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.     

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.     

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.     

Structure and energy demand of the benthic soft-bottom communities in the Java Sea and around the islands of Madura and Bali,Indonesia

In July 1984 benthic research was carried out in the Java Sea and around the islands of Madura and Bali.The aim was to describe the structure of the benthic communities in terms of numerical density (numbers of dominant macrofaunal groups and meiofauna per square metre) and biomass (g ashfree dry weight (AFDW) per square metre), and in relation to geographical position and main ambient factors, i.e. water depth, bottom temperature and sediment characteristics.In addition attempts were made to estimate the activity of these benthic ecosystems, and to show their relation to the pelagic.Box-core samples, collected along west-east axis in the Java Sea and in the Strait of Madura, showed relatively poor benthic communities. The average densities of macrofaunal organisms (> 1 mm) did not exceed 250 specimens·m⁻²; the total biomass was below 1 g AFDW·m⁻²; small polychaetes and crustaceans were the dominant groups.A number of larger organisms have developed very remarkable adaptations to the muddy environment. Meiofaunal organisms (nematodes being the dominant group) numbered 0.06·10⁶ to 0.46·10⁶ specimens·m⁻². In general there is a trend from west to east towards somewhat richer communities. The actual carbon demand of the benthic ecosystem in the area investigated is tentatively estimated at an average of 38 g C·m⁻²·y⁻¹, which amounts to about 40% of the primary production.Both the very soft fluid-mud bottoms in the central Java Sea and in Strait Madura, and the relatively low amount of energy available for growth, probably prevent the establishment of well developed benthic communities.     

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.     

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.     

Tidal and residual flows in the western Dutch Wadden Sea II: An analytical model to study the constant flow between connected tidal basins

In a one-dimensional analytical model the origin of constant flows between connected tidal basins, as well as the origin of the associated residual levels, is examined. Linearized shallow water equations are used to describe the propagation and damping of a tidal wave in schematized (uniform width and depth) connected basins. Analytical expressions are derived for the tidal stress terms, including the contribution of the non-linear bottom-friction term, which serve as the forcing functions in the equations for the mean field. It is shown that in a first approximation the residual levels in the tidal inlets, which give boundary conditions for the tidally-averaged equations, are dependent on the tidal velocities in the inlet because of a “Bernoulli effect”. The model shows that in general differences between the fluctuating water levels at the inlets influence the residual flow more than morphological differences between two connected basins. The tidally-driven mass transport in the western Dutch Wadden Sea, directed southwards from the Vlie basin towards the Marsdiep basin, can be explained from the larger water-level amplitude at the inlet of the Vlie basin.     

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.     

Abundance,growth and food demand of the scyphomedusa Aurelia aurita in the western Wadden Sea

Medusae of Aurelia aurita are found in the western Wadden Sea from the beginning of May till August with maximum numbers of 250 to 500 individuals per 10³ m³ during May-June. The existence of a continuous ebb surplus suggests an origin from polyps living in the inner parts of the estuary and a transport or migration of the released medusae towards the North Sea. Growth is fast; a bell size of 20 cm diameter is reached within 3 to 4 months. The species is important as a predator from May to July, reaching maximum carbon biomass values of 12 to 18 g C·10³ m⁻³. Predation by A. aurita may affect the recruitment of one of its food sources, viz. fish larvae.     

Consumption of eelgrass,zostera marina,by birds and invertebrates during the growing season in lake grevelingen (SW Netherlands)

During the growing season May through August 1976 living eelgrass (Zostera marina L.) in brackish Lake Grevelingen (108 km²) provided quantifiable amounts of food to 6 species of animals, viz. 5 bird species (Cygnus olor, Branta bernicla, Fulica atra, Aythya ferina and Anas platyrhynchos) and 1 invertebrate species, the isopod Idotea chelipes. Consumption of eelgrass amounted to 0.839 g C·m⁻² for the entire Lake Grevelingen, 23% for the birds and 77% for Idotea respectively. In a realistic estimate this means that only 3.7% of the living eelgrass production, estimated at 23 g C·m⁻²·y⁻¹ for the entire Lake Grevelingen, was consumed by macroconsumers during the growing season. A maximum estimate of eelgrass consumption leads to 1.486 g C·m⁻², i.e. 6.5% of the eelgrass production during the period May through August.     

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.     

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.     

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.     

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.     

Microbial growth in turbulent suspension and its relation to marine aggregate formation

Microbial growth and respiration responses to seston of differing organic composition revealed two different mechanisms of marine aggregate formation: an almost immediate physicochemical aggregation of fine (<8 μm) particles and a longer-term (48 hrs) production of large, robust aggregates, mediated by bacterial activity. Suspensions of fine Spartina alterniflora fragments and a glacial till from Nova Scotia were used to stimulate bacterial and protozoan growth while maintained in turbulent suspension. Under high (6 to 11 mg·dm⁻³) DOC concentrations bacterial responses were rapid and, after 48 hrs, resulted in large, robust flocculated aggregates. With only the till in suspension, DOC levels were low (<3 mg·dm⁻³) yet the bacterial reponse was still rapid. Initial flocculation due to physical factors was evident in the till-only experiment and was either masked or not important in the high DOC experiments. Bacterial respiration per cell was 2 to 3 times as high under low DOC conditions as under high DOC and initial bacterial attachment to particle surfaces was significantly higher. Significant aggregate formation occurred after 48 hrs and appeared to be due to relict exudates from attached bacteria, in spite of low numbers of attached bacteria at the time of flocculation.     

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.     

Free-living plathelminthes in sheep-grazed and ungrazed supralittoral salt marshes of the North Sea: Abundance,biomass, and their significance in food chains

The supralittoral salt marshes of the North Sea are marked by high halophyte primary productivity. The environmental factors are strongly fluctuating. Despite these features the metazoan meiofaunal abundance is equal to that found in other littoral habitats. On average 1250 marine metazoans are found per 10 cm² in ungrazed and 770 per 10 cm² in sheep-grazed supralittoral salt marshes. Nematoda dominate in numerical abundance, Oligochaeta in biomass. Plathelminthes account for 15% of marine metazoans in ungrazed and 5% in grazed salt marshes.Total plathelminth abundance increases with halophyte density, whereas the abundance of diatom-feeding Plathelminthes decreases. In ungrazed marshes on average 104 Plathelminthes are found per 10 cm², accounting for a biomass of 0.65 g DW·m⁻². In sheep-grazed marshes the average abundance is only 32 individuals per 10 cm², accounting for a biomass of 0.1 g DW·m⁻². Average individual weight is 3.2 μg DW or 2.5 μg AFDW.In grazed salt marshes, 30% of plathelminthes feed on diatoms, 66% are predators, and 4% feed on bacteria (gut analysis). In ungrazed salt marshes only 3% are diatom-feeders, and 90% are predators feeding on Nematoda, Copepoda, Oligochaeta, and smaller Plathelminthes. Presumably plathelminthes are top predators on the salt marsh meiofauna.     

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.     

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.     

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

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