Zooplankton distribution and diversity in a frontal area of the Aegean Sea

This study was carried out in the northern Aegean Sea during late summer and spring. The aim was to examine the spatial and temporal distribution of the zooplankton community across a dynamic frontal area and to investigate how the oceanographic heterogeneity structures the composition of the zooplankton assemblages. The low-salinity and cold Black Sea water coming from the Dardanelles Strait is modified by mixing with the underlying warm and saline Aegean water. These hydrological features result in a pronounced thermohaline front in the northern Aegean Sea throughout the year. In both seasons, zooplankton was collected using both 45 and 200 µm mesh plankton nets. A high abundance of zooplankton was observed in the surface layer at the stations closest to the Dardanelles Strait on the stratified side of the front. The zooplankton distribution and community structure in the northern Aegean Sea were strongly influenced by the hydrological features. The frontal structure acts as a boundary for the zooplankton community. The surface layer at the stratified stations had the lowest copepod diversity, from where it increased with depth and horizontally as the stratification weakened outside of the front. The total abundance of zooplankton collected with the 200 µm net was between two and 20 times lower than samples taken with the 45 µm net. The most pronounced differences were observed for the adults and copepodids of the small genera Oithona, Oncaea and Microsetella. Thus, to manage and understand the transfer of primary production up the food chain in the Aegean Sea, the smaller fraction of copepods should be taken into account in future investigations.     

The effect of zooplankton grazing on estuarine blooms of the toxic dinoflagellate Gonyaulax tamarensis

A series of short-term ^{in situ} experiments was conducted intwo Cape Cod embayments to estimate mortality rates of the toxicdinoflagellate, Gonyaulax tamarensis, resulting from grazingby zooplankton. Rates of grazing by the whole zooplankton communityand by specific zooplankton populations were measured at variouspoints in the G. tamarensis bloom cycle. The planktonic larvaeof the spionid polychaete Polydora ligni and the tintinnid ciliateFavella sp. were important grazers in the systems studied. Gonyaulax-specificclearance rates effected by Polydora ranged from 0.02 to 0.5ml individual^–1 h^–1; for Favella the range was aboutan order to magnitude lower. Peak population densities wereclose to 900 and 400 individuals 1^–1 for P. ligni andFavella, respectively. Whether measured directly or predictedas the product of individual clearance rates and numerical abundance,rates of grazing were often higher than estimated algal divisionrates in years when blooms failed to develop. A simulation modelcorroborated the results of the field study, demonstrating thatgrazing can be a significant source of mortality during blooms,and can suppress bloom development when grazers are abundant.     

Impact of temperature acclimation on photosynthesis in the toxic red-tide dinoflagellate Alexandriumfundyense (Ca28)

This study assessed the impact of temperature on the photosyntheticactivity in the dinoflagellate Alexandrium fundyense (Ca28)for cultures grown at 75 mol photons m^–2 s^–1 overa range of temperatures. Increasing light intensity under statictemperatures caused a 5-fold decrease in the maximum quantumyield for photosystern II (PSII) (F_vlF_m) Carbon fixation ratesmirrored high-light depressions in (F_vlF_m). Cells in the presenceof streptomycin showed an 83% recovery in (F_vlF_m); therefore,only a minor proportion of the decline in (F_vlF_m)was attributableto PSII damage by bright light. For cells transferred to highertemperatures, F. was less sensitive to high light, decreasingonly 20–40% compared to the 80–90% decrease observedfor cells incubated at their ambient growth temperature. Forcells shifted to higher temperatures, the rapid recovery phaseof (F_vlF_m) was not present; therefore, cells did not initiatedownregulation of PSII. Higher capacity to maintain electrontransport, as indicated by the quantum yields, was confirmedby enhanced carbon fixation. Shifts lower temperatures significantlyincreased PSII sensitivity to high light. Overall, these relationshipsreflect the synergy between photosynthetic light and dark reactionswhich are differentially impacted by changes in temperature.     

Photophysiological responses of the toxic red-tide dinoflagellate Gymnodinium breve (Dinophyceae) under natural sunlight

Because Gymnodinium breve Davis exhibits positive phototaxisand often accumulates at or near the air–water interface,the diurnal, in situ physiological responses of this red-tidetoxic dinoflagellate were investigated in both cultured andfield populations. Cultures were incubated outdoors in temperature-controlledwaterbaths, and allowed to acclimate to attenuated (i.e. withoutUV) natural irradiance. Five-liter aliquots of these photoacclimatedcultures were placed in cubitainers fitted with either a quartzwindow or a quartz window covered with acetate [providing exposureto photosynthetically available radiation (PAR) + UV or PAR-only,respectively], incubated in Sarasota Bay, Florida (USA), andthen assessed for diurnal variations in in vivo fluorescenceand in vitro pigmentation, lipid, carbohydrate and protein contentsover three sinusoidal photoperiods in weather varying from overcastto partly cloudy to extremely sunny. In addition, surface, fieldpopulations were assessed for diurnal variations in in vivofluorescence and in vitro pigmentation over two sinusoidal photoperiodsin extremely sunny weather. The maximum quantum yield for stablecharge separation at photosystem II (measured by variable fluorescence/maximumfluorescence) exhibited depressions that were roughly symmetricalabout solar noon on the overcast and partly cloudy days, butexhibited a pronounced hysteresis on the sunny days for boththe cultured and field populations. Induction and relaxationof the xanthophyll cycle over the course of the photoperiodduring the partly cloudy and sunny days resulted in stoichiometricallyinverse cellular accumulation of the xanthophyll cycle pigmentsdiadinoxanthin and diatoxanthin. Generally, only minor adjustmentsoccurred in the cellular chlorophyll a and fucoxanthin contents.No differences occurred between cultures exposed to PAR-onlyor PAR + UV treatments in the epoxidation state of the xanthophyllcycle pigments or in the maximum quantum yield for stable chargeseparation at photosystem II. Differences in oxygen productionrates and other biochemical parameters between cultures exposedto PAR-only or PAR + UV treatments were not directly attributableto UV, indicating that G. breve possesses an inherent UV resistanceand a robust photosynthetic capability.     

Evidence of a new toxin in the red-tide dinoflagellate Prorocentrum minimum

Several clones of Prorocentrum minimum were isolated from variousFrench Mediterranean and English Channel sites. Mouse testsperformed using methanolic extracts from cultures revealed aneurotoxic activity in four clones. The water-soluble toxindetected worked rapidly, killing mice within a few minutes athigh doses. Toxin production in cultures was observed duringthe phase of decline and appeared to be weak or nil during thegrowth phase. Toxin production was stimulated by associatedbacteria in the culture, but a clone rendered axenic remainedable to produce toxins. The potential risks of human poisoningfrom consumption of shellfish harvested during or after toxicblooms of P.minimum are discussed.     

Spatial and temporal distribution patterns of Cladocera in the Argentine Sea

In the marine sciences, continuous monitoring systems have been regarded as very useful tools to provide continuous high frequency measurements of many parameters. We analyse here a high frequency time series of temperature measurements recorded every 10 min between 1997 and 2004 in the macro tidal Seine estuary (France) by a Marel buoy, an automatic monitoring network for littoral environment. We have adapted multi-scale data analysis methods to deal with the many missing values present in the time series. A power spectral density analysis is performed over time scales spanning 5 decades, from 20 min to more than 7 years. A scale invariant behaviour of the form with β = 2.2 is revealed for scales below 5 h. Over this scaling range, we have performed structure functions analysis, and shown that the Seine river temperature data exhibit turbulent-like intermittent properties, with multifractal statistics. The multifractal exponents obtained possess some similarities with passive scalar turbulence results. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Phytoplankton distribution in the Caspian Sea during March 2001

Phytoplankton abundance, biomass and species composition of the Caspian Sea were evaluated by using samples collected from the Iranian (southern Caspian Sea) and southern Kazakhstan (eastern Caspian Sea) surface waters in March 2001. A total of 45 taxa were found in the samples (20 diatoms, 17 dinoflagellates and 8 others). Abundance and biomass of diatoms were high at the eastern stations, while dinoflagellates were dominant in terms of both biomass and abundance in the southern region. Average abundance and biomass were 40 000 ± 35 000 cell l⁻¹ and 580 ± 690 μg l⁻¹. The mean biomass value found here for the Middle and southern Caspian Sea in March are difficult to compare with the past due to limited information, but seems higher than previously registered values. Higher chlorophyll values were also apparent from the SeaWIFS images in 2001 compared to those in 1998. This is suggested to be due to decreased grazing of phytoplankton by zooplankton which is voraciously preyed by the recent invasive ctenophore Mnemiopsis leidyi.     

Phytoplankton distribution in the upwelling area off NW Africa

In spring 1968, the centre of the upwelling region off NW Africa was investigated using R. V. Meteor. The hydrographic structure in this area is characterized by irregular upwelling in space and time. The complicated distribution of the biological components (phytoplankton and dissolved plus particulate organic substances) is expressed only within the limits given by the resolving power of the system. Therefore, the basic values have been smoothed statistically to suppress accidental irregularities. The problematics of such manipulation are discussed. The phytoplankton distribution shows a pronounced maximum in the Cap Blanc region with extreme values in the range of > 50µg C · l^–1 in the surface layer. A decrease was observed not only in the direction of the open sea, but also along the coast. This distribution pattern is repeated (1) at the depth of the vertical phytoplankton maximum, which is near the surface in the Cap Blanc region; (2) at the large proportion of diatoms characteristic for upwelling water; (3) at the phytoplankton stock in the euphotic zone with maximum values up to > 0.5 g C · m^–2. It is calculated that (minimum) daily production is almost equal to the stock. Processes of decomposition demonstrate the upwelling by regionally different relations between O₂ consumption and PO₄ liberation. An anchor station situated near the centre of the upwelling area indicates another small-scale development of phytoplankton as demonstrated by distribution charts of the total area. Pronounced daily rhythms are revealed after extensive smoothing procedures.     

The importance of small-sized copepods in a frontal area of the Aegean Sea

Distribution, production and grazing of the copepod communitywere investigated in the northern Aegean Sea, which is characterizedby a permanent thermohaline front. Cruises were conducted alonga transect crossing the frontal area during spring and latesummer. Biomass and production of autotrophs were measured bysize fractionation and heterotrophic nanoflagellates and ciliateswere also studied. Copepod biomass, production and grazing impacton the phytoplankton and ciliate populations were estimated.The copepod community was sampled with a 45 µm net toinclude the smallest species and their developmental stages.The size, structure and distribution of the phytoplankton implythat most carbon was fixed by picoplankton during both seasonsand throughout the study area. The partitioning of carbon amongthe different plankton compartments was not a broad-based pyramidand the biomass of heterotrophs was higher than that of autotrophs,except in the non-frontal region during spring. Copepod biomasswas substantially higher in the frontal area. Our results showedthat the small-sized copepods (calanoids and cyclopoids) dominatedin terms of biomass and production, but also had a greater influenceon the efficiency of the trophic coupling between the primaryproducers and the protozooplankton than the larger species,stressing their importance in the northern Aegean Sea and theEastern Mediterranean in general.     

Circadian rhythms of cell cycle processes in the marine dinoflagellate Gonyaulax polyedra

The circadian expression of several growth properties was examined in the dinoflagellate Gonyaulax polyedra under constant light and light-dark conditions. The cell concentration, mean cell volume and rate of DNA synthesis varied in a circadian rhythm, with the primary maximum of cytokinesis and DNA synthesis at about dawn. High rates of cell mortality also occurred during phases related to events of cytokinesis, and may be important in the expression of the other rhythms and in "red tide" generation. Flow-cytofluorimetric analysis indicated that cells of a population contain either a relatively high or a low amount of DNA, but the proportion of cells in each of these classes and the absolute amount of DNA in each cell varied rhythmically depending on the circadian time. This DNA-distribution pattern was unlike the usual G1-S-G2+M pattern typical of eukaryotic cell populations. Isotopically labelled thymidine, used as a marker of DNA synthesis, was continuously incorporated; but the incorporation rate fluctuated in a regular pattern that repeated each circadian period.     

Growth rates and elemental composition of Alexandrium monilatum, a red-tide dinoflagellate

The combined effects of temperature and salinity on growth of Alexandrium monilatum were studied in laboratory cultures. This toxic, red-tide dinoflagellate grew faster with higher temperatures, up to a maximum of approximately 1 division per day at 31 °C. Salinities above 15 psu had a lesser effect on growth rate, as might be expected for an estuarine species. Growth rates of cultures exposed to natural light and temperature fluctuations were comparable to laboratory cultures. The minimum N cell quota suggested that high N flux would be required to support bloom development. A literature survey of documented A. monilatum blooms indicated that within US waters, blooms occur in July–September in nearshore or estuarine regions of the Gulf of Mexico and the Florida Atlantic coast. Temperature and salinity measured during blooms correspond to the optimal growth conditions of the laboratory cultures. Nevertheless, the occurrence of A. monilatum blooms is sporadic compared to the occurrence of seemingly optimal growth conditions. Laboratory growth experiments predict when blooms of this species are unlikely due to low growth rates, but so far cannot predict individual blooms.     

Identification of a poison in toxic scallops from a Gonyaulax tamarensis red tide

The poison in stored extracts of the hepatopancreas of scallops that became toxic during a red tide caused by $\text{Gonyaulax tamarensis}$ has been extensively purified and the product shown to be identical to saxitoxin in its toxicity to mice and TLC behavior in six solvent systems.     

Discovery of the toxic dinoflagellate Pfiesteria in northern European waters.

Several dinoflagellate strains of the genus Pfiesteria were isolated by culturing techniques from sediment samples taken in the Oslofjord region of Norway. Pfiesteria piscicida, well known as a fish killer from the Atlantic coast of America, was identified by genetic methods and light microscopy. The related species Pfiesteria shumwayae was attracted from the sediment by the presence of fish, and has proved toxic. This present survey demonstrates the wide distribution of these potentially harmful species, but so far they have not been connected with fish kills in Europe.     

Immunogold labeling of organelles in the bioluminescent dinoflagellate Gonyaulax polyedra with anti-luciferase antibody

A polyclonal antibody directed against the luciferase of the luminous dinoflagellate Gonyaulax polyedra labels both dense vesicles and trichocyst sheaths, as visualized in the electron microscope after treatment of antibody-reacted sections with an immunogold probe. Because of their similar size, shape and localization, the dense vesicles seen with the electron microscope are postulated to correspond to autofluorescent particles seen with the fluorescent microscope, which are known to be the origin of bioluminescent flashes in this alga. The explanation for the trichocyst sheath-specific labeling is less evident. The possibility that a second antibody of different specificity is involved has not been excluded but seems unlikely. Alternatively, it could be due to a different but antigenically cross-reacting protein. But the possibility that luciferase itself occurs in two different organelles is intriguing and consistent with previous biochemical studies of cell extracts.     

Comparative culture and toxicity studies between the toxic dinoflagellate Pfiesteria piscicida and a morphologically similar cryptoperidiniopsoid dinoflagellate

A series of fish bioassays using cultures of the toxic dinoflagellate, Pfiesteria piscicida and a cryptoperidiniopsoid dinoflagellate indicated various degrees of toxicity for Pfiesteria piscicida and no toxicity by the cryptoperidiniopsoid. P. piscicida maintained toxicity in the presence of live fish, and this toxicity was perpetuated following a series of inoculations to other culture vessels. Differences in the onset and magnitude of the fish deaths occurred, requiring 16 days for the initial fish death when using P. piscicida from a culture that had previously been maintained on algal cells, to kills within hours when using a culture that had recently (previous day) killed fish. Autopsies of moribund fish from the test and control fish bioassays indicated a general lack of bacterial infection, which ensued following death of other autopsied fish. Moreover, bacterial comparisons of waters in the fish bioassay and control fish cultures indicated that similar bacterial concentrations were present. Neither oxygen or ammonia levels were determined to be factors in the fish death. Life stages of a cryptoperidiniopsoid dinoflagellate from Virginia estuaries were also identified, including motile zoospore, gametes, planozygote, amoebae, and cyst stages. The cryptoperidiniopsioid did not initiate fish deaths in bioassays conducted over a 14-week period at zoospore concentrations of ca. 700-800 cells ml(-1). Elemental X-ray analysis of the scales from cysts of this dinoflagellate and P. piscicida indicate that they both contain silicon. Overall, the data from this study demonstrate that the cryptoperidiniopsoid possesses several similar life stages and feeding patterns as P. piscicida, but was not toxic to fish.     

Ordered distribution of modified bases in the DNA of a dinoflagellate.

In DNA of the dinoflagellate Crypthecodinium cohnii, 38% of the thymine is replaced by the modified base 5-hydroxymethyluracil, and approximately 3% of the cytosine is replaced by 5-methylcytosine. Both of the modified bases are non-randomly distributed in the DNA. Determinations of 3' nearest neighbors show that HOMeU is preferentially located in the dinucleotides HOMeUpA and HOMeUpC. Pyrimidine tract analysis shows that HOMeU is also greatly enriched in the trinucleotide purine-HOMeU-purine. As in other eukaryotes, methylcytosine in C. cohnii DNA occurs predominantly in the dinucleotide MeCpG. By analysis of restriction endonuclease digestion patterns of C. cohnii total DNA and ribosomal DNA, we have found that the central CpG dinucleotides in the sites for the enzymes Hpa II (CCGG) and Hha I (GCGC) are extensively methylated in both total DNA and ribosomal DNA. Results of digestion with Ava I, however, indicated that not all CpG dinucleotides in the sequence CCTCGGAG are methylated in C. cohnii DNA.     

Phase of the circadian clock is accurately transferred from mother to daughter cells in the dinoflagellate Gonyaulax polyedra.

In the first cycle following transfer from a 12 h light-12 h dark cycle (LD12:12) to constant darkness (DD), the standard deviation in circadian phase among individual clocks in populations of Gonyaulax polyedra is approximately 60 min. When a culture is transferred to constant light conditions (LL) from an LD 12:12 cycle, the standard deviation increases in the first 2-3 d, but then remains unchanged, suggesting a lack of observable desynchronization in LL after the transient period. The synchrony in a cell population is preserved even after several cell divisions. The results indicate that variations in period among cells are small, that the period of an individual clock does not fluctuate randomly from day to day, and that the circadian phase of a mother cell is faithfully passed to the clocks of the daughter cells.