Increased phytoplankton primary production in the Marsdiep area (Western Dutch Wadden Sea)

Annual phytoplankton primary production in the Marsdiep tidal inlet increased from ca 150 gC·m⁻² in the period 1964 to 1976, to ca 300 gC·m⁻² in 1981/1982 and 1985. This increase is considerable, but comparable to that observed in the outer Ems estuary, eastern Wadden Sea, from 240 gC·m⁻²a⁻¹ in 1972/1973 to 400–500 gC.m⁻².a⁻¹ in 1976/1980. Although the increase is most probably due to eutrophication, as illustrated by the regular increase in phosphate in the Marsdiep area since 1950, it is difficult to explain why this affected primary production not earlier than the late seventies. Primary production has probably not increased in the more turbid inner parts of the Wadden Sea, where light is the limiting factor, and P values were already higher than in the inlet areas.     

Particulate and dissolved organic carbon and chlorophyll A in the Zaire river,estuary and plume

Data were collected on POC, DOC and phytoplankton in the Zaire river, estuary and plume. Mean river value for POC was 1.1 mg·l⁻¹, 4.7% of the suspended matter. Average DOC content of the river water was 8.5 mg·l⁻¹. These values are in accordance with the calculations of TOC input from rivers to the world's ocean. Within the estuary POC and chlorophyll decreased regularly up to a salinity of 20. Between salinities of 20 and 32 small phytoplankton bloom occurred resulting also in higher POC values.DOC mixed conservatively up to a salinity of 25; at salinities above 25, values indicate DOC production. This DOC production occurred partly in the bottom water of the canyon where low oxygen values indicated mineralization and conversion of the accumulated POC into DOC. Another area of DOC production observed inside and outside the surface waters of the plume, was partly related to autolysis and degradation of the phytoplankton bloom.This study shows that the influence of rivers on the organic carbon in the ocean will not be confined to the amount introduced directly, but that we have to add the amounts of POC and DOC resulting from enhanced phytoplankton primary production by nutrient input from rivers and by river induced upwelling.     

Phytoplankton primary production,chlorophyll and composition in an inlet of the Western Wadden Sea (Marsdiep)

From February 1974 to April 1976, estimates for in situ phytoplankton primary production in the Marsdiep tidal inlet area of the western Dutch Wadden Sea were made from incubator primary production data using a constant light source. Low values were estimated in winter with a minimum in January (average 0.016 g C·m⁻²·d⁻¹. Values increased rapidly from the middle of March during the spring phytoplankton bloom to a maximum in May (average 1.1 g C·m⁻² ·d⁻¹, maxima of 3 g C·m⁻²·d⁻¹ or more). During summer these estimates showed large short-term variations in in situ primary production from incubator data as well as in in situ measurements during 1 to 3 days in June and August in the years 1971 to 1979. This variability forms a problem in estimating annual phytoplankton primary production in the area. It also prevents a reliable comparison of the present estimate with earlier estimates of annual primary production based on fewer data (one per month or less). It is likely that also in other areas, in particular in coastal regions, similar difficulties in estimating primary production will be encountered.     

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.     

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.     

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.     

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.     

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.     

The spring bloom of phaeocystis pouchetii (haptophyceae) in Dutch coastal waters

A Phaeocystis pouchetii (Harlot) Langerh. bloom, which contained up to 22×10⁶ cells ·dm⁻³, occurred in the spring of 1984 along the Dutch coast of the North Sea. The largest cell numbers of P. pouchetii were observed near the coast, with cell numbers decreasing towards the open sea. In contrast to a shark peak in the cell numbers along the coast in May, P. pouchetii cell numbers in the open sea slowly increased towards June.During a preceding bloom of diatom, P. pouchetii was already present from an early stage onwards while at the end of the P. pouchetii bloom a high percentage of the cells was observed as microflagellates.The distribution of dissolved nutrients (Si, N, P) showed a typical pattern with highest values near the coast, apparently due to the Rhine outflow, and a gradual decrease towards the open water. This distribution pattern coincided with particular stages of the P. pouchetii bloom. Whereas growth of the P. pouchetii population may be due to increasing temperature and irradiance, its decline is probably caused by phosphate limitation.Total primary production measured at one station and based on short term ¹⁴C incubation experiments was 121 gC·m⁻² for the period 17 March to 20 May with a daily rate of production of up to 4.8 gC·m⁻². The mean proportion of extracellular carbon release based on 4 h incubation periods was 7.5%.     

Production of Crangon crangon in the tidal zone of the Dutch Wadden Sea

The density and size of shrimps on an extensive tidal flat in the western Wadden Sea was followed through 4 successive years. The population consists of juvenile shrimps (< 35 mm) that enter the area in May and remain in the tidal zone till October–November. During these months there is a constant shift of shrimps that reached a length of 30 to 35 mm towards the subtidal, whereas at the same time new larvae settle on the tidal flats throughout the warmer part of the year with peak settlement during May and June. Shrimps that did not reach 30 to 35 mm by October do enter the tidal zone again the next spring.With aid of independant growth data the production of juvenile shrimps in the tidal zone was estimated at 2.35 g ADW·m⁻²·a⁻¹, of which about 60% is consumed within the area by crabs and fishes. The average food consumption of the juvenile shrimps was estimated at 6.7 g ADW·m⁻²·a⁻¹, which makes them the most important carnivore population of the Wadden Sea's tidal area.     

Primary production and import of particulate organic matter on a salt marsh in the Netherlands

The macrophyte production and the transport of particulate organic matter between marsh and adjacent estuary have been investigated for a 0.3 km² salt marsh along the Oosterschelde estuary, The Netherlands.The primary production of macrophytes, measured with Smalley's (1959) method was 837 to 1030 g dry organic matter·m⁻²·y⁻¹. Measurements of amounts of particulate organic matter transported through one of the main tidal creeks in the salt marsh resulted in a net import. On an average 31% of the material brought in by the flood settled in the marsh. The majority of this material is smaller than 63 μm. On the other hand large floating material is exported during storm tides, although the quantity seems to be smaller than that of the suspended material imported.The differences between various marshes with regard to export and import of organic matter are explained in terms of marsh level, primary production, turbulent diffusion, sinking and resuspension of particulate matter and biotic transformations.     

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.     

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.     

Soft-bottom benthic community in the estuarine waters of East Java

During the Snellius-II Expedition, both in the period July–August and in the period November–December 1984, the structure of the macrobenthic communities in the estuaries of the rivers Porong, Wonokromo and Solo was investigated. The numerical densities of the benthic organisms (>0.5 mm) from the Porong, Wonokromo, and Bengawan Solo estuaries during the Snellius-II Expedition varied between 87 and 5787 ind·m⁻²; biomasses between 0.4 and 602 g DW·m⁻². The high numerical density (1573 ind·m⁻²) found at st.2 of the Porong estuary in period I (July–August 1984) was attributed to the dense population of the bivalve Varicorbula rotalis. Consequently a very high biomass (602 g DW·m⁻²) was measured at this station.The mollusc Theora lata and the polychaetes Ancistrosyllis parva, Nepthys dibranchis, Paraprionospio pinnata, and Sternaspis laevis occurred at almost every station during the investigation periods.Paraprionospio pinnata and Theora lata, which are considered indicator species of disharmonic environments, were found at almost every station of the study areas in both periods. Species diversity at all the stations was low, ranging form 0.1 to 1.6.     

Diurnal oxygen rhythm and primary production in the mixed layer of the Atlantic Ocean at 20°N

A diurnal oxygen rhythm with an amplitude of less than 0.6% of ambient oxygen concentration was measured at 5 stations in the mixed layer of the Atlantic Ocean at 20° N through the application of a high precision Winkler titration method.At anyone time and location the mixed layer showed a very constant vertical oxygen concentration. The estimated gross primary production in the mixed layer was high and had a mean value of 800 mg C·m⁻²·d⁻¹.     

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 water column and its sedimentation,fladen ground (North Sea), May 1983

POC, DOC and sedimentation were studied during the spring phytoplankton bloom from April 27 to May 25. POC in the euphotic layer consisted largely of phytoplankton, maximum values of 0.5 mg·dm⁻³ were found at the height of a bloom of the colonial microflagellate Corymbellus aureus. DOC increased during the waxing of this C. aureus bloom to a maximum of 2.4 mg·dm⁻³ after the bloom. Data indicate that most of the primary produced material ended up in the DOC pool. Sedimentation accounted only for small losses from the euphotic zone: ca 1% of the daily primary produced material sank out, except at the end of the C. aureus bloom when values increased to almost 10%. Daily sedimentation amounted to no more than 0.1 to 1% of the amounts present in the euphotic layer of POC, chlorophyll and phytoplankton cells.     

The river varde Å: Processes affecting the behaviour of metals and organochlorines during estuarine mixing

Measurements of dissolved and suspended metals and organochlorine compounds were made in conjunction with hydrographic and sedimentological parameters at three fixed positions in different salinity regimes of the estuary of the Varde Å at regular intervals during a tidal cycle and during a survey in the adjacent Wadden Sea in August 1976.In the river, low concentrations of dissolved pentachlorobenzene (0.1 ng·1⁻¹), hexachlorobenzene (0.1 ngsd1⁻¹), polychlorinated biphenyls (1 ngsd1⁻¹ Aroclor 1254 equivalent), α-hexachlorocyclohexane (2 ng·1⁻¹), γ-hexachlorocyclohexane (2 ng·1⁻¹) and a few yet unidentified compounds at simililarly low concentrations were detected. In the estuary, the only compounds present in concentrations above the limits of detection in 1-litre samples were the hexachlorocyclohexane isomers. In all samples, the amount of each isomer in solution was larger than in suspension. No evidence was found for transitions of these compounds between solution and suspension during estuarine mixing.The metals Cd, Zn and Fe were removed from solution in the early stages of estuarine mixing by sorption to freshly eroded sediment particles in the mud reach. Conservative behaviour was found for Cu. Manganese is cycled between particulate and dissolved forms. It is removed from solution in the lower estuary and produced in the upper part, mainly in response to variations in pH and resuspended sediment concentration, resulting in a maximum in the dissolved Mn concentration at low salinities.Elemental contents in seston in the estuary were determined primarily by mixing of particles from the lower estuary, the river and resuspended bottom sediments. These have different contents of organic C, organic N and metals. The relative contribution from originally dissolved metals is small. The variations in elemental contents could be explained in terms of a model involving a continuously suspended fraction, dominating at low seston concentrations, consisting of smaller, less dense particles with relatively high contents of Cd, Zn, Cu, Pb, Ca, Mg, organic C and organic N, and a fraction introduced by resuspension, consisting of larger, denser particles with higher contents of Mn, Al and Ti. Elemental contents and elemental/A1 ratios are nearly constant at seston concentrations above 20 mg·1⁻¹. Elementaa content of Fe and Si did not vary with seston concentration. It is suggested that in addition to A1 silicates, other minerals such as Fe and Mn oxides and organic matter are important sites for particulate trace metals, especially in the continuously suspended fraction.     

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.     

Primary production and nitrogen assimilation in the North sea during July 1987

An extensive survey of the whole of the North Sea, carried out in July 1987, is described. The study concentrated on the measurement of surface water inorganic nutrient concentrations and concomitant rates of primary production (¹⁴C) and nitrogen assimilation (¹⁵N). Primary production was investigated using size fractionation techniques. Three vertical profiles of primary production and nitrogen assimilation were also investigated.Much of the North Sea exhibed thermal stratification. Surface nutrient concentrations were low and chlorophyll concentrations typically <1 mg m⁻³. More than 75% of the primary production was attributable to cells <5 μm in diameter. Ammonium assimilation accounted for most of the nitrogen assimilation.The water column was vertically well mixed in the coastal zones. Here, inorganic nitrogen concentrations were high (e.g. up to 25 μmol NO₃⁻ with chlorophyll concentrations up to 10 mg⁻³, and organisms >5 μm diameter accounted for most of the primary production. As in offshore regions ammonium accounted for the major part of the nitrogen assimilated.A 115 km section obtained using an undulating oceanographic recorder showed that in certain regions of the North Sea physical features acted to increase the dependence of the phytoplankton on nitrate.