter consist mostly of introduced non-native species originating from warmer coasts, with some epibenthic species acting as ecosystem engineers that create novel habitats on the tidal flats. Warming also changes interactions between species by decoupling existing predator–prey dynamics, as well as forming new interactions in which mass mortalities caused by parasites and pathogens can play an understudied but essential role. However, Wadden Sea organisms can adapt to changing abiotic and biotic parameters via genetic adaptation and phenotypic plasticity, which can also be inherited across generations (transgenerational plasticity), enabling faster plastic responses to future conditions. Important research advances have been made using next-generation molecular tools (-omics), mesocosm experiments simulating future climate scenarios, modelling approaches (ecological network analysis), and internet-based technologies for data collection and archiving. By synthesising these climate change impacts on multiple levels of physical and biological organisation in the northern Wadden Sea, we reveal knowledge gaps that need to be addressed by future investigations and comparative studies in other regions in order to implement management, mitigation and restoration strategies to preserve the uniqueness of this ecosystem of global importance.

Extracting waterline and inverting tidal flats topography based on Sentinel-2 remote sensing images: A case study of the northern part of the North Jiangsu radial sand ridges

Muddy tidal flats with landforms vary widely over space and time. The evolution of tidal flats topography has had a significant impact on the region. Therefore, there is a need to develop methods for rapidly and accurately characterizing tidal flats topography. This study applied thresholding segmentation (TS), machine learning, and two image classification methods to Sentinel-2 remote sensing images to extract the instantaneous waterline of the northern part of the North Jiangsu Radial sand Ridges (NJRSR). Tidal level data at specific time intervals were obtained using the Delft3D and TPXO9 models, after which tidal flats topography inversion was conducted. As a result, varied among the different classification methods, with the K-Nearest Neighbors (KNN) algorithm producing the optimal waterline extraction. Furthermore, the accuracy of tidal level simulation was improved by employing Delft3D software and constructing a scaled-down model of the study area. Comparisons with Delft3D model simulations of tidal levels with observations at three tide gauge stations yielded accuracies of 17.5 cm, 18.5 cm and 17.9 cm, this result showed that the establishment of the tidal level model can enhance the accuracy of tidal flats Digital Elevation Model (DEM) inversion. In fact, the combined application of the KNN algorithm and Delft3D model within retrieval of tidal level provided the most accurate topographic inversion, with an average error of 0.297 m, which was reduced to 0.292 m after median filtering, and the results indicated a trend of southward erosion in the tidal flats in the study area, with a turning point roughly located in the southern part of Doulong Port. The results of this study can help improve the future monitoring of dynamic changes in intertidal wetland topography and the conservation and development of tidal flats areas.

Morphodynamics of macrotidal channels in Korean tidal flats: implications for the role of monsoon precipitation and the stratigraphic architecture of tidal point bars

Abstract Tidal channels exert a crucial control on sediment transport and drive geomorphic changes in the tidal environment. Despite their ubiquitous occurrence, long-term morphodynamics and processes driving the morphologic changes remain less well understood than fluvial counterparts. Spanning from straight to dendritic, Korean tidal channels become more sinuous and densely populated with elevation due to higher mud content. Mutually evasive current patterns resulted in a cuspate meander bend, where a flood barb develops at the seaward side of the bend. Multiannual observation revealed that tidal channels migrate from up to 80 m per year in the lower intertidal zone of open-coast sandy tidal flats to nearly stationary for several years in the upper intertidal zone of protected muddy tidal flats. Migration rates are temporarily pronounced during the summer monsoon, when heavy rainfall-induced surface runoff intensifies ebb tidal asymmetry and promotes headward erosion. Meander bends are mostly landward-skewed and shift downstream, implying that ebb currents primarily drive the long-term channel morphodynamics. Tidal point bars commonly display ebbward dipping, inclined heterolithic stratification (IHS) dominated by bedforms formed by subordinate flood tidal currents. An overall ebb dominance and mutually evasive current patterns account for the counterintuitive stratigraphic architecture of point bars in Korean tidal channels.

Salt dynamic changes between seawater and phreatic brine in muddy tidal flats under tidal influence

Evaporated salt accumulated on the surface of tidal flats is an important source of salt for supplying coastal phreatic brine resources. A clear understanding of the flux and process of evaporated salt recharge the shallow underground brine under tidal influence is a prerequisite for promoting the orderly development of coastal underground brine resources. This study established an in-situ numerical model for simulation analysis based on the results of ERT and hydrological parameter surveys conducted on muddy tidal flats on the south bank of Laizhou Bay to clarify the dynamic changes in salt transport between seawater and phreatic brine and to evaluate the recharge capacity of evaporated salt to phreatic brine. The results indicated that in the tidal flat on the sea side (from the “underground brine divide” to the sea) where evaporation is significant, the tidal effect can only periodically inhibit the loss of salt in phreatic brine, but cannot drive the effective accumulation of evaporated salt in the phreatic brine aquifer. During the flooding tide, a flow field from the tidal flat surface to the lower phreatic brine appeared in the aquifer. Salt from the burrows of the top layer entered the phreatic brine under the driving force of the flow field. These salt inputs only slowed the salt loss rate in the phreatic brine from 10.59 kg/h to 7.44 kg/h; During the ebb tide, when the seawater has not receded from the tidal flat, the direction of the groundwater flow field reverses from downward to upward and the driving force of water and salt migration disappears, the salt loss is inhibited in the phreatic brine, and the salt loss rate decreased rapidly to 0.95 kg/h. After the seawater receded from the tidal flat, the salt loss rate of the phreatic brine recovered to 9.89 kg/h, which was basically the same as the salt loss rate at the beginning of the flooding tide.

Salt transport dynamics across the sediment-underground brine interface driven by tidal hydrology and benthic crab burrowing in muddy tidal flats

Muddy coasts with burrows are widespread globally and most contain brine in underground aquifers. Salt evaporated on a tidal flat surface is an important source of shallow underground brine (phreatic brine and brine in the top layer). However, the salt accumulation/release process in shallow underground brine during tidal cycles remains unclear. To analyze the abovementioned process, sediment permeability and pore-water parameters are measured, and electrical resistivity tomography and ring electrode resistivity probe monitoring are conducted in a tidal flat on the south bank of Laizhou Bay, China, where underground brine occurs and biological activities are pronounced. The results show that evaporated salt on the surface of tidal flats and offshore high-salinity underground brine are the two salt sources of shallow underground brine in tidal flats. Dense burrows and large tidal ranges can accelerate the rate of salt cycling, and promoting the exchange of surface and deep salt through the top layer. During the flooding tide, shallow underground brine is primarily replenished by evaporated salt on the beach surface. In areas with dense burrows, some evaporated salt on the beach surface can pass through the top layer to recharge the phreatic brine. During the ebbing tide, the salt in layers U and B is released into seawater and phreatic brine, respectively. When seawater recedes from the beach surface, a trace amount of phreatic brine and salt in the top layer is released to the beach surface via an upward flow field. Layers U and B are the main areas of salt accumulation and release. The proportion of salt accumulation/release in layer M is the lowest; however, layer M is the most susceptible to salt accumulation and features the highest pore-water salinity among the top layers.

Populations of the Intertidal Crab Macrophthalmus banzai are Not Grouped by Latitude: Taiwan Population is Genetically Closer to the Populations in the Japanese Mainland Than the Ryukyu Islands.

Macrophthalmus banzai is an intertidal crab species of Macrophthalmidae inhabiting muddy tidal flats in the northwestern Pacific. A previous study on the population genetic structure of Japanese M. banzai based on the mitochondrial COI gene sequences revealed the presence of two genetically distinguished groups, i.e., the northern group (Honshu, Shikoku, and Kyushu Islands) and the southern group (the Ryukyu Islands). In the present study, we newly determined the COI gene sequences of M. banzai collected from Taiwan and conducted population genetic analyses of these sequences together with Japanese sequences obtained from GenBank to reveal the genetic relationship of this species between Japan and Taiwan. The SAMOVA and pairwise ΦST analysis showed that the Taiwan population is more closely related to the northern group than the southern group. This indicates that the populations of M. banzai are not genetically differentiated by latitude but probably by the pathway of the Kuroshio Current, resulting in the isolation of the population in the Ryukyu Islands. Such a pattern is consistent with the population genetic structure of the fiddler crab Tubuca arcuata shown by a previous study, whereas the pattern differs from those of other intertidal invertebrates. The difference in the larval durations may have influenced the difference in population genetic structures among species. The present study provides a further case of the genetic structure of intertidal species that are not simply regulated by geographic distances.

Effect of sediment on meiofauna and Nematoda structure in different freshwater Amazon environments under the influence of mesotides

Abstract The present study investigated the horizontal and vertical distribution of meiofauna community and Nematoda assemblages on two sandy beaches and two muddy tidal flats in a freshwater Amazon estuary governed by mesotides. Meiofauna density was significantly higher in muddy environments, explained by the higher food availability. On the one hand, there was no difference between sediment layers in both sandy environments, which suggests that individuals must have migrated to layers deeper than 10 cm. On the other, the highest densities and richness in muddy environments occurred in the upper layer due to higher food and oxygen availability in the surface sediment layer, which decreases with depth. Muddy environments differed on both beaches; they had the highest nematode genera densities and richness. This might be explained by the high nutrient enrichment in these environments. There was no significant difference in Nematoda density and richness between zones in any environment, likely due to the small mid-littoral extension (<40 m), which did not favour the high spatial variability of genera. Nematoda density was higher at a depth of 0–2 cm in both muddy environments, explained by the availability of oxygen and food. The high abundance of deposit-feeder genera in muddy environments indicates high organic enrichment, whereas the distribution of trophic types was more homogeneous on the beaches where abiotic factors tended to be more structuring than food availability.

Poleward mangrove expansion in South America coincides with MCA and CWP: A diatom, pollen, and organic geochemistry study

The projected warming trend in the 21st century is likely to alter the global distribution of mangroves. However, the migratory pattern of different mangrove species is still unclear, especially in the subtropical Brazilian littoral. This study utilizes pollen, diatom, and organic geochemistry to document the late Holocene morphological and ecological transformation in São Francisco do Sul Bay, the mangrove sub-range limit in south Brazil. This multi-proxy dataset indicates the establishment of saltmarshes and mangroves on muddy tidal flats at ∼1720 and ∼870 cal yr BP, and Laguncularia, Avicennia, and Rhizophora colonized the study area at ∼870, ∼390, and ∼70 cal yr BP, respectively. This stepwise succession of three mangrove species in São Francisco do Sul Bay, and poleward mangrove expansion toward the austral mangrove range limit, indicate that the migratory histories of different mangrove species were not synchronized during the late Holocene, and temperature is the primary climatic factor regulating the mangrove distribution in south Brazil. More importantly, the Medieval Climate Anomaly and the Current Warm Period were clearly registered in the pollen record, suggesting that the two climate anomalies likely facilitated the mangroves colonization into higher latitudes in south Brazil. Overall, our dataset indicates that mangrove expansion into more temperate zones will likely accelerate in South America.

Salt marsh establishment in poorly consolidated muddy systems: effects of surface drainage, elevation, and plant age

AbstractConservation and restoration of salt marsh ecosystems are becoming increasingly important because of the many ecosystem services they provide. However, the processes controlling salt marsh establishment and persistence, especially on bare tidal flats in muddy areas, remain unclear. As muddy sediments typically experience a restriction of soil drainage, we expect that a surface drainage relief due to a heterogeneity topography, as might occur on the edge of tidal channels, can facilitate the establishment of salt marsh vegetation on muddy tidal flats. By means of a manipulative field experiment, using “Mega‐Marsh Organ” mesocosms, we investigated the impact of surface drainage and elevation relative to mean sea level on (1) the survival of Spartina anglica seedlings from three different age classes: 1‐yr, 3‐month, and 1‐week; and (2) the growth performance of mature S. anglica marsh tussocks. S. anglica seedling survival, especially in the establishment phase, was positively affected by better surface drainage, increases of seedling age, and higher elevation relative to mean sea level. That is, the survival rate of S. anglica seedlings at the end of 6th week increased from 0% (at surface water undrained, 1‐week, 0 cm elevation) to 94.44% (at surface water drained, 1‐yr, 90 cm elevation). In contrast, surface drainage did not affect the performance of large S. anglica marsh tussocks, as only increased elevation relative to mean sea level was shown to affect S. anglica tussock growth in terms of plant height, shoot numbers, and dry biomass. Based on our findings, we proposed a conceptual model to understand how surface drainage‐driven feedbacks in a heterogeneous topography may be reinforced to induce salt marsh establishment in muddy systems. Further testing of present hypothesized model would be beneficial for insights into salt marsh establishment on tidal mudflats.

Effects of Beach Nourishment Project on Coastal Geomorphology and Mangrove Dynamics in Southern Louisiana, USA

Relative sea-level (RSL) rise associated with decreased fluvial sediment discharge and increased hurricane activity have contributed to the high rate of shoreline retreat and threatened coastal ecosystems in Port Fourchon, Louisiana, USA. This study, based on QuickBird/drone images (2004–2019) and LIDAR data (1998–2013), analyzed the impacts of shoreline dynamics on mangroves (Avicennia germinans) and marshes before and after the initiation of a beach nourishment project in 2013. The coastal barrier and dune crest migrated landward between 1998 and 2013. Meanwhile, the dune crest height increased between 1998 and 2001, then decreased in 2013, probably due to hurricane impacts. The total sediment volume along this sandy coastal barrier presented an overall trend of decline in the 1998–2013 period, resulting in a wetlands loss of ~15.6 ha along 4 km of coastline. This has led to a landward sand migration onto muddy tidal flats occupied by Avicennia germinans (1.08 ha) and Spartina (14.52 ha). However, the beach nourishment project resulted in the advancement of the beach barrier from Nov/2012 to Jan/2015, followed by a relatively stable period between Jan/2015 and Mar/2019. Additionally, both the dune crest height and sediment volume increased between 2013 and 2019. This set of factors favored the establishment and expansion of mangroves (3.2 ha) and saltmarshes (25.4 ha) along the backbarrier environments after 2013, allowing the tidal flats to keep pace with the RSL rise. However, waves and currents caused shoreline erosion following the beach nourishment project between Oct/2017 and Nov/2019, threatening wetlands by resuming the long-term process of shoreline retreat.

Sea-level, storms, and sedimentation – Controls on the architecture of the Andros tidal flats (Great Bahama Bank)

Integrated time-separated remote sensing, geographic information systems (GIS), microbiology, and carbonate sedimentology of the tidal-flat portion of Andros Island (Great Bahama Bank) quantify the geomorphologic change of tidal-flat facies through time and illustrate how sea-level rise, among other controls, is recorded within these deposits. In the 75 years spanned by the remote-sensing datasets, sea level has risen by approximately 10 cm. In the face of this change: a) the seaward margin of the tidal flats has remained static, while b) the internal boundary, which separates the channelized and supratidal marsh deposits, retrograded up to 220 m, broadening the channelized zone, and c) in unison to the broadening of the channelized zone, the abundance of laminated (Scytonema) cyanobacterial fabrics decreased, d) accompanied by the lengthening and avulsion of the network of tidal channels that traverse this zone. Given the amount of sea level rise during the period of observation, such broad-scale restructuring of the Andros tidal-flat architecture is surprising. The muddy tidal flats did not prograde, but instead aggraded and locally retrograded. Whereas retrogradation was minimal at the coastline, the channelized zone retrograded substantially over the more landward supratidal inland marsh. These results question aspects of the autocyclic model for the development of peritidal carbonate cycles, which is underpinned by a dominant behavior of tidal-flat progradation. Whereas other controls cannot be explicitly excluded, sea-level oscillations seemingly exerted considerably more impact on the Andros tidal flat in the last decades than did major storm events.

Sediment Characteristics and Intertidal Beach Slopes along the Jiangsu Coast, China

Tidal flats play an important role in promoting coastal biodiversity, defense against flooding, land reclamation and recreation. Many coastal tidal flats, especially the tide-dominant ones, are muddy. However, the number of studies on the profile shape and surficial sediment distribution of muddy tidal flats is small compared to sandy beaches. Based on high spatial-resolution measurements along the tide-dominant Jiangsu Coast, China, we analyzed the morphology and sediment characteristics of the unvegetated intertidal flats along the Jiangsu Coast. The Jiangsu Coast can be divided into an eroding northern part (north coast) and an accreting southern part (south coast). The beach slope of the north coast shows a southward flattening trend, apart from some outliers related to rocky parts of the coastline. We found alternating very fine and coarse sediment (depending on the local clay content) for different locations along the north coast, which can be explained from consolidation and armoring-induced erosion resistance. In the south coast, we found gradual coarsening of bed surface sediment and gradual flattening of beach slopes to the south. This seemingly unexpected pattern is explained by the flood-dominant current causing landward sediment transport, larger tidal range in the south part, sheltering effect of the Radial Sand Ridges, and contribution of different sediment sources, viz. the Abandoned Yellow River Delta and the Radial Sand Ridges. In the cross-shore direction, the sediment grain size decreases landward. Waves are only of secondary importance for the sediment dynamics at the unvegetated tidal flats along the Jiangsu Coast.

Impact of a high rainfall event on the water level, current velocity, and total suspended solids in tidal flats environments of the estuarine Cruces River wetland, south-central Chile

The effects of a sudden rainfall (40 mm d-1) event on the surface waters covering muddy tidal flats were studied during April 2016 at the estuarine Cruces River wetland in south-central Chile (~40ºS). The study area included flooded vestigial tree trunks, which is evidence of coseismic subsidence associated with the 1960 Valdivia earthquake as a source of environmental variability. The tidal flat with vestigial tree trunks registered the fastest and highest depth of inundation. In contrast, the tidal currents velocity and total suspended solids' concentrations were higher at the flat without trunks. Sudden rainfall events can significantly modify the characteristics of surface waters above sedimentary intertidal surfaces, where structures such as flooded trunks are present.

Quantifying annual land-cover change and vegetation greenness variation in a coastal ecosystem using dense time-series Landsat data

Land-cover change may affect water and carbon cycles when transitioning from one land-cover category to another (land-cover conversion, LCC) or when the characteristics of the land-cover type are altered without changing its overall category (land-cover modification, LCM). Given the increasing availability of time-series remotely sensed data for earth monitoring, there has been increased recognition of the importance of accounting for both LCC and LCM to study annual land-cover changes. In this study, we integrated 1,513 time-series Landsat images and a change-updating method to identify annual LCC and LCM during 1986–2015 in the coastal area of Zhejiang Province, China. The purpose was to quantify their contributions to land-cover changes and impacts on the amount of vegetation. The results show that LCC and LCM can be successfully distinguished with an overall accuracy of 90.0%. LCM accounted for 22% and 40.5% of the detected land-cover changes in reclaimed and inland areas, respectively, during 1986–2015. In the reclaimed area, LCC occurred mostly in muddy tidal flats, construction land, aquaculture ponds, and freshwater herbaceous land, whereas LCM occurred mostly in freshwater herbaceous land, Spartina alterniflora, and muddy tidal flats. In the inland area, both LCC and LCM were concentrated in forest and dryland. Overall, LCC had a mean magnitude of normalized difference vegetation index (NDVI) change similar to that of LCM. However, LCC had a positive effect and LCM had a negative effect on NDVI change in the reclaimed area. Both LCC and LCM in the inland area had negative impacts on vegetation greenness, but LCC resulted in larger NDVI change magnitude. Impacts of LCC and LCM on vegetation greenness were quantified for each land-cover type. This study provided a methodological framework to take both LCC and LCM into account when analyzing land-cover changes and quantified their effects on coastal ecosystem vegetation.

Feasibility of Elevation Mapping in Muddy Tidal Flats by Remotely Sensed Moisture (RSM) Method

Li, H.; Gong Z.; Dai, W.Q.; Lu, C.Z.; Zhang, X.Y; Cybele, S., and Guo, H.T., 2018. Feasibility of elevation mapping in muddy tidal flats by remotely sensed moisture (RSM) method. In: Shim, J.-S.; Chun, I., and Lim, H.S. (eds.), Proceedings from the International Coastal Symposium (ICS) 2018 (Busan, Republic of Korea). Journal of Coastal Research, Special Issue No. 85, pp. 291–295. Coconut Creek (Florida), ISSN 0749-0208.It is hard for traditional ways to measure muddy intertidal topography because of its difficult access and limitation of exposure time. As a result, this paper proposes a new way, called remotely sensed moisture (RSM) method, to retrieve elevation in muddy tidal flats. When the tide ebbs, the intertidal flat exposes gradually from the land to the sea, thus forming a phenomenon of ‘low moisture in high-lying areas and high moisture in low-lying areas’. It can be seen that there may exist the negative correlation between the moisture and elevation of intertidal flat. By means of TM Image at low tide, this paper retrieved the moisture of muddy intertidal flat and found out that there does exist the negative correlation, but relatively low (R2=0.55). Further analysis showed that the remnant water is a critical factor to affect the correlation. After excluding regions nearby waterlines, the correlation between moisture and elevation is improved significantly (R2=0.78). The research demonstrates that RSM method has a potential to map topography of muddy tidal flats.

Sedimentary dynamics of a subtropical tidal flat sheltered inside a coastal channel (Araçá Bay, SE Brazil)

The sedimentary patterns of Araçá Bay and the adjacent sector of the São Sebastião Channel (SSC) were determined by grain size parameters and carbonate content. The inner bay contains two intertidal flat facies, with very fine to coarse sand and very coarse silt respectively. The outer bay is a subtidal flat facies with unimodal very well sorted and very fine sand. In the adjacent area of the SSC, the central and northern sectors correspond to a sublittoral muddy facies, composed of poorly to very poorly sorted and very coarse to medium silt, and the southern sector corresponds to a sublittoral sandy facies, with very poorly sorted and very fine to coarse sand. The source of sand and very coarse silt are relict terrigenous deposits; coarse silt and medium silt are supplied to the inner bay and the SSC by the intense summer rainfall; and the input of fine and very fine silt is related to suspended sediment from the inner shelf due to storm waves coming from the south. In contrast to seasonal grain size variations in the SSC and the intertidal flat, grain size of the subtidal flat is homogeneous throughout the whole year, denoting an intense sediment transport by local waves and tidal currents. In summary, Araçá Bay is a particular case of sheltered tidal flats, controlled by tides and waves with a minor fluvial influence, but mainly composed of terrigenous sediments. According to this sedimentary characterization, the sensitivity of Araçá Bay to oil spills is reclassified. The practical absence of fine sediments in the subtidal flat determines its low potential to accumulate pollutants, while the intertidal flat and the adjacent SSC present a higher potential. Consequently, this study shows that sensitivity to oil spills of sandy tidal flats and muddy tidal flats must be classified separately.

A pilot study on remediation of muddy tidal flat using porous pile

In order to prove that porous piles are effective in remediating muddy tidal flat sediments and increasing the biomass, field experiments were carried out at the tidal flat of a brackish river located in Hiroshima City, Japan. Porous piles with a diameter of 16cm and height of 50cm were installed in the muddy sediment that covers the sand layer of the tidal flat. After installation, concentrations of dissolved oxygen in interstitial water in and around the porous piles increased to a maximum concentration of 6mg/l due to enhancement of the groundwater flow. The increase of dissolved oxygen in the interstitial water produced a decrease in the concentration of ammonia and an increase in the individual number of benthos at the porous pile site. From these results, we concluded that the porous pile is an effective technology for remediation of muddy tidal flats.

The Removable-Cap Suction Corer: An Inexpensive and Durable Device To Extract Unconsolidated, Wet Sediments

Obtaining sediment samples in unconsolidated sediment presents many challenges. The locale substrate can be soft, penetration of the sediment may be limited, preservation of the sedimentary fabric may be incomplete, and extraction of the sample may be difficult. Some of these issues are overcome with vibracore or suction-core techniques. Vibracoring requires heavy, bulky, and relatively expensive equipment which is costly to ship to the remote field-study locations and difficult to use in muddy settings such as muddy tidal flats or coastal marshes. The Van der Staay suction corer (Van de Meene et al. 1979) is cost-effective and fixes logistical problems, but sample extraction has to be done in the field. The TESS-1 suction corer (Mendez et al. 2003) was designed on the basis of the Van der Staay suction corer and allows recovering intact sediment samples for further laboratory analysis, but it requires custom machinery with more man-hours and uses small-diameter core tubes with a high possibility of disturbance of sedimentary structures in unconsolidated and waterlogged deposits. Here we present a new removable-cap suction corer that is inexpensive to construct, light in weight, highly portable, and can be designed to extract any core diameter. The corer can be used for coarse- and fine-grained wet sediments (tidal flats, riverine and estuarine bars, coastal marshes) and shallow subaqueous environments (lakes, rivers, shallow shoreface).

Classifications, sedimentary features and facies associations of tidal flats

Significant progress has been achieved in the research of tide-dominated environments in the past two decades. These studies highlight both the importance and diversity of tidal flats in modern coastal environments. Based on their developing settings, tidal flats are subdivided into nine types, which are in turn grouped into sheltered or exposed spectrums according to the magnitude of exposure to waves. The ternary coastal classification model is revised with an embedded triangle to highlight non-open coast tidal flats as major second-order morphological elements to the first-order coastal environments including deltas, estuaries and lagoons. A new continuous spectrum of open coast depositional settings is proposed from muddy tidal flats of tide dominance with wave influence, through sandy tidal flats of mixed energy (tide-dominated), and tidal beaches of mixed energy (wave-dominated), to beaches of wave-dominance with tide influence. It is worth noting that no open coast setting is absolutely exempt from wave or tide influence. Three diagnostic criteria for the intertidal-flat deposits are proposed. Upon an upward-fining succession, (1) regular changes vertically from flaser bedding, through wavy bedding and to lenticular bedding are diagnostic of most of intertidal flats; (2) cyclical tidal rhythmites point to sheltered intertidal flats typically at the inner part of macrotidal estuaries; (3) rhythmic alternations of storm and tidal deposition are diagnostic of exposed intertidal flats, especially the open coast types. Intertidal-flat deposits are generally topped by saltmarsh deposits, but underlain by different subtidal successions, like thick subtidal channel-fills, sand-bar complexes (sheltered coastal settings), and upwards coarsening successions of subtidal flats or thick subtidal sand ridge/bar complexes (exposed coastal environments).