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.     

Oxygen profiles and organic carbon fluxes in Laurentian Trough sediments

Sediment trap samples and undisturbed cores of bottom sediment were obtained during spring and summer from a 350-m-deep site in the Laurentian Trough, Gulf of St. Lawrence, and used to determine the flux of organic carbon to the sediment surface, and oxygen and organic carbon concentration profiles close to the sediment-water interface. Oxygen uptake rates by the sediments, calculated from micro-electrode profiles and a one-dimensional molecular diffusion transport model, can only account for 20% of the organic carbon that is mineralized in the sediment. Mechanisms other than molecular diffusion must dominate the transport of oxygen across the sediment-water interface in these deep coastal sediments.     

On sulphide and the distribution of Arenicola marina in a tidal mud flat in the Dutch Wadden Sea

The concentrations of free sulphide in the upper 30 cm of the sediment of a tidal mud flat in the Dutch Wadden Sea were measured periodically during one and a half year. Also grain size composition and iron sulphide content were determined. In sandy sediments free sulphide values were as a rule below 5 mg S·l⁻¹ pore water throughout the year; at stations with fine sediment much higher values were found.The depth to which Arenicola marina burrowed in the sandy sediments was 22 cm at maximum. Generally sulphide concentrations at this depth were below 0.1 mg S·l⁻¹ but local contact of the animals to concentrations of 50 mg S·l⁻¹ may occur. The density of the lugworms was found to be inversely related to the free sulphide concentration at the depth of the burrows. Whether the sulphide was responsible for this distribution or other parallel changes in sediment characteristics is not clear.Iron sulphide appeared to be of no importance for the distribution of Arenicola marina although the worms were often found in contact with concentrations as high as 450 mg S·l⁻¹ sediment.     

Sediment reworking by the polychaete heteromastus filiformis on a tidal flat in the Dutch Wadden Sea

Sediment reworking by Heteromastus filiformis was studied in the field for several years. Heteromastus feeds 10 to 20 cm below the sediment surface and deposits its oval faecal pellets on the sediment surface. The pellets are relatively resistent against bacterial and mechanical breakdown. Numerical density of Heteromastus varied both during the seasons and from year to year. The size distributions of the faecal pellets produced by the population could be used as a measure of the population structure. Recruitment did not occur every year on the locality studied. Numerical densities largely controlled the amount of sediment reworked. On average the Heteromastus population amounted to 500 m⁻² and the amount of sediment brought to the surface to ca 100 ml·m⁻² ·d⁻¹ corresponding with a sediment layer of 4 cm annually. A heavy settlement in summer and autumn 1978 resulted in densities up to 9000 m⁻² and a sediment reworking rate of 2.51·m⁻²·d⁻¹, corresponding with a sediment layer of 200 1·m⁻², or 20 cm in total in the period August to November inclusive 1978.     

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.     

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.     

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.     

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.     

Dissolved and particulate organic carbon in the North Equatorial current of the Atlantic Ocean

As part of a study of biological systems in the Atlantic North Equatorial Current measurements were made of carbon dioxide and dissolved and particulate organic carbon along a section off the African coast chiefly following the parallel of 20°N. Particulate organic carbon (POC) in the mixed layer showed a decrease from east to west from an average of 100 μg·1⁻¹ to 35 μg·1⁻¹ and a difference between day and night from 70 to 20 μg·1⁻¹. From this difference a minimum primary production of 450 to 1000 g C·m⁻²·a⁻¹ was calculated. These production values are much higher than earlier data obtained by ¹⁴C measurements, but they support observations made by Gieskes, Kraay & Baars (1979) and Tijssen (1979) during the same programme but with different methods.All three components measured show distinct vertical gradients. On the basic of the CO₂ gradient an average downward loss of organic metter through a reference level of 200 m was calculated of about 60 g·m⁻²·a⁻¹. For dissolved organic matter (DOC) a downward transport of 8 g·m⁻²·a⁻¹ was derived from the vertical gradient.     

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.     

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.     

Methane production from screened dairy manure using a fixed-film reactor

A laboratory scale comparison of two anaerobic digester designs-conventional and fixed film—was performed. Screened, 1:1 manure/water slurry was fermented in 4 litre reactors at 30°C, with intermittent mixing. Feed material (4·0% Total Solids, 2·8% Volatile Solids) was added once per day at loading rates representing hydraulic retention times (HRT) of 16 to 1 days. For HRTs less than 1 day (i.e. 1h), all of the waste was drained (once each hour) and the digester refilled with fresh feed. Maximum methane productivity (litre gas litre⁻¹ digester day⁻¹) of 6·33 litre CH₄ litre⁻¹ day⁻¹ was obtained from the fixed-film reactor at a loading rate of 672 g VS litre⁻¹ day⁻¹ (1 h HRT). Anaerobic digestion efficiency (litre gas g⁻¹ Volatile Solids added) was highest in the fixed-film reactor operated at a 3·6 day HRT, when COD and VS reductions reached 38·4% and 27·6%, respectively. At HRT of less than 6 days, the conventional digester would not sustain a high gas-production rate because of bacterial biomass washout. Data indicate that the increased gas production capability of the fixed film becomes evident only at HRT of less than 6 days. The maximum film loading rate (gVS m⁻² day⁻¹) and methane productivity (litre CH₄ m⁻² day⁻¹) were not attained within the HRT range studied. Average biogas methane content from the fixed-film reactor (68·6%) was slightly higher than from the conventional digester (63·5%). Results indicate that a high potential reduction of required digester volume is possible through application of the fixed-film concept.     

Does the influence of the epibenthic predator crangon crangon L. (brown shrimp) extend to sediment microalgae and bacteria?

The effect of juvenile Crangon crangon L. (brown shrimp) on the microbial part of the food web of marine shallow-water sandy sediment was investigated in two experiments using an outdoor flow-through system. Biomass, composition and productivity of microalgae and bacteria, POC and PON content in the sediment, as well as nutrient and oxygen fluxes, were measured in the absence and presence of two natural densities (50 and 100 ind·m⁻²) of juvenile Crangon. Stimulating effects of the presence of juvenile Crangon on both microalgae and bacteria were observed after three weeks. However, the number of statistically significant effects was low, despite the conspicuous change in sediment characteristics caused by Crangon activity. To explain the possible trophic effect of Crangon via meiofaunal grazing, the results were compared with meiofaunal grazing rates (dual labelling by ¹⁴C-bicarbonate and ³H-thymidine) and changes in the meiofaunal and macrofaunal community. The reasons for the low number of significant trophic effects of Crangon on algae and bacteria were that the overall meiofaunal grazing pressure on microbiota was low, and that only part of the meiofauna (mainly harpacticoid copepods) was significantly affected by the predator. Meiofauna thus appeared to be a weak mediating link in the sandy sediment system investigated. No effect of Crangon on oxygen fluxes, or POC and PON content in the sediment was observed. Neither were the nutrient concentrations in the overlying water affected, but increased nutrient levels in the pore water were observed. According to theoretical calculations, excretion from the shrimp may imply a considerable nutrient addition which can be rapidly re-utilized by the microbes directly at the sediment/water interface. Bioturbation by Crangon appeared to counteract flaking of the microalgal mat.     

Recycling of poultry droppings through tilapia fish culture. Effect of monosex and mixed-sex culture on production

Tilapia fish production in tanks manured with poultry droppings and stocked either with tilapia males and females separately or together is compared and discussed.The mixed-sex group gave the highest production rate of 34·2 kg ha⁻¹ day⁻¹ with a manure coefficient of 2·2. The ‘all female’ group ranked last with a production rate of 22·9 kg ha⁻¹ day⁻¹ and a manure coefficient of 3·3. Sixty per cent of the tilapia biomass produced in the ‘mixed-sex’ tanks was contributed by reproduction. While monosex culture is better for the production of large fish for human consumption, where small fish do not matter, as in the case of the preparation of fishmeal for animal feeding, mixed-sex culture may be adopted without curtailing production.     

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.     

Effects of river discharge and vertical circulation on aquatic primary production in a turbid Louisiana (USA) estuary

Aquatic primary production was measured in stationary and moving bottles with the light/dark oxygen method at two sites in Fourleague Bay, a shallow, turbid estuary on the central Louisiana coast receiving flow from the Atchafalaya River. Riverflow strongly influenced spatial and seasonal patterns of production. Annual net production increased from 382.5 g O₂·m⁻² (119.5 g C·m⁻²) at an upper bay site near the river mouth to 1015.7 g O₂·m⁻² (317.4 g C·m⁻²) at a lower bay site distant from the river. Net production was negatively correlated with seasonal changes in riverflow at both sites. Maximum production rates occurred at intermediate salinities. At low sainities, production was apparently light-limited because of the extreme turbidity of the riverwater. At high salinities, production declined despite greater water clarity, apparently due to nitrogen limitation. At the upper bay areal production estimates from moving incubations were significantly lower than estimates from stationary incubations, but no consistent differences were found at the lower bay and in the bay as a whole. This is the first report of moving incubations yielding significantly lower estimates most of the time. The productivity differences between moving incubations and stationary incubations were significantly correlated with relative light penetration and we suggest that this was because of at least two distinct time-dependent production-versus-irradiance phenomena whose influences varied as conditions changed.     

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.     

Production and growth dynamics of eelgrass (Zostera marina) in brackish lake grevelingen (The Netherlands)

Biomass changes of overground eelgrass (Zostera marinaL.) parts in a permanent sample plot in brackish Lake Grevelingen showed a unimodal seasonal pattern over the years 1974–1976, with maxima of 75 to 94 g ash-free dry weight ś dm⁻² in July–August and minima between 0 and 5 g ash-free dry weight·m⁻² in December–April. Biomass changes of living underground parts showed a faint peak in the period July–October (35 to 50 g ash-free dry weight · m⁻²) and a minimum in the period December–April (about 10 g ash-free dry weight·m⁻². Concerning the vertical biomass distribution, eelgrass reached maxima in July–August of about 130 g ash-free dry weight·m⁻² (over- and underground parts) at 1 to 1.5 m water depth, and approached zero at 4 m water depth. Biomass increase of overground and underground parts was estimated at 39.4 g C.m ·⁻² and 12.2 g. C.m ·m⁻² respectively in 1976. With a leaf-marking technique net leaf production and rhizome production were estimated at 207.5 g C.·m⁻²·a⁻¹ and 27.4 g C·m^-2. a⁻¹ respectively in 1976. Production of stems and inflorescences was estimated at 45.2 g C·m⁻² ·a⁻¹. The P/B ratio of overground parts was estimated at 6.4, of underground parts at 2.2, and of both together at 5.4. The data from Lake Grevelingen show good agreement with Danish results. Considering the standing stock of eelgrass in Lake Grevelingen in 1973, and 1975, an annual production of 200 to 300 g C·m⁻² within the Zostera beds seems a realistic estimate.     

Behaviour of PCB,pentachlorobenzene, hexachlorobenzene, α-HCH, γ-HCH, β-HCH,dieldrin, endrin and p,p′-DDD in the rhinemeuse estuary and the adjacent coastal area

Organochlorines (pentachlorobenzene, hexachlorobenzene, PCB, α-HCH, γ-HCH, β-HCH, dieldrin, endrin, p,p′-DDD) have been quantitated in water, seston, bottom sediment and interstitial water in samples from the fresh water and estuarine region of the Rhine-Meuse system obtained in 1974, 1976 and 1979.In the estuarine region, the amounts in solution are smaller than in suspension for pentachlorobenzen, hexachlorobenzene and PCB, dissolved concentrations dominating for the hexachlorocyclohexane isomers. In river water, PCB is roughly equally distributed over dissolved and suspended forms while pentachlorobenzene, hexachlorobenzene and the HCHs dominate in solution. Dissolved dieldrin, endrin and p,p′-DDD were below the limits of detection in most river water samples. However dissolved dieldrin and endrin are produced in the upper part of the estuary resulting from local pollution. During estuarine mixing, dissolved concentrations of all compounds were found to be linearly related to salinity (1974 data).Concentration data in seston and sediment in the estuarine region can be explained in terms of mixing of fluviatile and marine derived particles. It is suggested that in particular PCBs are associated specifically with finer particles.Dissolved concentrations in the central part of the Southern Bight are at the very lower end of the range of values reported for various coastal and open ocean areas. Concentrations detected in large volume samples are: penta- and hexachlorobenzene 0.1 ng·1⁻¹; PCB (Aroclor 1254) 0.7 ng·1⁻¹; α-HCH 0.9 ng·1⁻¹; γ-HCH 1.0 ng·1⁻¹; β-HCH 0.04 ng·1⁻¹; dieldrin 0.1 ng·1⁻¹ and endrin and p,p′-DDD < 0.01 ng·1⁻¹.Temperature programmed GLC techniques demonstrate the contribution of lower chlorinated PCB components in the central part of the Bight. This has not been observed in other reports. Adoption of a PCB formulation with a lower degree of chlorination than the usual Aroclor 1254 as the standard for quantitation results in a significant shift of the PCB concentrations from 0.7 (Aroclor 1254 or the equivalent Clophen A50) to 4.1 ng·1⁻¹ (Clophen A30). It is suggested that the lower chlorinated fraction is derived from atmospheric input. River water could not be excluded as a source however.Concentrations in the coastal area are higher than in the central part of the Southern Bight. Analyses of large volume water samples in 1979 are consistent with continuing conservative behaviour in the coastal area. A large fraction of the organochlorine load of the Dutch Wadden Sea is derived from the Rhine-Meuse estuary.