Egil Sakshaug

Chemical composition of Skeletonema costatum (Grev.) Cleve and Pavlova (Monochrysis) lutheri (Droop) Green as a function of nitrate-, phosphate-, and iron-limited growth.

The variation in chemical composition of Skeletonema costatum (Grev.) Cleve and Pavlova lutheri (Droop) Green was studied in batch cultures with N-, P-, and Fe-deficient media under continuous light. In vivo fluorescence, chlorophyll a, ATP, cellular nitrogen, carbon, and phosphorus, and cell density were measured. The exponential growth rate was 2.9–3.5 doubl, day−1 for Skeletonema and 1.2–1.9 doubl, day−1 for Pavlova, with the lower rates pertaining to cultures in Fe-deficient media. In exponentially growing cultures nitrogen and carbon per cell increased with an increasing N/P ratio of the media, whereas chlorophyll a, ATP, and organic phosphorus per cell remained relatively constant. In Fe-deficient cells nitrogen and carbon per cell was high, as for cells in media with a high N/P ratio. The total variation in exponential cells was contained within a factor of three. Larger differences in chemical composition were found between exponentially growing cells and nutrient deficient cells. For all types of nutrient deficiency the chl/N and chl/C ratios decreased when cells went from the exponential phase to the starvation phase. The ratio between in vivo fluorescence and chlorophyll a increased 4–5 fold. The CN ratio (atomic) of both species increased from 5–7.5 for exponentially growing cells up to 35 during N-deficiency and up to 13 during P-deficiency. For cells in Fedeficient medium values were scattered in the 7–11 range. ATP and organic phosphorus followed each other closely and had values which were about three times higher in Skeletonema than in Pavlova when using any other parameter as a frame of reference for identical media and growth stages. In N-deficient media the NATP ratio was virtually the same for exponentially growing cells and starved cells. The CATP ratio reached high levels in nutrient deficient cells, with a maximum of 9000 for P-deficient cells of Pavlova. Both species had a minimum value of polyphosphates corresponding to ≈ 10 % of the total phosphorus. In P-rich media Pavlova had storage polyphosphates up to 70 % of the total P. Storage polyphosphates were not observed in Skeletonema. With the set of factors used it is possible to identify growth stages and type of nutrient deficiency for a given culture. It is indicated that, with certain limitations, this approach may be applicable in field studies.

Structure, biomass distribution, and energetics of the pelagic ecosystem in the Barents Sea: A synopsis

Biomass distribution and energetics of trophic levels in the pelagic ecosystem of the Barents Sea are presented as averages over several years for the whole Barents Sea using data from the research programme Pro Mare in 1984–1989 and mathematical ecosystem models. Average biomasses range from more than 3 tonnes carbon km−2 (zooplankton) to 0.1 kg C km−2 (polar bears) and P/B ratios from 300 (bacteria) to 0.035 (minke whales). However, the Barents Sea ecosystem is in a far from steady state with, for instance, capelin stocks ranging from 30–700 kg C km−2 between years and cod stocks from 150–700 kg C km−2. As a general rule, the various fish stocks grow adequately, albeit at different rates, in “warm” years characterized by large influxes of Atlantic water and high zooplankton productivity. The skewed populations distribution which arises in “warm” years may lead to grave imbalances in “cold” years and even to the “collapses” of stocks, such as of capelin in the eighties. The food requirements of average-sized stocks of cod, seabirds and marine mammals correspond to more than twice the average productivity of capelin. Thus other species of pelagic fish (herring, polar cod) and zooplankton obviously play major roles as prey for these animals.

Studies on the phytoplankton ecology of the Trondheimsfjord. I. The chemical composition of phytoplankton populations

Abstract Samples of phytoplankton populations from the Trondheimsfjord, collected in 1970 and the first five months of 1971, have been analysed for carbohydrate, protein, lipid, and phosphorus. Lipid was in all cases less than 10% of the organic dry matter. The N P ratio was remarkably constant, but the ratio protein/carbohydrate varied between wide limits. For samples consisting mainly of dinoflagellates, the protein/carbohydrate ratio was always low, due to a large amount of insoluble polysaccharides, probably corresponding to material in the cell walls. For diatoms, the carbohydrates may conveniently be divided into three fractions: 1) an acidsoluble glucan of the β-1, 3-linked type; 2) an alkali-soluble fraction giving a complex mixture of monosaccharides on hydrolysis and, 3) an insoluble glucan. The amounts of acid-soluble glucan varied from 7.7 to 36.5% of organic dry matter and these changes are the main cause of the variation of the protein/carbohydrate ratio of diatom samples. For diatom samples this ratio is a valuable indicator of the physiological state of the population. The variations observed in this study are discussed.

Alternate Carbon Pathways at Lower Trophic Levels in the Antarctic Food Web

A large percentage ( >50%) of total chlorophyll found in waters surrounding Antarctica is contained in the pico- and nano-plankton ( <20 μm) size-fraction. About 30% of the total nano-plankton biomass consists of heterotrophic flagellates. The total phytoplankton standing stock is low, averaging <0.6 μg Chl 1−1 in the East Wind Drift area. Higher concentrations are found in the shallow portions of the Weddell Sea and over the Scotia Ridge. In routine incubations pre-screened with a 200 μm Nitex net there is a measurable loss of chlorophyll and cells in the nanoplankton size category. This decrease in autotrophic biomass, as compared to the control incubations, apparently results from feeding by heterotrophic microplankton. Ciliate populations in Antarctic environments can attain high celldensities, which suggests they may be an important food reserve for animals requiring microplankton sized food particles. This new information requires that we revise the classical concept of the Antarctic ‘diatom-krill’ food chain to one which incorporates the feeding by Krill (Euphausiacea) on many large-volumed particles (protozoans as well as diatoms), with protozoans acting as a link which couples pico-and nano-plankton production to higher trophic levels.

Biooptical characteristics of PSII and PSI in 33 species (13 pigment groups) of marine phytoplankton, and the relevance for pulse-amplitude-modulated and fast-repetition-rate fluorometry1

We studied the variability of in vivo absorption coefficients and PSII‐scaled fluorescence excitation (fl‐ex) spectra of high light (HL) and low light (LL) acclimated cultures of 33 phytoplankton species that belonged to 13 different pigment groups (PGs) and 10 different phytoplankton classes. By scaling fl‐ex spectra to the corresponding absorption spectra by matching them in the 540–650 nm range, we obtained estimates for the fraction of total chl a that resided in PSII, the absorption of light by PSII, PSI, and photoprotective carotenoids. The in vivo red peak absorption maxima ranged from 673 to 679 nm, reflecting bonding of chl a to different pigment proteins. A simple approach is presented for quantifying intracellular self‐shading and evaluating the impact of photoacclimation on biooptical characteristics of the different PGs examined. In view of these results, parameters used in the calculation of oxygenic photosynthesis based on pulse‐amplitude‐modulated (PAM) and fast‐repetition‐rate (FRR) fluorometers are discussed, showing that the ratio between light available to PSII and total absorption, essential for the calculation of the oxygen release rate (using the PSII‐scaled fluorescence spectrum as a proxy) was dependent on species and photoacclimation state. Three subgroups of chromophytes exhibited 70%–80%, 60%–80%, and 50%–60% chl a in PSII‐LHCII; the two subgroups of chlorophytes, 70% or 80%; and cyanobacteria, only 12%. In contrast, the mean fraction for chromo‐ and chlorophytes of quanta absorbed by PSII was 73% in LL‐ and 55% in HL‐acclimated cells; thus, the corresponding ratios 0.55 and 0.73 might be used as correction factors adjusting for quanta absorbed by PSII for PAM and FRR measurements.

BIO-OPTICAL CHARACTERISTICS AND PHOTOADAPTIVE RESPONSES IN THE TOXIC AND BLOOM-FORMING DINOFLAGELLATES GYRODINIUM AUREOLUM, GYMNODINIUM GALATHEANUM, AND TWO STRAINS OF PROROCENTRUM MINIMUM1

Photoadaptive responses in the toxic and bloom‐forming dinoflagellates Gyrodinium aureolum Hulbert, Gymnodinium galatheanum Braarud, and two strains of Prorocentrum minimum (Pavillard)Schiller were evaluated with respect to pigment composition, light‐harvesting characteristics, carbon and nitrogen contents, and growth rates in shade‐ and light‐adapted cells. The two former species were grown at scalar irradiances of 30 and 170 μmol · m −2 at a 12‐h daylength at 20° C. The two strains of P. minimum were grown at 35 and 500 μmol. m−2· s−1 at a 2‐h daylength at 20° C. For the first time, chlorophyll (chl) c3, characteristic of several bloom‐forming prymnesiophytes, was detected in G. aureolum and G. galatheanum. Photoadaptional status affected the pigment composition strongly, and the interpretation of the variation depended on whether the pigment composition was normalized per cell, carbon, or chl a. Species‐specific and photoadaptional differences in chl a‐specific absorption (°ac, 400–700 nm) and chl a‐normalized fluorescence excitation spectra of photosystem II fluorescence with or without addition of DCMU (°F and °FDCMU 400–700 nm) were evident. Gyrodinium aureolum and G. galatheanum exhibited in vivo spectral characteristics similar to chl c3‐containing prymnesiophytes in accordance with their similar pigmentation. Prorocentrum minimum had in vivo absorption and fluorescence characteristics typical for peridinin‐containing dinoflagellates. Species‐specific differences in in vivo absorption were also observed as a function of package effect vs. growth irradiance. This effect could be explained by differences in intracellular pigment content, cell size/shape, and chloroplast morphology/numbers. Light‐ and shade‐adapted cells of P. minimum contained 43 and 17% of photoprotective carotenoids (diadino + diatoxanthin) relative to chl a, respectively. The photoprotective function of these carotenoids was clearly observed as a reduction in °F and °F DCMU at 400–540 nm compared to °ac in light‐adapted cells of P. minimum. Spectrally weighted light absorption (normalized to chl a and carbon, 400–700 nm) varied with species and growth conditions. The use of quantum‐corrected and normalized fluorescence excitation spectra with or without DCMU‐treated cells to estimate photosynthetically usable light is discussed. The usefulness of in vitro absorption and fluorescence excitation spectra for estimation of the degradation status of chl a and the ratio of chl a to total pigments is also discussed.

Factors controlling the development of phytoplankton blooms in the Antarctic Ocean — a mathematical model

A mathematical model describing the development of phytoplankton blooms as a function of the depth of the wind-mixed layer, spectral distribution of light, passage of atmospheric low-pressure systems, size of the initial phytoplankton stock and loss rates is presented. Model runs represent shade-adapted, large-celled, bloom-forming diatoms. Periodic deep mixing caused by strong winds may severely retard the development of blooms and frequently abort them before macronutrients are completely exhausted. Moderate depths of mixing (40–50 m) in combination with a moderately large total loss rate ( about 0.013 h−1 ) can prevent blooms from developing during the brightest time of the year. Complete exhaustion of macronutrients in the upper waters is likely only if the wind-mixed layer is less than 10 m deep, i.e. in very sheltered waters, and also in the marginal ice zone when ice is meiting. We do not exclude the possibility of control of phytoplankton biomass by iron in ice-free, deep-sea parts of the Antarctic Ocean, but the implied enhancement of export production through addition of iron might be restricted because of limitation by light, i.e. vertical mixing.

The taxonomic identity of the cosmopolitan prymnesiophyte Phaeocystis: a morphological and ecophysiological approach

Abstract Phaeocystis species diversity has been reviewed by comparing the morphological and physiological characteristics of Phaeocystis cells and colonies of different geographical origin. These analyses gave evidence for four Phaeocystis species: P. globosa, P. scrobiculata, P. pouchetii and one undefined antarctic species, distinguishing themselves by colony and single cell morphology and temperature tolerance. Typical colonial shape constitutes the most apparent morphological characteristics distinguishing P. pouchetii from P. globosa. Differences between colonies referable to pouchetii and globosa can be confirmed on the basis of variation in temperature and light requirements, as well as morphological descriptions of palmelloid stages, e.g. colony shape and size, organisation of the cells inside the colonies. The most striking features of the motile single cell are the thread-like appendages, which are much longer than the cell itself, the organic scales covering the cells, varying in shape and size, the haptonema and the flagella. On this basis, previous Phaeocystis records were analysed and the geographical distribution of the genus reported. There was no evidence for strain specific elemental composition or photosynthesis or growth performance of cells and colonies. This indicates that more elaborate molecular and biochemical analyses are required to identify different species. Possible opportunities available through modern chemical and molecular biological advances are described.

Modeling of light-dependent algal photosynthesis and growth: experiments with the Barents sea diatoms Thalassiosira nordenskioldii and Chaetoceros furcellatus

Abstract The models by Sakshaug et al (1989, Limnology and Oceanography, 34, 198–205) and Webb et al. (1974, Oecologia, 17, 281–291), for prediction of the gross growth rate of phytoplankton and short-term photosynthesis, respectively, have been modified on the basis of experiments with cultures of the centric diatoms Thalassiosira nordenskioeldii and Chaetoceros furcellatus grown at 0.5°C at combinations of two irradiances (25 and 400μmol m−2s−1) and two day-lengths (12 and 24 h). The models have one spectrum, °σ, which represents chlorophyll a (Chla) specific absorption of photosynthetically usable light, and introduces a factor q which represents Chla per PSU, functionally defined. The models describe phytoplankton growth in terms of physiologically relevant coefficients. A properly scaled fluorescence excitation spectrum (°F) represents a more appropriate estimate for °σ than the Chla-specific absorption spectrum °ac judging from calculations of Φmax (=αB/°σ). On the basis of °F, Φmax is 0.04 g-at C(mol photons)−1 for gross growth and about 0.05–0.08 for short-term carbon uptake (unfiltered samples). Calculations based on °ac yield values for Φmax which on average are 44% lower. P vs I (photosynthesis vs irradiance) parameters are relatively independent of day-length and highly dependent on growth irradiance. The product of q [mg Chla (mol PSU)−1] and τ (the minimum turnover time of the photosynthetic unit, h) increases 2–3-fold from high to low irradiance, thus PmB (=Φmax/qτ) and Ik(=1/qτ°σ)decreased. °F decreases from high to low irradiance. Carbon-specific dark respiration rates are Pigment ratios vary inversely with irradiance and day-length. The Chla:C ratio is particularly low under high, strong continuous light; Chlc: Chaa ratios are higher for shalde- than for light-adapted cells, while the converse is true for the ratio of the sum of the photoprotective pigments diadinoxanthin and diatoxanthin to Chla. The fucoxanthin: Chla ratio is virtually independent of the light regime. The two species are similar with respect to variations in growth rate (0.09–0.33 day−1 and Ik (31–36 vs 49–100 μmol m−2 s−1 at low and high irradiance, respectively). PmB and aB for growth as well as °F are systematically higher for C. furcellatus than for T. nordenskioeldii, while the product qτ is lower. C. furcellatus is considerably more plastic than T. nordenskioeldii with respect to pigment composition.

Studies on the phytoplankton ecology of the trondheimsfjord. III. Dynamics of phytoplankton blooms in relation to environmental factors, bioassay experiments and parameters for the physiological state of the populations

Abstract Quantitative phytoplankton sampling was carried out at weekly intervals at one station in the central part of the Trondheimsfjord and at irregular intervals at one station near Trondheim Harbour during March–October, 1970 and 1971. Various developmental stages of diatom blooms have been observed, which have been related to variations in freshwater discharge, hydrography, nutrients (nitrate, orthophosphate, and reactive silicate in sea water and river water), light, the results of bioassay experiments, parameters for the physiological state of natural phytoplankton populations, and to data on phytoplankton and hydrography collected during 1963–1969. Two spring blooms of diatoms are persistent from year to year in the area. The first one starts in March, triggered by an increase in the incident radiation and culminates in early April. It develops analogously to a batch culture and is nourished mainly by nutrients accumulated during the winter. The second takes place in brackish waters during May–June concomitant with floods in rivers. The magnitude of its populations corresponds to discharge maxima unless disturbed by hydrographical irregularities and heavy grazing by Calanus finmarchicus (Gunnerus). This bloom is analogous to a continuous culture and is nourished by nutrients in entrainment water and to a lesser extent by those in the river water. Furthermore, the unpredictable development of diatom blooms in the autumn seems to follow peaks in the discharge unless prevented by too low salinity and poor incident light. In autumns of little discharge and with turbulence in the upper 5–10 m dinoflagellates predominate. In high salinity waters nitrogen seems generally more limiting than phosphorus for phytoplankton growth The N/P atomic ratio of such waters with no phytoplankton growth was 10–12 in contrast to 13–18 in the phytoplankton. Due to the high N/P ratio of 40–50 in river water, phosphorus was more limiting than nitrogen in some brackish waters. On two occasions trace metals seemed to be the most limiting.