Dinoflagellate cyst production at a coastal Mediterranean site

To assess the diversity and seasonality of dinoflagellate cystproduction, surface sediment and trap samples were studied inthe Gulf of Naples (Mediterranean Sea). A total of 59 differentcyst morphotypes were recorded. At the stations within the 70m isobath, sediment assemblages were dominated by calcareousPeridiniales (66–79%), while at the deepest stations non-calcareousPeri-diniales attained the highest percentages (40–49%).The sediment trap sampling, carried out fortnightly over twoannual cycles, revealed high production rates (up to 1.7 x 10⁶cysts m^–2 day^–1) from spring to late autumn of bothyears, with a distinct seasonal production pattern. Althoughrather similar in species composition, the total cyst flux differedmarkedly between the 2 years (1.26 and 0.55 x 108 cysts m^–2year^–1, respectively). Species-specific production patternswere observed: some species formed cysts over several months,others in restricted periods of the year. Cyst-forming speciesconstituted a small part of the planktonic dinoflagellate populationsrecorded in the area. A coupling between the trap material andsurface water plankton was observed for calcareous Peridiniales.This sampling approach allowed the detection of some speciesnever recorded before in the gulf, including two potentiallytoxic species: Alexandrium andersoni and Gymnodinium catenatum-likespecies.     

Relationship between vegetative cells and cyst production during Alexandrium minutum bloom in Arenys de Mar harbour (NW Mediterranean)

A recurrent Alexandrium minutum bloom in the Arenys de Mar harbour(Catalan coast, North Western Mediterranean) was monitored inorder to establish the relationship between vegetative cellsand cyst production. The bloom lasted from January 21 to February24, 2002 and reached cell concentrations of up to 47 x 10⁶ cellL^–1. Two aspects related to the resting cysts depositionwere studied: (i) production of resting cysts during the bloomperiod (by means of sediment traps) and (ii) distribution ofresting cysts in the sediment after the bloom (May 2002). Cystformation in Arenys clearly started in a period with high vegetativecell densities in the water column. Once production was initiatedencystment fluxes remained constant for two weeks, and coveringthe periods of maintenance and decline of the bloom. High cystfluxes (up to 6000 cysts cm^–2 day^–1) were quantifiedas a result of the high vegetative cell concentration. Moreover,encystment occurring in less than 1% of the total populationindicates that most of the cells are not involved in restingcysts formation. A comparison of the resting cyst flux valuesobtained from the sediment traps and the resting cyst concentrationsin surface sediment (628–3270 cysts cm^–3) threemonths later, revealed that the number of cysts in the sedimentdecreased during that time. The studies of excystment showeda high germination percentage (91%) and germling viability (100%).These data, together with the resting cyst distribution in thesediment, are important in assessing the role of resting cystsin the bloom dynamics of A. minutum in confined waters.     

Distribution, dynamics and in situ seeding potential of Scrippsiella hangoei (Dinophyceae) cyst populations from the Baltic Sea

The distribution and seasonal dynamics of cyst populations ofthe spring bloom dinoflagellate Scrippsiella hangoei were studiedin surface sediments on the southwest coast of Finland, BalticSea. In situ germination was assessed by monitoring the fractionof empty cysts and chlorophyll a fluorescence in cyst populationsat different coastal sites throughout the annual cycle. Scrippsiellahangoei resting cysts were widely distributed in the study areaand occurred in exceptionally large numbers (magnitudes of 10⁴–10⁶cysts cm^–3) at all sampling locations between the innermostparts of the coastal archipelago and the open Gulf of Finland.The decreases in cyst number in winter and the increases occurringin late spring reflected the dynamics of germination and encystmentof the species. Chlorophyll fluorescence appeared in mid-winterin ~40% of cysts from well-aerated basins and 6–15% ofcysts from temporarily anoxic sediments. A generally low increasein the proportion of empty cysts indicated that only a partof the potentially germinable cysts actually germinates. Giventhe high cyst concentrations in the sediments, the potentialfor germination is considerable, despite the environmentallyand physiologically determined losses. In contrast, the sizeof the vegetative inoculum is very low, indicating that thesurvival of germlings is problematic under harsh winter conditions.This is an unusual life cycle strategy; however, the early releaseof cells into the water column provides a high probability forsuccessful bloom initiation under the unpredictable meteorologicalconditions in winter and early spring, which often lead to thesudden onset of favourable growth conditions.     

Development and fate of Eutreptiella gymnastica bloom in nutrient-enriched enclosures in the coastal Baltic Sea

Response of the phytoplankton community to bottom-up (nutrients,organic carbon source) and top-down (fish) manipulations, bothsingly and together, were studied daily during a 3 week periodin July 1993 by using eight 50 m³ mesocosms in the coastal northernBaltic Sea. Nutrient additions (once per week) invoked a seriesof blooms of Eutreptiella gymnastica Throndsen (Euglenophyceae)(up to 13 x 10³ cells ml^–1) which formed the major part(60–90%) of the total autotrophic biomass. After rapiddepletion of nutrients (2–3 days) from the surface layer(0–6 m) downwards migration and a subsequent peak of E.gymnasticain the lower part of the water columns (6–12 m) followed.Settled material collected from the bottom of the enclosurescontained a considerable amount of E.gymnastica cells and restingcysts. Nevertheless, sinking loss rates of E.gymnastica wereestimated to be less than 1% day^–1 of the suspended cellnumbers. The fate of E.gymnastica blooms was estimated to begrazing through mesozooplankton. However, provided the nutrientsare plentiful in the water column, the growth potential of E.gymnasticaappears to exceed the ambient grazing pressure. If the nutrientsbecome depleted, it seems to be effectively controlled by mesozooplanktongrazing, which is probably limiting the likelihood of massiveE.gymnastica blooms in the coastal Baltic Sea. Our study suggeststhat E.gymnastica appears to be a fast-growing fugitive (bloom)species with flexible behavioural (vertical migration) and lifehistory (cyst formation) adaptations which is able to exertdominant role and direct trophic relations similar to otherbloom species adapted for decaying turbulence and high nutrientenvironments.     

Bloom of Emiliania huxleyi (Prymnesiophyceae) in the western South Atlantic Ocean

A monospecific bloom of the coccolithophorid Emiliania huxleyi(Lohmann) Hay & Mohler (Prymnesiophyceae) was detected offRio de La Plata at 36°S and 54°W in November 1989. Thecell densities observed were up to 6x10⁵ cells I^–1. Thisis the first record of a bloom of E.huxleyi in the area.     

The effects of temperature, growth medium and darkness on excystment and growth of the toxic dinoflagellate Gymnodinium catenatum from northwest Spain

The chain-forming dinoflagellate Gynmodinium catenatum Grahamcauses recurrent outbreaks of paralytic shellfish poisoning(PSP) in the Galician Rias Bajas (northwest Spain). A sedimentsurvey in Ria de Vigo in April 1986 indicated that the highestconcentrations of cysts of this species were located in themiddle sections of the ria, with maximum abundance of 310 cystscm^–3. The effects of temperature, growth medium compositionand irradiance on the germination of laboratory-produced restingcysts were investigated. Newly formed cysts required very littletime for maturation, as excystment was possible within 2 weeksof encystment. Growth media did not affect germination success.In contrast, the excystment rate was retarded signifiantly indarkness. Germination was also strongly affected by temperature,with {small tilde}75% excystment success at 22–28°Cand little or no germination below 11°C after 1 month ofincubation. In culture, the optimum growth rate of vegetativecells was between 22 and 28°C, the highest rate being 0.53divisions day^–1 at 24°C. Growth did not occur at temperatures< 11°C or >30°C. These results are important withrespect to the different hypotheses proposed to explain theinitiation of G.catenatum blooms in the Galician Rias Bajasand Northern Portugal. The pattern of G.catenatum bloom developmentalong this coast has been related to seasonal upwelling in thearea, with major blooms occurring during the autumn as warmeroffshore surface water is transported towards the coast whenupwelling relaxes. The landward transport of established offshorepopulations of G.catenatum with the warm surface layer remainsa viable explanation for the observed blooms within the rias,but alternatively, our data suggest that cysts within the riascan provide the inoculum population at times conducive to growthand bloom formation. Even though newly formed G.catenatum cystshave a very short maturation time and can germinate in darknessacross a wide temperature range, bloom development will be significantonly during the late summer and early autumn, since in othermonths light levels at the sediment surface and temperaturesthroughout the water column are too low for significant germinationor growth.     

Comparison of east and west chlorophyll a standing stock and oceanic habitat along the Transition Domain of the North Pacific

Chlorophyll (Chl) a was measured every 10 m from 0 to 150 min the Transition Domain (TD), located between 37 and 45°N,and from 160°E to 160°W, in May and June (Leg 1) andin June and July (Leg 2), 1993–96. Total Chl a standingstocks integrated from 0 to 150 m were mostly within the rangeof 20 and 50 mg m^–2. High standing stocks (>50 mg m^–2)were generally observed westof 180°, with the exceptionof the sporadic high values at the easternmost station. Thetotal Chl a standing stock tended to be higher in the westernTD (160°E–172°30'E) than in the central (175°E–175°W)and eastern (170°W–160°W) TD on Leg 1, but thesame result was not observed on Leg 2. It was likely that largephytoplankton (2–10 and >10 µm fractions) contributedto the high total Chl a standing stock. We suggest that thehigh total Chl a standing stock on Leg 1, in late spring andearly summer, reflects the contribution of the spring bloomin the subarctic region of the northwestern Pacific Ocean. Thedistribution of total Chl a standing stock on Leg 2 was scarcelyaffected by the spring phytoplankton bloom, suggesting thattotal Chl a standing stock is basically nearly uniform in theTD in spring and summer. Moreover, year-to-year variation inthe total Chl a standing stock was observed in the western TDon Leg 1, suggesting that phytoplankton productivity and/orthe timing of the main period of the bloom exhibits interannualvariations.     

Seasonal dynamics in the abundance of Micromonas pusilla (Prasinophyceae) and its viruses in the Gulf of Naples (Mediterranean Sea)

To assess the role of viruses in the bloom dynamics of Micromonaspusilla in the Gulf of Naples (Mediterranean Sea), variationsof host and virus abundance were followed over one annual cycleand in late winter–spring of three consecutive years.Micromonas pusilla was recorded from autumn to spring, withpeak values up to 6.6 x 10³ cells ml^–1, but was undetectablein summer. Free M.pusilla viruses were detectable in all seasons,with concentrations from 0.02 viruses ml^–1 to 1.9 x 10³viruses ml^–1, exceeding host abundances only in one case.We found a great intraspecific variability in host susceptibilityto viruses present in natural samples, with viral titres rangingover one or two orders of magnitude for the same samples incubatedon different M.pusilla strains. Over the winter–springperiods, a highly dynamic situation was evident, with wide fluctuationsfor both host and virus abundances from one week to another.In some cases, peak host concentrations were accompanied byan increase in viral numbers, whereas in other cases the respectivefluctuations were uncoupled. Although fluctuations of M.pusillaabundance could be influenced by viral infection, there wasno evidence that viruses were able to terminate host blooms.The summer decline of M.pusilla populations did not appear tobe related to the impact of viral infection.     

A recurrent and localized dinoflagellate bloom in a Mediterranean beach

A recurrent, prolonged and singular bloom of Alexandrium tayloriBalech in an open beach (La Fosca, Spain, NW Mediterranean)is described. Alexandrium taylori appears at several placesalong a wide area of the NW Mediterranean (Costa Brava) duringthe summer, reaching concentrations up to 10⁵ cells l^–1,but it only proliferates persistently, massively (densities>10⁶ cells l^–1) and recurrently during August in LaFosca beach. The A.taylori bloom can be considered a manifestationof large-scale proliferation in a restricted area, where couplingbetween resting cysts in the sediment and bloom outbreak isnot a major factor compared to the interaction of local environmentalconditions with the planktonic organism's life history. Fromobservations of environmental conditions (the environmentalwindow) and the multiscale spatio-temporal distributions andlife history of A.taylori, we describe the bloom dynamics andanswer some critical questions about the different phases ofthe bloom. Some of these answers are: (i) the source of theA.taylori population is widespread offshore and is not locateddirectly at the beach; (ii) high cell densities are reachedand maintained with a moderate in situ growth and low loss rates;(iii) temporary cysts act as a reserve of the population.     

Density and distribution of bacterioplankton and planktonic ciliates in the Bering Sea and North Pacific

The total number of planktonic bacteria in the upper mixed layerof the Bering Sea during the late spring-early summer periodranged between 1 and {small tilde}4 x 10⁶ ml^–1 (biomass10–40mg C m^–3). In the northern Pacific, along 47–52⁶N,the corresponding characteristics of the bacterioplankton densityin the upper mixed water layer were: total number 1–2x 10⁶ cells ml^–1 and biomass 15–46mg C m^–3Below the thermocline at 50–100 m, the density of bacterioplanktonrapidly decreased. At 300 m depth, it stabilized at 0.1–0.2x 106 cells ml^–1. The integrated biomass of bacterioplanktonin the open Bering Sea ranged between 1.2 and 3.6 g C m^–2(wet biomass 6–18 g m^–2) Its production per dayvaried from 2 to 23 mg C m^–3 days^–1 in the upper0–100 m. The numerical abundance of planktonic ciliatesin this layer was estimated to be from 3 to l0 x 10³ cells l^–1,and in the northern Pacific from 0.4 to 4.5 x 10³ l^–2.Their populations were dominated by naked forms of Strombidium,Strombilidium and Tontonia. In some shelf areas, up to 40% ofthe total ciliate population was represented by the symbioticciliate Mesodinium rubrum. The data on the integrated biomassof basic groups of planktonic microheterotrophs are also presented,and their importance in the trophic relationships in pelagiccommunities of subarctic seas is discussed.     

The seasonal abundance of the phototrophic ciliate Mesodinium rubrum in Southampton Water, England

The abundance of the marine phototrophic planktonic ciliateMesodinium rubrum was monitored throughout an annual cycle attwo stations in the Southampton Water estuary. Seasonal changesin the concentration of nitrate, ammonia and phosphate weremonitored both at the inner estuary station (NW Netley) andouter estuary station (Calshot). Nutrient levels in the winterwere similar at both stations, and were diminished during sequentialdiatom blooms dominated initially by Sketetonema costatum andthen by Rhizosolenia dclicatula. Nitrate was reduced to a seasonalminimum in the outer estuary following these spring diatom blooms,but in the inner estuary was sustained >500 µg I^–1until the onset of the M.rubrum bloom. During the developmentof a visible red tide of M.rubrum in June/July at NW Netley,nutrient concentrations were considerably reduced. Cell numbersof M.rubrum varied between 2 and 3 cells ml^– during winterto >400 cells ml^–1 during the bloom at NW Netley, whereasat Calshot cell abundance did not increase above 25 cells ml^–1at any time of the year. At NW Netley, dense accumulations ofthe ciliate occurred over restricted depth intervals duringthe bloom and possible factors influencing the observed verticaldistribution of cells are considered. ¹Present address: Biology Department, Faculty of Science, AddisAbaba University, PO Box 1176, Addis Ababa, Ethiopia     

L-Leucine Transport in Isolated Protoplasts of Vinca Suspension Culture: Characterization of Uptake

The uptake of L-leucine into Vinca protoplasts was studied undervarious conditions. The uptake was highly pH-dependent, withthe optimal pH between 3.0 and 4.0. The uptake was also energydependent, since azide, 2,4-dinitrophenol (DNP), carbonyl cyanidem-chlorophenyl hydrazone (CCCP), and iodoacetate inhibited theuptake. Oligomycin, N,N'-dicycIohexyI carbodiimide (DCCD) andvanadate, but not ouabain, inhibited the uptake, suggestingthat ATPase for H⁺ electrogenic extrusion was necessary to theuptake of L-leucine. The uptake showed stereospecificity, butwas partially inhibited by other L-amino acids. A kinetic studyof the uptake showed that the uptake was multiphasic with threesaturable phases and one unsaturable phase which occurred atconcentrations of L-leucine over 1 mM. The K_m values of thethree affinity sites were 1.4 x 10^–3 M, 1.3 x 10^–4M, 4.3 x 10^–5 M; the maximum velocity values were 3.3x 10^–8, 4.5 x 10^–9, 1.8 x 10_–9 mol/10 min/4x 10⁶ cells. (Received April 18, 1981; Accepted August 25, 1981)     

Photosynthetic characteristics of Dinophysis norvegica Claparede & Lachmann, a red-tide dinoflagellate

The relationships between photosynthesis and photosyntheticphoton flux densities (PPFD, P-l) were studied during a red-tideof Dinophysis norvegica (July-August 1990) in Bedford Basin.Dinophysis norvegica, together with other dinoflagellates suchas Gonyaulax digitate, Ceratium tripos, contributed {small tilde}50%of the phytoplankton biomass that attained a maximum of 16.7µg Chla 1^– and 11.93 10⁶ total cells I^–1.The atomic ratios of carbon to nitrogen for D.norvegica rangedfrom 8.7 to 10.0. The photosynthetic characteristics of fractionatedphytoplankton (>30 µm) dominated by D.norvegica weresimilar to natural bloom assemblages: o (the initial slope ofthe P-l curves) ranged between 0.013 and 0.047 µg C [µgChla]^–1 h–1 [µmol m^– s^–1]^–1the maximum photosynthetic rate, p^B_m, between 0.66 and 1.85µg C [µghla]^–1 h^–1; l_k (the photoadaptationindex) from 14 to 69 µ,mol m^–2 s^–1. Carbonuptake rates of the isolated cells of D.norvegica (at 780 µmolm^–2 s^–1) ranged from 16 to 25 pg C cell^–1h^– and were lower than those for C.tripos, G.digitaleand some other dinoflagellates. The variation in carbon uptakerates of isolated cells of D.norvegica corresponded with P^B_mof the red-tide phytoplankton assemblages in the P-l experiments.Our study showed that D.norvegica, a toxigenic dinoflagellate,was the main contributor to the primary production in the bloom.     

The Effects of Light Intensity and External Potassium Level on Root/Shoot Ratio and Rates of Potassium Uptake in Perennial Ryegrass (Lolium perenne L.)

Loliun perenne L. (cv.S. 23) was grown on vermiculite in winterin a heated greenhouse for 8 weeks under factorial combinationsof two potassium regimes (nominally 6 parts/10⁶ and 156 parts/10⁶in Hewitt's solution) and three densities of artificially supplementedvisible radiation flux (36.1, 7.3, and 2.2 W m^–2). Growthand potassium uptake were studied through the calculation ofvarious growth functions from fitted curves. There was little effect of potassium treatment but the experimentalmaterial responded markedly to light. Leaf-area ratio in thethree treatments showed extreme plasticity in increasing from2–3 x 10^–2 through 6 x 10^–2 to 8–9 x10^–2 m² g^–1 as light intensity decreased. Correspondingdecreases in unit leaf rate, however, caused over-all reductionsin relative growth rate. Specific absorption rates for potassium (A_K, dry-weight basis)were strongly reduced at the lower light intensities but alsodisplayed complex ontogenetic drifts. Values of the allometricconstant, k (the ratio of root and shoot relative growth rates),decreased from c. 0.7 at 36.1 W m^–2 through c. 0.3 at7.3 W m^–2 to a value not significantly different fromzero (P < 0.05) at 2.2 W m^–2. In material grown under the two higher light intensities a constantinverse relationship was found between the mass ratio of rootand shoot and the corresponding activity ratio. The resultsconform to this model: Mass ratio = –0.001+45.0 (1/activityratio) where activity ratio is expressed as specific absorptionrate for potassium (in µg g root^–1 h^–1)/unitshoot rate (rate of increase of whole-plant dry weight per unitshoot dry weight, in mg g shoot^–1 h^–1). The implicationsof this relationship are discussed.     

Floodwater Oxygen Content, Ethylene Production and Lenticel Hypertrophy in Flooded Mango (Mangifera indica L.) Trees

A system was developed to test the effects of floodwater O₂concentration on ethylene evolution and stem lenticel hypertrophy,and the effects of exogenous ethylene on stem lenticel hypertrophyin mango (Mangifera indica L.) trees. Dissolved O₂ concentrationsof 1–7x10^–9 m³ m^–3 generally resulted in hypertrophyof stem lenticels within about 6 d of flooding, whereas floodwaterO₂ concentrations of 13–15 x 10^–9 m³ m^–3 delayedhypertrophy until about day 9. After 14d of flooding, therewere more than twice the number of hypertrophied lenticels pertree with floodwater O₂ concentrations of 1–7 x 10^–9m³ m^–3 than with floodwater O₂ concentrations of 15 x10^–9 m³ m^–3. Ethylene evolution from stem tissueimmediately above the floodline increased 4- to 8-fold in treesexposed to floodwater O₂ concentrations of 1–2 x 10^–9m³ m^–3, increased 2-fold for trees exposed to floodwaterO₂ concentrations of 6–7 x 10^–9 m³ m^–3, butremained constant with floodwater O₂ concentrations of 13–15x 10^–9 m³ m^–3. Plants maintained in highly oxygenatedfloodwater (13–15 x 10^–9 m³ m^–3), and givenexogenous ethylene developed many hypertrophied lenticels, whereasplants in highly oxygenated water and not given ethylene developedfewer or nohypertrophied lenticels. These data suggest thatethylene plays a role in promotion of stem lenticel hypertrophyin flooded mango trees, and that floodwater dissolved oxygenconcentration can regulate stem lenticel hypertrophy and ethyleneevolution in this species. Key words: Flooding, hypoxia, hypertrophic cell swelling     

Horizontal distribution of Thetys vagina Tilesius (Tunicata, Thaliacea) in the Japan Sea during spring 2004

The occurrence of the salp Thetys vagina was observed in theJapan Sea during spring 2004. Catches up to 187 kg wet weight(WW) per 2.18 x 10⁵ m³ (equal to 0.9 g WW m^–3) were collectedwith 10-m diameter surface-water otter trawl nets. The horizontaldistribution indicated that the high biomass was related tothe area with high chlorophyll a (Chl a) concentration, whichwas located around the subarctic front with the warm TsushimaCurrent. Five prey taxa were identified from the gut contentsof individuals from the high Chl a area. The diatom Coscinodiscusspp. (13–55 µm in diameter) dominated numerically.Another significant prey was the large diatom Coscinodiscuswailesii (219–313 µm) that is an indicator of thespring bloom in this area. The mass occurrence of T. vaginathus appears related to phytoplankton availability, though themechanisms remain uncertain.     

Modelling of autumn plankton bloom dynamics

A simple system of parametrically forced ordinary differentialequations is used to model autumn phytoplankton blooms in temperateoceans by a mechanism involving deepening of the upper mixedlayer. Blooms are triggered provided the increase in nutrientsin the mixed layer is rapid within the first few days of deepeningand provided light-limited phytoplankton growth rate is relativelyhigh. Blooms exist as transient trajectories between quasi-equilibriumstates, rather than as bifurcations of steady states; thereforevery gradual deepening cannot trigger blooms. Very rapid deepeningalso prevents blooms due to the deleterious effect on phytoplanktongrowth rate. The mechanisms identified by this simple modelare vindicated by considering alternative grazing and deepeningregimes and by comparison with a more ecologically complex model(Fasham, 1993, in The Global Carbon Cycle, Springer-Verlag).Modelled estimates of primary productivity from both the simplemodel and the complex model parameterized for Ocean WeatherStation ‘India’ are around 0.5 g C m^–2 day^–1during the autumn bloom, therefore comprising a significantcomponent of annual production in temperate areas.     

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.     

Dinoflagellate cyst records in recent sediments from Daya Bay, South China Sea

Nine sediment cores of 8–26 cm in length were collected from two basins of Daya Bay, the South China Sea, by Tokyo University Fisheries Oceanography Laboratory core sampler in August 2001 to investigate the distribution of dinoflagellate resting cysts. In the present study, 51 different cyst morphotypes representing 22 genera were identified from 65 sediment samples. Among them, there were 21 autotrophic species and 30 heterotrophic ones. Cyst species richness in each sample varied from 12 to 29, while the values of Shannon‐Weaver diversity index (H′) were between 0.15 and 4.13. There were an obvious increase in both species richness and values of H′in 2–6 cm sediments. Cyst concentrations varied from 154 to 113 483 cysts per gram dry weight sediment, and were much higher in upper sediments. Scrippsiella trochoidea was the most dominant cyst type, which took up over 90% of cyst assemblages in the upper sediments. The abrupt increase of S. trochoidea cysts in the surface sediments reflected the bloom of this species in Daya Bay in 2000. The results from cyst assemblages showed some trend of changes in water quality in this area, and indicated a typical type of pollution caused by cultural eutrophication, which started in the 1980s and greatly accelerated in the middle of 1990s. Cysts of Alexandrium, mainly those of Alexandrium catenella and Alexandrium tamarense complex, occurred frequently and abundantly in this area, with the highest concentration and relative frequency of 503 cysts per gram dry weight sediment and 22.3%, respectively. The high abundance of Alexandrium cysts provided rich ‘seed bed’ for Alexandrium blooms and was also an important source of paralytic shellfish poisoning toxins, especially in winter.     

Summer phytoplankton succession in Ellis Fjord, eastern Antarctica

Spring phytoplankton communities in the water column of EllisFjord are characterized by diatoms originating from the bottomsea-ice strand community. Upon ice breakout in early summer,these are replaced by blooms of the phytoflagellates, Phaeocystispouchetii, Cryptomonas cryophila, Pyramimonas gelidicola, silicoflagellatcsand dinoflagellates. The narrow entrance of the fjord and thedevelopment of summer stratification is probably limiting theavailability of nutrients and containing the magnitude of thesmall bloom (maximum 2.8 10⁶ cells l^–1).