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.     

Physical aspects of the flushing of the east Indonesian basins

The flushing of the deep basins in the east Indonesian archipelago is studied by means of water mass analysis, current observations and simple theoretical models. The Banda Sea and its secondary basins appear to be flushed from the Pacific Ocean with a typical flushing time of the order of 40 years. This fast flushing is made possible due to the high vertical diffusivity which is estimated to be of the order of 10⁻³ m²·s⁻¹. Theoretical models to descrive the vertical distribution of tracers due to mixing and basin wide upwelling turn out to fit the data very well. The observations cannot be used to confirm the horizontal structure of the deep circulation which results from a simple β-plane model. The vertical structure of the Aru Basin and the Timor Trench is more complicated due to the presence of water from the Indian Ocean, the Pacific Ocean and the Banda Sea. But also here simple models can be applied.     

Deep-water transit times in the eastern Indonesian basins,calculated from dissolved silica in deep and interstitial waters

The flushing of the deep basins in the eastern Indonesian archipelago is studied by means of the dissolved silica (Si) distribution, using water column data from the Snellius-II Expedition as well as from the Indopac Expedition. The linear θ-Si relationships below 500 to 1000 m permit the use of Si as a water-mass tracer. The main outflow of deep water from the Banda Sea into the Timor Trench is found east of Timor, above 1250 m depth, and the deeper parts of this outflow are partly recirculated through the Aru Basin and the Seram Sea into the north Banda Sea. Below the sill depth of the various basins, θ remains about constant while Si gradually increases towards the bottom. The time needed to create this Si-excess is obtained with the Si-flux out of sediments, calculated from pore-water gradients. Deep-water transit times are 20 years for the Banda Sea, 2 to 15 years for smaller basins and 60 years for the intermittently flushed Weber Deep. Modelling the Si-excess with error functions in some small basins gives high diffusion coefficients of 45 to 150 cm²·s⁻¹. The flux of Si from the sediments, 0.5 to 2 mol·m⁻²·a⁻¹, is higher than in other deep-sea basins, it amounts to roughly 30% of the biogenic silica production in the euphotic zone. Dissolved Si at 20–40 cm depth in the sediments ranges from 450 to 590 mmol·m⁻³; about 2% of Al in biogenic silica from sediments might cause this apparently low solubility for sediments rich in biogenic silica.     

Reactive phosphate in the eastern Indonesian seas

Vertical profiles and horizontal sections of the reactive phosphate concentration in deep-sea basins in eastern Indonesia, based on data obtained during the Snellius-II and Indopac Expedition are given.Below 1000 m depth the concentration ranged from 2.7 to 2.9 mmol·m⁻³, slightly larger than in the waters flowing in from the Pacific Ocean. Below sill depth of isolated basins a small increase of phosphate is always found. This increase is related to decomposition of organic material, which shows that about 2% of the surface production of organic material is recycled in the lowermost 500 m, probably for a substantial part in the surface sediments.     

Recent sediment deposition rates in the oyster ground,North Sea

Recent sediment accumulation rates were calculated from downcore ²¹⁰Pb-excess activity profiles obtained from box cores taken in the Oyster Ground, North Sea. The rates ranged from 0.04 to 0.94 cm·yr⁻¹ with an average of 0.39 cm·yr⁻¹. In the top sediment layer the activities were rather low, the highest being only 1.15 dpm·g⁻¹, and the average value 0.92 dpm·g⁻¹.As shown by X-ray radiographs, downcore variations in texture were small: most samples being homogeneous and lacking primary sedimentary structures. Burrowing was evident in all cores, and most of them had a surface mixed layer of at least 5 cm, below which the ²¹⁰Pb activity rapidly dropped to background value.Downcore deviations from the logarithmic decrease in ²¹⁰Pb with depth could generally be attributed to biological disturbance or physical processes causing reworking of the sediment. Only in one core did the ²¹⁰Pb activity decrease regularly with depth. The activities of ¹³⁷Cs in this core and another demonstrated recent accumulation rates of 0.44 and 0.88 cm·yr⁻¹, respectively, which are in good agreement with the rates obtained from the ²¹⁰Pb method in the same cores (viz. 0.37 and 0.94 cm·yr⁻¹). From these sedimentation rates the total amount of mud yearly deposited in the muddy part of the Oyster Ground is estimated to be in the order of 2×10⁹ kg·yr⁻¹, which is 4 to 5% of the total amount of mud yearly deposited in the North Sea.     

Temporal and lateral fluctuations in production and biomass of bacterioplankton in the western Dutch Wadden Sea

To contribute to the validation of a recently developed ecosystem model of the western Wadden Sea (EON, 1988), data on bacterial biomass and production were acquired. Seven field stations, spread over the two main basins of the estuarine system, were sampled monthly in 1986. Between these basins significant differences were found in counts, biovolume, biomass and production of bacteria (measured by the ³H-thymidine method) with consistently higher mean values of bacterial variables in the Vlie basin. Bacterial production rates of 2 to 175 mg C· m⁻³· d⁻¹ were obtained for the Vlie basin, with an annual production of 10 to 11 g C· m⁻³, while the production in the Marsdiep basin did not exceed 45 mg C·m⁻³ ·d⁻¹, with an annual production of g C·m⁻³. Bacterial biomass varied over the year from 2 to 140 mg C·m⁻³ in the study area, with a mean biomass of 39 mg C·m⁻³ in the Vlie basin and 23 mg C·m⁻³ in the Marsdiep basin. Blooms of bacteria occurred in May and July–August. Spatial and temporal fluctuations in bacterial variables are discussed, taking into account different environmental factors and the availability of food for bacteria in relation to transport and exchange of water masses between the two basins and the North Sea. Results are compared with the results as simulated by the ecosystem model.     

Preliminary studies on the marine chemistry of ruthenium

An upper limit for the concentration of ruthenium in surface sea waters off the California coast is estimated to be 5 pg·dm⁻³ on the bases of laboratory analyses and of theoretical considerations. Ruthenium contents follow those of platinum in ferromanganese nodules. Like the latter element, the former appears to be oxidized from a dissolved state in sea water upon incorporation in the solid phases. Finally, the Pt/Ru ratios in pelagic sediments from the Pacific Ocean cluster around 3 to 4. The platinum metals are associated with the detrital phases and their concentrations are a measure of crustal values.     

Settlement,abundance, growth and mortality of juvenile flatfish in a subtropical tidal estuary (Georgia,U.S.A.)

The population dynamics of juveniles of some flatfish species were studied in the Duplin River, a tidal creek in a subtropical salt-marsh area in Georgia, U.S.A. from April until September 1990. Seven species were found. Paralichthys dentatus, Paralichthys lethostigma, Paralichthys oblongus and Trinectus maculatus were relatively rare. Etropus crossotus, Citharichthys spilopterus and Symphurus plagiusa were abundant and settled during the period studied. E. crossotus was the most abundant species with a mean abundance of 18 ind·10⁻² m⁻² (max. 287). Demersal settlement of E. crossotus took place in shallow areas and over sandy bottoms from mid-May to August. Prolonged settlement hampered the calculation of growth rate and instantaneous mortality rate. However, laboratory growth experiments indicated a mean growth of about 0.50 mm·d⁻¹ at 24–28°C. Juveniles of C. spilopterus were already present in the Duplin River in March. Settling continued until the end of April with a mean abundance of 3.5 ind·10⁻² m⁻² (max. 183). With increasing size the juveniles of this species tended to migrate to deeper waters and to the mouth of the river, possibly as a reaction to increasing water temperatures. Maximum growth rate was 1.4 mm·d⁻¹ at about 26°C. The mean instantaneous mortality rate (z) was estimated at 0.03·d⁻¹. Settling of S. plagiusa occurred from mid-May onwards. The mean abundance was 10.3 ind·10⁻² m⁻² (max. 98.3). Newly settled juveniles were most abundant on muddy sediments in the shallow river areas. The maximum growth rate was 1.3 mm·d⁻¹ at about 28°C. The mean instantaneous mortality rate (Z) decreased from 0.04·d⁻¹ in April to 0.01·d⁻¹ in August. At all sites the abundance of juveniles of this species decreased with increasing water depth. Predation experiments indicated that blue crabs (Callinectes similis and C. sapidus) and sea robins (Prionotus sp.) are potential predators on juvenile flatfish. The high abundances of juvenile flatfish indicate that the tidal creeks are an important nursery area. The correspondence between growth rates estimated from field data and those observed in the laboratory suggests that growth in the nursery is mainly related to water temperature and not food limited.     

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.     

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.     

Light grazing of crop residues by sheep in a Mediterranean-type environment has little impact on following no-tillage crops

Crop residue is often grazed by sheep after harvest, over the dry summer period from December to March in Mediterranean environments. However, soil cover provided by crop residues is a key component of conservation agriculture for maintaining favourable soil structure and high yields.A series of 31 site × year experiments was conducted to assess the effect of summer stubble grazing on residue levels and following crop yields. Relatively light grazing, with stocking rates below 10 dry sheep equivalent (DSE) and between 90 and 471 DSE days ha⁻¹, had no significant effect on the amount of residue, soil properties, soil water, weeds or yield in the following crop. The main effect of grazing was to knock down and scatter the standing crop residues. However, longer term grazing at relatively high intensity (956 DSE days ha⁻¹) on heavy soil, over both summer and winter, as in a pasture phase, did significantly reduce residue levels, infiltration and yield (by 59%). The effect of summer grazing on soil mineral N was small and inconsistent, with increased mineral N, by about 3–7 kg N ha⁻¹, following grazing at two of the 13 sites. By contrast, higher mineral N, by 2–15 kg N ha⁻¹, was measured in the un-grazed plots at three of the 13 sites. This was due to increased growth of legume pastures in the absence of grazing.More research is needed to confirm the yield effects when cropping after an annual pasture/fallow that is grazed over summer and winter, particularly on different soil types.     

Recirculation in the Banda throughflow,traced with dissolved silica

The change of water-mass properties during the flow of waters originating in the Pacific Ocean through the eastern Indonesian archipelago causes an increase of dissolved silica in the layers between 100 and 1500 m depth. The time needed to build up this increase is estimated as 5 years, with contributions of basin-wide upwelling, dissolution of biogenic siliceous debris at depth and vertical diffusion. The dissolved silica distribution suggests a recirculation of Banda Sea Intermediate Water leaving the southern Banda Sea at around 1000 m depth through the Timor Sea and the Seram Sea, and flowing back into the northern Banda Sea. This recirculation is also detectable in the Molucca Sea between 250 and 500 m depth.     

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.     

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.     

Screening chickpea for adaptation to water stress: Associations between yield and crop growth rate

Robust associations between yield and crop growth rate in a species-specific critical developmental window have been demonstrated in many crops. In this study we focus on genotype-driven variation in crop growth rate and its association with chickpea yield under drought. We measured crop growth rate using Normalised Difference Vegetative Index (NDVI) in 20 diverse chickpea lines, after calibration of NDVI against biomass accounting for morphological differences between Kabuli and Desi types. Crops were grown in eight environments resulting from the combination of seasons, sowing dates and water supply, returning a yield range from 152 to 366 g m⁻². For both sources of variation – environment and genotype – yield correlated with crop growth rate in the window 300 °Cd before flowering to 200 °Cd after flowering. In the range of crop growth rate from 0.07 to 0.91 g m⁻² °Cd⁻¹, the relationship was linear with zero intercept, as with other indeterminate grain legumes. Genotype-driven associations between yield and crop growth rate were stronger under water stress than under favourable conditions. Despite this general trend, lines were identified with high crop growth rate in both favourable and stress conditions. We demonstrate that calibrated NDVI is a rapid, inexpensive screening tool to capture a physiologically meaningful link between yield and crop growth rate in chickpea.     

Dynamics of dissolved silicium and nitrogen-nutrients at low temperature in the Ems-dollard estuary

With salinity and natural fluorescence as conservative tracers to discriminate between fresh water contributions from two different fresh water sources (the river Ems and the river Westerwoldse Aa), deviations from conservative behaviour for silicate, ammonia and nitrate during winter conditions (4–6°C) are reported and discussed. For silicate a net removal from overlying water during transport through the estuary of 15% was found. Nitrification in overlying water (260 μmol·m⁻³·h⁻¹) was the only important process in nitrogen cycling; sediment-water exchange was of no importance during this period.     

Organic carbon in the upper 100 m and downward flux in the banda sea; monsoonal differences

Organic carbon was measured in the upper 100 m and its downward flux from the euphotic zone recorded, as part of a multidisciplinary study in the Banda Sea on monsoonal differences in the upper water layers.All 35 stations occupied showed on average 1.5 times higher POC values in August 1984 (4.9 g·m⁻², SE monsoon, upwelling period) than in February/March 1985 (3.3 g·m⁻², NW monsoon, downwelling period) for the 0 to 100 m water column. Monsoonal variation in POC was confined to the surface layer and related to a variation in phytoplankton content; it can be explained by — local— upwelling during the SE monsoon. At 100 m no seasonal variation occurred.Contrary to POC, average figures for DOC in February/March were almost twice the average observed in August (66.5 against 37.5 g·m⁻² for the 0 to 100 m water column). In February/March spatial variation was more pronounced than in August with highest values occurring in the NE part (>100 g·m⁻², 0 to 100 m), with a subsurface maximum. In this period the NW monsoon causes a surface current to the east. This might cause a westward subsurface countercurrent of Arafura Sea water rich in riverine or benthic layer DOC, influencing the eastern Banda Sea.At 100 m depth daily sedimentation amounted to ∼50 mg C·m⁻². The limited number of data do not indicate a difference between the 2 periods. On average the daily downward flux at 100 m depth amounted to 1.2% of POC and 0.8% of chlorophyll present in the 0 to 100 m water column, and equalled 5% of the daily primary production.     

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.     

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.