The IJsselmeer and its phytoplankton--with special attention to the suitability of the lake as a habitat for Oscillatoria agardhii Gom

The IJsselmeer (surface area 1200 km², mean depth 4.5 m, residencetime 0.4 year, phosphorus load 7 g m^–2 year^–1) isa very important conservation area. Regular summer bloomingof Oscillatoria spp. can depreciate this value, so the boundsof possibility of this kind of blooming have been investigated.Therefore samples were taken along the shore and in the openwater from 1974 to 1982, continuous temperature profile measurementswere made in the same period in the middle of the lake and insitu primary production was measured in 1976 and 1977. The phytoplanktonconsists of green algae throughout the year, diatoms in spring,and blooms of Microcystis aeruginosa in summer. Blooming ofOscillatoria agardhii Gom occurs regularly in summer along theFrisian shore. In 1976, however, a heavy bloom of this algaoccurred in the whole lake. Comparing the IJsselmeer with shallowerOscillatoria-lakes in the Netherlands distinct differences arepresent concerning biomass, chlorophyll a content, relativevolume of the euphotic zone and light-dark cycle. Not only thelarger depth and extensiveness of the IJsselmeer are unfavourablefactors for Oscillatoria, but also the separation by land reclamationof many of the shallow littoral regions from the main body ofthe lake. High temperature and microstratification are neededto develop a bloom in the whole lake.     

The synergistic effect of carbon concentration and high temperature on lipid peroxidation in Peridinium gatunense

Lipid peroxidation in Peridinium samples taken from two differentdepths in Lake Kinneret fluctuated throughout the spring withan overall increasing trend. Samples from 0.5 and 5 m showeda similar peroxidation pattern, which was maximal after thefall off in algal biomass. The rapid decline in Peridinium biomasscoincided with ambient lake temperatures of 21–23C. Fattyacid composition profiles were similar at both depths, althoughafter the peak of the bloom, a significant increase in polyunsaturatedfatty acids and oleic acid was only found at 0.5 m, togetherwith a decrease in the percentage of polyunsaturated fatty acids.These effects were related to ambient light stress rather thana result of lipid peroxidation. Lake samples taken at differentperiods of the bloom and incubated at various temperatures showeddifferential peroxidation. Higher temperatures caused increasedlipid peroxidation, but this appeared to be dependent on thesampling period. Samples withdrawn from the lake at the beginningof the bloom showed little peroxidation after a 5 day incubationat 14C, room temperature (25C) or ambient lake temperature(16C) compared to mid-bloom samples in which there was a significantincrease in peroxidation when they were incubated at room temperature(25C) or ambient lake temperature (22C). Incubation at 14Cinhibited peroxidation; however, samples from mid-bloom againshowed enhanced peroxidation compared with those from the beginningof the bloom. These in situ results suggested a relationshipbetween temperature, another environmental variable during thebloom and lipid peroxidation in Peridinium. As total dissolvedinorganic carbon (DIC) concentrations fall significantly duringthe progress of the bloom and represent an important sourceof environmental stress, laboratory experiments were establishedto investigate the synergistic effect of temperature and carbonnutrition on lipid peroxidation in Peridinium cultures. Increasedtemperature alone caused a slight increase in lipid peroxidation,but this was greatly augmented by carbon limitation. Althoughcarbon limitation induced increased catalase activity, at highertemperatures activity declined after 48 h, allowing for thesubstantial increase in lipid peroxidation.     

Growth and decline of a diatom spring bloom phytoplankton species composition, formation of marine snow and the role of heterotrophic dinoflagellates

During the spring of 1994, we determined the factors responsiblefor the decline of the seasonal diatom bloom in the Gullmarfjord, on the west coast of Sweden. Four species constituted>75% of the biomass—Detonula confervacea, Chaetocerosdiadema, Skeletonema costatum and Thalassiosira nordenskioeldii—reachingconcentrations of 4900, 350, 8200 and 270 cells ml^–1,respectively. Growth of phytoplankton was exponential (growthrate = 0.12 day^–1) from 3 to 21 March, after which a galewith winds >15 m s^–1 caused massive aggregation. Amaximum of 130 p.p.m. (v/v) of marine snow aggregates was observedby in situ video at the peak of the bloom. Critical concentrations(Jackson, Deep-Sea Res., 37, 1197–1211, 1990) were similarto observed showing that coagulation theory could explain thesudden decline of the bloom. The heterotrophic dinoflagellateGyrodinium cf. spirale increased exponentially after the peakof the bloom with maximum (temperature-adjusted) growth rates.After the rapid aggregation and sedimentation of the bloom,they were able to control any further growth of diatoms. Nitrateand silicate were never depleted, but phosphate may have beenlimiting by the end of the study period. We conclude that massaggregation during a gale marked the end of the bloom, and thatintense grazing by heterotrophic dinoflagellates prevented anysubsequent increase of diatoms.     

The Changes in the Composition of the Diatom Assemblages Found in the Surface Sediments of Blelham Tarn in the English Lake District During 1973

A study has been made of the accumulation of diatom frustulesin the surface layers of lake sediments, taking advantage ofthe appearance of a new diatom bloom in Blelham Tarn in theEnglish Lake District. Results from monitoring the top 5 cmof sediment and seston trap material, and comparison with phytoplanktonrecords show that there is good agreement between all threeand that sediment diatom assemblages change in response to theliving populations so that the record of large ‘blooms’can be quickly diluted. The centric diatom, Stephanodiscus astraeavar. minutula appeared to sink out of the phytoplankton quicklyand mix down into older sediments fairly readily, circumstanceswhich may be assisted by the compact shape of its frustule.     

Changes in the elemental composition of Asterionella formosa during the diatom spring bloom

Changes in the elemental composition of the diatom Asterionellaformosa within mixed phytoplankton samples were determined overthe spring bloom period using energy dispersive X-ray microanalysis.Using a 10 kV electron microprobe, X-ray information from thetop 1–2 µm of the cell revealed overall mean concentrationsof: Si (4636 mmol kg^–1 dry wt), P (82), S (54), Cl (71)K (94) and Ca (90). Concentrations of all elements showed widevariation within each date sample, with unimodal frequency distributionsapproximating to a Normal distribution. Correlation of the entiredata set demonstrated clear statistical associations betweenelements, including Si, P and K. Uptake of Si during the courseof the diatom bloom led to a major fall in lake water concentration(1.0 to 0.07 mg l^–1), which correlated with a decreasein the mean cell Si concentration (6735 to 3107 mmol kg^–1dry wt) during the final phase of the bloom. Mean cell concentrationsof P and K also showed marked decreases with time, in closeparallel with cell Si. These changes in P and K were attributedto the high level of internal correlation with Si and not toany significant decrease in P and K availability in the environment.     

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.     

An unusual bloom of Gyrodinium cf. aureolum in the Argentine sea: community structure and conditioning factors

In October 1988 a bloom of Gyrodinium cf. aureolum was recordedalong a transect across the continental shelf reaching celldensities of 1300 ml^–1 and chlorophyll a concentrationsup to 16 µg 1^–1. The bloom was restricted to a nutrient-richthin surface layer of 10 m depth at the beginning of seasonalpycnocline. Hydrological conditions showed a particular salinitydistribution. The phytoplankton community was dominated by G.cf. aureolum but the coexisting flora presented a great diversity.UV-absorbing pigments were recorded in the community.     

Microbial dynamics during the decline of a spring diatom bloom in the Northeast Atlantic

The microbial dynamics during a spring diatom bloom declinewas monitored in the Northeast Atlantic during a 5-day Lagrangianstudy (8–12 April 2002). Phytoplankton abundance, compositionand health status were related to viral and bacterial abundance,zooplankton abundance and grazing rates, as well as bacterialproduction. Phytoplankton reached maximum concentration on Day3 (Chl a >5 µg L^–1) and declined on Day 5 (Chla 2 µg L^–1) and was dominated (70% of Chl a) bydiatoms. Bacterial production increased substantially to >20µg C L^–1 day^–1 on Day 3 and concomitantlylarge viruses decreased in number by half to <10 x 10³ mL^–1.This was followed by a 5-fold increase in large viruses on Day5, indicating infection and subsequent lysis on Days 3 and 5,respectively. Micro- and mesozooplankton grazing were not theprincipal cause for the decline of the bloom and pheophorbide-ashowing little variation in concentration from Days 1–4(100 ng L^–1) although doubled on Day 5. The poor physiologicalstatus of the diatoms, indicated by the high chlorophyllide-aconcentrations (50–480 ng L^–1), likely promoteda series of closely interrelated events involving bacteria andviruses leading to the demise of the diatom bloom.     

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     

Evolution and structure of a shelf coccolithophore bloom in the Western English Channel

The physical factors associated with the development, advectionand disappearance of a shelf bloom of the coccolithophore Emilianiahuxleyi were investigated in the Western English Channel inJune 1992 from an early reflective stage to a mature and dissipativephase (3–4 weeks lifetime) in conjunction with thermaland visible satellite imagery [Advanced Very High ResolutionRadiometer (AVHRR)]. The physical processes that appeared importantin patch evolution and structure were differential stratificationin an area of weak tidal currents, initial zero resultant windconditions (allowing local bloom development), later strengtheningNE winds (driving a coastal warm surface current). entrainmentof the bloom water into the anticyclonic tidal circulation aroundthe Isles of Scilly, and finally bloom dispersal by mixing andflow divergence. Sea-truth results simultaneous with the satelliteimages on 25 and 26 June were examined in relation to the bloomintensity and vertical structure (E.huxleyi cells, detachedcoccoliths and empty coccospheres). Bloom conditions were characterizedby sections of temperature, chlorophyll a, inorganic nutrients,particulate organic and inorganic carbon, and carbon to nitrogenatom ratio. The phytoplankton composition of non-coccolithophorespecies presented significant variations inside and outsidethe bloom. Measurements of beam attenuation coefficient (660nm),diffuse attenuation coefficient [Photosynthetically active radiation(PAR)] and reflectance spectra (415–660 nm) were usedto analyse the extent to which the bloom changed the local opticalproperties. Optical measurements at sea were correlated withcoccolith concentration in order to quantify concentration estimatesderived from remote sensing reflectance measurements.     

Temporal changes in the ciliate assemblage and consecutive estimates of their grazing effect during the course of a Heterocapsa circularisquama bloom

Temporal changes in ciliate assemblages during the course ofa bloom of the harmful microalga Heterocapsa circularisquama(Dinophyceae) were investigated and consecutive estimates ofspecies-specific maximum grazing losses were analyzed from Augustto September 1998 at a site in western Hiroshima Bay, the SetoInland Sea of Japan. Temporal increases of the H. circularisquamamean concentration in the water column were observed twice (25–29August and 7–10 September) with the maximum concentration(ca. 4000 cells mL^–1) being recorded on 25 August. Themain ciliate genera during the bloom were Favella, Tontonia,Eutintinnus, Tintinnopsis and Amphorellopsis. Increases of Favellaand Tontonia were observed when the concentration of H. circularisquamaranged from 260 to 1170 cells mL^–1. Total maximum grazingloss estimated from the abundance and ingestion rate of eachciliate species on H. circularisquama ranged from 1 to 75% standingstock removed d^–1 of the H. circularisquama concentration.High grazing losses mainly due to the genera Favella and Tontoniaoccurred during the period when the H. circularisquama concentrationwas decreasing. These results suggest that grazing by ciliateassemblages can influence the population dynamics of H. circularisquamadespite the potentially toxic nature of the phytoplankter.     

Dissolved organic carbon released by zooplankton grazing activity-a high-quality substrate pool for bacteria

Experiments were designed to investigate whether processes relatedto zooplankton feeding have a positive effect on bacterial growth.Bacterial abundance and [³H]thymidine incorporation rates werefollowed in grazer-free batch cultures originally containingeither Scenedesmus quadricauda or Rhodomonas lacustris as foodsources, and Daphnia cucullata or Eudiaptomus graciloides asgrazers. Compared with controls lacking either animals or algae,a significantly higher bacterial abundance and productivityoccurred in cultures which contained both phyto- and zoo-plankton.The same experimental methodology was tested during the declineof a diatom spring bloom in a eutrophic, temperate lake. A significantincrease in bacterial biomass was observed due to the grazingactivity of in situ mesozooplankters during the clear-waterphase. Our results demonstrated that the dissolved carbon pathwayfrom mesozooplankton to bacteria averaged 57% (26–78%)of the algal carbon filtered from suspension.     

Differences in nitrate and ammonia uptake among components of a phytoplankton population

We examined the NO₃^– and NH₄⁺ uptake capabilities of thedinoflagellate Peridinium cinctum and of the accompanying nanoplanktonduring the spring P. cinctum bloom in Lake Kin-neret. Throughoutthe Peridinium season, the smaller algae had greater affinitiesand faster specific uptake rates for both NO₃^–and NH₄⁺It appears that P. cinctum cannot directly compete with nanoplanktonfor nitrogen nutrients. Other factors such as the ability ofdinoflagellates to swim freely in the water column and low grazingpressures may explain their dominance in the lake.     

Sedimentation of phytoplankton populations dominated by Microcystis in a shallow lake

The sedimentary flux of phytoplankton was measured using sedimenttraps in a shallow hypertrophic lake (Lake Kasumigaura), whereMicrocystis bloomed, from June to November 1983 The sedimenttraps were set at 0.5, 1.5 and 3.0 m depth in Takahamairi Bay(3.5 m depth). Microcystis spp. (including M.aerugmosa and M.viridis)in the traps were rare until early August, but increased thereafter.Sinking rates of Microcystis were 0.0045, 0.020 and 0.24 m day^–1in June–August, September and October respectively, whichwere far lower than those of Melosira (0.2–1.7 m day^–1)and Syncdra (0.2–1.0 m day^–1). The total sedimentaryfluxes of POC and that of algal carbon during the study periodwere 283.2 and 96.7 gC m^–2 which were 59.5% and 20.3%of the gross primary production (475.8 gC m^–2) respectively.The sedimentary flux of living algae measured by algal countswas large in June but small in August and September. On theother hand, the flux of detritus obtained by subtracting totalalgal carbon from POC was small in June and July but large inAugust and September. Therefore diatoms, which appeared mostlyin June, tended to sink as live algae, while Microcystis sankas detritus after being decomposed or consumed in the waterIt was concluded from the results of carbon budget calculationsand the respiration rate of the 1- to 20-µm fraction thatthe activity of decomposers or consumers increased greatly inthe short period at the end of the bloom of Microcystis.     

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.     

Grazing by the calanoid copepod Neocalanus cristatus on the microbial food web in the coastal Gulf of Alaska

Neocalanus cristatus feeding on phytoplankton and microzooplanktonwas measured in the coastal Gulf of Alaska during spring andearly summer of 2001 and 2003. Neocalanus cristatus CV fed primarilyon particles >20 µm. Particles in the 5- to 20-µmsize range were ingested in some experiments under nonbloomconditions but not under bloom conditions. Particles <5 µmwere not ingested but increased during incubations because N.cristatus consumed their microzooplanktonic predators. Neocalanuscristatus are sufficiently abundant in nature to induce sucha cascade effect in situ. Microzooplankton provided >70%of the carbon ingested by N. cristatus under nonbloom conditionsbut only     

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.     

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.     

Ecological studies on the phytoplankton of Korsfjorden, western Norway. The dynamics of a spring bloom seen in relation to hydrographical conditions and light regime

The investigation was carried out at one station in Korsfjorden,a typical deep silled fjord of western Norway. During 14 cruisesfrom 4 February to 30 June 1977 ATP, chlorophll a, phaeopigmenta, and in situ ¹⁴C-assimilation were measured in the net (>30µm), nano and ultraplankton (<5 µm). Sampleswere collected from five light depths within euphotic zone.The impact of hydrographical conditions and light regime onthe bloom dynamics was also studied. In the periods 4 February-7 March and 13 April-30 June, ultraplankton contributed >60%to the total primary production while net and nanoplankton dominatedfrom 7 March to 13 April. The diatoms Skeletonema costatum,Chaetoceros compressits and C. debilis, and Rhizosolenia hebetatavar. semispina made up the main part of the biomass on 21 March,28 March and 4 April respectively. A shade adapted diatom societywas located at the top of the nutricline in late June with S.costatum, Chaetoceros spp., and Thalassiosira spp. as the dominantspecies. The highest assimilation number of eight for the netplankton and four for the ultraplankton were found at the depthof 32% light intensity on 28 March and 24 May respectively.Linear relationships were found between chlorophyll a and ATPfor the different size fractions with regression slopes rangingfrom 4.3 to 5.8. The total primary production for the periodof investigation was calculated to 74 g C m^–2. Light regimeand water column stability were decisive factors for the outburstof the first diatom bloom in late March. Grazing on net planktondiatoms increased during late March-early April. Changes inthe longshore wind-stress component were found to be essentialfor the understanding of the bloom dynamics.     

Size and species-specific primary productivity and community structure of phytoplankton in Tokyo Bay

Combined methods of size fractionation and single-cell isolationwere used to investigate the seasonal variation of phytoplanktondynamics in Tokyo Bay with an emphasis on primary productivity.Red tides occurred in Tokyo Bay from spring to autumn; a diatom,Skeletonema costatum, and a raphidophycean, Heterosigma akashiwo,were the most important primary producers. Small diatoms andflagellates, including these species, were dominant and showedrapid changes of phytoplankton community structure within severaldays in summer. The nanoplankton (3–20 µm) fractioncontributed most to chlorophyll a concentration and primaryproductivity during spring to autumn, whereas the microplankton(>20 µm) contribution was remarkable in winter. Picoplankton(<3 µm phytoplankton) remained relatively constantthroughout the year. A significant reverse relationship wasobtained between assimilation rate and chlorophyll a contentfor the total and nanoplankton population; the assimilationrate was high at the initial phase of the bloom, then decreasedto a minimum level at the peak of the bloom. Factors controllingthe reduction of assimilation rates at the peak, and changesin phytoplankton community structure, are discussed.