A fine-grained sediment budget for the Sylt-Rømø tidal basin

A budget for net accumulation of fine-grained sediment (<63 μm) has been set up for the Sylt-Rømø tidal basin. Net accumulation within the basin was computed from²¹⁰Pb core dating and mapping of the intertidal and supratidal surface sediments. It was found that a yearly mean value of 58·10³ tons of sediment was deposited in the tidal basin. The largest sediment source for the net input of fine-grained sediment is the North Sea contributing about 64% to the net budget; the fluvial input and primary production contribute 14% and 15%, respectively. Local salt marsh erosion accounts for about 5% of the budget and atmospheric deposition for only 2%. The total amount of sediment deposited in the investigated area was low compared with earlier investigations in the Wadden Sea. This is explained partly by the intensive diking of the natural salt marshes fringing the area in the past, and partly by the exposed conditions of most of the intertidal flats. An index describing the trapping efficiency of the water exchanged between the North Sea and the Sylt-Rømø tidal area is defined as the ratio between yearly net sediment input from the North Sea and yearly exchanged water volume between the tidal basin and the sea. This index shows that in the Sylt-Rømø tidal basin, fine-grained suspended sediment “filters” out of the exchanged sea water at a rate that is 12 times lower than in the Gr»dyb tidal basin. It is concluded that the net deposition of fine-grained sediment in a tidal basin is mainly a function of physiographical and hydrodynamical parameters and to a lesser degree of sediment availability     

VERTICAL MIGRATION OF A MIXED‐SPECIES EUGLENA (EUGLENOPHYTA) ASSEMBLAGE INHABITING THE HIGH‐INTERTIDAL SANDS OF NYE BEACH,OREGON1

Comparatively little is known about the vertical migration of the microphytobenthic community forming visible patches on high‐energy beaches. We collected surface and cored samples to evaluate the timing and extent of downward migration of a multispecies Euglena assemblage inhabiting Nye Beach, Oregon. Euglena density at the surface was highly variable and was not correlated with the time of low tide or instantaneous irradiance measurements; however, triplicate cores collected at low and high tides revealed a tidal rhythm in mean depth. On average, 95% of the assemblage occurred within 1 cm of the surface during low tide, but 54% of the assemblage was collected between 1 and 8 cm below the surface during high tide. A midday shading experiment revealed that short‐term changes in irradiance levels altered the Euglena density at the sediment surface by inducing vertical migration. This response to short‐term fluctuations in light may explain the weak correlation between cell density at the surface and time of day. The high‐intertidal location of these patches prevented the removal of nonmigrating cells by daily high tides, which increased the variability in surface samples and obscured the tidal migration rhythm detected in the core samples. Due in part to the semidiurnal nature of Oregon tides, this study provides in situ confirmation of past mesocosm research indicating that sediment disturbance during daily submersed periods is an important process in maintaining the quasi‐tidal rhythm in the appearance and disappearance of Euglena spp. from the surface of beaches and intertidal sandflats.     

Untersuchungen über die Auswirkung kommunaler Abwässer auf das benthische Ökosystem mariner Watten

The influence of domestic sewage on intertidal sand and mud flat benthic communities has been studied at three minor outfalls around the North Sea island of Sylt (F. R. Germany) and at the main outfall of Vancouver, B. C. (Canada). During high tide, domestic waste water was found at the water surface due to its lower specific gravity. Consequently, O₂ depletion and salinity decrease occured mainly in surface waters. Oxygen deficiency prevailed in the sediment-water layer, when settled sewage particles were re-suspended by strong tidal currents. This resulted in anaerobic conditions at the sediment surface for up to 40% of the day. Substantial pH decrease and increase in seston load of the water were observed only along the sewage channels and around the outlets when large quantities of sewage were discharged. The amount of particulate organic material increased up to 10% and more in muddy sediments due to sedimentation of sewage sludge. Considerable eutrophication of the mud flats resulted from nutrient load of the sewage and from decomposition of faecal material deposited on the sediments. The vertical distribution of microalgae in the sediment was not influenced by sewage. Primary productivity of microalgae on the sediment surface was positively correlated with the degree of pollution. In heavily and mediumly polluted zones, heterotrophic bacterial production exceeded primary production rates at the sediment surface. Compared with unpolluted intertidal flats the number of macrobenthic species was not reduced in polluted mud flats. In regard to overall species abundance, the macrofauna associations of the disposal area showed three distinct zones: (1) Heavy pollution around the outfall resulted in a degradation zone either without benthic invertebrates or with a very low density of individuals; (2) medium polluted areas were characterized by a zone of maximum density; (3) with decreasing or diminishing pollution the number of individuals returned to normal values (background values). The distribution of species within these zones indicated a significant difference between the response of the channel fauna and the flat fauna to pollution: while the number of species increased continously inside the channels with decreasing pollution, the species variety of the tidal flats was positively correlated with the number of individuals. The diversity of the benthos inside the channels was negatively correlated with the degree of pollution, whereas the diversity indices for the fauna on the tidal flats increased quickly from the centre of pollution, remained at a high level in the medium polluted zone, and decreased again to the area of diminishing pollution. The number of dominating species in the sewage channel increased from the outlet towards the low-tide level, with ratios of 0.9 to 0.33; on the flats the number remained constant throughout the three zones, with maximum ratios of 0.5, demonstrating that no qualitative changes for the macrobenthos of intertidal flats result from domestic sewage disposals. In polluted intertidal flats the macrobenthos is dominated by muddy-sand species whose adaptability to varying sedimentary factors, such as grain size distribution and content of organic material (coefficients of variability: 50.7 and 45.7%, respectively), is relatively high compared with mud and sand species. A special indicator community for polluted intertidal areas does not exist. Intensified waste-water treatment prior to discharge reduced the size of the degradation zone around the outfalls while it simultaneously caused a lower density of the macrobenthos in the medium polluted zone.

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Dissertation aus dem Fachbereich Biologie der Universität Hamburg.     

Contribution of unvegetated tidal flats to coastal carbon flux

Unvegetated flats occupy a large area in the intertidal zone. However, compared to vegetated areas, the carbon sequestration of unvegetated tidal flats is rarely quantified, even though these areas are highly threatened by human development and climate change. We determined benthic maximum gross primary production (GPP_m), net primary production (NPP) and total respiration (TR) during emersion on seven tidal flats along a latitudinal gradient (from 22.48°N to 40.60°N) in winter and summer from 2012 to 2016 to assess the spatial and temporal variability of carbon dioxide flux. In winter, these processes decreased by 89%–104% towards higher latitudes. In summer, however, no clear trend was detected across the latitudinal gradient. Quadratic relationships between GPP_m, NPP and TR and sediment temperature can be described along the latitudinal gradient. These curves showed maximum values of GPP_m and NPP when the sediment temperatures reached 28.7 and 26.6°C respectively. TR increased almost linearly from 0 to 45°C. The maximum daily NPP across the latitudinal gradient averaged 0.24 ± 0.28 g C m^?2 day^?1, which was only 10%–20% of the global average of NPP of vegetated coastal habitats. Multiplying with the global area of unvegetated tidal flats, our results suggest that the contribution of NPP on unvegetated tidal flats to the coastal carbon cycle is small (11.04 ± 13.32 Tg C/year). If the land cover of vegetated habitats is continuously degraded to unvegetated tidal flats, the carbon sequestration capacity in the intertidal zone is expected to reduce by at least 13.10 Tg C/year, equivalent to 1% of global carbon emissions from land‐use change.     

Influence of sediment on burrowing by the soldier crab Mictyris longicarpus Latreille

This study analyses the spatial distribution of burrowing by the soldier crab Mictyris longicarpus (Mictyridae) in relation to the topography of intertidal sandflats. Four different locations were sampled in Port Jackson and Botany Bay (Sydney, New South Wales, Australia). In each location, abundances of crabs and the proportion of juvenile crabs varied among sites 50 m² and tens of metres apart, but not between high- and lowshore levels, nor among shores. Topographic formations, referred to as humps and depressions, were common on the flats and were of the same order of size and spacing as the sites, which showed variation in numbers/sizes of crabs. Manipulative experiments were done in one of the locations to distinguish between the hypotheses that the spatial distribution of the crabs varied according to (1) topography, or (2) the composition of sediment in the different topographic features. In the first experiment, small plots (0.09 m² and 20 cm deep) of sediment were transplanted between humps and depressions, including appropriate controls for disturbance and translocation. The numbers and sizes of crabs colonizing the experimental plots were then measured. Differences in abundance among plots were found higher on the shore. Therefore, the experiment was repeated at highshore levels with increased replication. There were more crabs in depressions than in humps. The proportion of juveniles varied according to the type of sediment, but only in depressions. Heterogeneity and sediment were, therefore, important for explaining some of the spatial variability of the abundances of the crabs.     

Physical habitat attribute mediates biotic resistance to non-indigenous species invasion

A soft-shelled non-indigenous clam, Nuttallia obscurata, has invaded coastal soft-sediment habitats of the northeastern Pacific. In a survey of 35 sites within the San Juan Islands, Washington, USA, Nuttallia was found almost exclusively in sandy substrates, higher in the intertidal than most native clams (>1 m above mean lower low water). The distinctive distribution of Nuttallia suggested that tidal height and sediment composition may be important physical factors that control its refuge availability, regulating its exposure to predation and ultimately the success of its invasion. I tethered Nuttallia for 24 h in the high intertidal where it is typically found and in the low intertidal at an elevation where it was never found. Clams restrained to the surface suffered high mortality from crab predation at both tidal heights, whereas control clams with unrestricted burrowing movement exhibited high mortality rates only in the low intertidal. In a second experiment, I transplanted sediment within and between the two intertidal heights to measure effects of tidal height and sediment type on survival and burial depth of Nuttallia. At both tidal heights all clams placed on mud-cobble substrate, naturally common in the low intertidal, suffered high mortality rates (>60% in 24 h). Nuttallia on loosely packed sand substrate, naturally found in the upper intertidal, survived much better, however, because they buried deeper than in the tightly packed mud. Caged control clams at both tidal heights suffered no mortality. Apparently native predators are mitigating community level impacts of an invader by excluding Nuttallia completely from some beaches with improper sediment characteristics or relegating it in others to a zone not often inhabited by native species, thereby reducing potential competitive interactions. These findings show that a physical habitat characteristic can mediate biotic resistance to an invader and thus control invasion success and community-level impacts. Generally, such physical-biological interactions may explain some of the reported site-to-site variability in invasion success, as well as the patchy distribution of many soft-sediment infaunal species.     

Recruitment of shore crabsCarcinus maenas on tidal flats: Mussel clumps as an important refuge for juveniles

During the late summer and early fall, juvenile shore crabs (Carcinus maenas L.) occurred in high abundances in mussel clumps scattered on tidal flats of the Wadden Sea. Abundances were much lower on bare tidal flats without mussel clumps and decreased substantially from July to November, whereas numbers in mussel clumps remained high. Large crabs left the tidal flats in early fall, whereas juveniles undertook tidal migrations only in the late fall. In March very few shore crabs were found in the intertidal area. The size of juvenile shore crabs living between mussels did not increase significantly during fall. On the bare tidal flats surrounding the mussels, a size increase was observed. Mussel beds and mussel clumps serve as a spatial refuge for the early benthic phases of juvenile shore crabs. Between mussels they can hide effectively from their epibenthic predators. Juvenile shore crabs do not leave the intertidal area and the mussel habitats before their major predators have left the area. Mussel clumps scattered over the tidal flats may be a critical refuge for juvenile shore crabs settling on tidal flats. Intensified efforts in mussel culturing in the European Wadden Sea during recent decades may have caused an increased abundance of mussel clumps on tidal flats, thus enhancing habitat availability for some mussel-clump inhabitants.     

泉州湾蟳埔潮间带大型底栖动物群落的时空分布

为了比较泉州湾蟳埔潮间带沙滩、互花米草滩和牡蛎石泥滩3种生境(3个潮层)的大型底栖动物群落,2011年4月至2012年1月对3种生境的大型底栖动物进行了季度定量取样。在3种生境共获得85种大型底栖动物,其中环节动物39种,软体动物20种,节肢动物21种,刺胞动物、扁形动物、纽虫动物、星虫动物和脊索动物各1种。多维标度排序(MDS)分析表明,春季和冬季泉州湾蟳埔潮间带3种生境的大型底栖动物群落相似性较低;夏季和秋季互花米草滩与牡蛎石泥滩的大型底栖动物群落相似性较高,而与沙滩的大型底栖动物群落相似性较低。沙滩大型底栖动物群落的季节变化较明显,其次是牡蛎石泥滩,而互花米草滩大型底栖动物群落的季节变化较不明显。大型底栖动物栖息密度和生物量随着潮层降低而增加。单变量双因素方差分析(Two-way ANOVA)表明,不同生境之间的大型底栖动物物种数、栖息密度、多样性指数、均匀度指数和丰度指数有显著差异,但生物量无显著差异,这是因为沙滩的物种数较少,栖息密度较低,但优势种弧边招潮蟹(Uca arcuata)个体较大,互花米草滩和牡蛎石泥滩的优势种为加州中蚓虫(Mediomastus californiensis),个体相对弧边招潮蟹小。不同季节之间大型底栖动物物种数、栖息密度、生物量和丰度指数有显著差异,但多样性指数和均匀度指数元显著差异,这是因为沙滩物种数少,但个体分布比较均匀,而互花米草滩和牡蛎石泥滩物种数较多,个体分布较不均匀。以上结果表明,潮汐、沉积物粒径和生境是影响潮间带大型底栖动物群落的主要因素。潮汐导致潮间带的空间异质性,空间异质性导致大型底栖动物群落的差异。     

Predation on 0-group and older year classes of the bivalve Macoma balthica: interaction of size selection and intertidal distribution of epibenthic predators

The bivalve Macoma balthica is a common species in the Wadden Sea and North Sea. Juveniles temporarily use nurseries in the high intertidal. To explain this nursery use, predation pressure was examined for both juvenile and adult Macoma at low and high tidal flats. The study was carried out in the eastern Dutch Wadden Sea.Shrimps Crangon crangon, adult crabs Carcinus maenas, gobies Pomatoschistus and juvenile flatfish were more abundant and larger on low than on high tidal flats, but 0-group Carcinus was more abundant on the high tidal flats. Crangon and 0-group Carcinus stomachs frequently contained Macoma remains. These predators selectively preyed on small 0-group Macoma, both in the field and in laboratory experiments. The effect of predation by epibenthic animals and birds, on the low and high tidal flats, was examined in exclosure experiments (2 mm mesh). There was no effect of epibenthos exclosure on adult Macoma. For 0-group Macoma, densities were higher in exclosures than in the controls where predators had normal access. The density reduction by epibenthic predators was much larger in the low than in the high intertidal. We found no effect of bird predation on densities of 0- and 1+group Macoma.Thus, 0-group Macoma is under high predation pressure by epibenthos in the low intertidal, especially by shrimps, while they are relatively safe in the high intertidal. However, most of the shellfish outgrow their epibenthic predators during their first summer. Therefore, it becomes safe for the bivalves to redistribute to locations where epibenthic predators are abundant, during their first winter. On the other hand, it did not become clear from this study why many of the larger Macoma leave the high intertidal. Concluding, the nursery use of Macoma-spat in the high intertidal is probably, at least partly, an adaptation to avoid epibenthic predation.     

Manometric Assessment of interstitial microalgae production in two estuarine sediments

Summary The potential production of estuarine interstitial microalgae from two tidal flats differing in sediment texture was determined manometrically. Sediment cores from the Southbeach tidal flat showed a maximum amount of productivity in the lower intertidal zone whereas sediment cores from Sally's Bend showed no production in this zone. The upper cm³ of sediment from both tidal flats had a higher amount of gross potential production than subsurface core sections. the sandy substratum from Southbeach had an estimated gross annual production of 275–325 g C m^-2 yr^-1 whereas the silty substratum of Sally's Bend had an estimated value of 0–125 g C m^-2 yr^-1. Respiration/Photosynthesis (R/P) ratios indicate that respiration often exceeds photosynthesis on the Sally's Bend tidal flat.This investigation was supported by a predoctoral traineeship from the National Aeronautics and Space Administration and is part of a dissertation submitted by the senior author in partial fulfillment of the requirements for the Ph. D. degree to the Graduate School of Oregon State University, Corvallis.     

Meiofaunal emergence from intertidal sediment measured in the field: significant contribution to nocturnal planktonic biomass in shallow waters

Field studies on the occurrence of meiobenthos in the water column above intertidal sandflats have been performed near the Island of Sylt in the northern Wadden Sea. Swimming meiobenthos was strongly dominated by harpacticoid copepods. Many of them have a semiplanktonic life-style. They rest in superficial sediment layers at low tide and swin in the water column at high tide. Swimming activity correlated negatively with light. The abundance in the water column was one order of magnitude higher during the night. Strong currents caused by storm tides significantly decreased meiobenthic abundance in the water column. Light and flow being constant, no significant changes of meiobenthic abundance per unit area occurred over a tidal cycle. Since holoplankton and meroplankton abundances correlated positively with the height of the water column, semiplanktonic meiobenthos may dominate the mesozooplankton in shallow waters. On an average, emergence of meiobenthos increased the mesozooplanktonic biomass by about 2% during diurnal high tides over the entire tidal cycle, and by about 50% during nocturnal high tides. Because of seasonal cycles of the dominant harpacticoids, this high contribution to planktonic biomass may be a summer phenomenon.     

Prediction of macrozoobenthic species distribution in the Korean Saemangeum tidal flat based on a logistic regression model of environmental parameters

This study aims to contribute to the development of heuristic statistical models, which are able to predict benthic macrofaunal responses to environmental gradients in coastal areas, such as tidal flats. Ecological response surfaces were derived for 15 intertidal macrobenthic species, using logistic regression based on three separate environmental parameters (shore level, mud content, and organic content) measured on the tidal flats of the Saemangeum estuary, Korea. The presence/absence of the 15 intertidal macrofauna was accurately predicted from each separate environmental factor in the environmental models, with a prediction accuracy of ~65–92%. Subsequently, geographical comparison was made between the mapped probability surfaces and maps of observed species occurrence. The results indicated that the models developed for different species exhibited a wide variety of functional forms, highlighting potential variability in species response to changes in habitat conditions, even for closely associated species with a similar trophic type. Our modeling approach was capable of predicting macrobenthic species distributions with a relatively high degree of accuracy, although the ecological processes controlling intertidal macrobenthic distribution could not be fully determined. Overall, good agreement between modeling results and field observations, with relatively high concordance regardless of target species, emphasized that such an approach would be of practical use in terms of ecosystem approach to tidal flat management.     

Macrofauna on flood delta shoals in the Wadden Sea with an underground association between the lugwormArenicola marina and the amphipodUrothoe poseidonis

Living conditions for macrofauna on flood delta shoals are determined by surf, strong currents and sediment mobility. Thus, a unique assemblage of invertebrate species colonize these far off-shore, low intertidal flats. We here describe the macrobenthic fauna of emerging shoals in the Wadden Sea between the islands of Römö and Sylt. Besides ubiquitous macroinvertebrates of the intertidal zone and species which attain their main distribution in the subtidal zone, the flood delta shoals are characterized by organisms adapted to live in these highly unstable sediments, like the polychaetesSpio martinensis, Streptosyllis websteri, Magelona mirabilis, Psammodrilus balanoglossoides, the pericarid crustaceansCumopsis goodsiri, Tanaissus lilljeborgi, Bathyporeia sarsi and a few others. Average abundance (1440 m^?2 of ind >1 mm) and biomass (12.9 g AFDW m^?2) were low compared to other intertidal habitats in the Wadden Sea. Biomass was dominated by largesized individuals of the lugwormArenicola marina. The U-shaped burrows of these polychaetes were inhabited by high numbers ofUrothoe poseidonis. Maximum densities of these amphipods occurred in the deepest parts of the burrows. Sampling at approximately montly intervals revealed no apparent seasonality ofU. poseidonis abundance. Together with smallCapitella capitata, these amphipods constitute a deep-dwelling component of the macrofauna associated with lugworms, which is separated from all other macrofauna living at the sediment surface. As a response to rising sea level and increasing tidal ranges, we expect the unstable sandy shoals, inhabited by numerousSpio martinensis andUrothoe poseidonis, to expand within the Wadden Sea at the cost of stable sandy flats with abundant macrofauna.     

Holocene reef evolution in a macrotidal setting: Buccaneer Archipelago,Kimberley Bioregion,Northwest Australia

This study uses information derived from cores to describe the Holocene accretion history of coral reefs in the macrotidal (up to 11 m tidal range) Buccaneer Archipelago of the southern Kimberley coast, Western Australia. The internal architecture of all cored reefs is broadly similar, constituting well-preserved detrital coral fragments, predominantly branching Acropora, in a poorly sorted sandy mud matrix. However, once the reefs reach sea level, they diverge into two types: low intertidal reefs that maintain their detrital character and develop relatively narrow, horizontal or gently sloping reef flats at approximately mean low water spring, and high intertidal reefs that develop broad coralline algal-dominated reef flats at elevations between mean low water neap and mean high water neap. The high intertidal reefs develop where strong, ebb-dominated, tidal asymmetry retains seawater over the low tide and allows continued accretion. Both reef types are ultimately constrained by sea level but differ in elevation by 3–4 m.     

Major Changes in the Ecology of the Wadden Sea: Human Impacts, Ecosystem Engineering and Sediment Dynamics

Shallow soft-sediment systems are mostly dominated by species that, by strongly affecting sediment dynamics, modify their local environment. Such ecosystem engineering species can have either sediment-stabilizing or sediment-destabilizing effects on tidal flats. They interplay with abiotic forcing conditions (wind, tide, nutrient inputs) in driving the community structure and generating spatial heterogeneity, determining the composition of different communities of associated species, and thereby affecting the channelling of energy through different compartments in the food web. This suggests that, depending on local species composition, tidal flats may have conspicuously different geomorphology and biological functions under similar external conditions. Here we use a historical reconstruction of benthic production in the Wadden Sea to construct a framework for the relationships between human impacts, ecosystem engineering and sediment dynamics. We propose that increased sediment disturbances by human exploitation interfere with biological controls of sediment dynamics, and thereby have shifted the dominant compartments of both primary and secondary production in the Wadden Sea, transforming the intertidal from an internally regulated and spatially heterogeneous, to an externally regulated and spatially homogenous system. This framework contributes to the general understanding of the interaction between biological and environmental control of ecosystem functioning, and suggests a general framework for predicting effects of human impacts on soft-bottom ecosystems.     

The distribution and interconversion of ammonium and nitrate in the Skallingen salt marsh (Denmark) and their exchange with the adjacent coastal water

The potential nitrogen sources for the primary production in the intertidal area are nitrogen compounds obtained from mineralization in the sediment and the water column, nitrogen fixation, outflow from rivers and groundwater seeping from the mainland. The available inorganic nitrogen in the adjacent coastal waters decreases from 50–80 μmol NO₃ ^-/l and 6–15 μmol NH₄ ⁺/l in early spring to ca one tenth during the growing season. In the sediment of the tidal flats available ammonia and nitrate vary between 50 and 100 μmol/1 pw. In the salt marsh available ammonia increases from 200–300 nmol NH₄ ⁺/g fwt to approximately double the amount, and the available nitrate varies from 100–300 nmol NO₃ ^-/g fwt (250–750 μmol NO₃ ^-/l pw) to ca one third during the growing season. The exchange of NH₄ ⁺, NO₂ ^- and NO₃ ^- across the sediment water interface has been estimated during tidal cycles under light and dark conditions on the tidal flats. The flux of nitrogen was dependent on the flora and fauna as well as the time of the year. The tidal activity, frequency and length of inundation are considered the driving force in a two-way process between salt marshes and adjacent coastal waters. The role of marsh sediment, tidal water and sediments of the tidal flats as sites of accumulation, consumption and remineralization of organic matter is emphasized. The possible exchange of ammonia and nitrate between the salt marsh and the different compartments of the tidal water is discussed.     

On the importance of sediment-water exchange processes of ammonia to primary production in shallow areas of the Sado estuary (Portugal)

Coupling between the seasonal phytoplankton growth cycle and resuspension mechanisms in low depth areas was studied. Tidal flood in an estuarine inlet with a large area of intertidal flats was investigated with respect to nutrients and suspended particulate matter, during two different periods of the year. A prespring bloom period in March and another during the productive summer term in August were picked. Results indicate that tidal resuspension mechanisms are important in mobilizing NH₄ ⁺ from the sediment surface pool, and that this additional input of the limiting nutrient is rapidly consumed in the flooding water column, in benefit of microphytes resident in the nutrient-poor, incoming,coastal waters.     

Sulphate,dissolved organic carbon,nutrients and terminal metabolic products in deep pore waters of an intertidal flat

This study addresses deep pore water chemistry in a permeable intertidal sand flat at the NW German coast. Sulphate, dissolved organic carbon (DOC), nutrients, and several terminal metabolic products were studied down to 5 m sediment depth. By extending the depth domain to several meters, insights into the functioning of deep sandy tidal flats were gained. Despite the dynamic sedimentological conditions in the study area, the general depth profiles obtained in the relatively young intertidal flat sediments of some metres depth are comparable to those determined in deep marine surface sediments. Besides diffusion and lithology which control pore water profiles in most marine surface sediments, biogeochemical processes are influenced by advection in the studied permeable intertidal flat sediments. This is supported by the model setup in which advection has to be implemented to reproduce pore water profiles. Water exchange at the sediment surface and in deeper sediment layers converts these permeable intertidal sediments into a “bio-reactor” where organic matter is recycled, and nutrients and DOC are released. At tidal flat margins, a hydraulic gradient is generated, which leads to water flow towards the creekbank. Deep nutrient-rich pore waters escaping at tidal flat margins during low tide presumably form a source of nutrients for the overlying water column in the study area. Significant correlations between the inorganic products of terminal metabolism (NH₄ ⁺ and PO₄ ³⁻) and sulphate depletion suggest sulphate reduction to be the dominant pathway of anaerobic carbon remineralisation. Pore water concentrations of sulphate, ammonium, and phosphate were used to elucidate the composition of organic matter degraded in the sediment. Calculated C:N and C:P ratios were supported by model results.     

Seagrass recovery in the Northern Wadden Sea?

Aerial surveys on seagrass (Zostera spp.) indicate a three to fourfold increase in bed area from 1994 to 2006 with up to 100 km² or 11% of intertidal flats in the Northfrisian Wadden Sea (coastal eastern North Sea), observed at seasonal maximum in August when flying during low tide exposure 300 to 500 m above ground. When viewed from the air, difficulties in distinguishing between seagrass and green algae and a lack of contrast on dark-coloured mudflats are sources of error in areal estimates. Particularly the positioning of beds remote from shores was imprecise. However, the consistency in method over time gives confidence to the inferred positive trend which is opposite to the global pattern. Both, the spatial pattern and a recent decrease in storminess suggest that sediment stability is the key factor for seagrass dynamics in this tidal area. On exposed sand flats, high sediment mobility may be limiting and along the sheltered mainland shore land claim activities with high accretion rates may cause a scarcity of seagrass. The potential area of seagrass beds may be twice as large as the realized maximum in 2006 but eventually the rising sea level will reverse the observed seagrass expansion.     

Composition and abundance of shorebird and macroinvertebrate communities differ according to densities of burrowing fiddler crabs in tropical intertidal flats

The burrowing activity of fiddler crabs inhabiting intertidal flats creates visually distinct patches within these habitats. However, differences in the composition and abundance of shorebirds and their macroinvertebrate prey between areas inhabited or not by crabs are yet to be studied. Here, we compare the macroinvertebrate and shorebird assemblages in low and high crab density areas in the intertidal flats of the Bijagos archipelago, Guinea-Bissau. High crab density areas are associated with lower richness and densities of macroinvertebrates. Shorebird assemblages were also less rich at high crab density areas and the differences in species composition occurred according to prey type preferences. Fiddler crab density was the most important variable explaining macroinvertebrate abundance, after accounting for the effects of fine fraction of sediment and distance to coast. Nonetheless, a controlled experimental setup would be required to attribute differences found to the engineering activity of fiddler crabs rather than other unaccounted habitat features. Our findings suggest that crab patches should be taken into account when assessing the distribution and abundance of macroinvertebrates and shorebirds in intertidal areas. Since low and high crab density areas differ markedly in terms of shorebird carrying capacity, monitoring variations in their extent will be important to interpret past and present population trends.