Accumulation rates of estuarine sediment in the dollard area: comparison of 210Pb and pollen influx methods

Profiles of excess ²¹⁰Pb activity in sediment cores collected on the muddy tidal flats of the Dollard indicate deposition rates ranging from 0.14 to 0.27 cm·y⁻¹. One profile was also analysed for pollen: the pollen profile in the core provided a record of the sedimentation rate because two independent historical events were found recorded in the profile. The first, a sudden abundance of an Aster-type pollen, reflected, at a depth of 33 cm in the sediment, the large land reclamation of 1862 A.D. The second, the appearance of Zea mays (corn) at a depth of 6 cm, reflected the increase of the area in which corn was cultivated: from 44 ha in 1970 up to 552 ha in 1973. The pollen data gave an estimated sedimentation rate of 0.25 cm·y⁻¹. Excess ²¹⁰Pb analysis of the same core yielded a sedimentation rate of 0.27 cm·y⁻¹.     

Radon profiles in the Indonesian archipelago

A series of near-bottom excess radon profiles was measured in the Indonesian archipelago during the Snellius-II Expedition. The results show considerable variations in structure, ²²²Rn concentrations and integrated excess ²²²Rn inventory in the water column. Near-bottom vertical eddy diffusion coefficients vary from 46.4 to 63.6 cm²·s⁻¹ in the basins; they are in general much higher on slopes and sills. The standing crop (the integrated amount of excess ²²²Rn) ranges from 0.9 to 49.6 dpm·cm⁻². ²²⁶Radium supported ²²²Rn concentrations range from 9 to 3888·10⁻² dpm·kg⁻¹ Compared to mid-ocean basins the vertical mixing processes of near-bottom water masses in the Indonesian archipelago is fast, especially in the water layer from 50 to 200 m above the bottom, with diffusion coefficients in the order of 10 times higher than in the Pacific Ocean. With the exception of two stations standing crop values were comparable with those found in the Pacific Ocean but higher than those from the Atlantic Ocean. ²¹⁰Pb analyses show high sedimentation rates, up to 75 cm·10⁻³·a⁻¹. This does not agree with related standing crops. One explanation can be that erosion plays an important role on the sills and slopes.     

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.     

An experiment on the relative importance of denitrification,nitrate reduction and ammonification in coastal marine sediment

Simultaneous determinations of ¹⁵N-nitrogen gas production, ¹⁵N-ammonia formation by nitrate reduction and ammonia production from organic nitrogen were carried out after addition of ¹⁵N-nitrate to anaerobically incubated sediment slurries from a location in the Dutch Wadden Sea. The rate of nitrogen gas production was 1.57 μmol N·cm⁻³ wet sediment·d⁻¹, nitrate reduction to ammonia occurred at a rate of 0.79 μmol N·cm⁻³·d⁻¹ and ammonification at 1.17 μmol N·cm⁻³·d⁻¹. At the end of the incubation experiment (after 30 h) 63% of the originally added ¹⁵N-nitrate was present as nitrogen gas, 32% ended up as ammonia and ∼4% as particulate organic nitrogen. Ammonia production by degradation of organic nitrogen compounds was the most important process in building up the ammonia pool in the anaerobic sediment incubation.     

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.     

Buoyant river plumes and mud deposition in a rapidly extending tropical delta

The Solo river, East Java, Indonesia, is a typical monsoonal river. The major part of river discharge and sediment transport is realized during the four or five months of the wet season. River outflow essentially has the character of a buoyant jet. The buoyancy of the effluents gives rise to the development of buoyant river plumes. The properties and behaviour of these plumes are of fundamental importance in the depositional processes which govern delta growth.Deposition rates near the river mouth have been calculated by modelling river outflow and plume dimensions. Simulations of ‘banjir’ events (periods of high discharge) suggest that the average, areal deposition may range from 1 to 10 cm·d⁻¹. The estimated annual changes in bed level exceed values of 1.0 m, even if consolidation is taken into account. Measured changes in bed level, as deducted from echosoundings, show good agreement with model calculations.     

Soil loss from road batters in the Karuah state forest,Eastern Australia

Open plot sediment traps 0.8 m wide recorded flux rates up to 56 kg y⁻¹ mwidth⁻¹ for cut batters and 3.2 kg y⁻¹ mwidth⁻¹ for fill batters. The rates for cut batters were ten times higher than the sediment flux rates for hillslopes above them. The actual erosion rates for cut batters and fill batters are probably 100 times and 10 times respectively larger than those for the natural hillslopes. The erosion rates are significantly correlated to batter height and slope; positively correlated for cut batters and negatively correlated for fill batters. The rate of erosion of batters decreases exponentially over time.     

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 determination of the sizes and settling velocities of estuarine flocs by an underwater video system

The in situ underwater video system VIS is described. VIS determines sizes and settling velocities of suspended mud flocs without destroying the large fragile aggregates. The settling process of the flocs and aggregates can be observed directly on a screen on board the research vessel. Analysis of the videorecordings was possible up to 600 mg·dm⁻³, which is of importance in turbid estuarine systems. Experiments with VIS in the Ems estuary showed macroflocs with settling velocities of 0.5 to 2.0 mm·s⁻¹ and sizes of 200 to 500 μm. These flocs were observed at relatively low suspended-sediment concentrations (20 to 50 mg·dm⁻³). During certain phases of the tidal cycle (around mid-tide) even larger aggregates were observed with dimensions in the range of 400 to 1100 μm and settling velocities of 1 to 7 mm·s⁻¹, surviving relatively high current velocities. The large aggregates are shown to play an essential role in the phenomenon of rapid settling, as observed in various estuaries.     

Actual flux of the Huanghe (yellow river) sediment to the Western Pacific ocean

More than 90% of the Huanghe sediment load is deposited in the lower reaches of the river and within the shallow estuarine area. Additional sedimentation occurs in the southern part of Bohai so that the actual flux of sediment to the Yellow Sea (Huanghai) is very limited. We show that this flux takes place via a nepheloid layer across the Bohai Sea strait, transporting ∼ 6·10⁹ kg·y⁻¹, i.e. less than 1% of the Huanghe sediment discharge.     

Biomass and activity of benthic fauna on the fladen ground (northern North Sea)

During May 1983 the abundance and biomass of macrobenthos and meiobenthos populations, as well as the metabolic activity (shipboard incubated and in situ measured respiration, and potential respiration activity by ETS) were studied in the Fladen Ground area.Macrofauna showed an abundance of 4500 specimens per m²; the corresponding biomass was 10.7 g ash-free dry weight per m². The bivalve Arctica islandica, with 12 specimens per m² contributed 7.2 g to this biomass. Average meiofauna numbers, 0.9·10⁶ per m² were low; a meiofaunal biomass of only 0.3 g was estimated. Depending on the method applied, large variations in metabolic activity were found. Results from shipboard incubation experiments probably present a large underestimate. The in situ bell-jar experiments showed an average oxygen uptake of 1010 μmol O₂·m⁻²·h⁻¹, equalling a carbon demand of 7.3 gC·m⁻²·month⁻¹ in May. ETS measurements gave a 3 times higher estimate. Taking into account that macrofaunal and meiofaunal abundance and biomass, as well as bottom temperatures, show minimum values in spring, an annual carbon mineralization by the benthic system of 50 to 70 gC·m⁻² is suggested. This would mean that about one third of the total primary produced organic matter is channelled into the benthic system.     

Mean currents and current variability in the iceland basin

Long-term (>6 months) current measurements from five moorings in the Iceland Basin have been analysed for the mean currents and the structure of the variable current components. The time-averaged flow at all five moorings had a strong baroclinic character. The mean circulation in the upper layers with relatively warm Sub-Polar Mode Water appears to have a general north-eastward direction with maximum mean velocities of 6 to 7 cm·s⁻¹. In the bottom layer south of Iceland, where the cold Iceland-Scotland Overflow Water flows westwards along the topography in a Deep Northern Boundary Current, mean velocities of the order of 10 to 20 cm·s⁻¹ have been observed. Over the deep slope of the Hatton Bank, water enters the Iceland Basin in a branch of the Deep Northern Boundary Current which has a cyclonic rotation sense in the Iceland Basin. The variable part of the current has been analysed by means of principal-component analysis. The current variations in the central Iceland Basin appear to have a mainly barotropic character while variations in the baroclinic flow of Iceland-Scotland Overflow Water contributed 10% or less to the total energy of the variable deep flow. Over the slope of the Hatton Bank the variable currents had a mainly baroclinic character with shear in both current speed and direction. Comparison of the geostrophic velocity with the mean Eulerian velocity has revealed that the σ_θ=27.725 kg·m⁻³ surface can be used adequately as level of non-motion for the geostrophic modelling of the flow along the Iceland and Hatton slopes. The mean westward geostrophic transport of ISOW south of Iceland relative to this reference surface amounted to 3.5 Sv, in agreement with existing independent estimates.     

Radon anomaly as a possible precursor of a submarine earthquake

In the framework of investigating the ventilation of deep-sea basins in the Indonesian archipelago, ²²²Rn was determined in near bottom water samples. In one of these basins, the Halmahera Basin, excess ²²²Rn concentration and the related standing crop was unexpectedly high. At this station excess Radon has an almost uniform concentration of about 2 dpm·kg⁻¹ up to 120 meters above the bottom. In this paper the possibility is discussed that the high standing crop in this basin is connected with three submarine earthquakes with a magnitude of resp. 4.4, 5.4 and 5.9 mb that took place four days after we took the samples, possibly in combination with submarine faults, which are typical for the Indonesian archipelago. The epicentres were within 100 km from the station. All over the world variations in ²²²Rn concentrations have been observed prior to earthquakes (up to 500 km from the epicentre). This can be regarded as an important non-seismic precursor to earthquakes, but it has to our knowledge never been observed in deep-sea basins.     

Production and consumption of tropical seagrass fields in Eastern Indonesia measured with bell jars and microelectrodes

During the Indonesian-Dutch Snellius-II Expedition the production and consumption of tropical seagrass species were measured with bell jars at four sampling stations in eastern Indonesia. Applying a conversion factor of 0.29, the amount of carbon fixed and mineralized was calculated from the recorded oxygen evolution. The gross production of the different seagrass communities was found to be between 1230 and 4700 mg C·m⁻²·d⁻¹. The consumption lay between 860 and 3860 mg C·m⁻²·d⁻¹. From these data a relatively low net production of 60 to 1060 mg C·m⁻²·⁻¹ could be calculated. At one sampling station incubations were carried out at different depths in a sloping Halodule uninervis (Forssk.) Aschers meadow, which indicated that seagrasses above a depth of about 2 m may become subject to photoinhibition. A linear correlation between biomass and measured production was found for Thalassia hemprichii (Ehrenb.) Aschers. Above a biomass of 100 g DW·m⁻² the production per unit of biomass decreased due to self shading.Using microelectrodes the oxygen production of epiphytes was found to be 230 mg C·m⁻² leaf surface·d⁻¹. at 1900 μE·m⁻²·s⁻¹ assuming an epiphyte coverage of 40%. This indicated that up to 36% of the primary production in a seagrass community may be attributed to epiphytes.The seagrass fields in Indonesia were found to be healthy ecosystems with a high primary production, but organisms within the communities use the abundance of organic matter very efficiently, creating net production rates of 100 to 300 mg C·m⁻²·d⁻¹ which are similar to barren areas.     

Examination of the connection between the patchiness of benthic nutrient efflux and epiphytobenthos patchiness on intertidal flats

Intertidal epiphytobenthos activity (expressed in mmol oxygen evolution·m⁻²·h⁻¹) and nutrient efflux out of the sediment (computed from pore water nutrient concentration profiles and expressed in μmol·m⁻²·h⁻¹) were studied at the same places. Increased ammonia or phosphate efflux stimulated epiphytobenthos activity, and an optimum of epiphtobethos activity was observed when the molar N:P ratio in the efflux agreed with the N:P ratio in diatom biomass. It is concluded that epiphytobenthos patchiness depends on the spatial distribution of nutrient efflux and on the N:P ratio in the efflux. The conditions of nutrient efflux were further studied by fertilizing the sediment with urea and phosphate. Phosphate could be traced in the pore water over two months; ammonia over more than two months. The formation of a diatom bed at the fertilized are was observed once, but efforts to reproduce it failed, indicating the importance of other growth-determining conditions. Nutrient efflux was prevented by a plastic foil to stimulate a possible efflux inhibition by diatom mats. Concentrations below the foil were examined and compared with results from an untreated area. An increase of nutrient concentrations in the pore water was observed.     

Strategies to optimize nitrogen efficiency when fertilizing with pig slurries in dryland agricultural systems

In dryland agricultural systems, pig slurry (PS) is usually applied to cereal crops only at sowing, and slurries accumulate for the rest of the year in pits. In this context, a four-year experiment was established in order to evaluate the feasibility of PS applications at the barley or wheat tillering stage. The main treatments were PS either applied at sowing (25 Mg ha⁻¹) or not, but they alternated after a two-year period. Both were annually combined with eight side-dressing treatments at cereal tillering: mineral N as NH₄NO₃ (M; 60 or 120 kg N ha⁻¹ yr⁻¹), PS from fattening pigs (PSf; 17, 30, 54 Mg ha⁻¹ yr⁻¹), PS from sows (PSs; 25, 45, 81 Mg ha⁻¹ yr⁻¹) and a treatment without N. The combined fertilization treatments were 18 plus a control (no N applied). In the context of crop rotation, the biennial alternation of PS applied at sowing allowed the control of soil nitrate increments, while PS side-dressing improved N recovery compared with a unique application at sowing. The highest yields (>3.6 Mg ha⁻¹ yr⁻¹) were obtained with an annual average (4-yr) N rate close to 173 kg N ha⁻¹ (±40 kg N ha⁻¹). The best overall strategies corresponded to PSs side-dressings of 50–90 kg N ha⁻¹. These PSs rates also recorded the highest values on the five calculated N-efficiency indexes, which were higher than or similar to results from M side-dressings or those recorded in the literature. These similarities (M vs. PSs) were also shown by the reduction of unaccounted-for N inside the overall N balance. Thus, split PS application during the crop cycle is a sound fertilization option in dryland systems.     

Flux of particulate inorganic matter through the Niger River into the Atlantic ocean

The solid load level in the Niger River proved to be subject to distinct seasonal variations, the highest concentration being attained well before peak river discharge.A total of 25.4×10⁹ kg of sediment was flushed into the Atlantic through the drainage system in 1980–1981, representing an upward review of a previous estimate statedin Martins (1982). The specific erosion rate then becomes 20.5·10³kg·km⁻²·y⁻¹, the highest so far measured in Africa.The composition of the transported sediment reflects the chemical disintegration of silicate rocks predominant in the basin.     

Exposure,vegetation and sediment as primary factors for mobile epibenthic faunal community structure and production in shallow marine soft bottom areas

Mobile epibenthic fauna was quantitatively assessed in 22 shallow-water (0–1.5 m), soft-bottom areas on the Swedish west coast. Relationships between faunal structure, density, biomass and annual production on the one hand, and physical factors such as exposure, vegetation cover and sediment structure on the other, were investigated. The areas were grouped according to their physical characteristics into 4 categories: exposed, semi-exposed and sheltered with little or no vegetation and vegetated.Species number, density, biomass and annual production were shown to be significantly higher in vegetated areas (Zostera marina L.) than in mainly unvegetated ones. The epibenthic production in Z. marina meadows was 6 g (AFDW)-m⁻²·y⁻¹. Species composition was also different and Carcinus maenas L. was the only dominant species occurring in both vegetated and unvegetated areas. In all unvegetated areas the number and composition of species were about the same, regardless of exposure, whereas biomass and production varied with the degree of exposure. Highest production (3 to 5.5 g (AFDW)-m⁻²·y⁻¹) in these areas was found in semi-exposed areas. In exposed and sheltered areas production was 1.1 to 3.3 g and 0.6 to 1.7 g (AFDW)-m⁻²·y⁻¹, respectively.Degree of exposure, vegetation cover and sediment structure are suggested as the primary factors affecting the composition and quantity of mobile epibenthic fauna in the investigated areas. Biotic factors e.g. predation and competition are generally considered to be subordinate to these physical factors.     

Seasonal variation in growth and biomass of an intertidal Zostera noltii stand in the Dutch wadden sea

To assess relationships between the life cycle of the seagrass Zostera noltii and light conditions in its habitat, the seasonal dynamics of a seagrass-dominated community on a tidal flat off Terschelling were studied. The main components of this community were seagrass, periphyton and the periphyton grazing mudsnail Hydrobia ulvae. Total biomass of the seagrass stand showed a unimodal curve with a maximum of more than 110 gADW·m⁻² in August and a minimum of less than 10 gADW·m⁻² in January. Chlorophyll density of periphyton on the seagrass leaves followed a more or less similar pattern, ranging from 0.4 μg chlorophyll·cm⁻² in May to more than 3 μg chlorophyll·cm⁻² at the end of August. Periphyton biomass was, however, already maximal in May with almost 1.6 mgADW·cm⁻² and subsequently decreased to less than 0.6 mgADW·cm⁻² in August. The total weight of H. ulvae was more or less stable, varying between more than 150 and less than 400 gDW·m⁻², although significant changes were observed within size classes.Light is assumed to be the primary limiting factor for seagrass distribution in the Wadden Sea. The light conditions of seagrass in the study area were influenced by periphyton and mudsnails. Leaf growth rates and biomass development appeared to be related with light conditions in the seagrass habitat. Shading caused by periphyton during the growing season was estimated at 10 to 90% of incident light, resulting in a reduction of about 2 to 80% of the yearly period during which the light compensation point (LCP) of the seagrass is exceeded. The mudsnails were found to be theoretically able to ingest daily 25 to 100% of the standing stock of periphyton and microphytobenthos. It is concluded that shading by periphyton and grazing by mudsnails play an important role in the seasonal biomass development and survival of Z. noltii in the seagrass-dominated community on a tidal flat off Terschelling.     

Food intake and growth in Macoma balthica (mollusca) in the laboratory

Groups of Macoma balthica were kept during 4-week periods in an experimental set up at a constant temperature and food concentration. Food concentrations (expressed in particulate organic carbon) for the different groups ranged from 0 to 16 mg C·I⁻¹. The experiment was repeated 6 times, viz. in different months, and the temperatures were changed accordingly to correspond with levels found in the field. The rates of food intake, water clearance and growth were followed throughout the experiment. The flagellate Isochrysis galbana served as food.With increasing food concentration all 3 rates (food intake, water clearance and growth) increased up to maximum to decrease again at high food concentrations. Such bell-shaped relationships were observed in all seasons. The dependence of growth on food concentration was similar in all seasons. Zero growth or weight losses were observed at food concentrations below 1.3 mg C·I⁻¹, and maximum growth rates were reached at food concentrations between 5 and 7 mg C·I⁻¹. The daily maintenance ration amounted to about 1.2% of the body weight.Seasonal differences were observed in the relationships of the rates of both water clearance and food intake with food concentration. During the winter and spring the optimum curves for these relationships reached their maximum at food concentrations of 8 to 10 mg C·I⁻¹. During summer and early autumn the optimum curves were shifted to lower food concentrations, around 2 to 4 mg C·I⁻¹, probably as an adaptation to low food concentrations observed in the field during these periods.