Grazing on green algae by the periwinkleLittorina littorea in the Wadden Sea

On sedimentary tidal flats in the Wadden Sea near the Island of Sylt, the periwinkleLittorina littorea occurred preferentially on clusters and beds of mussels and on shell beds (100 to 350 m⁻²), achieved moderate densities on green algal patches or mats (20 to 50 m⁻²), and remained rare on bare sediments (<5 m⁻²). Green algae covering>10% of sediment surface appeared in summer on approximately one third of the tidal zone, mainly in the upper and sheltered parts and almost never on mussel and shell beds. In feeding experiments,L. littorea ingested more of the dominant alge,Enteromorpha, than ofUlva, irrespective of whether or not algae were fresh or decaying. The tough thalli ofChaetomorpha were hardly consumed. Snails feeding onEnteromorpha produced fecal pellets from which new growth ofEnteromorpha started. In the absence of periwinkles,Enteromorpha developed on mussels and the attached fucoids. Experimentally increased snail densities on sediments prevented green algal development, but the snails were unable to graze down established algal mats. It is concluded that natural densities ofL. littorea hardly affect the ephemeral mass development of green algae on sediments. However, where the snails occur at high densities, i.e. on mussel beds, green algal development may be prevented.     

Sporulation ofEnteromorpha spp. (Chlorophyta) and overwintering of spores in sediments of the Wadden Sea,Island Sylt,North Sea

For the last two decades dense mats of species of the filamentous green algaeEnteromorpha spp. have regulary occurred on tidal flats of Köningshafen Bay (island Sylt, North Sea, FRG). In calm areas overwintering of adult plants or plant fragments is a common process to guarantee the mass development during the next season. In contrast, the distribution ofEnteromorpha on exposed sandy tidal flats depends on recruitment by juvenile stages. In 1993Enteromorpha spore settlement was recorded regularly in the field. Polyethylene dishes were placed in the field and left for a period of seven days and lateron cultivated in the laboratory to checkEnteromorpha germling development. During summer 1993 — at a minimum distance of 200 m to the nearest adultEnteromorpha populations — a total of at least 82×10⁶ spores m^–2 settled. During winter the number of spores attached to the collecting dishes was close to zero and the adjacent sand flats were free of any visibleEnteromorpha plants. In further experiments it was shown that the development ofEnteromorpha juveniles in the next spring depended on the overwintering capacity of spores. More than 2×10⁶ spores m^–2 attached to large sand grains and other substrata (e.g. Hydrobia ulvae) survived the winter. In a laboratory experiment several species ofEnteromorpha were able to survive in total darkness for at least 10 months.     

Historical changes and inventory of macroalgae from Königshafen Bay in the northern Wadden Sea

Knowledge on the distribution, abundance and species richness of intertidal macroalgae occurring on sandy and muddy flats of the German Wadden Sea is still incomplete. We summarize published and unpublished information available on the presence of macroalgae on the tidal flats of Königshafen Bay (island of Sylt, North Sea), one of the more extensively studied areas of the Wadden Sea. A total of 46 green algal species, 36 brown algal species and 26 red algal species has been recorded within the last 120 years on soft and hard substrata of Königshafen Bay (disregarding species found unattached or drifting). Several of these species were only temporarily resident on the tidal flats. Today, at least 35 green, 15 brown and 12 red algal species occur within or close to Königshafen Bay. Significant long-term changes in species abundances have occurred in all three major groups of algae: Since the late 1970s, dense green algal mats dominated byEnteromorpha flexuosa, E. radiata andE. prolifera have occurred regularly on the intertidal flats, whereas a general decrease of brown and red algae has been documented. Two red algal species,Gracilaria verrucosa and its epiphyteCallithamnion corymbosum, were conspicuous members of the macroflora until the middle of this century. Although still present in the 1980s, they have now disappeared completely. On the other hand, the brown algaSargassum muticum has begun to colonize mussel beds. The causal background of long-term changes in the macroalgal flora of Königshafen Bay is discussed. Owing to substantial nomenclatural changes during the last 120 years, a revised species list with authors’ names and synonyms is included.     

Temporal and spatial variations of biodeposits—A preliminary investigation of the role of fecal pellets in the Sylt-Rømø tidal area

The fecal pellets ofHeteromastus filiformis andHydrobia ulvae are abundant on the sediment surface of the Sylt-Rømø tidal flats. AlthoughCerastoderma edule is present in most parts of the study area, only small amounts of its pellets could be found. Fecal pellets are non-randomly distributed in the study area. Large amounts ofH. ulvae pellets are found in the mud flats and smaller amounts in the mised flats and sand flats, whereH. filiformis pellets predominate as biogenic component. This distribution pattern varies with time under the influence of environmental factors. Marked temperature fluctuations provide seasonal signals reflecting distinct patterns. The results show a net balance of biodeposit components in the sediment. This could be a result of the sheltered location of the area and this fits in well with the mostly stable sedimentological picture.     

Sewage,green algal mats anchored by lugworms,and the effects on Turbellaria and small Polychaeta

On sandy tidal flats at the Island of Sylt (North Sea) ephemeral mats of green algae covered wide areas in the vicinity of sewage outflows. Algae became anchored in the feeding funnels of lugworms (Arenicola marina) and thus were able to resist displacement by tidal currents. Below the algal mats anoxic conditions extend to the sediment surface. After about one month a rough sea removed all algae. Polychaetes endured this short-term environmental deterioration, while the more sensitive Turbellaria decreased in abundance and species richness. Diatom-feeders were affected most, predators to a medium extent, and bacteria-feeders the least affected. Rare and very abundant species were more affected than moderately abundant ones. None of the turbellarian species increased in abundance and none colonized the algal mats above the sediment. In a semicontrolled experiment with daily hand-removal of drift algae from a 100-m² plot within an extensive field of algal mats, this cleaned "island" served as a refuge to Turbellaria escaping from their algal covered habitat. Here abundance doubled relative to initial conditions and was 5-times higher than below algal mats.     

An actualistic perspective into Archean worlds - (cyano-)bacterially induced sedimentary structures in the siliciclastic Nhlazatse Section, 2.9 Ga Pongola Supergroup, South Africa

Extensive microbial mats colonize sandy tidal flats that form along the coasts of today's Earth. The microbenthos (mainly cyanobacteria) respond to the prevailing physical sediment dynamics by biostabilization, baffling and trapping, as well as binding. This biotic-physical interaction gives rise to characteristic microbially induced sedimentary structures (MISS) that differ greatly from both purely physical structures and from stromatolites. Actualistic studies of the MISS on modern tidal flats have been shown to be the key for understanding equivalent fossil structures that occur in tidal and shelf sandstones of all Earth ages. However, until now the fossil record of Archean MISS has been poor, and relatively few specimens have been found. This paper describes a study location that displays a unique assemblage with a multitude of exceptionally preserved MISS in the 2.9-Ga-old Pongola Supergroup, South Africa. The 'Nhlazatse Section' includes structures such as 'erosional remnants and pockets', 'multidirected ripple marks', 'polygonal oscillation cracks', and 'gas domes'. Optical and geochemical analyses support the biogenicity of microscopic textures such as filamentous laminae or 'orientated grains'. Textures resembling filaments are lined by iron oxide and hydroxides, as well as clay minerals. They contain organic matter, whose isotope composition is consistent with carbon of biological origin. The ancient tidal flats of the Nhlazatse Section record four microbial mat facies that occur in modern tidal settings as well. We distinguish endobenthic and epibenthic microbial mats, including planar, tufted, and spongy subtypes. Each microbial mat facies is characterized by a distinct set of MISS, and relates to a typical tidal zone. The microbial mat structures are preserved in situ, and are consistent with similar features constructed today by benthic cyanobacteria. However, other mat-constructing microorganisms also could have formed the structures in the Archean tidal flats.     

Upward cascading effects of nutrients: shifts in a benthic microalgal community and a negative herbivore response

We evaluated the effects of nutrient addition on interactions between the benthic microalgal community and a dominant herbivorous gastropod, Cerithidea californica (California horn snail), on tidal flats in Mugu Lagoon, southern California, USA. We crossed snail and nutrient (N and P) addition treatments in enclosures on two tidal flats varying from 71 to 92% sand content in a temporally replicated experiment (summer 2000, fall 2000, spring 2001). Diatom biomass increased slightly (~30%) in response to nutrient treatments but was not affected by snails. Blooms of cyanobacteria (up to 200%) and purple sulfur bacteria (up to 400%) occurred in response to nutrient enrichment, particularly in the sandier site, but only cyanobacterial biomass decreased in response to snail grazing. Snail mortality was 2–5 times higher in response to nutrient addition, especially in the sandier site, corresponding to a relative increase in cyanobacterial biomass. Nutrient-related snail mortality occurred only in the spring and summer, when the snails were most actively feeding on the microalgal community. Inactive snails in the fall showed no response to nutrient-induced cyanobacterial growths. This study demonstrated strongly negative upward cascading effects of nutrient enrichment through the food chain. The strength of this upward cascade was closely linked to sediment type and microalgal community composition.     

Effects of green algal mats on bivalves in a New England mud flat

Concurrent with the spread of green algal mats on tidal flats, reports of macrofauna dieoffs under dense algal mats have increased in numbers. Bivalves seem to be particularly affected by persistent dense algal mats. Bivalve species with a long extendible siphon seem to be less affected underneath algal mats, but no distinction has been made in the past between species with short and those with long siphons,Mya arenaria andMacoma balthica, on an intertidal mudflat in New England. Abundances ofM. arenaria declined substantially during the study period when a thick green algal mat covered the mudflat for several months. Numbers of the small bivalveGemma gemma also decreased substantially, whereas abundances ofM. balthica showed minimal variation during the time of algal coverage. In algae removal/addition experiments numbers ofM. arenaria decreased, but effects were only significant in an algal addition to previously algal-free mudflat areas. Abundance ofM. balthica did not change significantly in the algal removal/additition experiments. Over the time period of the experiment (9 weeks),M. arenaria showed measurable size increase in uncovered mudflat areas, but not underneath algal mats. Similarly,M. balthica only increased in size in the uncovered mudflat area. From these results it is concluded thatM. balthica can survive time periods of dense algal coverage because it is able to penetrate through the algal mat with its long extendible siphon, and thus can reach well-oxygenated water layers above the mat.M. arenaria with its thick, less extendible, siphon cannot push through dense algal mats and therefore is more likely to die underneath persistent algal mats.     

Non-trophic Interactions Control Benthic Producers on Intertidal Flats

The importance of positive effects of ecosystem engineers on associated communities is predicted to increase with environmental stress. However, incorporating such non-trophic interactions into ecological theory is not trivial because facilitation of associated species is conditional on both the type of engineer and the type of abiotic stress. We tested the influence of two allogenic ecosystem engineers (lugworms, Arenicola marina L. and cockles, Cerastoderma edule L.) on the main primary producers (microphytobenthos) of the tidal flats, under different abiotic stresses controlled by reefs of blue mussels (Mytilus edulis L.). We added 25,000 cockles or 2,000 lugworms to 5 × 5 m plots, both in a muddy site with high sedimentation rates located coastward of a mussel bed, and in a sandy site without mussels and characterized by high hydrodynamic stress. After a year, cockles increased algal biomass in the sandy area, but not in the mussel bed site, where high values were measured in all plots. However, lugworms did not affect algal biomass in any of the sites. Field measurements suggest that cockles outweighed negative effects of water currents in the site without mussels by locally increasing sediment stability, whereas mussels overruled the effects of cockles in the wake of the reefs through hydrodynamic stress alleviation and/or biodeposition. Our results suggest that non-trophic interactions by ecosystem engineering bivalves control primary production of intertidal areas, and that the sediment-stabilizing effect of cockles plays a crucial role where the overruling effects of mussel beds are not present.     

Sampling Design and Enumeration Statistics for Bacteria Extracted from Marine Sediments

The spatial and temporal distributions of marine bacteria were studied at both a muddy and a sandy subtidal site in North Inlet, S.C. The sampling design was hierarchical, since subsampling (by a dilution series) of the sediments was necessary to count bacterial cells using acridine orange epifluoresence microscopy. The cell count data fit a log-normal distribution. The abundance of bacteria was 10¹¹ g⁻¹ (dry weight) of mud and 10⁹ g⁻¹ (dry weight) of sand. Variance component analyses demonstrated that variation due to the subsampling procedures was always statistically significant. Thus the common practice of counting 20 fields from one filter preparation is inadequate for estimating the true bacterial population variance in marine sediments. It is recommended that replication of the subsampling level be performed. Standardization of data (by dry weight of sediment) decreased sampling variance at the mud site but not at the sand site, implying that bacteria are more homogeneously distributed in sand than in mud.     

The evolution of the fine sediment regime of the Severn Estuary and Bristol Channel

Fine sediment forms tidal flats along most of the Severn Estuary coastline but subtidal deposits are localized, mainly confined to Newport Deep and Bridgwater Bay. The estuary is experiencing a period of sea level rise which has led to coastal mud erosion in recent centuries. Sediment exchanges with the sea and river inputs are negligible compared to the large-scale exchanges within the estuary itself. Measurements of tidal flat level changes, archaeological finds and anthropogenic chemical concentrations confirm that mud flat erosion is long term and typical of the entire estuary. Sea level rise and coast erosion would normally lead to much of this sediment being redeposited inshore at the landward limit of the alluvium. Other than in narrow salt marsh strips, such deposition is prevented in the Severn by the river walls. In cores of subtidal muddy sediment, the number and thickness of sand layers decreases upwards. The top metre or so is entirely mud and exhibits thousands of layers of varying thickness and origin. Radiochemical analysis confirms that mud is accumulating in the subtidal zone of Newport Deep and the seaward periphery of the Bridgwater Bay mud patch and is increasing in proportion to sand. Circumstantial evidence strongly suggests that for at least the last 600 years the dominant pattern has been one of erosion of fine sediment from the coastal margin and its accumulation in subtidal sinks.     

Food items of the grey mullet Mugil cephalus in the Banc d'Arguin area (Mauritania)

The stomach and gut content of 22 Mugil cephalus from the Banc d'Arguin shallows (Mauritanian coast, West Africa) is examined. Quartz grains are the predominant material found in the stomach together with small portions of benthic diatoms and flakes, aggregates of fine-grained inorganic and organic particles. As the composition of this material is modified by a sorting procedure during feeding it is difficult to conclude, whether seagrass stands (muddy and mixed sediments with sand fraction) or sandy flats are preferred as feeding habitats. The animals studied did not or not relevantly utilize the non-diatom microphytes, though the environment of the Banc d'Arguin is rich in microbial mats (cyanobacteria), Vaucheria beds and epiphytic vegetation of seagrass leaves.     

Experiments on epibenthic predation in the Wadden Sea

Field experiments were designed to evaluate the role of predators in the Wadden Sea, small predators like shore crabs, shrimps and gobies, and large ones like flatfish and birds. Exclosures, maintained in aCorophium volutator bed, an eelgrass bed, in a sandy and a muddy flat, protected the infauna from such epibenthic predation. The resulting changes in the macrofauna were recorded and compared with an unaffected control area. In sandy and muddy flats of the lower intertidal zone, cages (mesh size5 mm) altered abundance and composition of the infauna almost entirely. Nearly all species achieved higher population densities than in the control area, and in addition, the number of species increased as well. In the the scarcely populated mud flat a dense suspension-feeder assemblage emerged, associated with numerous tube-building polychaetes. The sand flat, normally dominated by deposit feeders, also became occupied by a dense suspension-feeder assemblage, mainly cockles. In contrast, narrowly meshed cages had only little effect in the beds of eelgrass and ofCorophium volutator. Both are positioned in the upper intertidal zone. Although a number of species still responded with significant increases in abundance, many remained indifferent or even tended to be less abundant within cages. Cages provided with a 20-mm mesh nylon net, excluding only birds, flatfish and the biggest crabs, increased significantly the survival of large-sized infaunal members. This was only apparent in the upper intertidal zone. It was concluded, that small sized epibenthic predators are the major determinants of the dynamic species abundance pattern of the lower intertidal flats. In the eelgrass bed, the meshwork of rootlets constitutes an important spatial refuge from these predators. The complex habitat structure causes a diversified faunal assemblage. TheCorophium bed in the uppermost intertidal zone is less accessable to predators like crabs, shrimp and gobies. The monotonous appearance of this faunal assemblage is assumed to be the outcome of competitive exclusion and of occasional harsh physical conditions.     

Rapid colonization of new habitats in the Wadden Sea by the ovoviviparousLittorina saxatilis (Olivi)

The intertidal periwinkleLittorina saxatilis completely lacks larval dispersal and adult vagility is low. Although this suggests a low dispersal rate,L. saxatilis is frequently found in recently established habitats “exotic” to the Wadden Sea. Populations occur on man-made structures like dikes, breakwater and groynes, some of which are not older than several years. Furthermore,L. saxatilis is found on marsh grassSpartina anglica, introduced to the Wadden Sea in the 1920s and 1930s, as well as on mats of green macroalgae, which have become an abundant feature on the tidal flats since the late 1970s. Seagrass beds are likely to be the original habitat ofL. saxatilis in the Wadden Sea. Since seagrass populations have dramatically declined over the last decades, colonization of new habitat types enabledL. saxatilis to maintain its Wadden Sea populations despite a changing environment. Colonizers can reach new habitats by means of passive transport, especially by rafting on macrophytes and by aerial dispersal attached to birds. In thew Wadden Sea, the ovoviviparously reproducingL. saxatilis has demonstrated its ability to successfully found new populations with only a few individuals. No reduction of genetic variablility (founder effect) was observed in recently established populations.     

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.     

CONSUMPTION OF ULVA LACTUCA (CHLOROPHYTA) BY THE OMNIVOROUS MUD SNAIL ILYANASSA OBSOLETA (SAY)

Marine invertebrate grazing on temperate macroalgae may exert a significant “top-down” control on macroalgal biomass. We conducted two laboratory experiments to test (1) if consumption by the omnivorous mud snail Ilyanassa obsoleta (Say) on the macroalga Ulva lactuca Linnaeus was a function of food quality (nitrogen content) and (2) if grazing on benthic macroalgae occurred at significant rates in the presence of alternative food sources in the sediment (detritus, larvae, benthic microalgae). Grazing rates were higher for N-enriched macroalgae; however, all snails lost weight when grazing on macroalgae alone, indicating that U. lactuca was a poor food source. The presence of sediment from two sites, a sandy lagoon and an adjacent organic-rich muddy tidal creek, did not affect consumption of macroalgae in microcosm experiments, and the grazing snails were capable of significantly reducing macroalgal biomass associated with both sediment types. Grazing rates by this omnivore were as high as 10.83 mg wet weight·individuals ^{− 1}·d ^{− 1} and were similar to those recorded for herbivorous species. In situ loss rates calculated from average grazing rates per individual and snail abundances (up to 3.5 g dry weight·m ^{− 2}·d ^{− 1}) also were comparable with those calculated for herbivorous species. This level of grazing could remove up to 88% of new macroalgal growth at the lagoon site where the N supply was relatively low but had a much smaller effect (18% of new growth) at the high-nutrient creek site. Snails facilitated macroalgal growth at both sites by increasing tissue N content by 40%–80%. Consumption and digestion of macroalgae aided in the recycling of nutrients temporarily bound in the algae and resulted in enrichment of surficial sediments. Increased N sequestration in the sediments also was associated with an interruption of snail burrowing behavior due to persistent anoxia in sediments rich in decaying algal material. Our data suggest that in shallow lagoons where mud snails and benthic macroalgae coexist, grazing may influence N retention in macroalgal biomass.     

底栖生物影响下的潮滩微地貌演化数值模拟

潮滩是海岸带湿地的主要类型之一,其中分布的底栖生物对生态环境具有重要的调节作用。潮滩底栖微藻、泥沙与水动力之间存在相互作用,影响潮滩微地貌形态,明晰底栖生物对潮滩微地貌的演化机制至关重要。以黄河三角洲潮滩湿地为研究区,通过构建潮滩微地貌动力模型,探究底栖生物对微地貌格局演化的作用机制,分析底栖生物对微地貌系统稳定性的影响。结果表明(1)底栖微藻生长与泥沙扩散、水流再分配过程交互作用驱动下,潮滩上可形成底栖微藻覆盖的高丘与积水洼地交替分布的规则性微地貌斑图;(2)微地貌斑图的形成提高了潮滩生态系统初级生产力和泥沙淤积高度;(3)底栖微藻与泥沙、水流的交互作用使得潮滩微地貌系统对侵蚀扰动呈现非线性响应行为,系统存在临界点,且在一定侵蚀率范围内存在双稳态;(4)黄河口泥螺入侵使得微地貌系统抵抗侵蚀扰动能力减小,且系统稳定性随泥螺生物量的增加而降低。     

The role of sediment type in growth and fecundity of mud snails (Hydrobiidae)

Summary We test the hypothesis that body size and population density of the deposit-feeding gastropod, Hydrobia truncata, are greater in muddy than in sandy habitats as a result of faster growth on fine- compared to coarse-grained sediments. We refute this hypothesis using a combination of field measurements and laboratory experiments. Three out of three populations tested had higher maximal growth rates and two of three populations approached their asymptotic size more quickly on sand than on silt-clay fractions of natural sediment. Growth decreased with increasing snail density and was as high or higher on sand as on silt-clay at all densities. Two populations were more fecund on sand than on silt-clay, and fecundity of the third population was not affected by sediment type. We show that the smaller body sizes observed in snails from the sandiest habitat result from late recruitment of these snails, relative to the other populations.     

POTOS: a portable topographic system for measuring inaccessible muddy creek areas

Tidal flats and marshes in intertidal environments and estuaries generally cover large areas. The access to these places is usually complicated. The area suffers cycles of flooding and drying under the influence of the tide, which in conjunction with soft and muddy soils makes every movement very difficult. In particular, in tidal channels mud is softer and, many times, it is impossible to move around these areas. Furthermore, muddy tidal flat creeks are often located far away from conventional vehicle access. Then, to perform a topographic study using traditional methodology is not feasible; therefore, an alternative methodology has been developed to deal with these problems. POTOS is a light metallic frame developed to measure creek topography indirectly with good precision. The tensed frame, up to 12 m long, simply supported and weighting only 40 kg allows making relative height measurements of the surface between the supporting posts. The field methodology is described and some results obtained with the instrument developed are presented. Errors were calculated by comparing measurements obtained with the instrument and those gathered by traditional methodologies. Field results are a series of measurements obtained in Bahía Blanca Estuary tidal flats and marshes at different periods of the year.     

Distribution of candidate division JS1 and other Bacteria in tidal sediments of the German Wadden Sea using targeted 16S rRNA gene PCR-DGGE

The bacterial candidate division JS1 dominates a number of 16S rRNA gene libraries from deep subseafloor sediments, yet its distribution in shallow, subsurface sediments has still to be fully documented. Sediment cores (down to 5.5 m) from Wadden Sea tidal flats (Neuharlingersieler Nacken and Gr?ninger Plate) were screened for JS1 16S rRNA genes using targeted PCR-denaturing gradient gel electrophoresis (DGGE), which also detects some other important Bacteria. Bacterial subpopulations at both sites were dominated by Gammaproteobacteria in the upper sediment layers (down to 2 m) and in deeper layers by members of the Chloroflexi. The deeper layers of Neuharlingersieler Nacken consisted of grey mud with low sulphate (0.1-10 mM), elevated total organic carbon (TOC) ( approximately 1-2%) and JS1 sequences were abundant. In contrast, the deeper sandy layers of Gr?ninger Plate, despite also having reduced sulphate concentrations, had lower TOC (<0.6%) with few detectable JS1 sequences. Results indicated that JS1 prefers muddy, shallow, subsurface sediments with reduced sulphate, whereas Chloroflexi may out-compete JS1 in shallow, sandy, subsurface sediments. Bacterial population changes at both sites ( approximately 2 m) were confirmed by cluster analysis of DGGE profiles, which correlated with increased recalcitrance of the organic matter. This study extends the biogeographical range of JS1. The presence of JS1 and Chloroflexi in Wadden Sea sediments demonstrates that subsurface tidal flats contain similar prokaryotic populations to those found in the deeper subseafloor biosphere.