Biosynthetically active glutamine synthetase in the marine diatom Phaeodactylum tricornutum: optimization of the forward-reaction assay

The activity of glutamine synthetase (GS) was measured in the marine diatom Phaeodactylum tricornutum Bohlin (Strain SME) by a biosynthetic assay, based on -glutamyl hydroxamate synthesis, and referred to as the forwardreaction assay. The effects of pH, temperature and different homogenizing buffer preparations on enzyme activity, linearity of reactions, and substrate-saturation kinetics were investigated. The resultant data provide the basis for establishing optimum experimental conditions for a standard assay. Affinities of P. tricornutum GS for glutamate, ATP and Mg²⁺ were similar to those recorded elsewhere for a variety of other phytoplankton species using true biosynthetic assays based on release of inorganic phosphate, whereas the affinity for hydroxylamine was two orders of magnitude lower than that for ammonium, with an apparent K _m value in the millimolar range. This, together with negative results obtained during earlier attempts to detect GS activity in P. tricornutum using the true biosynthetic assay, indicates that the GS of this alga has a lower affinity for ammonium than that of other phytoplankton species. Dual substrate kinetics demonstrated that apparent K _m and V _m values for glutamate were directly proportional to the concentration of ATP, thus giving indirect evidence of a correlation between GS activity and the adenylate energy charge. Comparisons between synthetase activities obtained with the optimized forward-reaction assay and transferase activities reported from other studies on various phytoplankton species revealed discrepancies which, to a great extent, probably arise from differences in the growth conditions of the organisms.     

Marine food-web analysis: An experimental study of demersal zooplankton using isotopically labelled prey species

Short-term incubations in seawater containing H¹⁴CO₃ ^- or ³H₂O in place of the naturally predominant isotopes can yield highly radioactive preparations of living phytoplankton or zooplankton. Subsequent in situ incubation of these labelled organisms with the community from which they were taken results in the rapid transfer of radioisotope to those species which prey upon them. This technique has been employed to map a portion of a marine food web involving demersal zooplankton; experiments were conducted in summer and autumn on a coral reef and in a subtropical estuary. Similar results were obtained from these initial experiments at each study site during both seasons. Prey supplied as zooplankton (124 to 410 m nominal diameter), which consisted mainly of Oithona oculata, was fed upon by zooplankton size classes ranging from 410 to 850 m and containing amphipods, ostracods, cumaceans and polychaetes. In experiments employing labelled phytoplankton as prey a wide size spectrum was used (10 to 106 m) in order to include representative samples of most of the available planktonic autotrophs as estimated by primary production measurements. In two separate experiments, only 7 out of 63 samples evidenced grazing of phytoplankton by demersal zooplankters. In contrast, labelled diatom auxospores, employed in one experiment as they constituted the most numerically abundant species in the water column, were found to be grazed upon in nearly half the samples examined.     

Phosphate uptake behavior of natural phytoplankton during exposure to solar ultraviolet radiation in a shallow coastal lagoon

The effect of solar UV radiation (UVR: 290–400 nm) on the ³²P-phosphate uptake rates of natural phytoplankton from a southern Atlantic Ocean coastal lagoon was studied during two consecutive summers at one station located in the marine-influenced area. Due to the shallowness of this lagoon and also to the generally high UV water transparency in this area, phytoplankton are exposed to high UV irradiances. The ³²P-phosphate uptake rates measured at several phosphate concentrations were inhibited up to 59.2% by UVR, although uptake stimulation was also observed in four of nine experiments (up to 28%). The effect of UVR on the apparent maximum velocity of ³²P-phosphate uptake (V _uptake) ranged from an inhibition of 49% to a stimulation of 31%. Although the highest inhibition values were associated with the maximum registered incident UV irradiance, a significant correlation between these two parameters was not observed. Changes in microalgal community structure were not related to the observed UV effect; however, a significant relationship was found between the inhibition of ³²P-phosphate uptake rates and V _uptake used as a proxy for phosphate deficiency. This relationship suggests that the phytoplankton phosphorus nutritional status modulates their sensitivity to UV exposure. Overall, our results suggest that solar UVR has the potential to affect phosphorus cycling.Communicated by O. Kinne, Oldendorf/Luhe     

Photosynthetic characteristics of nanoplankton and picoplankton from the surface mixed layer

Nanoplankton and picoplankton primary production has been studied at two oceanic stations in the Porcupine Sea-bight and at one shelf station in the Celtic Sea. At both sites, low wind conditions in June and July 1985 resulted in greatly reduced vertical turbulent mixing and a secondary, temporary thermocline developed in what is usually a well-mixed surface layer; as a result, there was physical separation of the phytoplankton within two zones of the surface mixed layer. The photosynthetic characteristics of three size fractions (>5 m, <5 to >1 m and <1 to >0.2 m) of phytoplankton populations from the two zones have been measured. Phytoplankton was more abundant at the oceanic stations and chlorophyll a values were between 1.3 and 2.2 mg chlorophyll a m^-3, compared with 0.3 to 0.6 mg chlorophyll a m^-3 at the shelf station; at both stations, numbers of cyanobacteria were slightly higher in the lower zone of the surface mixed layer. There was no effect of the temporary thermocline on the vertical profiles of primary production and most phtosynthesis occurred in the surface 10 m. Photosynthetic parameters of the three size fractions of phytoplankton have been determined; there was considerable day-to-day variation in the measured photosynthetic parameters. Assimilation number (P _m ^B ) of all >5 m phytoplankton was lower for the deeper than for the surface populations, but there was little change in initial slope (a ^B ). The small oceanic nanoplankton (<5 to >1 m) showed changes similar to the >5 m phytoplankton, but the same size fraction from the shelf station showed changes that were more like those shown by the picoplankton (<1 m) viz, little change in P _m ^B but an increase in a ^B with depth. Values of a ^B were generally greater for the picoplankton fraction than for the larger phytoplankton, but values of adaptation parameter (I _k )(=P _m ^B /) were not always less. There was little evidence to support the hypothesis that these populations of picoplankton were significantly more adapted to low light conditions than the larger phytoplankton cells. When photosynthetic parameters of the picoplankton were normalised to cell number (P _m ^C /a ^C ) rather than chlorophyll a, P _m ^C was comparable to other published data for picoplankton, but a ^C was much lower. The maximum doubling time of the picoplankton at saturating irradiance is calculated to be ca. 8.5 h for the oceanic population and ca. 6.2 h for the shelf population.     

Mesocosm studies of DCMU-enhanced fluorescence as a measure of phytoplankton photosynthesis

When measurements of in-vivo fluorescence are used to estimate photosynthesis in the field, the marked temporal and spatial variations in phytoplankton populations, and their nutrient and light histories, have produced varied results. Natural phytoplankton populations in large, flow-through mesocosms with different controlled nutrient and sewage sludge additions were sampled weekly from June to September 1984. Good correlations were observed between the increase in fluorescence upon the addition of DCMU (F) and both in-situ production and the parameters (P_m and ) of the photosynthesis-irradiance curve for these phytoplankton populations. Good correlations were also obtained between DCMU-enhanced fluorescence (F_DCMU) and chlorophyll a concentrations. The relationship between F and in-situ ¹⁴C production was consistent among mesocosms even in the face of major shifts from diatom-dominated to dinoflagellate-dominated populations. On the other hand, the F_DCMU:Chl a relationship was significantly different between mesocosms and related to species composition. It was concluded that F offers the possibility of rapidly and accurately indexing both in-situ production and the photosynthetic capacity of mixed phytoplankton populations.This study was supported by EPA cooperative agreement 810265-03 and NOAA grant NA-83-ABD-00008     

Carbon fixation in marine phytoplankton: carboxylase activities and stable carbon-isotope ratios; physiological and paleoclimatological aspects

We measured the activity of three carboxylases: RuBP carboxylase, PEP carboxylase and PEP carboxykinase of marine phytoplankton species in culture and in natural communities. Activities of the three carboxylases were measured simultaneously with stable carbon-isotope ratios. The enzymatic activities have been used to estimate the importance of carboxylation and its impact on the ¹³C:¹²C ratio (expressed as ¹³C). The marine phytoplankton species in culture were Fragilariopsis kerguelensis, Nitzschia turgiduloides, Skeletonema costatum, Phaeodactylum tricornutum, Isochrysis galbana, Dunaliella marina, and Prorocentrum micans, and the field samples were collected from different depths off the coast of Portugal (August/September, 1981). Our results indicate that, as in terrestrial plants, the ¹³C value is a good indicator of the extent of carboxylation. RuBP carboxylase activity was always predominant, whereas the ¹³C value never reached values typical of the C₄ pathway. The carboxylases could be PEP carboxylase (in dinoflagellates) or PEP carboxykinase (in diatoms). carboxylation increased at the end of the exponential growth phase in a diatom culture and with increased biomass in natural samples. We interpret these increases as an adaptative response mechanism to poor environmental conditions, especially to low light intensity.     

Some experiments on phosphate assimilation by coastal marine plankton

A study of phosphate assimilation by coastal marine plankton revealed that both phytoplankton and microheterotrophs incorporated radioactive phosphorus (³³P). Size fractionation of the particulate matter (using 1 m pore diameter Nucleopore^® membrane filters), antibiotic treatment (using garamycin), and independent estimaties of photoautotrophic (¹⁴CO₂ uptake) and heterotrophic (³H-glucose uptake) activities were employed to separate phyto- and bacterioplankton phosphate uptake. Results indicated that phytoplankton ³³P-uptake was best estimated by the fraction of particulate matter retained on the 1 m membrane filters. Usually, less than 10% of the phytoplankton (based on chlorophyll a measurements) passed the 1 m pore-diameter filters, whereas about 90% of the heterotrophic activity passed. At least 50% of the ³³P-uptake was associated with the <1 m fraction. It may be possible to resolve the phytoplankton and bacterial contributions to ³³P-uptake by comparing the percent of total ³³P-uptake with the percent of total ³H-glucose uptake associated with the >1 m fraction.     

Phytoplankton exudate release as an energy source for the growth of pelagic bacteria

The release of dissolved organic carbon (DOC) from phytolankton during photosynthesis, and the utilization of this carbon by planktonic bacteria, was studied using ¹⁴CO₂ and selective filtration. Natural sea water samples from a coastal area of the Northern Baltic Sea were incubated in the laboratory for detailed studies, and in situ for estimation of annual dynamics. In a laboratory incubation (at +1°C) the concentration of ¹⁴C-labelled dissolved organic carbon increased for about 2 h and then reached a steady state, representing about 0. 1% of the total DOC. Labelled organic carbon in the phytoplankton and bacterial fractions continued to increase almost linearly. The continuous increase in the bacterial fraction is thought to represent almost instantaneous utilization of the DOC released from the phytoplankton during photosynthesis. As an annual average, in 4 h in situ incubations, about 65% of the labelled organic carbon was found in the phytoplankton fraction (>3 m), about 27% in the bacterial fraction (0.2 to 3 m) and the remaining 8% as DOC (<0.2 m). Large variations in these percentages were recorded. The measured annual primary production was 93 g C m^-2 (March to December), and the estimated bacterial production due to phytoplankton exudates 29 g C m^-2. This represents a release of DOC of about 45% of the corrected annual primary production of 110 g C m^-2 (assuming a bacterial growth efficiency of 0.6).     

Seasonal study of phytoplankton pigments and species at a coastal station off Sydney: Importance of diatoms and the nanoplankton

Phytoplankton pigments and species were studied at a coastal station off Sydney (New South Wales, Australia) over one annual cycle. Sudden increases in chlorophyll a (up to 280 mg m^-2), due to short-lived diatom blooms, were found in May, July, September, January and February. These were superimposed upon background levels of chlorophyll a (20 to 50 mg m^-2), due mostly to nanoplankton flagellates, which occurred throughout the year. The nanoplankton (<15 m) accounted for 50 to 80% of the total phytoplankton chlorophyll, except when the diatom peaks occurred (10 to 20%). The annual cycle of populations of 16 dominant species-groups was followed. Possible explanations as to alternation of diatom-dominated and nanoplankton-dominated floras are discussed. Thin-layer chromatography of phytoplankton pigments was used to determine the distribution of algal types, grazing activity, and phytoplankton senescence in the water column. Chlorophyll c and fucoxanthin (diatoms and coccolithophorids) and chlorophyll b (green flagellates) were the major accessory pigments throughout the year, with peridinin (photosynthetic dinoflagellates) being less important. Grazing activity by salps and copepods was apparent from the abundance of the chlorophyll degradation products pheophytin a (20 to 45% of the total chlorophyll a) and pheophorbide a (10 to 30%). Chlorophyllide a (20 to 45%) was associated with blooms of Skeletonema costatum and Chaetoceros spp. Small amounts of other unidentified chlorophyll a derivatives (5 to 20%) were frequently observed.     

Photoadaption in marine phytoplankton: Response of the photosynthetic unit

Some species of phytoplankton adapt to low light intensities by increasing the size of the photosynthetic unit (PSU), which is the ratio of light-harvesting pigments to P700 (reaction-center chlorophyll of Photosystem I). PSU size was determined for 7 species of marine phytoplankton grown at 2 light intensities: high (300 E m^-2 s^-1) and low (4 E m^-2 s^-1); PSU size was also determined for 3 species grown at only high light intensity. PSU size varied among species grown at high light from 380 for Dunaliella euchlora to 915 for Chaetoceros danicus. For most species grown at low light intensity, PSU size increased, while the percentage increase varied among species from 13 to 130%. No change in PSU size was observed for D. euchlora. Photosynthetic efficiency per chlorophyll a (determined from the initial slope of a curve relating photosynthetic rate to light intensity) varied inversely with PSU size. In contrast, photosynthetic efficiency per P700 was enhanced at larger PSU sizes. Therefore, phytoplankton species with intrinsically large PSU sizes probably respond more readily to the rapid fluctuations in light intensity that such organisms experience in the mixed layer.Contribution No. 1180 from the Department of Oceanography, University of Washington, Seattle, Washington, USA     

Production of DMSP and DMS during a mesocosm study of an Emiliania huxleyi bloom: influence of bacteria and Calanus finmarchicus grazing

We investigated the influence of bacteria and metazooplankton on the production of dimethylsulfoniopropionate (DMSP) and dimethylsulfide (DMS) during blooms of Emiliania huxleyi (Lohmann) Hay and Mohler in seawater mesocosms. The phytoplankton succession was marked by the rapid collapse of an initial Skeletonema costatum (Greville) Cleve bloom followed by a small E. huxleyi bloom. The collapse of the diatom bloom was accompanied by an increase in concentrations of dissolved DMSP (DMSP_d) and bacterial abundance and activity (as determined by the thymidine incorporation technique). The increase in bacterial activity was followed by a rapid decrease in DMSP_d concentrations which remained low for the rest of the experiment, even during the subsequent collapse of the E. huxleyi blooms. The absence of DMSP_d and DMS peaks during the declining phase of the E. huxleyi blooms was attributed to the high bacterial activity prevailing at that time. The influence of metazooplankton grazing on DMSP and DMS production was investigated by adding moderate (24 mg dry weight m^-3) and high (520 mg dry weight m^-3) concentrations of Copepodite Stage V and adults of Calanus finmarchicus to two of four filtered (200 m mesh net) enclosures during the E. huxleyi blooms. The addition of C. finmarchicus, even in high concentrations, had no apparent effect on the dynamics of E. huxleyi, suggesting that the copepods were not grazing significantly on nanophytoplankton. The addition of copepods in high concentrations favored an accumulation of chlorophyll a and particulate DMSP. These results suggest that copepods were preying on the herbivorous microzooplankton which, in turn, was controlling the biomass of nanophytoplankton. DMS production was also enhanced in the enclosure with maximum metazooplankton biomass, suggesting that the grazing of C. finmarchicus on microzooplankton containing DMSP may contribute to DMS production. These results provide strong support to the emerging idea that bacteria and metazooplankton grazing play a dominant role in determining the timing and magnitude of DMS pulses following phytoplankton blooms.     

Diel periodicity of photosynthesis in marine phytoplankton

Short-term changes in photosynthesis were documented for 17 of 24 marine phytoplankton species, representing a range of taxonomic groups. Periodicity in phytoplankton photosynthesis on light-dark cycles (diel periodicity) was widespread but not universal for the species studied. The centric diatoms Lauderia borealis, Ditylum brightwellii, Stephanopyxis turris, Coscinodiscus rex, Chaetoceros gracile, and Biddulphia mobiliensis had strong diel periodicity in photosynthetic capacity (P _max). Amplitudes of the daily variations ranged from 2.9 to >50, with maxima in the morning or near midday, and with minima during the dark period, and these variations were not dependent on changes in cell pigmentation. There was some evidence for sustained photosynthetic periodicity in constant conditions in several diatoms, and an endogenous rhythm may have been present. The photosynthesis-irradiance (P-I) relationship was time-dependent for representative marine diatoms, with both the initial slope () and the asymptote (P _max) of P-I curves exhibiting significant synchronous diel oscillations. Moreover, detailed studies of the amplitude and timing of photosynthetic periodicity for the diatoms L. borealis and D. brightwellii demonstrated large temporal variations in photosynthesis with morning maxima. These P-I oscillations are discussed with reference to models of primary production which use the relationship between photosynthesis and light as a component of predictive equations for phytoplankton growth in the sea.     

Tidal variations in the photosynthesis of estuarine phytoplankton isolated in a tank

In 1981 two large (1 200 1) seawater samples from the St. Lawrence Estuary were kept under constant temperature and light conditions for periods of 50 and 68 h, respectively. In both tank experiments, semidiurnal variations in NH₄ were observed that could be related to cyclical NH₄ uptake by the phytoplankton. Semidiurnal cycles in photosynthetic efficiency (^B) and intracellular chlorophyll a in the tank, phased on tides at sea, were also evidenced in both experiments. These results support the hypothesis that variations in phytoplankton photosynthetic activity, which are possibly endogenous, can be phased on semidiurnal variations in vertical tidal mixing (variations of the mean light conditions in the mixed layer). In addition, observed variations in intracellular chlorophyll a suggest the possibility of endogenous cycles of phytoplankton light and shade adaptation.Contribution to the program of GIROQ (Groupe interuniversitaire de recherches océanographiques du Québec)     

Patterns of photosynthetic carbon metabolism in light-limited phytoplankton

Patterns of phytoplankton carbon (C) metabolism were examined in å combined laboratory and field study to assess the influence of light conditions on ¹⁴C assimilation into photosynthetic end-products. Laboratory studies with three species representing distinct size classes and taxonomic groups tested the influence of low light on patterns of C flow. Prorocentrum mariae-lebouriae (dinoflagellate) and Ditylum brightwellii (diatom) showed decreased movement of photoassimilated ¹⁴C into protein following a shift to low light ¹⁴C assimilation into lipids and photosynthetic pigments increased in low light and was paralleled by increased chl a per cell. The proportion of ¹⁴C fixed into protein returned to the pre-shift level upon return to initial light conditions. Monochrysis lutheri (chrysophyte) did not show this pattern of reduced % ¹⁴C protein. Incubations of 12 and 24 h demonstrated significant rearrangements in labeling patterns at night, wherein ¹⁴C flow into protein in darkness was favored. % ¹⁴C protein at night was lower for M. lutheri than for the other species, suggesting some interspecific differences in the low light response. Measurements of ¹⁴C assimilation in phytoplankton assemblages from Chesapeake Bay demonstrated movement of a higher proportion of photo-assimilated C into protein in samples collected in the surface mixed layer than in those below the pycnocline. In comparison, phytoplankton collected below the pycnocline fixed a higher proportion of ¹⁴C into lipids, photosynthetic pigments, and low molecular weight metabolites, as was observed in low light laboratory cultures. A comparison of 12- and 24-h incubations for measuring patterns of C flow into photosynthetic end-products confirmed the inadequacy of short-term measurements, as significant changes in ¹⁴C allocation occurred in the dark phase of the photocycle. Together, these results suggest that ¹⁴C assimilation into photosynthetic end-products can be a useful measure of adaptive state in changing light conditions, but point out some difficulties in applying this approach in situ.     

Photosynthetic characteristics of marine phytoplankton from contrasting physical environments

Two suites of phytoplankton samples have been collected in consecutive years at various times over a day from selected depths within vertically mixed and stratified water columns in the western Irish Sea, in order to provide a range of possible light histories within the populations collected. Values for the maximum rate of ¹⁴C retention (P _max) and the initial slope of the ¹⁴C retention: light intensity curve () were obtained. Supra-thermocline samples from the stratified water exhibited higher P _max values than corresponding subthermocline samples. Higher values of were also generally associated with samples from the supra-thermocline zone of the stratified region. Differences in the depth distribution of P _max and in the mixed water were small, except in the presence of a shallow thermocline. In one suite of samples from the stratified water, a diurnal increase in the P _max values of the supra-thermocline samples was observed. P _max values obtained from the samples from the mixed water were interpreted in relation to the distribution obtained from the samples from the stratified zone. Data from both the contrasting sites visited for one sample suite demonstrated a two-phase relationship between the chlorophyll a concentration and both P _max and . The rates of ¹⁴C retention of the first suite of samples were estimated by two techniques. The average differences in the retention were greater in samples from the sub-as opposed to suprathermocline zone. No trends were apparent in the smaples from the mixed waters.     

Effect of inorganic phosphorus on the growth rate of diatoms

The effect is studied of different inorganic phosphorus concentrations on changes in growth rates of 7 diatom species isolated from the plankton of the Black Sea. All species examined increase their cell-division rate with increasing phosphorus concentration in the medium. The phosphorus concentrations have been determined above which division rate is not limited by phosphorus content in the medium. The non-limiting concentration amounts to V/V _m =0.9. In the species studied, non-limiting concentrations ranged from 1 to 30 g P/1. The highest values were obtained for relatively large-sized species. The ratio of cell surface to volume tended to be inversely related to the growth-rate-limiting concentration. Minimal values of phosphorus content in cells have been calculated to range from 0.6 to 4.0·10^–17 g.at–P/^–3. Based on a comparison of phosphate levels in the Black Sea with experimentally-derived rate-limiting concentrations, it is concluded that phosphorus does not limit the reproduction rate of phytoplankton in the more productive regions of the Black Sea or in the lower strata of the euphotic zone.     

Regional studies of daily,seasonal and size fraction variability in ammonium remineralization

Rates of ammonium remineralization were determined using a ¹⁵N isotope dilution technique for two oceanic regions, one coastal region, and one estuarine region, covering a wide range of ambient nutrient, light, and temperature conditions. Results showed that NH ₄ ⁺ assimilative and regenerative fluxes were primarily in balance, even when the ambient nitrogenous pool was completely dominated by NO ₃ ^- . Variations in uptake and remineralization rates relative to time of day and season were also determined. Size fraction studies at several of the sites showed that the smallest size fraction (<10 m) was usually the most important in remineralizing NH ₄ ⁺ , and the importance of the apparent bacterial fraction (<1 m) may increase following blooms. The results support the concept that, over a wide variety of conditions, the fluxes of NH ₄ ⁺ remineralization and uptake are tightly coupled; phytoplankton are able to utilize NH ₄ ⁺ at the rate that it is produced by heterotrophic processes.     

Relationship between phytoplankton cell size and the rate of orthophosphate uptake: in situ observations of an estuarine population

Orthophosphate uptake by a natural estuarine phytoplankton population was estimated using two methods: (1) ³²P uptake experiments in which filters of different pore sizes were used to separate plankton size-fractions; (2) ³³P autoradiography of phytoplankton cells. Results of the first method showed that plankton cells larger than 5 m were responsible for 2% of the total orthophosphate uptake rate. 98% of the total uptake rate occurred in plankton composed mostly of bacteria, which passed the 5 m screen and were retained by the 0.45 m pore-size filter. There was no orthophosphate absorption by particulates in a biologically inhibited control containing iodoacetic acid. Orthophosphate uptake rates of individual phytoplankton species were obtained using ³³P autoradiography. The sum of these individual rates was very close to the estimated rate of uptake by particulates larger than 5 m in the ³²P labelling experiment. Generally, smaller cells were found to have a faster uptake rate per m³ biomass than larger cells. Although the nannoplankton constituted only about 21% of the total algal biomass, the rate of phosphate uptake by the nannoplankton was 75% of the total phytoplankton uptake rate. Results of the plankton autoradiography showed that the phosphate uptake rate per unit biomass is a power function of the surface: volume ratio of a cell; the relationship is expressed by the equation Y=2x10^-11 X ^1.7, where Y is gP m^-3 h^-1 and X is the surface: volume ratio. These results lend support to the hypothesis that smaller cells have a competitive advantage by having faster nutrient uptake rates.     

The development of a natural plankton population in an outdoor tank with nutrient-poor sea water. I. Phytoplankton succession

Due to complex hydrodynamic and biological inhomogeneities, the phytoplankton species succession cannot be satisfactorily observed, apart from the seasonal blooms which occur in temperate waters. Large flexible plastic tanks have proved to be useful for such observations. In 1972, for 28 days, a phytoplankton succession in nutrient-poor water in the outer harbor of Helgoland was observed in a flexible plastic tank (3 m³). During this period, 3 phytoplankton biomass maxima were formed with many significant correlations. In the first 7 days the ammonia concentration decreased from over 6 to 2 mol 1^-1 for 14 days. The nitrate concentration remained in the range of 3–6 mol 1^-1 and then fell abruptly to 0–2 mol 1^-1. The phosphate concentration was about 0.1 mol 1^-1. Lauderia borealis dominated the first period, and its increase was significantly correlated with the decrease in ammonia. The diatom was succeeded by two dinoflagellates, Dinophysis acuminata and Prorocentrum micans. The last period of the experiment was characterized by a stronger development of Rhizosolenia species. The rapid recovery of the crop in the nutrient-poor water points to intensive remineralization processes. The irregular occurrence of ammonia near the surface was correlated with the appearance of Noctiluca miliaris at this depth. It is expected that repetitions of this type of experiment will permit further explanations of statistical correlations which are not yet clear. As a first step, in order to test hypotheses, a correlation analysis was employed to eliminate the statistically non-significant correlations.This work was supported by the Sonderforschungsbereich 94, Hamburg, of the Deutsche Forschungsgemeinschaft.     

Influence of irradiance on the nutritional value of two phytoplankton species fed to larval Japanese scallops (Patinopecten yessoensis)

Japanese scallop (Patinopectin yessoensis Jay) larvae grew faster and were larger after 18 d when fed a diet of high-light(HL)-grown Chaetoceros simplex or HL Pavlova lutheri relative to diets of the same phytoplankton species grown at low light (LL). When provided as saturating rations to larval scallop, these diets could be ranked: HL C. simplex>LL C. simplex>HL P. lutheri>LL P. lutheri. In both phytoplankton species, HL-grown cells contained more of the short-chain saturated fatty acid (FA), 16:0 than LL-grown cells. Scallop growth rates were a significant function of the amounts (mg g^-1 dry wt) and the proportions (as percentage of total FAs) of the FAs 14:0 and 14:0+16:0 (total saturated FAs) in their diet. The proximate biochemical composition of HL- versus LL-grown phytoplankton showed no significant differences in protein, total lipid, carbon, carbohydrate or nitrogen per cell which were consistently associated with the greater nutritional value of HL cells. In spite of this high variability in proximate composition, the larval growth rate was a significant function of the average carbon content, nitrogen content and cell volume of the phytoplankton cells. Increased amounts of the essential polyunsaturated FAs 20:5 3 and 22:6 3 in the phytoplankton were negatively correlated with larval scallop growth rates. Thus HL-grown phytoplankton cells were nutritionally superior to LL-grown cells. This nutritional superiority seems to be determined by the fatty acid composition of the cells which, in turn, is controlled by variation in irradiance. The general tendency of predator FA profiles to resemble that of their prey was not observed in larvae fed P. lutheri. The much greater amounts of 18:4 3, 20:5 3, and 22:6 3 FA in P. lutheri relative to C. simplex were not evident in the scallop larvae fed these cells.