Reproductive Ecology of the Dominant Dinoflagellate ... · Ceratium fusus is a dominant red tide species in Sagami Bay; high cell numbers are observed frequently from April to September,

35

Journal of Oceanography, Vol. 63, pp. 35 to 45, 2007

Keywords:⋅⋅⋅⋅⋅ DinoflagellateCeratium fusus,

⋅⋅⋅⋅⋅ reproductivestrategy,

⋅⋅⋅⋅⋅ bloom,⋅⋅⋅⋅⋅ growth rates,⋅⋅⋅⋅⋅ Sagami Bay, Japan.

* Corresponding author. E-mail: [email protected]

Copyright©The Oceanographic Society of Japan/TERRAPUB/Springer

Reproductive Ecology of the Dominant Dinoflagellate,Ceratium fusus, in Coastal Area of Sagami Bay, Japan

SEUNG HO BAEK*, SHINJI SHIMODE and TOMOHIKO KIKUCHI

Graduate School of Environmental and Information Sciences, Yokohama National University,Tokiwadai, Hodogaya-ku, Yokohama 240-8501, Japan

(Received 12 March 2006; in revised form 1 September 2006; accepted 1 September 2006)

The seasonal abundance of the dominant dinoflagellate, Ceratium fusus, was investi-gated from January 2000 to December 2003 in a coastal region of Sagami Bay, Japan.The growth of this species was also examined under laboratory conditions. In SagamiBay, C. fusus increased significantly from April to September, and decreased fromNovember to February, though it was found at all times through out the observationperiod. C. fusus increased markedly in September 2001 and August 2003 after heavyrainfalls that produced pycnoclines. Rapid growth was observed over a salinity rangeof 24 to 30, with the highest specific rate of 0.59 d–1 measured under the followingconditions: salinity 27, temperature 24°C, photon irradiance 600 µµµµµmol m–2s–1. Thegrowth rate of C. fusus increased with increasing irradiance from 58 to 216µµµµµmol m–2s–1, plateauing between 216 and 796 µµµµµmol m–2s–1 under all temperature andsalinity treatments (except at a temperature of 12°C). Both field and laboratory ex-periments indicated that C. fusus has the ability to grow under wide ranges of watertemperatures (14–28°C), salinities (20–34), and photon irradiance (50–800µµµµµmol m–2s–1); it is also able to grow at low nutrient concentrations. This physiologicalflexibility ensures that populations persist when bloom conditions come to an end.

2003). In addition, red tides occurred from March to Julyin 1997 along the Pacific coast of central Japan fromWakayama to Ibaraki Prefecture (Machida et al., 1999).Furthermore, C. fusus has a seasonality similar to that ofC. furca. The cell density of C. fusus at the peak of pro-liferation exceeds an average value for red tides (Mulford,1963; Dodge and Marshall, 1994). There have been fieldand laboratory studies of factors that control seasonalchanges in C. furca, and optimal environmental condi-tions for bloom outbreaks have been determined (Baek etal., 2006). There is no similar information for C. fusus,though a few authors have reported on cell tolerance tochanges in temperature and salinity. For example, C. fususis able to survive at temperatures ranging from 1.7 to 27°Cand salinities between 14.4 and 34.8 in water from Vir-ginia, USA (Mulford, 1963).

Due to difficulties in isolating C. fusus from naturalseawater and subsequent laboratory cultivation, there havebeen few ecological or physiological studies of the spe-cies, and there is a shortage of information obtained un-der controlled laboratory experiments on the life cycle(including cyst populations), and on nutrient require-ments, light intensities, salinities and water temperaturesfor optimal growth.

1. IntroductionThe dinoflagellate genus, Ceratium, is an important

component of marine phytoplankton communities. It hasan extraordinary biogeographical range through all of theworld’s oceans, from the warmest waters of the tropics tothe cold polar seas (Graham, 1941). Within the NorthAtlantic Ocean and adjacent seas, distribution dependssignificantly on water temperature (Dodge and Marshall,1994). Some species of the genus Ceratium frequentlydominate coastal phytoplankton communities, where theycontribute substantially to annual primary production(Nielsen, 1991; Dodge and Marshall, 1994).

Ceratium fusus and Ceratium furca have recentlybeen recognized as dominant red tide species in easternAsian areas, such as Chinese coastal water, Hong Kong,the Philippine Sea and the Gulf of Thailand etc (Lu, 2003;Yin, 2003; Lirdwitayaprasit, 2003). Although both spe-cies are frequently observed in coastal areas of Korea andJapan, red tides of C. furca have been especially frequenton the southern coast of Korea since 1995 (Suh et al.,

36 S. H. Baek et al.

Ceratium fusus is a dominant red tide species inSagami Bay; high cell numbers are observed frequentlyfrom April to September, after the spring diatom bloom.The species sometimes occurs as a red tide under rela-tively low nutrient conditions. Population densities in thewater column decrease from November to February. Thisseasonal pattern of occurrence has not been as rigorouslyassessed as those of other dinoflagellates. In this study,the natural population of C. fusus was monitored to pro-vide information on the relationship between cell abun-dance and environmental factors (water temperature, sa-linity, nutrients) in the coastal waters of Sagami Bay.Laboratory cultures were used to examine optimum physi-ological requirements for growth. The experimental re-sults were compared to observations of the natural popu-lation to better understand the reproductive ecology ofthe species in Sagami Bay.

2. Materials and Methods

2.1 Field investigationSampling was conducted monthly from 2000 to 2003

at two coastal stations, designated St. 40 and 70 (ca. 40m and 70 m depth, respectively) in the north-western partof Sagami Bay, Central Japan (Fig. 1). Sagami Bay facesthe Pacific Ocean and its hydrography is related prima-rily to fluctuations of the Kuroshio Current axis. It is alsoinfluenced by the fresh water discharged from the Sagamiand Sakawa Rivers as well as the water from Tokyo Bay(Hogetsu and Taga, 1977). Interaction between the cur-rent and river discharges results in stratified waters inSagami Bay. The surface layer consists of a mixture ofwaters from the Kuroshio Current and fresh waters (Iwata,1985).

Water samples were collected with a bucket (surface

layers only) and with 6 L Niskin bottles. The samplingdepths at the two stations were 0 m, 5 m, 20 m and 35 mat St. 40, and 0 m, 5 m, 20 m, 40 m and 65 m at St. 70.Debris and large-sized plankters in the collected waterswere removed by filtering through 330 µm mesh on boardship immediately after measurement of water tempera-ture with a mercury thermometer. Each filtered water sam-ple was kept in a dark bottle and taken to the laboratoryfor determination of salinity, inorganic nutrients, chloro-phyll a (Chl.a) concentrations, and phytoplankton assem-blages. Salinity was measured using an inductive salinom-eter (Model 601 MK-IV, Watanabe Keiki MFG. Co. Ltd.).Subsamples for the estimation of phytoplankton abun-dances were fixed immediately with 2.5% (final concen-tration) glutal aldehyde solution after filtration throughTTTP type 2.0 µm membranes, and stored at 4°C in thedark until cells were counted (using a Sedgwick-Rafterchamber).

Duplicate subsamples of >100 ml were filtered ontoWhatman GF/F glass fiber filters for analysis of Chl.a.Each filter was extracted in the dark at 4°C for 24 h in a10 ml brown vial containing 10 ml N,N-Dimethylformamide (DMF) (Suzuki and Ishimaru, 1990).Chl.a concentration was determined fluorometrically ona Turner Design fluorometer according to the method ofHolm-Hansen et al. (1965). Water samples for determi-nation of dissolved inorganic nutrients were filteredthrough Millipore Milex filters (pore size: 0.45 µm). Thefiltered water was transferred into plastic tubes and keptin a freezer (–20°C) for later measurement of nutrients.The water samples were thawed to room temperature, andnitrite + nitrate-N (NO2

– + NO3–), and phosphate-P

(PO43–) concentrations were analyzed using an auto-

analyzer (Bran Luebbe, AACS-II), following analyticalmethods based on Parsons et al. (1984).

Fig. 1. The sampling stations.

Reproductive Ecology of Ceratium fusus 37

Rainfall was measured every day during the sam-pling period with rain gauges located on the roof of theManazuru City Hall (35°09′15″ N, 139°08′26″ E) and theOdawara office of the Japan Meteorological BusinessSupport Center (35°15′01″ N, 139°09′03″ E).

2.2 Isolation and culture of Ceratium fususIndividual cells of C. fusus were isolated from natu-

ral assemblages in waters of Sagami Bay during July 2003,when water temperature and salinity were approximately22°C and 32.5, respectively. Each of the isolated C. fususcells was washed by serial transfer through three drop-lets of T1 medium containing H2SeO3 (Ogata et al., 1987;Baek et al., 2006), and then pipetted into one 5 ml wellof a 12-well tissue culture plate. For acclimation to labo-ratory conditions, the cells were cultured for a period of

one month at 22°C under a photon fluence rate of 180µmol m–2s–1 with a 12 L: 12 D cycle (using cool whitefluorescent lamps). Enriched seawater (modified T5 me-dium, i.e., T1 medium concentrated five-fold) with soilextract was used in the acclimation period. Preliminaryexperiments showed that T5 medium (N: 5 µM, P: 0.5µM) promoted better cell population growth than T1 me-dium. The following experiments were conducted afterT5 medium-acclimation.

2.3 Specific growth rate experimentsWe tested the effects of varying light, temperature

and salinity on the growth rate of C. fusus populationsmaintained in 100 ml flasks in T5 medium with soil ex-tract. Stock cultures containing 3000 cells ml–1 of C. fususwere concentrated by gentle reverse filtration through a

Fig. 2. Seasonal changes in vertical profiles of water temperature and salinity at St. 70, Sagami Bay, Japan (2000–2003). Blackdots indicate sampling depths.


Fig. 3. Variation in rainfall in the north-western part of Sagami Bay, Japan. White and black bars indicate average rainfall inmonth and total rainfall over five days preceding sampling date, respectively.

20 µm Nitex mesh. Elevated concentrations were neededbecause densities did not increased above 1500 cellsml–1, even under optimal conditions. One ml of the cellconcentrate was transferred into each 100 ml conical flask.Thus, the initial cell density was 30 ml–1. The densitieswere determined as means of three replicate cell countsusing a 1 ml cell counts using a 1 ml Sedgwick-Raftercounting chamber under a microscope. The experimentswere run three times.

Five temperature (12, 16, 20, 24 and 28°C), six sa-linity (17, 20, 24, 27, 30 and 34) and six photon irradi-ance regimens (0, 58, 180, 230, 600 and 800µmol m–2s–1) were established. Cells were incubated at20, 24 and 28°C under all six salinity conditions. Theywere also incubated at 12 and 16°C at a salinity of 34.Experiments ran four days under a 12L: 12D cycle. Thespecific growth rate (µ) was estimated from:

µ = ln(Nt/N0)/t,

where N0 and Nt represent the initial and final cell densi-ties and t represents incubation time (day).

2.4 Data analysisWe examined relationships among cell densities and

environmental parameters (Temperature, Salinity, Chl.a,NO2

– + NO3– and PO4

3–) in the field using Pearson’scorrelation analysis. A significance level of P < 0.05 wasused in all statistical analyses.

3. Results

3.1 Abiotic factorsSeasonal changes in environmental factors were al-

most identical at the two stations. Therefore, we showonly the results obtained at St. 70.

Water temperature varied from 12 to 29°C during thesampling period (Fig. 2). In each sampling year, the high-est temperature was recorded in August or September and

the lowest in March. The water column was well mixedvertically from November to March, and gradually strati-fied thereafter. Salinity varied from 23.0 to 34.7 duringthe sampling period, and low salinities were frequentlyrecorded in summer due to rainfall (Figs. 2 and 3). Inparticular, salinity decreased drastically from 34.5 to 24.5in September 2001, and from 34.5 to 23 in August 2003,probably due to heavy rain two to four days prior to sam-pling in both months. In contrast, we did not find salinitiesof less than 30 after relatively high rainfall (>100 mm) inJune of 2000 and 2002. Large rainfall events were re-corded from May to October during all 4 years (Fig. 3).The largest annual rainfall during the study period oc-curred in 2003. Average rainfalls five days prior to eachsampling day were in excess of 100 mm on four occa-sions during summer (June to September) in each sam-pling year.

Concentrations of nitrate + nitrite-N ranged from>0.02 µM on July 2003 to 20.49 µM on August 2003,and the mean values during four sampling years were3.61 ± 2.86 µM (Fig. 4). The highest (20.49 µM) nitrate+ nitrite-N value was recorded in surface water on Au-gust 2003 after heavy rainfall. The second highest con-centration was observed on July 2001, although there wasno rain in the day (July 7 to 13) preceding the samplingdate. Phosphate-P concentrations ranged from >0.02 µMon May 2001 to 1.62 µM on July 2001, with a mean valueof 0.30 ± 0.23 µM over the four sampling years (Fig. 3).

3.2 Chl.a concentrations and phytoplankton assemblagesChl.a concentrations at 5 m depth in the two station

are shown in Fig. 5. The Chl.a concentrations ranged from0.02 mg m–3 on November 2002 to 9.66 mg m–3 on March2001 at St. 40. The concentrations remained at low val-ues through September to January in each of four years,and tended to increase from February to July. Eight peaks(>5 mg m–3) of Chl.a concentration were observed at bothstations. Seasonal changes in Chl.a concentrations werealmost identical at the two stations. Spring blooms, rec-


Fig. 4. Seasonal changes of vertical profiles of nutrients (NO2– + NO3

–-N and PO43–-P) at St. 70, Sagami Bay, Japan (2000–

2003). Black dots indicate sampling depths.

ognizable by high Chl.a concentrations (>5 mg m–3) fol-lowing mixing of the water column in winter, were mostlydominated by diatoms such as Eucampia spp.,Rhizosolenia spp., and Chaetoceros spp. Thephytoplankton assemblages during the summer periodsconsisted mainly of dinoflagellates, such as Ceratiumfurca and C. fusus. When the two species bloomed at bothstations in September 2001 and May 2002, C. fusus madeup >90% of total phytoplankton cell density. In additionto Ceratium species, other dinoflagellates, such asProrocentrum spp. and smaller diatoms, such as Nitzschiaspp., increased in abundance after heavy rainfall duringsummer.

3.3 Temporal variation of C. fusus abundancePopulation densities of C. fusus were measured at

Fig. 5. Variation in Chl.a concentrations at 5 m depth duringthe study period. White and black bars indicate St. 40 andSt. 70, respectively. Arrows indicate point in time when C.fusus accounted for more than ca. 80% of totalphytoplankton abundance.


Fig. 6. Seasonal changes in cell density (cells l–1) of Ceratium fusus by depth at St. 40 (a) and St. 70 (b) in Sagami Bay, Japan(2000–2003). Black dots indicate sampling depths.

Depth Temperature Salinity Chl.a NO2– +NO3

– PO43–

Temperature –0.152Salinity 0.455* –0.492*Chl.a –0.478* –0.024 –0.304NO2

– + NO3– 0.249 –0.567* 0.137 –0.229*

PO43– 0.325* –0.634* 0.342* –0.316* 0.776*

C. fusus –0.276* 0.144 –0.160 0.222* –0.299* –0.244

Table 1. Pearson correlation coefficients (r) indicating relationships between environmental factors and cell density of Ceratiumfusus between 2000 and 2003.

*Significant correlations (P < 0.01).

both stations in each sampling year (Fig. 6). Total abun-dances at St. 70 were slightly lower than those at St. 40.Populations remained at low densities through Octoberto January, and increased from April to September.

Marked seasonal blooms of the species were observedduring the periods from April to August in 2000–2002. In2003, abundance was exceptionally high in both summerand winter in comparison with the previous three years.


During the study period, individual blooms persisted forno more than one month at both stations.

Maximum densities of C. fusus occurred between thesurface and 20 m depth. Subsurface maxima were fre-quently observed at 5 m depth, with sharply decreasingabundances with increasing depth at each station duringthe summer period. In contrast, during winter periods of2002 and 2003 at both stations, cells were observedthroughout the water column (as a result of vertical mix-ing). The annual average density calculated from 0 to 5

m depth was always higher at St. 40 than at St. 70, exceptin 2000 when the average value was higher at St. 70.

The relationship between environmental factors andabundances of C. fusus during the 4 years are shown inTable 1. The abundance of the species was not signifi-cantly correlated with water temperature and salinity. Incontrast, the abundance was significantly negatively cor-related with water depth (r = –0.276, p < 0.01), nitrate +nitrite-N concentrations (r = –0.299, p < 0.01), and phos-phate concentration (r = –0.244, p < 0.01).

Fig. 7. Changes in growth rates of Ceratium fusus with increasing photon irradiance at different salinity (a: 34, b: 30, c: 27, d: 24,e: 20, f: 17) and temperature conditions. Error bars are SD.


3.4 Specific growth rateLaboratory experiments were conducted to investi-

gate effects of temperature, salinity and irradiance on thespecific growth rate of C. fusus (Fig. 7). Five tempera-tures between 12 and 28°C were tested at salinity value34 only (Fig. 7a). At 12°C, the specific growth rates of C.fusus decreased gradually with increasing photon irradi-ance between 53 to 183 µmol m–2s–1, after which therewas no growth up to the highest photon irradiance of 796µmol m–2s–1. Although the growth rates increased withincreasing photon irradiance from 58 to 796 µmol m–2s–1

at all temperatures above 16°C, the highest growth oc-curred at 24 and 28°C. Accordingly, in salinity treatmentsbelow 34 salinity, we measured growth at the followingtemperatures: 20, 24 and 28°C. The specific growth ratesof C. fusus increased gradually with increasing photonirradiance from 58 to 216 µmol m–2s–1 in all salinity treat-ments (except at 12°C and a salinity of 34). Growth ratesreached a plateau between 216 and 796 µmol m–2s–1.Photoinhibition did not occur, even at 796 µmol m–2s–1,the maximum photon irradiance used in this study.

In salinities from 24 to 30, high specific growth ratesoccurred at >24°C and the highest rate was 0.59 d–1 inthe following treatment combination: 27, 24°C and 600

µmol photons m–2s–1. However, growth rates at salinitiesof 34, 20 and 17 were lower than those in the range from24 to 30. At a salinity of 17, morphologically abnormalcells without apical horns were observed during incuba-tion. At salinity <14 (data not shown), we also observedcells damaged by phenomena such as cytolysis (Fig. 8).

4. DiscussionDonaghay and Osborn (1997) stated that, ecologi-

cally, bloom dynamics seem to be dominated by interac-tions between biological and physical processes that oc-cur over a broad range of temporal and spatial scales.Morse (1947) reported that Ceratium furca reached anespecially high density in warm water above thepycnocline in Patuxent River, Maryland, USA. Densepopulations of C. fusus have been observed mostly nearpycnoclines in stratified water columns, in accordancewith the general observation that the pycnocline is a nec-essary precondition for the development of dinoflagellatepopulations (Donaghay and Osborn, 1997). Pycnoclinesalso play an important role in the occurrence of subsur-face populations and their occurrence has often been in-terpreted as an underlying factor in phytoplankton patchi-ness (Rasmussen and Richardson, 1989; Horner et al.,1997). At our sampling sites, stratification of the watercolumn developed gradually from spring to summer. Redtides of C. fusus broke out 5 days after heavy rainfall on20 August 2003 (Kinoshita, personal communication).Three days later (8 days after the rain), high abundancesof C. fusus were observed when surface water was at rela-tively low salinity (24), and strong pycnocline layers ap-peared, especially near the surface. The results suggestthat the development of the dinoflagellate populationsprobably requires a stabilized water column under thepycnocline, subsequent to nutrient addition by naturalrainfall inputs during the rainy season from late spring tosummer.

Cell densities of C. fusus decreased gradually fromthe late summer to autumn. Factors that may be related tothe decline are: (1) breakdown of summer stratificationwith decreasing temperature, and (2) continuous high sa-linity condition of more than 34 during the period of re-duced rainfall after October. Elbrächter (1973) reportedthat water temperature clearly influenced generation timefor Ceratium species. Our results from the field surveyand laboratory experiments indicate that the growth ratesof C. fusus in the field are probably limited by a gradualwater temperature decline (to <16°C) and continuous highsalinity (>34) during the fall (though the doubling timeof C. fusus in the field is likely shorter in the laboratory).Our results suggest that, in addition to the low growthrate caused by falling temperatures in autumn, the reduc-tion of the field populations results from vertical and hori-zontal diffusion induced by vertical water mixing after

Fig. 8. Cell morphology of Ceratium fusus. (a) Normal veg-etative cell; arrow points to flagella. (b) and (c) show ab-normal forms; white arrow indicates cytolysis in yellow-brown chloroplasts of and the nucleus.


breakdown of stratification in the latter part of year.Nutrient concentrations are often regarded as impor-

tant in determining bloom scale and period. Relativelyhigh abundances of Ceratium in oligotrophic conditionsare closely related to the occurrence of phagotrophy,which compensates for low nutrient levels (Norris, 1969;Weiler, 1980). In this study, high abundances of C. fususwere usually observed during months when nutrient con-centrations were low. In addition, there were significantnegative correlations between the densities of the spe-cies and nutrient concentrations (nitrate + nitrite-N andphosphate-P; Table 1). We successfully isolated the spe-cies from natural assemblages with T1 medium, whichhas a fairly low nutrient level (nitrate ≤1.0 µM and phos-phate ≤0.1 µM). The medium concentration was similarto the seawater level during the bloom period of the spe-cies. According to Dortch and Whitedge (1992) and Justicet al. (1995), growth of phytoplankton species may beconsidered limited when concentrations of dissolved in-organic nitrogen (DIN; nitrate, nitrite and ammonium) andphosphate are <1.0 and 0.2 µM, respectively. However,because Ceratium species are motile, their relatively highdivision rates might also reflect an ability to change ver-tical positions to find an optimal depth for nutrient avail-ability; this would be an advantage over non-motile forms.Downward nocturnal migration to nutrient-rich water lay-ers allows uptake of nitrogen (Cullen and Horrigan, 1981)and phosphorus (Watanabe et al., 1988), giving an ad-vantage to photosynthetic cells that subsequently migrateupward to the surface layer during the day (Eppley et al.,1968; Heaney and Eppley, 1981). We found that nutrientconcentrations in the deeper water layers in the field werecomparatively high, even when values at the surface werelow (nitrate + nitrite-N: <0.5 µM, phosphate-P: <0.1 µM)in early summer.

Because of difficulties in isolation and culture fromnatural seawater, there is limited information on thegrowth rate of C. fusus. Our laboratory experiments pro-mote a better understanding of the physiology and lifestrategies of the species. The results of growth measure-ments at various temperature and irradiance combinationsindicate an ability to tolerate a wide range of temperature(16 to 28°C), with highest growth rates recorded at 24and 28°C (Fig. 7). There is clearly growth stimulation atthe temperatures encountered in temperate or tropicalocean water, and vegetative cells have an obvious abilityto overwinter when water temperature is >12°C.

C. fusus in culture was able to tolerate a wide rangeof salinity (17 to 34), with higher growth rates at 24, 27and 30 (Figs. 7b to d). Nordli (1953) reported rapid growthof this species in the field at temperatures over 24°C andsalinities from 20 to 25. In contrast, Smalley and Coats(2002) reported that C. furca appeared to be restricted tolow salinities of >10, and was most abundant at ca. 14 in

the Chesapeake Bay, USA. However, we found that spe-cific growth rates of C. fusus decreased at salinity 17. Inaddition, cells of both C. furca and C. fusus were irre-versibly damaged at salinity below 14. Baek et al. (2006)found that growth rates of C. furca in culture were simi-lar and relatively high at salinities between 17 to 34. Al-though there are some variations between the studies, theresults indicate that high growth rates of C. fusus arestimulated by relatively low salinities (24–30), as com-monly encountered in coastal waters.

Most dinoflagellates dominating coastal waters pro-duce two different types of non-motile cells, i.e., a tem-porary cyst and/or a resting cyst, in their life cycle. Theresting cysts in particular have an important ecologicalrole as a seed for recurrent blooms. However, there areno reports of resting cyst formations by C. fusus. Themechanism by which vegetative cells recruit from theinitial stage before the bloom has not been well under-stood. We found that C. fusus cells adapt to a wide-rangeof variations in water temperature, salinity, irradiance andreduced nutrient concentrations. Observations on seasonalvariations in abundance suggest that C. fusus is able tosustain a population in the water column throughout theyear. Small numbers of cells in the water column, par-ticularly during the winter, may play an important sur-vival role, allowing the population to sustain adequatelevels (without cyst formation) to initiate the next bloom.

Ceratium species (C. furca, C. fusus and C. tripos)have been considered primarily photosynthetic, but, foodvacuoles were observed by Bockstahler and Coats (1993)and Li et al. (1996). Smalley and Coats (2002), andMouritsen and Richardson (2003) noted that distributionsof these potentially mixotrophic Ceratium species (C.furca, C. fusus and C. tripos) are strongly influenced bythe vertical and horizontal distribution of ciliate prey in astratified estuary. Smalley et al. (2003) also found thatthe feeding of C. furca in the culture occurred when cellshad been growing under N- or P-depleted conditions,while nutrient-replete cells did not ingest prey. There is aneed for research into the detail of mixotrophy in C. fusus.

In conclusion, C. fusus can survive throughunfavorable environment changes and small survivingpopulations may play an important role in seeding thenext bloom (C. fusus does not have cyst stages). Our re-sults also indicate that the species probably requires strati-fication in the water column for remarkable populationgrowth subsequent to nutrient addition by natural rain-fall inputs during the rainy season from late spring to sum-mer.

AcknowledgementsWe are grateful to Profs. S. Taguchi and T. Toda, and

Dr. A. Shibata of Soka University for their invaluablediscussion on this study and permission to use instruments


for the field sampling and laboratory experiments. Drs.V. S. Kuwahara, T. Fujiki and A. Kuwata are thanked forreviewing an earlier version of this manuscript. Mr. Y.Asakura of the Manazuru Marine Station and colleaguesfrom the Yokohama National University and Soka Uni-versity are thanked for their assistance in the present study.We also thank Manazuru City Hall and Odawara officethe Japan Meteorological Business Support Center forsupplying data on rainfall. We are grateful to The 21stCentury COE Program “Environmental Risk Managementfor Bio/Eco-Systems” of the Ministry of Education, Cul-ture, Sports, Science and Technology of Japan for finan-cial support. We also appreciate the editor and anonymousreviewer for their thoughtful comments for improving thisarticle.

ReferencesBaek, S. H., S. Shimode and T. Kikuchi (2006): Reproductive

ecology of dominant dinoflagellate, Ceratium furca, in thecoastal area of Sagami Bay. Coast. Mar. Sci., 30, 344–352.

Bockstahler, K. R. and D. W. Coats (1993): Spatial and tempo-ral aspects of mixotrophy in Chesapeake Baydinoflagellates. J. Euk. Microbiol., 40, 49–60.

Cullen, J. J. and W. G. Horrigan (1981): Effect of nitrate on thediurnal vertical migration, Carbon to nitrogen ratio, andphotosynthetic capacity of the dinoflagellate Gymnodiniumsplendens. Mar. Biol., 62, 81–89.

Dodge, J. D. and H. G. Marshall (1994): Biogeographic analy-sis of the armored Planktonic dinoflagellate Ceratium inthe North Atlantic and adjacent seas. J. Phycol., 30, 905–922.

Donaghay, P. L. and T. R. Osborn (1997): Toward a theory ofbiological-physical control of harmful algal bloom dynam-ics and impacts. Limnol. Oceanogr., 42, 1283–1296.

Dortch, Q. and T. E. Whitledge (1992): Does nitrogen or sili-con limit phytoplankton production in the Mississippi Riverplume and nearby regions? Cont. Shelf Res., 12, 1293–1309.

Elbrächter, M. (1973): Population dynamics of Ceratium incoastal waters of the Kiel Bay. Oikos, 15, 43–48.

Eppley, R. W., O. Holm-Hansen and J. D. H. Strickland (1968):Some observation on the vertical migration ofdinoflagellates. J. Phycol., 4, 333–340.

Graham, H. W. (1941): An oceanographic consideration of thedinoflagellate genus Ceratium. Ecol. Monogr., 11, 99–116.

Heaney, S. I. and R. W. Eppley (1981): Light, temperature andnitrogen as interacting factor affecting diel vertical migra-tion of dinoflagellates in culture. J. Plankton Res., 3, 331–344.

Hogetsu, K. and N. Taga (1977): Suruga Bay and Sagami Bay.p. 31–172. In JIBP Synthesis14, Productivity of Biocenosesin Coastal Region of Japan, Vol. 14, ed. by K. Hogetsu, M.Hatanaka, T. Hanaoka and T. Kawamura, University of To-kyo Press, Tokyo.

Holm-Hansen, O., C. J. Lorenzen, R. N. Holmes and J. D. H.Strickland (1965): Fluorometric determination of chloro-phyll. J. Cons. Perm. Int. Explor. Mer., 30, 3–15.

Horner, R. A., D. L. Garrison and F. G. Plumley (1997): Harm-

ful algal blooms and red tide problems on the U.S. westcoast. Limnol. Oceanogr., 42, 1076–1088.

Iwata, S. (1985): Chapter 10 Sagami Bay, physics. p. 401–409.In Oceanography of Japanese Islands, ed. by Oceanographi-cal Society of Japan Coastal, Tokai University Press, To-kyo (in Japanese).

Justic, D., N. N. Rabalais, R. E. Turner and Q. Dortch (1995):Changes in nutrient structure of river-dominated coastalwaters: stoichiometric nutrient balance and its conse-quences. Estuar. Coast. Shelf Sci., 40, 339–356.

Li, A., D. K. Stoecker, D. W. Coats and J. E. Adam (1996):Ingestion of fluorescently-labeled and phycoerythirin-con-taining prey by mixotrophic dinoflagellates. Aquat. Microb.Ecol., 10, 139–147.

Lirdwitayaprasit, T. (2003): Red tide in the inner gulf of Thai-land. p. 53–56. In Workshop on Red Tide Monitoring inAsian Coastal Waters (Program and Extended Abstracts),University of Tokyo, Tokyo.

Lu, D. (2003): Status of HAB monitoring in China with em-phasis on the East China Sea. p. 30–34. In Workshop onRed Tide Monitoring in Asian Coastal Waters (Program andExtended Abstracts), University of Tokyo, Tokyo.

Machida, M., M. Fujitomi, K. Hasegawa, T. Kudoh, M. Kai, T.Kobayashi and T. Kamiide (1999): Red tide of Ceratiumfurca along the Pacific coast of central Japan in 1997. Nip-pon Suisan Gakkaishi., 65, 755–756 (in Japanese).

Morse, D. C. (1947): Some observations on seasonal variationsin plankton population Patuxent River, Maryland.Chesapeake Biol. Lab. Publ., 65, 1–31.

Mouritsen, N. T. and K. Richardson (2003): Vertical microscalepatchiness in nano- and microplankton distributions in astratified estuary. J. Plankton Res., 25, 783–797.

Mulford, R. A. (1963): Distribution of the dinoflagellate genusCeratium in the tidal and offshore waters of Virginia.Chesapeake Sci., 4, 84–89.

Nielsen, T. G. (1991): Contribution of zooplankton grazing tothe decline of a Ceratium bloom. Limnol. Oceanogr., 36,1091–1106.

Nordli, E. (1953): Salinity and temperature as controlling fac-tors for distribution and mass occurrence of ceratia. Blyttia,2, 16–18.

Norris, D. R. (1969): Possible phagotrophic feeding in Ceratiumlunula Schimper. Limnol. Oceanogr., 14, 448–449.

Ogata, T., T. Ishimaru and M. Kodama (1987): Effect of watertemperature and light intensity on growth rate and toxicitychange in Protogonyaulax tamarensis. Mar. Biol., 95, 217–220.

Parsons, T. R., Y. Maita and C. M. Lalli (1984): A Manual ofChemical and Biological Methods for Seawater Analysis.Pergamon Press, Oxford, 173 pp.

Rasmussen, J. and K. Richardson (1989): Response ofGonyaulax tamarensis to the presence of a pycnocline inan artificial water column. J. Plankton Res., 11, 747–762.

Smalley, G. W. and D. W. Coats (2002): Ecology of the red-tide dinoflagellate Ceratium furca: distribution, mixotrophy,and garzing impact on ciliate populations of ChesapeakeBay. J. Eukaryot Microbiol., 49, 64–74.

Smalley, G. W., D. W. Coats and D. K. Stoecker (2003): Feed-ing in the mixotrophic dinoflagellate Ceratium furca is in-


fluenced by intracellular nutrient concentrations. Mar. Ecol.Prog. Ser., 262, 137–151.

Suh, Y. S., N. K. Lee and L. H. Jang (2003): Feasibility of redtide detection around Korean waters using satellite remotesensing. p. 53–56. In Workshop on Red Tide Monitoring inAsian Coastal Waters (Program and Extended Abstracts),University of Tokyo, Tokyo.

Suzuki, R. and T. Ishimaru (1990): An improved method forthe determination of phytoplankton chlorophyll using N,N-dimethylformamide. J. Oceanogr. Soc. Japan, 46, 190–194.

Watanabe, M., K. Kohata and M. Kunugi (1988): Phosphateaccumulation and metabolism by Heterosigma akashiwo(Raphidophyceae) during diel vertical migration in a strati-fied microcosm. J. Phycol., 24, 22–28.

Weiler, C. S. (1980): Population structure and in situ divisionrates of Ceratium in oligotrophic waters of the North Pa-cific central gyre. Limnol. Oceanogr., 25, 610–619.

Yin, K. (2003): Influence of monsoons and oceanographic proc-esses on red tides in Hong Kong waters. Mar. Ecol. Prog.Ser., 262, 27–41.

Documents

Reproductive Ecology of the Dominant Dinoflagellate ... · Ceratium fusus is a dominant red tide species in Sagami Bay; high cell numbers are observed frequently from April to September,