Zooplankton feeding ecology: does a diet of Phaeocystis support good copepod grazing, survival, egg production and egg hatching success?

The bloom-forming alga Phaeocystis is ingested by a varietyof zooplankton grazers, but is thought to be a poor source offood. We examined copepod grazing on solitary Phaeocystis cellsby adult females of Temora stylifera, and survival, fecal pelletproduction, egg production and egg hatching success in Calanushelgolandicus and T. stylifera over periods of 15 consecutivedays. Phaeocystis cell concentrations were high (1.2–3.6x 10⁴ cells ml^–1 for C. helgolandicus and 2.5–7.9x 10⁴ cells ml^–1 for T. stylifera), but within the rangeof maxima recorded for natural blooms. Both copepods survivedwell and continuously produced fecal pellets (indicating continuousgrazing) on a diet of Phaeocystis. However, egg production ratesfor both copepods were low, even though hatching success ofthe few eggs produced was high. Clearance rates for T. styliferawere higher than for most previous measurements of other copepodsfeeding on Phaeocystis solitary cells at lower cell concentrations.We conclude that even though copepods feed well upon Phaeocystis,resulting poor fecundity on this diet may inhibit copepod populationincreases during blooms, thereby contributing to the perpetuationof blooms. However, the high egg hatching success on this dietargues against Phaeocystis containing chemical compounds thatact as mitotic inhibitors reducing copepod egg viability, suchas those found in some other phytoplankters.     

Energetics and feeding dynamics of Euphausia superba in the South Georgia region during the summer of 1994

Measurements of adult Antarctic krill (Euphausia superba) gutcontents, evacuation and egestion rates, as well as digestiveefficiency, were carried out during February-March 1994 in thevicin ity of South Georgia to estimate in situ daily ration.These were combined with acoustically derived biomass data tocalculate the grazing impact of Antarctic krill and its contributionto the carbon flux in the region. Individual levels of gut pigmentconcentrations and evacuation rates ranged from 27 to 1831 ngchlorophyll a-eq. ind.^–1 and from 0.133 to 0.424 h^–1,respectively. Losses of pigment fluor escence during digestionwere very high, ranging from 58 to 98% of the total pigmentdigested. Daily carbon consumption estimated using the gut fluorescencemethod varied from 0.234 to 0.931 mg C ind.^–1 day^–1(or 0.4–1.7% of body carbon), compared to {small tilde}2.73mg C ind.^–1 day^–1 (or {small tilde}5% of body carbon)using the faecal pellet production data. The 3-fold higher dailyration estimated using egestion rate data may be explained bypredation on micro-and mesozooplankton. Maximum krill grazingimpact ranged from 0.4 to 1.9% of the total phytoplankton stockor from 10 to 59% of the total daily primary production. However,grazing impact on the microphytoplankton (>20 µm) wassubstantially higher, at times exceeding 100% of the daily microphytoplanktonproduction. It is suggested that to meet its energetic demands,kriil must consume a substantial proportion of heterotrophiccarbon. ³Present address Zoology Department, University of Fort Hare,P/Bag X1314, Alice, 5700, South Africa     

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.     

The production and ingestion of faecal pellets by nauplii of marine calanoid copepods

The downward transport of organic matter as zooplankton faecalmaterial is influenced by copepods which fragment, ingest andrecycle some of the pellet contents. Most of this activity hasbeen attributed to the later copepodite stages and the adults,but little is known about the role of nauplii. Stage-relateddefaecation rates during the naupliar development of two speciesof copepod, Calanus helgolandicus and Pseudocalanus elongatus,were quantified in a series of laboratory experiments. The productionof faecal material commenced soon after the appearance of theNIII in both species and increased throughout naupliar development.The causes of the increase were the formation of larger pelletsby later stages in Calanus and an increased rate of productionby Pseudocalanus. Calanus nauplii, when supplied with algalfood at concentrations that would support full naupliar development,ingested or broke up the pellets of the smaller Pseudocalanusspecies at rates of 1.15 pellets nauplius^–1 h^–1This consumption increased to 2.96 pellets nauplius^–1h^–1 when the concentration of algal food was reduced toa limiting level. Pseudocalanus was not able to consume thepellets of Calanus. Ingestion of Pseudocalanus faecal pelletsby Calanus could supply a nutritional benefit to a food-limitednauplius.     

Phytoplankton studies in the fjords of West Spitzbergen: physical environment and production in spring and summer

The phytoplankton in the fjords of West Spitzbergen was investigatedfrom 1978 to 1985. The spring bloom lasted longer than at theNorwegian coast: from mid March to early June. There is no delayin the onset of the spring bloom in Spitzerbergen fjords relativeto fjords of northern Norway. This is probably related to therapid daylength increase at high latitudes. The phytoplanktonspecies composition during spring was comparable to that alongthe coast of northern Norway. Annual primary production in theinvestigated area was calculated to be 150 g C m^–2 year^–1.     

The role of faecal material in the particulate organic carbon flux in the northern Humboldt Current, Chile (23{degrees}S), before and during the 1997-1998 El Nino

Sedimentation rates of faecal material, phytoplankton and microzooplanktonand production rates of faecal material from crustaceans andpelagic tunicates were estimated during the austral summer andwinter 1997, and summer 1998, in the northern Humboldt Current(23°S, off Antofagasta, Chile). Sampling periods coveredpre-El Niño (January 1997) and El Niño 1997–98(July 1997 and January 1998). Samples were collected using floatingsediment traps deployed at 65, 100, 200 and 300 m depth in oceanicand coastal areas. Sedimentation rates during January 1997 were,on average, 152 ± 23 and 85 ± 57 mg C m^–2day^–1 at 65 and 300 m depth, respectively. During July,these rates averaged 93 ± 56 mg C m^–2 day^–1at 65 m depth and 35 ± 12 mg C m^–2 day^–1at 300 m depth, while in January 1998 they were 98 and 109 ±37 mg C m^–2 day^–1 at 65 and 200 m depth, respectively.Recognizable faecal material made up the bulk of the sedimentingmatter, accounting for 8 ± 5% (n = 14), 31 ± 26%(n = 16) and 8 ± 5% (n = 5) of the average total organiccarbon recorded from all sediment trap samples collected duringJanuary and July 1997 and January 1998, respectively. However,at300 m depth, the contribution of recognizable faecal materialto total sedimented organic carbon increased to 43 ±33% (n = 4) during July 1997. The remaining sedimenting particlesconsisted mainly of tintinnids, crustacean exuviae, heterotrophicdinoflagellates (both thecated and athecated) and diatom cells.During this study, we estimated that only a minor fraction (average± SD = 5 ± 8%) of the copepod faecal materialproduced within the photic zone sedimented down to 300 m depth,suggesting an efficient recycling within the overlaying watercolumn. On the other hand, an important fraction (47 ±30%) of the euphausiid faecal strings was collected in the 300m depth trap, suggesting that this material would enhance thedownward flux of particulate organic matter (POC). POC fluxesto 65 and 300 m depth traps were in the range of 4–20%and 3–8% of the estimated primary production during thewhole study period. It is postulated that the overall verticalflux of particulates and, in particular, faecal pellets wasdetermined by a combination of three factors. The first wasthe composition of the zooplankton assemblages in the studyarea. When the dominant group was calanoid copepods, their faecesseemed to contribute poorly to the vertical flux of particulates.On the other hand, when the dominant group was euphausiids,a significant proportion of their faecal material was collectedin the sediment trap located at 300 m depth. The second wasthe relatively high abundance of cyclopoid copepods from thegenera Oncaea, Corycaeus and Oithona, which are reported tofeed on aggregates of phytodetritus and faecal pellets producedby calanoid copepods, suggesting that they may act as a naturalfilter to sedimenting particulates. The third was the compositionand size spectrum of the phyto- and microzooplankton assemblageswhich are potential food sources for the meso- and macrozooplankton.These factors were partially modulated by both the 1997–1998El Niño and seasonality.     

Bacterial dynamics and distribution during a spring diatom bloom in the Hudson River plume, USA

Bacterial abundance and [³H]thymidine incorporation rates wereused to characterize bacterial distributions and dynamics duringa spring diatom bloom in the coastal plume of the Hudson Riverduring March, 1981. Bacterial abundance did not decline significantlyaway from the plume or across the continental shelf. However,a pronounced gradient was observed for [³H]thymidine incorporation. Following a northeast gale, a bloom of Skeletonema costatumdeveloped in response to coastal upwelling. As the plume becamesaltier, warmer and larger, bacterial abundance averaged 1.3x 10⁹ cells 1^–1. Bacterial incorporation of [³H]thymidineinto cold 5% TCA insoluble materials, bacterial production andspecific growth rates averaged 80–120 pmol 1^–1 d^–1,1.4–2.0 x 10⁹ cells 1^–1 and 1.2–1.5 d^–1.respectively. The mean density of bacteria in the plume didnot change even though growth rates were higher than expectedlosses from sinking, mixing and export. The abundance and production levels of attached bacteria inthe plume were significantly higher than they were in coastalwater outside the plume. In contrast, free bacterial levelswere similar in all regions of the shelf and plume. Bacterialparameters in the plume were not correlated with phytoplanktonpigment concentrations, while significant positive correlationswere found in shelf waters >33 salinity. Thus bacteria werecoupled to phytoplankton in shelf waters but not in the plumewhere allochthonous dissolved matter was probably supportingthe bacteria. ^*Lamont-Doherty Geological Observatory Contribution No. 3459 ²Present address: Department of Microbiology, University ofGeorgia, Athens, GA 30602, USA.     

Colony growth and evidence for colony multiplication in Phaeocystis pouchetii (Prymnesiophyceae) isolated from mesocosm blooms

Using well plates of Phaeocystis pouchetii colonies isolatedfrom experimental mesocosms in western Norway, increases incolony size and division were documented. Median longest lineardimensions increased 0–7 µm h^–1; literaturePhaeocystis globosa values are 0.9–4.7 µm h^–1.Ten to twelve percent of colonies divided at rates of 0.21–0.28divisions day^–1. Daughter colonies were 100 µm smallerthan mother colonies. Colonies delayed 3.5–4.9 days tofirst division, compared with literature values of 4–5days for P. globosa. This study provides the first experimentalevidence for colony division of wild P. pouchetii.     

Planktonic nitrogen transformations during a declining cyanobacteria bloom in the Baltic Sea

The uptake of ¹⁵N-labelled nitrogen nutrients (ammonium, urea,nitrate) was studied during the decline of a bloom of nitrogen-fixingcyanobacteria in the Baltic Sea. This was done by sampling anorth-south transect of stations, representing different stagesof the bloom. Comparison with nitrogen fixation data showedthat this process was of minor importance, and that the nitrogenuptake was dominated by regenerated nitrogen, mainly ammonium.From time series incubations for studying nutrient uptake, itappears that the regeneration of ammonium was substantial, butthat the production of urea or nitrate was slow. The integrateddaily uptake was calculated for the 0–15 m interval atfour stations and values ranged between 6 and 21 mmol N m^–2day^–1, of which the regenerated nutrients, ammonium andurea, constituted 71–93%. Nitrate was of minor importanceand the highest nitrate uptake rates were found close to thethermocline (at 15 m) and in the southern part of the Baltic.Comparison with carbon fixation data reported from simultaneousmeasurements at two stations gave C/N uptake ratios of 4.9 and2.1 for integrated daily uptake. Contrary to earlier findings,the concentration of DON increased with increasing salinity(from 15 to 17 µmol l^–1). This was correlated withthe declination of the bloom and is suggested to be a resultof a gradual release of less easily utilized DON from the degradationof cyanobacteria. The C/N ratio of DOM was high, 21–23.     

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.     

Zooplankton fecal pellet fluxes and vertical transport of particulate organic material in the pelagic environment

Fecal pellet fluxes were determined using a series of multireplicatetraps set at 35, 65, 150, 500, 750 and 1500 m in the northeastPacific. Fecal pellets appear to be important contributors to total carbonfluxes. In near-surface waters (35–150 m), pellet fluxesranged from 2–3 x 10⁵ pellets m^–2 day^–1. Minimumpellet fluxes were observed at 500m({small tilde}0.4x 10⁵ pelletsm^–2day^–1). In contrast, the 1500 m pellet flux valueincreased to approximately 0.8 x 10⁵ pellets m^–2 day^–1relative to the 500 m depth, and is probably the result of insitu repackaging. Analyses of pellet content suggest multiplesources of "large" particle input throughout the water column.The implications of this phenomenon are discussed in terms ofthe detrital rain and ladder of migration theories.     

A Phaeocystis globosa bloom associated with upwelling in the subtropical South Atlantic Bight

We observed an unusual, subtropical bloom of Phaeocystis globosaScherffel during a strong upwelling event and confirmed itsidentity using morphological, physiological and genetic traits.This low-latitude bloom of P. globosa colonies occurred on theSouth Atlantic Bight continental shelf during the summer of2003. Maximum chlorophyll a concentration measured during thissubsurface bloom was 11.37 µg L^–1 at 31 m depth.Divers reported abundant flocculent material below the steepthermocline, and this material was identified as Phaeocystissp. using microscopy. Using morphological and physiologicaltraits as well as the sequence of the 18S small subunit ribosomalRNA gene (GenBank Accession # EF100712), this phytoplanktonwas identified as P. globosa. Interactions of physical and biologicalconditions may restrict low-latitude P. globosa blooms to strongupwelling events.     

Laboratory studies on egg and faecal pellet production of Centropages typicus: effect of age, effect of temperature, individual variability

The rates of egg production of individually reared females ofCentropager typicus have been observed from the moulting C5-adultto death under constant conditions of food (10 000 cells ml^–1of Hymenomonas elongara) and temperature (15 and 20°C).Preliminaiy experiments showed that virgin females could produceunfecunded eggs, but that the distinctive spines of the eggsof C.typicus were only obtained after fecundation occurred.High and continuous egg production needs several matings, andthe following experiments were run with a couple. In all experiments,both males and females fed and produced faecal pellets. Thefemales show a high variability in spawning life periods, inspawning rhythms, in daily rates and, consequently, in cumulatednumbers of eggs spawned during their life. The highest dailyrates of egg production were obtained in the first half of thefemale's life, and were as high at 20°C as at 15°C.Nevertheless, the females survived longer at 15°C than at20°C, and finally produced 47.6% more eggs during theirwhole life at 15°C than at 20°C, which can be partiallydue to the difference in size (almost 10% bigger at 15°Cthan at 20°C). The production of faecal pellets occurredduring the whole of life and was more regular than egg production,with a maximum daily rate in the first part of life, and showeda decrease until death. The decline in daily egg productionwith age was parallel to the decrease in daily faecal pelletproduction. The daily production of faecal pellets was higherat 15°C than at 20°C. At 20°C, a good correlationwas observed between the total egg production per female andthe total faecal pellet production per female. The individual-basedegg production rates of our experiments have been compared withrates obtained in different conditions by several authors, andwere lower than in other published studies, probably due tothe food offered. We emphasize the difficulties of comparingresults which have been obtained for the entire life of thefemales with those based on daily egg production. The totalegg production of a female is an important parameter in regardto the population d because it fixes the maximal rate of losses(mortality plus dispersion) for which a population can continueto develop.     

Copepod abundance in the western Irish Sea: relationship to physical regime, phytoplankton production and standing stock

The distinct patterns of stratification in the North Channeland stratified region of the western Irish Sea influence theseasonal abundance of phytoplankton. The 3–4 month productionseason in the stratified region was characterized by productionand biomass peaks in the spring (up to 2378 mg C m² day^–1and 178.4 mg chlorophyll m^–2) and autumn (up to 1280 mgC m^–2 day^–1 and 101.9 mg chlorophyll m^–2).Phytoplankton in the North Channel exhibited a short, late productionseason with a single summer (June/July) peak in production (4483mg Cm^–2 day^–1) and biomass (–160.6 mg chlorophyllm^–2). These differences have little influence on copepoddynamics. Both regions supported recurrent annual cycles ofcopepod abundance with similar seasonal maxima (182.8–241.810³ind. m^–2) and dominant species (Pseudocalanus elongatusand Acartia clausi). Specific rates of population increase inthe spring were 0.071 and 0.048 day¹ for the North Channel andstratified region, respectively. Increased copepod abundancein the stratified region coincided with the spring bloom, andwas significantly correlated with chlorophyll standing stock.Increased copepod abundance preceded the summer production peakin the North Channel. This increase was not correlated withchlorophyll standing crop, suggesting that a food resource otherthan phytoplankton may be responsible for the onset of copepodproduction prior to the spring bloom. Hetero-trophic microplanktonas an alternative food source, and advection of copepods fromthe stratified region, are proposed as possible explanationsfor copepod abundance increasing in advance of the summer peakin primary production.     

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.     

Determining the mass density of marine copepods and their eggs with a critical focus on some of the previously used methods

We collected Calanus finmarchicus copepodites CIV, CV and CVIfemales in a deep fjord on the west coast of Norway during April1996, May 1997 and November 1998. Eggs of C. finmarchicus andCalanus glacialis were collected during May 1989 in the BarentsSea. The sinking speeds of animals and eggs were measured ina homogeneous column with seawater of known density, and Stokeslaw was applied to estimate their mass density. Also the densitycontrast between the organisms and seawater was calculated.The mean mass density of C. finmarchicus ranged from 1.0274to 1.0452 g cm^–3. During spring copepodite stage CV hada significantly lower mean mass density (1.0345 g cm^–3)compared to CIV (1.0381 g cm^–3) and CVI females (1.0408g cm^–3). Copepods collected during winter had a distinctlylower mass density. The sinking speed of C. glacialis eggs followeda unimodal distribution, with a mean of 25.9 m day^–1,while sinking speeds of C. finmarchicus eggs were bimodal, thetwo groups of eggs having a mean sinking speed of 23.3 m day^–1and 35.4 m day^–1 respectively. Correspondingly the meanmass density was 1.0556 g cm^–3 for C. glacialis eggs andfor the two groups of C. finmarchicus eggs 1.0639 g cm^–3and 1.0812 g cm^–3. Results of earlier work, particularlyusing density gradient methods to determine mass density ofzooplankton, are critically reviewed, and it is suggested thatthis method should not be used to determine the mass densityof small organisms the size of C. finmarchicus.     

Dinoflagellate bloom development and collapse in Lake Kinneret: a sediment trap study

Warm monomictic Lake Kinneret, Israel, is characterized by awinter–spring water bloom of the large (~50 µm diameter)dinoflagellate Peridinium gatunense Nygaard. Usually the P.gatunense bloom declines in May–June and a less prominentbloom of smaller dinoflagellates (mostly Peridiniopsis spp.of ~20–30 µm diameter) develops. Water column abundancesand sedimentation losses to those dinoflagellates were followedthroughout 1994 and 1995. The objective was to quantify thevariables that describe population dynamics, that in turn willshed more light on the seasonal patterns of bloom dynamics.Sedimentation losses were measured by means of sediment trapswith and without a preservative (formaldehyde) that were exposedfor 24 h once every 2–3 weeks. Annual sedimentation lossesof Peridinium (hypolimnetic trap catches) were 209 g wet wtm^-2 year^-1 in 1994 and 187 g wet wt m^-2 year^-1 in 1995, whichconstituted 16 and 23% of Peridinium production in those years,respectively. This study revealed that increased death ratespreceded a mass sedimentation flux of Peridinium and causedthe decline of the bloom in Lake Kinneret. Annual sedimentationlosses of Peridiniopsis were 55 g wet wt m^-2 year^-1 in 1994and 34 g wet wt m^-2 year^-1 in 1995. In contrast to live Peridiniumcells, Peridiniopsis cells continued to swim to the lower trapafter the onset of thermal stratification, possibly taking advantageof the higher nutrient concentrations below the thermocline,at a time when the lake is already stratified and the epilimnionis nutrient depleted. This could be an important factor allowingPeridiniopsis spp. to peak after the decline of Peridinium.     

Growth and mortality of flagellates and non-flagellate cells of Phaeocystis globosa (Prymnesiophyceae)

Two cell types of the same clone of Phaeocystis globosa, solitarynon-flagellate cells and flagellates, were grown in batch culturesunder identical conditions. The non-flagellate cells had a shorterlag phase (1.4 versus 2.8 days) and a higher growth rate (0.72versus 0.65 day^–1) than flagellate cells. The flagellateshad a longer stationary phase (15.6 versus 9.5 days) and a lowerdeath rate (0.07 versus 0.52 day^–1) than non-flagellatecells. All differences were statistically significant. Biomassyield did not differ between the two cell types. The short lagphase and high growth rate of nonflagellate cells correspondsto field observations of rapidly developing non-flagellate Phaeocystisblooms that are typically observed in nutrient-rich environmentssuch as temperate seas in spring. The flagellate cell type,with its longer stationary phase and lower death rate than non-flagellatecells, is better equipped for survival in oligotrophic environments.This explains why the flagellates of Phaeocystis are abundantafter the spring phytoplankton bloom in temperate seas and inother nutrient-poor environments such as the open ocean.     

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.     

Egg production and hatching success in the peri-Antarctic copepod Calanus simillimus

In situ rates of egg production and hatching success are reportedfor Calanus simillimus, one of the most abundant calanoid speciesin peri-Antarctic regions, during the Italian ‘Italics’cruise in the Straits of Magellan in March-April 1995. Low fecundity(8.6 eggs female^–1 day^–1) and fecal pellet production(0.8 fecal pellets female^–1 day^–1) in this periodindicate that the species was feeding very poorly. Sixty-sixof the 126 females sampled did not produce eggs and 80 of thesedid not produce any fecal pellets during the 24 h period ofincubation. Striking abnormal naupliar and embryonic developmentwas recorded in 81.8% of the eggs spawned. Aberrant eggs didnot undergo normal cleavage, and failed to develop to hatching.Deformed nauplii were asymmetrical and presented strong anatomicalanomalies in the total body length and number of swimming appendages.These results are discussed in the light of recent findingson the causes of low hatching success in copepods.