Wim Admiraal

Observations and experiments on the population dynamics of epipelic diatoms from an estuarine mudflat

The kinetics of diatom growth on a mudflat in the Ems-Dollard estuary were analysed from field observations and experiments carried out during a spring bloom. Low-density natural populations of epipelic diatoms showed an exponential increase in cell number with an apparent cell division rate of 0·3 day−1. Similar division rates were measured in natural populations, incubated on their sediment in the laboratory and in unialgal cultures of dominant diatom species grown on interstitial water. The vertical migration of diatom cells in ‘dark’ millimeters deep sediment layers apparently did not decrease the mean cell division rate in the populations. The diatom populations on the mudflat reached densities of 1–×106 cells cm−2; these densities are comparable to peak values of chlorophyll a concentrations of 100–400 mg m−1 of sediment, as reported for microphytobenthos in the literature. In dense natural populations and in dense unialgal cultures similar patterns were found in the accumulation of biomass (as chlorophyll a or cell numbers), in the limitation of productivity (as mg O2 m−2 h−1) and in the decrease of cell division rate. The diffusion of inorganic carbon and oxygen between the diatom film and the overlying water limited the productivity of dense diatom populations to ca. 500 mg O2 h−1 or less depending on the external concentrations of inorganic carbon and oxygen, and the flow rate of the water. Correspondingly, the rate of cell production in dense culture reached an upper level and this resulted in linear growth curves. The rate of this linear growth was affected also by the length of the daily light period. At the highest densities of cultures and natural populations the population growth drops to zero, suggesting a per day balanced budget of carbon-limited photosynthesis and respiratory or excretory losses. Self-shading had no effect on the division rate in dense cultures. The supply of nutrients (as P, N or Si) was not a critical factor to the growth rate of natural populations on the nutriet-rich mudflat, nor to cultures under the conditions tested.

Nitrogen metabolism of marine planktonic diatoms; excretion, assimilation and cellular pools of free amino acids in seven species with different cell size

Abstract Marine diatom species with cell volumes between 135 and 4000000 μm 3 were grown in medium with a high concentration of nitrate and the transition to nitrogen starvation was studied. Nitrogen deprived cells of Cydotella cryptica , Reimann, Lewin & Guillard, Thalassiosira excentrica (Ehrenberg) Cleve, and Coscinodiscus granii Gough differed widely in their ability to absorb 10 amino acids in the concentration range of 10 to 400 nmol·l −1 . Cycltella cryptica rapidly assimilated aspartic acid, glumatic acid, and alanine, whereas Thalassiosira excentrica took up all amino acids except aspartic acid. Coscinodiscus granii assimilated arginine, valine, isoleucine, and leucine. A similar diversity in the excretion of amino acids was detected in nitrogen-sufficient cultures: Cydotella cryptica released traces only, whereas Coscinodiscus granii released 16 nmol of amino acids per μmol nitrate assimilated, of which valine, isoleucine, leucine, and arginine were apparently reabsorbed. Thalassiosira excentrica cultures released only significant amounts of amino acids in a single burst accompanying the transition to the stationary phase. Nitrate-sufficient cells of all the seven species studied contained 27–102 mmol of free amino acids per litre of cell sap, irrespective of their cell volume. The concentrations of total free amino acids in nitrogen-deprived cells were 3–50 times lower in most species, with a disproportional reduction of the nitrogen-rich amino acids, glutamine, histidine, and ornithine. It is suggested that the cellular pools of free amino acids may serve as a significant nitrogen buffer especially in large diatoms. On the other hand, these amino acids can be a sensitive indicator of the state of nitrogen nutrition as well as of the general synthetic activity of phytoplankton.

INFLUENCE OF ORGANIC-COMPOUNDS AND LIGHT LIMITATION ON THE GROWTH-RATE OF ESTUARINE BENTHIC DIATOMS

Ten species of benthic diatoms from the Eems-Dollard estuary were grown in axenic cultures under various combinations of irradiance and supply of organic substrates. Six species were capable of growth in the dark on yeast extract, casamino acids, or glucose. Four of these species grew best in the presence of glucose, whereas the growth of the other two species was supported only by yeast extract and casamino acids. The light limited growth rate of only those species that were also capable of heterotrophic growth in the dark was increased by organic substrates. The rate of this “mixed” growth together with the absence of a lag-phase upon change from autotrophic to heterotrophic conditions indicates the nutritional versatility of these diatom species. A positive relation between the organic matter content of the natural habitat and the heterotrophic capacities of the diatom species is suggested. All species with heterotrophic capacities were isolated from muddy sediments, whereas two species isolated from a...

SULFIDE TOLERANCE OF BENTHIC DIATOMS IN RELATION TO THEIR DISTRIBUTION IN AN ESTUARY

Benthic diatoms from the Eems-Dollard estuary were tested for their ability to survive in solutions of free sulphide. The unialgal cultures and mixed populations of diatoms tested were isolated from field stations on different parts of the estuary; some of these stations were exposed to organic waste water. Toxic concentrations of free sulphide were found in the range of 0·9–6·8 mm (5–48 h incubation in the dark); these concentrations of free sulphide are among the highest reported from marine sediments. Relatively tolerant benthic diatom species survived 6·8 mm of free sulphide during 5–24 h, whereas relatively sensitive species were strongly inhibited by 0·9 mm during more than 24 h. Two estuarine plankton diatom species were equally resistant, whereas two oceanic species were the most sensitive tested. The occurrence of a few benthic diatom species on mudflats with very strong organic pollution (and hence superficial black layers) was related to their relatively high sulphide tolerance. However, other ...

DISTRIBUTION OF DIATOM SPECIES ON AN ESTUARINE MUD FLAT AND EXPERIMENTAL-ANALYSIS OF THE SELECTIVE EFFECT OF STRESS

The distribution patterns of seven dominant diatom species on an estuarine mud flat transect were related to salinity gradients and organic pollution. The temporal distribution of species can be explained partly by the seasonal variation in irradiance and temperature, and partly by the large discharges of organically polluted fresh water during the late autumn. The selective effects of stress factors, such as low or high salinity, high concentrations of ammonia and free sulphide, and high temperatures were studied by measuring: (1) the tolerance of natural diatom populations isolated from the sediment; (2) the tolerance and capacities of unialgal cultures: and (3) the effects of the stress factors on the species composition of the populations in sediment samples incubated in the laboratory. A low salinity (1‰), but not a high salinity (20‰), markedly altered the species composition of natural diatom populations kept in the laboratory. Navicula salinarum Grunow and N. cf. cryptocephala Kutzing outcompete other species only at a salinity of 1‰, which is below the optimum salinity for growth of these species. High concentrations (2–4 mM) of ammonia inhibited the photosynthesis of N. phyllepta Kutzing and N. flanatica Grunow and decreased the cell numbers of these species in mixed populations kept in the laboratory. N. salinarum and Gyrosigma fasciola (Ehr) Cleve were relatively ammonia-tolerant and reached their highest abundance during periods of large discharges of organically polluted water. The populations on the mud-flat stations that had black, sulphide-containing layers 1 mm below the surface of the sediment, were dominated by the relatively sulphide-tolerant Navicula salinarum and N. pygmaea Kutzing. High values of irradiance and temperature were a selective factor explaining the absence of N. flanatica in summer. Uptake of organic substrates and the inhibitory effect of high population densities are discussed in regard to their possible modifying influence on the distribution of diatom species on the mud flat.

On the ecology of meiofauna in an organically polluted estuarine mudflat

Abstract The structure, distribution and seasonal changes of the benthic meiofauna in an organically polluted, tidal, brackish-water mudflat in the Ems-Dollart estuary were analysed. Towards the outfall of polluted fresh water, macrofauna disappeared, numbers of meiofauna increased but the diversity of the meiofauna decreased. In the area surrounding the outfall the numbers and biomass of nematodes and oligochaetes increased rapidly in spring and remained high until autumn ( c. 13 × 10 6 individuals m −2 ; c. 2gCm −2 ). The benthic fauna comprised small numbers of species, dominated by a few fast-growing diatom-feeding nematodes ( Eudiplogaster pararmatus and Dichromadora geophila ) and oligochaetes ( Amphichaeta sannio and Paranais litoralis ). Eudiplogaster pararmatus exhibits brood care and it tolerates low salinities. Dichromadora geophila which is oviparous, behaved similarly regarding tolerances, life cycle and feeding but this species was less successful than Eudiplogaster in colonizing the mudflats near the outfall. The success of the two naidid oligochaete species results from their method of reproduction by means of binary fission. Most organisms fed on benthic diatoms. In spite of intensive mineralization in the mudflat, only one bacteria-feeding organism was found in abundance (the nematode Leptolaimus papilliger ). The absence of macrofaunal organisms, e.g. Nereis diversicolor , is probably also responsible for the development of high densities of meiofauna in the upper sediment layers of the mudflats in the vicinity of the outfall. The heavy load of organic waste entering the Dollart reduced the diversity within the benthic ecosystem, but nevertheless a simple system remained, based on the recurrent reoxidation of the sediment surface.

CHEMICAL AND PHYSIOLOGICAL-CHANGES OF PHYTOPLANKTON DURING THE SPRING BLOOM, DOMINATED BY PHAEOCYSTIS-POUCHETII (HAPTOPHYCEAE) - OBSERVATIONS IN DUTCH COASTAL WATERS OF THE NORTH-SEA

Abstract During the spring bloom of phytoplankton in Dutch coastal waters of the North Sea the flux of 14 C-labelled products into pools of protein, polysaccharides, small metabolic intermediates and lipids was studied. The molecular size distribution of extracellular products, amounting to 28% of the particulate production, was determined by gel filtration. Concentrations of protein and carbohydrates were determined in the same phytoplankton samples. At an early stage of the bloom protein synthesis and protein concentration dominated over carbohydrate synthesis and carbohydrate concentration, but after the peak of the bloom carbohydrates were the predominant photosynthetic endproduct. These observations have been compared to similar observations on unialgal cultures of Phaeocystis pouchetii . The influence of phosphate depletion on the extracellular release of organic compounds and the impact of the colonial growth form of Phaeocystis pouchetii on the distribution of intra- and extracellular labelled compounds are also discussed.

Cycling of carbon and oxygen in layers of marine microphytes; a simulation model and its eco-physiological implications

SummaryA mathematical simulation model was used to ascertain the relation between the diffusion of oxygen and inorganic carbon into layers of marine microphytes and the carbon metabolism of these microphytes. The simulation model included physiological and physico-chemical parameters and was validated using the few data available from the literature on production determinations, on oxygen and pH values, and on growth dynamics of natural populations. The model was tested with various modifications to mimic experiments with suspended algae and algal films on inert substrates, and also to simulate microphytobenthos in sediment cores with or without grazing. The simulated variations in oxygen concentrations and pH values over time scales of min and days were consistent with field and experimental observations. The model predicted upper limits of primary production and biomass observed in well developed natural populations; these limits are caused by a combination of oxygen accumulation and depletion of inorganic carbon resulting from diffusion limitations and the recirculation of organic carbon in photosynthetic, respiratory and excretory processes. The model calculations were used to check on the adequacy of the various methods used to determine the primary production of benthic microphytes.

PRIMARY PRODUCTION IN A TURBID ESTUARY, THE EMS-DOLLARD - FIELD AND MODEL STUDIES

Abstract The Ems-Dollard Estuary has a steep axial gradient in turbidity which strongly regulates primary production of phytoplankton and microphytobenthos. A combination of the turbidity and nutrient gradients causes a high pelagic primary production in the outer, seaward part of the estuary, and a high benthic production on high-level mudflats (with long effective photoperiods) in the inner part. Model simulations were used to study phytoplankton succession and the factors influencing this succession. Light limitation was evaluated in a sensitivity run which used a lower compensation light intensity.