Biogenic sedimentary structures on a Korean mud flat: Spring-neap variations

Biogenic sedimentary structures created by dominant benthic animals of a mud flat were investigated in a small embayment at Panweol on the west coast of Korea. Burrows were replicated using the resin casting method on a sufficiently large scale to evaluate their variations statistically. The size and shape of burrows and other traces varied with locations and tidal phases. Burrows of the upper-intertidal crabs Helice tridens sheni and Ilyoplax dentimerosa were T- or I-shaped, vertical and deep whereas those of the mid-intertidal crab Macrophthalmus japonicus were U-shaped, oblique and shallow. The upper-intertidal polychaete Perineris aibuhitensis, a subsurface feeder, had sinuous and highly-branched burrows while the mid-internal one Periserrula leucophryna, a surface feeder, had vertical burrows without subsurface branches. Variations of crab burrows with tidal phases were distinct: The burrows of both H. tridens sheni and M. japonicus were significantly deeper and wider during neap tides than during spring tides. Sediment mounds were common in the upper intertidal zone, while crawling and feeding traces such as trails, trackways and cheliped scrapings were more prominent in the mid-intertidal zone. These traces were also observed in the upper intertidal zone during spring tides when surface sediments became watery. The size of the sediment mounds varied, being largest during neap tides. These biogenic sedimentary structures observed at Panweol were distinctly different from those found near Inchon only 50 km away, probably due to differences in hydrology and sedimentology.     

Nitrite and ammonia in the Tamar estuary

Seasonal variations in the axial distributions of nitrite and ammonia in the Tamar estuary, southwest England, have been recorded by continuous autoanalytical profiling at approximately monthly intervals together with measurements of salinity, turbidity and dissolved oxygen. In contrast to the persistent apparent conservativeness of nitrate in this estuary, these reduced species of nitrogen have shown marked non-conservative behaviour.Ammonia distributions generally showed a maximum in the middle to upper estuary which was variably located relative to salinity and had a magnitude which tended to vary with the spring-neap tidal condition. These maxima are attributed to releases from the sediment which are facilitated by tidal sediment disturbance.During the winter months, nitrite distributions reflected conservative mixing of riverine nitrite but internal inputs within the low salinity, turbidity maximum zone became increasingly significant from spring to late summer in phase with an increasing degree of local oxygen depletion. It appears that nitrite is generated within the water column by bacterially mediated oxidation of ammonia.Anthropogenic inputs significantly augment both constituents in the outer estuary.     

Community structure and biomass distribution of seagrasses and macrofauna in the flores sea,Indonesia

In several locations in the Flores Sea region the community structure and the biomass distribution of seagrasses were studied along transects perpendicular to the shoreline. The share of each species within a sample plot was estimated, divided in above- and below-ground biomass. Statistics regarding substrate coverage, shoot density and leaf-area index were sampled. A standard relation was calculated between seagrass dry weight, ash-free dry weight and organic carbon content.The biotic data were related to environmental factors: DOC and nutrients in the water, salinity, tidal amplitude, sediment composition. A relation was estimated between bottom coverage of seagrasses and standing stock. Further calculations of biomass-production ratios allow a quick and rough estimate of seagrass productivity. Maximum above-ground biomass values (500–700 g AFDW·m⁻²) together with qualitative data indicate resource (= space) partitioning among the component seagrasses within a community, and suggest a carrying capacity of the reefflat habitat for seagrass density and biomass.A tentative model was constructed, starting from a constant, non-distributed multispecies vegetation in the lower intertidal and subtidal zone on sand and coral rubble, and moving into several suboptimal situations. The upper shore carries an impoverished, constrained vegetation (irregular tides, desiccation, harvesting). Sediment reworking by animals and physical displacement of sand disturbs the vegetation and favours pioneer species. Muddy habitats bordering mangroves carry monospecific stands showing extremely high biomass (e.g. below-ground Enhalus acoroides 3500 g AFDW·m⁻²). Thalassia hemprichii and Enhalus acoroides are the most constant species in all habitats mentioned.Macrofauna biomass within the seagrass beds fluctuated widely (maximum values 50–70 g AFDW·m⁻² in mixed seagrass vegetations) and only a weak relation between benthic macrofauna biomass and seagrass community structure and biomass was found.     

Immigration and settlement pattern of Paralichthys dentatus and P. lethostigma in an estuarine nursery ground,North Carolina,U.S.A.

Immigration and settlement pattern of 0-group summer flounder (Paralichthys dentatus) and southern flounder (P. lethostigma) in an estuarine nursery are described. Flounders entering the estuary were transforming larvae. Despite their sympatric occurrence during larval immigration, the two species were found concentrated in different habitats. Sampling along transects within the estuary indicated that newly settled southern flounder were concentrated on tidal flats towards the head of the estuary while greatest numbers of summer flounder were captured on tidal flats in the middle reach of the system. Logistic regression analysis of presence/absence data indicated that salinity affected the distribution of southern flounder while substrate type was an important factor affecting the distribution of summer flounder. Numbers of both species on tidal flats declined in the spring, southern flounder appeared to move upstream to oligohaline riverine habitat and summer flounder appeared to move to high salinity salt marsh habitat. The segregated distribution apparent in juveniles is largely established during the immigration period. Differences in settling behaviour and subsequent movement and/or differential mortality occurring during settlement, appear important in determining the observed distribution of juvenile summer and southern flounder.     

Effects on the benthic fauna of embanking an intertidal flat area (the Markiezaat,Eastern Scheldt estuary,The Netherlands)

Effects of embanking an intertidal-flat area in the south-western part of the Netherlands, the Markiezaat, on abundance and condition of benthic animals were followed at 3 stations during spring, 1983. Part of the former intertidal flats became permanently submerged with stagnant salt water, whereas another part became permanently emerged.At the submerged sampling station, no clear changes were found during the first 3 months of submersion. However, within 6 months, most species died off.At the drained stations, all individuals of most species died off within 2 months. During some preliminary experiments with drained sediment cores, a high mortality of the animals was reached within much shorter periods. During these experiments the animals showed a retarded mortality after they had been returned to a normal tidal cycle. Species living in the lower part of the intertidal zone were found to be less tolerant to drainage than those living in the higher part. The observed mortality rates in the Markiezaat probably represent minimal values, because of the heavy rainfall and the absence of extreme temperatures during drainage. At high (summer) and low (winter) temperature and low precipitation, benthic animals may be less tolerant to drainage.Neither permanent submersion nor emersion affected the condition determined as dry weight per unit of volume. Therefore, the suitability of this condition-index as an indicator of stress for intertidal benthic animals may be questioned.     

Animal traces on a tidal flat in Hangzhou Bay,China

Despite prevailing low salinity (S ranging from 1 to 10) of the Hangzhou Bay, its extensive (up to 2 km wide) flats along the northern coast have a species-rich macrobenthic fauna of crustaceans, bivalves, gastropods and polychaetes. These produce a variety of crawling, feeding, dwelling and resting traces, some comparable to those observed on the flats of Korea and Taiwan. The characteristic starlike feeding traces of the bivalve Tellina (Moerella) iridescens are described here for the first time. Sediment reworking appears to be largely confined to the surface layer. The burrow-inhabiting crabs are also mainly surface sediment feeders. These Chinese flats lack organisms reworking deeper sediment layers such as lugworms or callianassid crustaceans. As compared to the Wadden Sea, sedimentation rate is higher and subsurface sediment reworking less intense, resulting in relatively few bioturbation structures in vertical sections of the Hangzhou tidal flat sediments.     

Path analyses of the influence of substrate composition on nematode numbers and on decomposition of stranded seaweed at an Antarctic coast

Large amounts of seaweed are deposited along the coast of Admiralty Bay, King George Island, Antarctica. The stranded seaweed partly decomposes on the beach and supports populations of meiofauna species, mostly nematodes. The factors determining the number of nematodes found in the seaweed packages were studied. Seaweed/sediment samples were collected from different locations, along the coast near Arctowski station, covering gradients of salinity, elevation and proximity of Penguin rookeries. On the same locations decomposition rate was determined by means of permeable containers with seaweed material. Models, including the relations between location, seaweed and sediment characteristics, number of nematodes and decomposition rates, were postulated and verified using path analysis. The most plausible and significant models are presented.The number of nematodes was directly correlated with the height of the location, the carbon-to-nitrogen ratio, and the salinity of the sample. Nematode numbers were apparently indirectly dependent on sediment composition and water content. We hypothesize that the different influences of melt water and tidal water, which affect both salinity and water content of the deposits, are important phenomena underlying these results.Analysis of the relation between decomposition rate and abiotic, location-related characteristics showed that decomposition rate was dependent on the water content of the stranded seaweed and sediment composition. Decomposition rates were high on locations where water content of the deposits was high. There the running water from melt water run-off or from the surf probably increased weight losses of seaweed.     

Adaptation to anaerobic metabolism in two mussel species,Mytilus edulis and Mytilus galloprovincialis,from the tidal zone at Arcachon Bay,France

Aspects of anaerobic metabolism were investigated in two sympatric mussel species, viz. Mytilus edulis and Mytilus galloprovincialis, living in the tidal zone in Arcachon Bay, France. Specific activities of pyruvate kinase (PK) and phosphoenolpyruvate kinase (PEP-CK) were remarkably similar in the two sympatric species and generally corresponded more closely to those observed in M. galloprovincialis in the Mediterranean than with M. edulis in the Dutch Wadden Sea. However, the values for the radio PK: PEP-CK for the two species in Arcachon Bay agreed with those of intertidal M. edulis from the Dutch Wadden Sea.Succinate accumulation during the first 24 h of anaerobicsis was about the same as in M. galloprovincialis in the Mediterranean, but decreased during the second 24 h, particularly in M. edulis, obviously due to propionate formation. Decrease in ATP concentrations in the tissues during anaerobiosis corresponded to that of intertidal M. edulis from the Dutch Wadden Sea. With the exception of specific activities of PK and PEP-CK, all properties investigated in both species were as expected in intertidal mussels.     

Food limitation in cockles (Cerastoderma edule (L.)): Influences of location on tidal flat and of nearby presence of mussel beds

Dilution of phytoplankton concentrations by suspension-feeding organisms, and local reduction in current velocity, may lower food supply for other suspension feeders, and consequently affect their growth. Influences of food supply on stomach contents and growth of the suspension-feeding bivalve Cerastoderma edule were studied in two situations: (a) inside and outside an experimental mussel bed in a basin with a natural seawater supply, and (b) along tidal creeks in the field. Cockles showed a lower increase in shell length, body weight and condition inside the experimental mussel bed than outside. In the field, phytoplankton concentrations in the water were lower at upper than at lower tidal-creek sites. At these upper creek sites, cockles were smaller, as were their growth rates and stomach contents, compared to the lower creek sites. Largest reductions of phytoplankton concentration in the water were observed at sites near a mussel bed. It is concluded that phytoplankton uptake by suspension feeders (particularly in mussel beds), as well as relatively low current velocities, negatively affects food supply, stomach contents and growth of C. edule.     

Environmental effects of the growth rate of intertidal invertebrates and some implications for foraging waders

The paper describes effects of intertidal height and sediment type on growth rate of the bivalves Cerastoderma edule, Macoma balthica, Mya arenaria, Mytilus edulis and Scrobicularia plana, and of the worms Arenicola marina, Nephtys hombergii and Nereis diversicolor in the eastern part of the Dutch Wadden Sea. In most species, exposure time was negatively correlated with length growth, although interfering effects of sediment type could not be ruled out. When controlled for the effects of exposure time, clay content of the sediment appeared to affect the growth of all species, but in different ways. The variation was related to the foraging methods of the invertebrates. Foraging waders may use the spatial variation in growth rate of the invertebrates to optimize the exploitation of individual cohorts.     

Physical and biotic aspects of fine-sediment import in the Oosterschelde tidal basin (The Netherlands)

The distribution of fine sediments inside the Oosterschelde (Eastern Scheldt) throughout the year has been investigated. The results have been interpreted in terms of physical transport processes and activity of biotic components. Evaluation of the mechanisms that may induce landward transport of fine sediment shows that an asymmetry in maximum ebb and flood currents is probably mainly responsible for the observed landward transport of fine sediment in the Oosterschelde. A theoretical formulation is presented which fits long-term sedimentation. Transport is higher during winter than during summer. Despite a lower transport rate during summer, filter feeders do not clear Oosterschelde water. Most of the biodeposits of filter feeders are resuspended by waves and currents, probably stimulated by bioturbation and fishery.     

How the food supply harvestable by waders in the Wadden Sea depends on the variation in energy density,body weight,biomass, burying depth and behaviour of tidal-flat invertebrates

For several reasons, waders in the Wadden Sea face a large seasonal and annual variation in their food supply. Observations on a tidal flat in the Dutch Wadden Sea have shown that:

• 1.- (1) The average energy density of ten invertebrate prey species varies between 21 and 23 kJ·g⁻¹ AFDW. In Scrobicularia plana and Mya arenaria, but not in Macoma balthica, the energy density is 10% lower in winter than in summer.
• 2.- (2) Depending on the species, body weights of prey of similar size are 30 to 60% lower in winter than in summer.
• 3.- (3) The year-to-year fluctuation in standing-crop biomass is larger in some species than in others, the difference depending mainly on the frequency of successful recruitment. The overall biomass of the macrobenthos in winter is half of that in summer, but the timing of the peak biomass differs per species.
• 4.- (4) The burying depth varies per species: Cerastoderma edule live just beneath the surface, while M. balthica, S. plana, M. arenaria, Arenicola marina and Nereis diversicolor bury more deeply and the majority of these prey live out of reach of the bird's bill. In all six species, burying depth increases with size. There is no seasonal variation in depth of C. edule and M. arenaria, but the four other species live at most shallow depth in early summer and most deeply in midwinter. Burying depths in winter vary from year to year, but are unrelated to temperature. Neither has temperature any effect on depth within months. For knot Calidris canutus feeding on M. balthica, the fluctuation in the accessible fraction was the main source of variation in the biomass of prey that is actually harvestable, i.e. the biomass of prey of suitable size that is accessible.Accordingly, the paper reviews the available data on the temporal variations in accessibility, detectability, ingestibility, digestibility and profitability of prey for waders. Only a small part of the prey is harvestable since many accessible prey are ignored because of their low profitability, while many profitable prey are inaccessible. The profitability of prey depends on their size and weight but also on their depth in the mud, since handling time increases with burying depth. A simple biomechanical rule explains why the handling time of small prey increases with bill length and why large, long-billed waders ignore a disproportionately larger part of the small prey. The fraction detectable for visually feeding waders is usually very low, especially when the temperature of the substrate is below 3–6°C. Waders vary their prey choice over the year in response to the changes in the availability and profitability of their different prey species. The food supply harvestable by waders is much lower in winter than in summer. For waders wintering in the Wadden Sea, the food supply may be characterized as unpredictable and usually meagre. Waders wintering in NW Europe are concentrated in coastal sites where the average surface temperature is above 3°C. This probably cannot be explained by a greater burying depth, and only partly by a lower body condition, of prey in colder areas. Yet the harvestable fraction is lower in colder sites, especially for sight-feeding waders, as invertebrates are less active at low temperatures. However, the lower energetic cost of living and reduced chances of the prey being covered by ice may also contribute to the waders' preference for warmer sites.

     

Seasonal variations and spatial gradients of nitrogenous nutrients in waters of Southeastern Vancouver Island,British Columbia,Canada

The study was conducted to begin testing the hypothesis that supplies of inorganic nitrogen are adequate to support rates of phytoplankton production observed in Saanich Inlet, a fjord in Vancouver Island, British Columbia, Canada. To test this hypothesis, rates of nitrogen transport into and within the euphotic zone by advection and diffusion are needed. Computation of these rates requires knowledge of spatial gradients of inorganic nitrogen, which were determined from spatial distributions of NO₃-N, NO₂-N, and NH₄-N measured during spring, summer and fall months of 1977, 1983 and 1984 in water of the southeastern coast of Vancouver Island. Of the nitrogenous nutrients, vertical gradients of NO₃-N were largest, usually exceeding 0.75 μM-m⁻¹ in the upper 10 m of the water column during late spring and summer months. During these same months, horizontal gradients were small, varying between 1.5 and 2.0 μM-km⁻¹. A model (Eppley et al., 1979) was used to estimate the relative importance of horizontal and vertical diffusion to nutrient supply in the euphotic zone and results indicated that vertical diffusion would dominate. Thus, it is likely that vertical advection and diffusion driven by winds, internal waves, and tidal currents were important sources of nutrients to phytoplankton in Saanich Inlet during late spring and summer months.     

Measuring the flux of oxygen to a muddy sediment with a cylindrical microcosm

We have examined the effects of changing the flow velocity and the oxygen concentratin in the water overlying a muddy sediment on the flux of oxygen across the sediment-water interface and on the distribution of oxygen within the pore water. The experiment was carried out on an intertidal sediment from the western Wadden Sea, using a cylindrical microcosm with a calibrated flow regime. Steady-state and transient-state models were used to estimate the values of the effective diffusion coefficient for oxygen in the pore water.Increasing the flow velocity caused a significant though small increase in the oxygen concentration in the pore water, but had little effect on the concentration gradient at the sediment-water interface. The concentration gradient in the boundary layer was too small at any of the flow velocities to account for the oxygen flux into the sediment via molecular diffusion. This is ascribed to a pressure gradient which exists in rotating flows, disrupting the diffusive boundary layer and augmenting the flux via advection. Model calculations indicate that about 25% of the flux can be attributed to irrigation by burrowing organisms, but in contrast to previous results with sandy sediments, irrigation of the pore water caused by the radial pressure gradient can be considered to be negligible. The effective diffusion coefficient ranged from 4·10⁻⁹ m²·s⁻¹ at a depth of 1 mm below the sediment-water interface to 1·10⁻⁹ m²·s⁻¹ deeper in the sediment. These estimates are within a factor of 1 to 3 of the modelular diffusion coefficient for oxygen, which is suprisingly close in view of the high numerical densities of meiofauna and macrofauna in this sediment.     

The effects of bioturbation on sediment transport on an intertidal mudflat

Laboratory flume studies were conducted to determine the relative importance of various biological and physical factors controlling sediment erosion threshold and transport rate on an intertidal mudflat in the Bay of Fundy. Studies were conducted in a July period of maximum solar exposure. The upper and mid-intertidal stations of the flat were dominated by silt-clay sediments, while the lower intertidal was dominated by very fine sand. The tube-dwelling amphipod Corophium volutator was the most abundant infaunal species with densities exceeding ∼1300 ind·m⁻² based on counts of burrow openings. Sediment-penetrometry and water-content measurements indicated no change in unconsolidated shear strength and porosity, respectively, along the intertidal transect. Despite the apparently cohesive nature of the sediment, erosion occurred as small ripples. Critical shear velocities (u_crit) for erosion determined with intact cores in a laboratory flume were relatively consistent between stations and sampling dates (mean = 2.1 cm·s⁻¹ ± 0.2 SD), with no relationship to Corophium density, sediment chlorophyll a, or physical variables. Field-treatment of sediment with formalin did not cause an obvious change in u_crit as determined by flume experiments. Corophium seemed to have little effect on erosion thresholds because incipient motion could be observed between tube burrows, beyond the local influence of the amphipod. In contrast to erosion thresholds, sediment-erosion rates measured with bedload traps were negatively correlated with density of small Corophium, probably due to binding of sediment into burrows and the ambient sediment microfabric, all of which reduce the availability of sediment for transport. Adult amphipods, which occurred at low density probably due to territorial/competitive interactions, had no obvious effect on erosion rate since only a small proportion of the sediment surface was impacted by their bioturbation. Although a portion of the amphipod population exerts a stabilizing influence on sediment-erosion rates, concurrent studies at the site indicate that Corophium seasonally reduces the sediment-erosion threshold by grazing on microflora which would otherwise inhibit the initiation of grain motion. Due to this decoupling of erosion rate and threshold, it is necessary to measure both processes in assessing the effects of benthic biota on sediment transport.     

滴灌条件下不同盐水平对棉花根系分布的影响

 通过盆栽试验研究膜下滴灌土壤盐分分布对棉花根系生长的影响。结果表明：受滴灌水分运移的影响,在0～30 cm土层形成脱盐区,土壤盐分明显下降,养分含量也很低,30 cm以下是积盐区。土壤盐分对棉花根系分布影响显著,0.32 dS·m^-1、1.12 dS·m^-1和1.90 dS·m^-1处理总根长分别为26.35 m、49.54 m和34.40 m,但在土层表现不同。在0～30 cm脱盐区内,1.12 dS·m^-1和1.90 dS·m^-1处理的根长密度明显高于0.32 dS·m^-1处理,而在40～50 cm却低于0.32 dS·m^-1处理。研究认为,在盐胁迫条件下,棉花根系分布进行适应性变化,产生补偿效应,通过显著的增加脱盐区（0～30 cm土层）根系数量,来获得更多的水分、养分保证棉花生长的需要。     

Distribution and abundance of the zooplankton of the Ems estuary (North Sea)

From 1974 to 1977 a study was made of the abundance and the distribution of the zooplankton species of the Ems estuary (The Netherlands and the Federal Republic of Germany), an area of about 500 km² with extensive tidal flats.The most important component of the zooplankton consisted of holoplanktonic calanoid copepods with, during the summer, a significant contribution of meroplankton, mainly consisting of polychaete and cirripede larvae. Zooplankton abundance showed a marked seasonality, with a pronounced spring peak and a smaller late summer/autumn peak. In the low salinity area the spring bloom was dominated by Eurytemora affinis, which persisted in the salinities below S=5 upriver during the whole year. In the polyhaline area Acartia bifilosa was the main component of the spring bloom. The late summer maximum from August to October was dominated by Acartia tonsa in the meso- and polyhaline area and by A. discaudata and Centropages hamatus in the euhaline part of the estuary bordering on the Wadden Sea proper. The number of abundant (> 1000 ind·m⁻³) species increased from the inner, fresher part of the estuary towards the Wadden Sea but, except for April, average zooplankton density in salinities <18 was similar (2768 ind·m⁻³) to the density in salinities >18 (2817 ind·m⁻³).The distribution patterns show that there are only a few autochthonous species and that the majority of species is allochthonous and penetrates more or less deeply into the estuary from the Wadden Sea and North Sea, varying with species and season.     

Denitrification and nitrous oxide in the North Sea

In situ sediment denitrification rates were determined in the major areas of deposition of the North Sea, using the acetylene block technique. In addition, nitrous oxide profiles of the water column were determined. Nitrous oxide production generally occurred in the photic zone possibly due to nitrification; and throughout the water column in the German Bight region. Consumption at depth was possibly due to reduction in the anoxic microzones of faecal pellets, concentrated at the thermocline. Saturation of surface waters was 102.2% compared to 130.3% in the German Bight region. Calculated flux of nitrous oxide to the atmosphere was 9.5 × 10⁶ kg yr⁻¹, over half of which was produced in the German Bight.Sediment denitrification rates varied through three orders of magnitude; the highest value of 150 μmol m⁻² d⁻¹ was recorded in the Norwegian Trench. Nitrous oxide production by the sediments was low (1.1 μmol m⁻² d⁻¹ max.), and was undetectable at half of the sites. Sediment nutrient profiles exhibited porewater nitrate concentrations exceeding that of the overlying water suggesting that denitrification was fuelled by nitrification, which, in turn was related to other environmental variables. A significant positive relationship existed between in situ denitrification rate and the nitrate content of the upper sediment. Extrapolation of the rate to the total area of deposition in the North Sea suggests that denitrification is responsible for a minimum loss of 7.5–12% of the total annual nitrogen contaminant input.     

Diagenesis in subrecent marine sediments in the Eastern Scheldt,Southwest Netherlands

The diagenesis in fine-grained sediments from a 300 to 400-years-old Dunkirk deposit, exposed on the intertidal flat, was studied at a site in the Eastern Scheldt. A new in situ pore water sampling technique that allowed repeated sampling at exactly the same place was used to monitor the seasonal fluctuations in interstitial water composition.Concentrations of organic carbon (1.5 to 2%), nitrogen (C/N = 19), phosphorus (500 μg·g⁻¹) and manganese (250 μg·g⁻¹) in the subrecent anoxic sediments were low, probably because they had already been depleted during earlier stages of diagenesis. Rates of organic carbon mineralization by sulphate reduction (0.1 Mole·m⁻²·y⁻¹) and rates of nutrient regeneration were 1 to 2 orders of magnitude lower than in recent fine-grained sediments elsewhere in the Eastern Scheldt.Pore water NH₄⁺ and ΣPO₄ concentrations were controlled by mineralization, uptake by Zostera noltii and sediment-seawater exchange. During the summer the uptake exceeded the mineralization rate at 0 to 5 cm. Mineralization and diffusional processes dominated the changes in the NH₄⁺ and ΣPO₄ profiles in the other seasons.Dissolved manganese and iron concentrations showed a typical subsurface maximum at 0 to 3 cm, and low (<5 μMole) concentrations below this depth. Dissolved iron concentrations were probably controlled by the solubility of iron sulphides, and manganese probably by the solubility of Mn, Ca-carbonate.