大亚湾海域锥状斯氏藻孢囊形成与萌发的季节变化

锥状斯氏藻(Scrippsiella trochoidea)是南海大亚湾海域优势甲藻。为了解该藻孢囊形成和萌发动态及其对营养细胞种群动态的影响,2001年1月-2002年1月在大亚湾澳头海域用沉积物捕捉器及TFO重力采泥器对其孢囊进行每月一次的周年监测,同时对浮游植物、水温、盐度、溶解氧等也进行了监测。孢囊形成和萌发分别以沉积物捕捉器中的孢囊形成率以及上表层沉积物中空孢囊的百分比来表示。钙质孢囊和非钙质孢囊年平均形成率分别为1.11×104 cysts m-2d-1和2.13×105 cysts m-2d-1。前者在冬季大量形成,而后者在夏季形成较多。孢囊多在春秋季节萌发,夏季萌发较少,而冬季几乎不萌发。在5月份和10月份营养细胞数量峰形成前,孢囊的萌发出现了高峰,而表层沉积物中的孢囊数量及孢囊形成率则在营养细胞数量峰后大幅度上升。由此可见,大亚湾沉积物中该藻孢囊的萌发给水体提供了丰富的营养细胞,反之水体中高密度营养细胞又促使孢囊的大量形成,从而造成了锥状斯氏藻赤潮在大亚湾海域接连发生。     

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

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

Cyst formation,sedimentation, and preservation: Factors affecting dinoflagellate assemblages in recent sediments from trondheimsfjord,Norway

Plankton records and 25 samples of Recent sediment from Trondheimsfjord and the adjoining shelf were studied to investigate production, sedimentation, and preservation of cysts, as factors which influence the eventual composition of dinoflagellate cyst assembleges. All sediment samples were examined for dinoflagellate cysts using routine semiquantitative palynological procedures. In addition, fjord sediments were subjected to a limited sediment analysis, and, for three samples, results from preparations both with and without acid treatments were compared. For the first time, cyst assembleges from Recent sediments were directly compared with extensive plankton records from overlying waters. Results indicate that approximately 20% of the 55 locally recorded dinoflagellate species contribute cysts to bottom sediments. Once formed, cysts behave as fine silt particles in the sedimentary regime, increasing in abundance as the percentage abundance of finer sediment increases, usually with increased water depth. Cyst-forming species are almost entirely restricted to a few genera, particularly Gonyaulax and Peridinium, within the order Peridiniales. For some groups, reasonably good correspondence was found between percentage abundances of dinoflagellates in plankton and their cysts in sediment, though plankton records covering at least five years were required to establish this. Gonyaulax grindleyi Reinecke (Von Stosch 1969) appeared to be consistently overrepresented by cysts in sediment relative to available plankton evidence; possible explanations are suggested. At least 30% of the cyst species present, including most Peridinium species, were eliminated, or rendered unreliable for semiquantitative palynology, by application of routine palynological preparation treatments. Such cysts may provide useful, non-quantitative, palynological information from Recent and possibly Quaternary sediments, but their persistence would seem unlikely. Thus, factors of preservation probably further restrict the dinoflagellate fossil record. Cyst assemblages from Trondheimsfjord are comparable with those previously recorded from the northeastern coast of U.S.A., and from Scotland and northeastern England. Fjord assemblages are dominated by small, simple, spinose cysts which would be regarded as acritarchs if culture experiments had not proved that they are dinoflagellate cysts. Much potential biogeographic and palaeoenvironmental information was included within the less abundant species.Attention is drawn to the role which future culture experiments may be expected to play in helping to resolve taxonomic difficulties currently affecting dinoflagellate studies. Palynological significance of results from the present study is discussed especially with reference to recent work by Von Stosch which strongly suggests that cysts may be hypnozygotes formed routinely in sexual cycles of dinoflagellates.     

广东大亚湾甲藻孢囊及其与锥状斯氏藻赤潮的关系

1999年12月至2001年1月,在大亚湾澳头海域用沉积物捕捉器(Sediment trap)及TFO重力采泥器对甲藻孢囊进行每月一次的周年监测,并同时研究了浮游植物的季节变化.结果显示,晚秋孢囊形成率最高(3.48105 cysts/m2d),冬季形成率较低,年平均为1.28105 cysts/m2d.锥状斯氏藻(Scrippsiella trochoidea)是大亚湾沉积物孢囊中的绝对优势种,除个别季节外,其形成率一般占孢囊总形成率的50%以上.2000年8月至9月,该海域发生了一次较大规模的锥状斯氏藻赤潮,最高细胞密度达4.0104 cells/mL.赤潮中后期,锥状斯氏藻孢囊包括暂时性孢囊和休眠孢囊大量形成,孢囊的形成减少了水体中营养细胞数量,是赤潮消退原因之一.       

Coupling planktonic and benthic shifts during a bloom of Alexandrium catenella in southern Chile: Implications for bloom dynamics and recurrence

Cell abundances and distributions of Alexandrium catenella resting cysts in recent sediments were studied along time at two locations in the Chilean Inland Sea exposed to different oceanographic conditions: Low Bay, which is much more open to the ocean than the more interior and protected Ovalada Island. The bloom began in interior areas but maximum cyst concentrations were recorded in locations more open to the ocean, at the end of the Moraleda channel. Our results showed a time lapse of around 3 months from the bloom peak (planktonic population) until the number of resting cysts in the sediments reached a maximum. Three months later, less than 10% of the A. catenella cysts remained in the sediments. Maximum cyst numbers in the water column occurred one month after the planktonic peak, when no cells were present. The dinoflagellate assemblage at both study sites was dominated by heterotrophic cysts, except during the A. catenella bloom. CCA analyses of species composition and environmental factors indicated that the frequency of A. catenella blooms was associated with low temperatures, but not with salinity, chlorophyll a concentration, and predator presence (measured as clam biomass). However, resting cyst distribution was only related to cell abundance and location. The occurrence of A. catenella cysts was also associated with that of cysts from the toxic species Protoceratium reticulatum. By shedding light on the ecological requirements of A. catenella blooms, our observations support the relevance of encystment as a mechanism of bloom termination and show a very fast depletion of cysts from the sediments (<3 months), which suggest a small role for resting cyst deposits in the recurrence of A. catenella blooms in this area.     

First report of fossilized cysts produced by the benthic Bysmatrum subsalsum (Dinophyceae) from a shallow Mexican lagoon in the Gulf of Mexico

Cysts belonging to the benthic dinoflagellate Bysmatrum subsalsum were recovered from palynologically treated sediments collected in the Alvarado Lagoon (southwestern Gulf of Mexico). The cysts are proximate, reflecting the features of the parent thecal stage, and their autofluorescence implies a dinosporin composition similar to the cyst walls of phototrophic species. This finding is important for our understanding of B. subsalsum life cycle transitions and ecology. Encystment may play an important role in the bloom dynamics of this species as it can enable the formation of a sediment cyst bank that allows reinoculation of the water column when conditions become favorable. This is the first report of a fossilized cyst produced by a benthic dinoflagellate recovered from sub‐recent sediments.     

Dinoflagellate resting cysts as factors in phytoplankton ecology of the North Sea

The occurrence and distribution of dinoflagellate resting cysts were investigated at 11 locations in the south-eastern part of the North Sea. Twenty-six known cyst species and 7 unknown cyst types, which may act as seed population for planktonic dinoflagellate blooms, have been recorded for the first time in the area. The most common cysts in recent sediments were those ofScrippsiella trochoidea, Zygabikodinium lenticulatum, Peridinium dalei, Scrippsiella lachrymosa, Protoceratium reticulatum, Protoperidinium denticulatum, andP. conicum. At all stations,S. trochoidea dominated the cyst assemblages with a maximal abundance of 1303 living cysts/cm³ in the uppermost half centimetre. Cysts of the potentially toxic dinoflagellatesAlexandrium cf.excavatum andA. cf.tamarense were scarce. In the upper 2-cm layer of sediment, dinoflagellate cysts were found in concentrations of 1.8 up to 682 living cysts/cm³. Empty cysts constituted 22–56% of total cyst abundance. The comparative distribution of the cysts showed a general increase in abundance from inshore sites to the offshore area, whereby sandy stations exhibited the lowest cyst abundance and diversity. The wide distribution of living and empty cysts ofScrippsiella lachrymosa suggests that its motile form, which has not been officially recorded in the area until now, is a common plankton organism in German coastal waters. The relatively high abundance of cysts in recent sediments demonstrates the potential importance of benthic resting stages for the initiation of dinoflagellate blooms in the study area.     

Dinoflagellate cysts from surface sediments of Saldanha Bay, South Africa: an indication of the potential risk of harmful algal blooms

The distribution and abundance of dinoflagellate cysts from recent coastal sediments in Saldanha Bay, was investigated, and compared to the cyst assemblages of the adjacent coastal upwelling system as reflected in the sediments off Lambert's Bay on the southern Namaqua shelf. Twenty-two cyst types were identified from three sample sites off Lambert's Bay with recorded abundances between 1726 and 1863 cysts ml⁻¹ wet sediment. At least 21 distinctive cyst types were identified from 32 sample sites within Saldanha Bay. Cyst abundance in Saldanha Bay was relatively low, averaging 116 cysts ml⁻¹ wet sediment. The region off Lambert's Bay is especially susceptible to the formation of harmful algal blooms attributed to high biomass dinoflagellate blooms. Owing to these blooms and the retentive circulation characteristics of this area, cyst formation and deposition is high. Blooms can be advected into Saldanha Bay, but their development and duration in the Bay is restricted by the system of exchange that operates between the Bay and the coastal upwelling system, in that there is a net export of surface waters from the Bay. Consequently, fewer cysts are formed and deposited within the Bay thereby reducing the likelihood of in situ bloom development initiated from the excystment of cysts.     

九龙江西陂库区沉积物甲藻孢囊的分布

孢囊在甲藻的生活史中发挥重要作用,福建省九龙江从2009年起,暴发多次拟多甲藻水华事件。采用显微镜观察和单细胞PCR技术,对九龙江西陂库区2012—2013年不同月份沉积物中的甲藻孢囊进行种属判定,并对甲藻孢囊的分布及其影响因素进行分析。结果表明,西陂库区沉积物中的甲藻孢囊主要为拟多甲藻属,约占80%,其次为裸甲藻属,发现了2009年水华的优势种佩式拟多甲藻(Peridiniopsis penardii)孢囊。库区沉积物中甲藻孢囊的丰度在(13.7±1.2)—(105.2±8.3)个/g干重之间。多元相关分析结果显示甲藻孢囊的丰度与含水率呈现显著正相关性(P0.05),反映了甲藻孢囊沿水流方向逐渐积累。本研究结果填补了国内水库甲藻孢囊鉴定和萌发的研究空白,为九龙江甲藻水华的防治提供科学参考。     

A comparative study of Alexandrium tamarense cyst distribution in Belfast Lough

Alexandrium tamarense is a cyst-forming dinoflagellate that can cause toxicity in shellfish. Belfast Lough, in northeast Ireland, has experienced toxicity events due to the presence of A. tamarense, which are monitored because of shellfish farms in the Lough. Since 1992 anthropogenic influences on the Lough have changed with the introduction of a ‘fast cat’ ferry service and an increase in mussel farming. In 2002 Belfast Lough was surveyed for A. tamarense cyst distribution in the sediments and the results are compared to a previous cyst survey carried out in the Lough in 1992. Cyst numbers were generally lower in 2002 than in 1992 (t-test, P < 0.01). The highest concentration found in 2002 was 1058 cysts g^?1 dry sediment compared with 3330 in 1992. Although sediment disturbance increased in the period between the surveys and plays a role in cyst distribution, A. tamarense cysts are still present in the seabed of the Lough. These cysts remain an important repository of inoculating cells for A. tamarense blooms that have not been removed by the recent anthropogenic activity in the Lough and therefore the requirement for monitoring remains. Comparisons between cyst counts per volume of wet sediment and per weight of dry sediment were carried out, and although the first is needed for ecological studies allowing a deeper analysis, it is also recommended that cyst counts per dry weight of sediment are always reported for wider comparative purposes.     

Species diversity and abundance of dinoflagellate resting cysts seven months after a bloom of Alexandrium catenella in two contrasting coastal systems of the Chilean Inland Sea

In Chile, 90% of the fish farms and major natural shellfish beds are located in the region surrounding the Inland Sea, where over the last few decades harmful phytoplankton blooms have often been observed. The onset and recurrence of bloom events are often related to the resuspension and germination of resting cysts that have accumulated in the sediments. The degree of cyst settling, accumulation and germination is highly variable between areas and depends on physical and environmental factors. To learn how differences in oceanographic exposure, amount of river runoff and bathymetry affect dinoflagellate cyst deposition, we examined the diversity and abundance of dinoflagellate resting cysts from two hydrographically contrasting coastal areas (oceanic Guaitecas Archipelago and estuarine Pitipalena Fjord) of the Chilean Inland Sea in September 2006, seven months after a bloom of Alexandrium catenella, a producer of paralytic shellfish toxin. Cyst species diversity consisted of 18 taxa, including A. catenella and the noxious species Protoceratium reticulatum, both of which have caused blooms in the study area. Our results revealed significant differences between the two study sites in terms of the abundance and diversity of resting cysts, suggesting that in the specific case of A. catenella, only Guaitecas stations have potential for cyst accumulation and successful growth of cells. However, there was no evidence of long-term resting cyst beds of A. catenella at either study site.     

Rapid molecular identification of the harmful freshwater dinoflagellate <Emphasis Type="Italic">Peridinium</Emphasis> in various life stages using genus-specific single-cell PCR

Cysts of the freshwater dinoflagellate Peridinium are typically different from vegetative cells in shape and remain largely undescribed. Molecular discrimination of such cysts would be useful to a number of research disciplines. A reliable method for the amplification of ribosomal DNA (rDNA) from Peridinium at different life stages is described. This genotyping strategy relies on whole cell PCR using Peridinium-specific primers designed from available 18S rDNA sequences. Here, we demonstrate the effectiveness of Peridinium-specific PCR for the rapid molecular identification of Peridinium cells in various life stages such as vegetative, planozygote, hypnozygote and cyst.     

Sulphate reduction in the hypolimnion and sediments of Lake Kinneret, Israel

1. Lake Kinneret is a warm (13–30°C) monomictic lake. Between January and June a heavy annual bloom of the dinoflagellate Peridinium gatunense dominates phytoplankton biomass (250 g m^?2). At the beginning of the summer, degradation and decomposition of the Peridinium biomass occurs, serving as a trigger for intense sulphate reduction in the hypolimnion and sediments. 2. The rates of sulphate reduction in the sediments varied seasonally from 12 to 1700 nmol SO₄.^?2 reduced cm^?3 day^?1 in December and July, respectively. The availability of organic matter and sulphate is high in June after the crash of the Peridinium bloom and the beginning of stratification and is lowest in December before overturn. 3. Sulphate concentrations in the hypolimnion range between 0.52 mM and 0.20 mM during mixing (January-April) and before overturn (December), respectively. The depletion in sulphate in the hypolimnion is stoichiometrically correlated to the increase in sulphide. The lake is not depleted of sulphate at any time, so the sulphate reduction process in Lake Kinneret is not limited by sulphate concentrations except in the sediments just before overturn.     

AN ENCYSTMENT STAGE,BEARING A NEW SCALE TYPE,OF THE ANTARCTIC PRASINOPHYTE PYRAMIMONAS GELIDICOLA AND ITS PALEOLIMNOLOGICAL AND TAXONOMIC SIGNIFICANCE1

Cysts of the Antarctic prasinophyte Pyramimonas gelidicola McFadden were found in water samples from a fjord and a saline lake in the Vestfold Hills, Antarctica Unialgal cultures of P. gelidicola from Ace Lake produced cysts. After ca. five weeks, tile cysts settled and adhered to the bottom of the culture flask. The cyst wall was covered by a scale type not seen on the flagellated cells; however, the base of the cyst scale was similar to the box scales of P. gelidicola motile cells. Cyst scales were also found off the continental shelf in Prydz Bay. In a 1.7 m sediment core taken from Ace Lake, both cyst scales and box scales of P. gelidicola occurred at most depths. Differences in the ratio of these two scale types at different depths in the core may indicate past ecological changes in the lake. Upper sediments of the core were dated at 5310 ± 90 yrs B.P., indicating that prasinophyte scales may be recognizably preserved for extended periods. P. gelidicola was widely distributed in saline lakes of the Vestfold Hills with salinities of 3.2–133% and temperatures ranging from – 5.0 to 10.4°C. This is the first report of encystment of P. gelidicola and, to our knowledge, is the first record of a prasinophyte with two distinctly different scale types occurring on cells during different stages of the life history.     

HIGH ENCYSTMENT SUCCESS OF THE DINOFLAGELLATE SCRIPPSIELLA CF. LACHRYMOSA IN CULTURE EXPERIMENTS 1

Close to 100% encystment efficiency and a yield above 10⁵ cysts·mL ^{? 1} were routinely achieved in full strength f/2 medium‐based batch cultures (883 μM NO₃ ^? and 36 μM PO₄ ^{? 3}) of the marine dinoflagellate Scrippsiella cf. lachrymosa Lewis. Increases in cell density led to nutrient depletion in this enriched medium, which was the most likely cause for initiation of cyst formation. Lowering the concentration of either nutrient to 1/10 the initial levels decreased the encystment efficiency, whereas use of ammonium as the N source resulted in both low cell yield and low encystment efficiency. The mandatory dormancy period was ca. 60 days and was not affected by cold dark storage of the cysts. Cysts produced in the initial phase of sexual reproduction were relatively large (length 47 μm, width 31 μm) with a heavy calcareous cover. Cysts produced thereafter lacked apparent calcareous cover and were smaller (length 29 μm, width 19 μm). The decrease of cyst volume (by a factor of 0.24–0.4) suggested strong resource limitation during the course of encystment. However, after the mandatory dormancy period, germination success of the smaller cysts was higher (80%), compared with the larger cysts that had been produced initially (50%). Germling survival (74%) was independent of cyst type but was enhanced by higher nutrient concentration during incubation. The ratio of initial nutrient concentration in the medium to the cyst yield was used as a proxy to estimate the cellular nutrient quota. The conservative estimates of 9 pmol N·cyst ^{? 1} and 0.4 pmol P·cyst ^{? 1} obtained in this manner are at the low end of the range of previous published estimates for other dinoflagellate cysts. Given the high encystment observed in laboratory experiments, we have no reason to assume an inherently lower encystment success in dinoflagellate field populations. Our results do not challenge the low nutrient paradigm for dinoflagellate sexuality. We believe that the high encystment success and cyst yield of this particular species is at least partly due to its ability to achieve very high cell densities in cultures, which evidently leads to nutrient depletion even in f/2 medium.     

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.     

ENCYSTMENT OF THE DINOFLAGELLATE GYRODINIUM UNCATENUM: TEMPERATURE AND NUTRIENT EFFECTS1

Sexual reproduction and encystment of the marine dinoflagellate Gyrodinium uncatenum Hulburt were induced in nitrogen and phosphorus-limited batch cultures. Sexuality did not occur under nutrient-replete conditions even when growth rate was reduced by non-optimal temperatures. Growth was optimal over a broader temperature range than encystment and virtually no cysts were produced at some low and high temperatures where growth occurred. Most cells initiated sexuality as intracellular pools of each limiting nutrient reached minimum or subsistence levels as much as four days after extracellular nutrients were exhausted. High nitrogen cell quotas during the phosphorus experiment indicate that sexuality was induced by a shortage of phosphorus and not by an indirect effect on nitrogen uptake. Total cyst yield corresponded to successful encystment of 9–13% of the motile populations, yet 60–85% of the plateau-phase motile cells were planozygotes (swimming zygotes formed from fusing gametes). Batch culture studies monitoring total cyst yield may thus seriously underestimate the extent of sexuality. More importantly, the number of cysts produced in a dinoflagellate population may be significantly reduced by environmental factors acting on the cells after sexual induction and fusion.     

ROLE OF ENCYSTMENT AND EXCYSTMENT OF PERIDINIUM BIPES F. OCCULATUM (DINOPHYCEAE) IN FRESH WATER RED TIDES IN LAKE KIZAKI,JAPAN1

The encystment flux of Peridinium bipes f. occulatum (Dinophyceae) was investigated with sediment traps from 1968 to 1990 in Lake Kazki. Cysts of P. bipes were formed throughout the blooms, Encystment flux of P. bipes in the pelagic zone was usually lower than those at shallow sites, and the density of P. bipes cysts in lake sediment was higher in the shallow region than in the pelagic zone. However, in the shallower region, The concentration of P. bipes cysts varied widely, possibly due to high rates of encystment and excystment. Peridinium bipes encystment occurred between 15° and 25° C in the laboratory, with very little cyst formation below 10°C. Though cyst formation was observed in continous darkness, the rate increased with irradiance. Under continuous darkness, no excystment was observed at any temperature from 5° to 25° C. Eighty-one percent of the cysts illuminated at 105 μE m^?2 s^?1 excysted after 13 days incubation at 15° C, and lower irradiances decreased germination success. Results from laboratory experiments suggest that light is a critical factor in the germination of P. bipes cysts. Bottom depth thus can have a significant effect on germination because cysts only can excyst from depths where light is sufficient. The shallow region of the lake is thus very important as a seed bed for P. bipes during early spring. Cyst deposited in deeper waters may not ever germinate unless they are resuspended and transported to shallow areas where light reaches the bottom.     

Generalist Life Cycle Aids Persistence of Alexandrium ostenfeldii (Dinophyceae) in Seasonal Coastal Habitats of the Baltic Sea

In seasonal environments, strong gradients of environmental parameters can shape life cycles of phytoplankton. Depending on the rate of environmental fluctuation, specialist or generalist strategies may be favored, potentially affecting life cycle transitions. The present study examined life cycle transitions of the toxin producing Baltic dinoflagellate Alexandrium ostenfeldii and their regulation by environmental factors (temperature and nutrients). This investigation aimed to determine whether genetic recombination of different strains is required for resting cyst formation and whether newly formed cysts are dormant. Field data (temperature and salinity) and sediment surface samples were collected from a site with recurrent blooms and germination and encystment experiments were conducted under controlled laboratory conditions. Results indicate a lack of seasonal germination pattern, set by an endogenous rhythm, as commonly found with other dinoflagellates from the Baltic Sea. Germination of quiescent cysts was triggered by temperatures exceeding 10°C and combined nutrient limitation of nitrogen and phosphorus or a drop in temperature from 16 to 10°C triggered encystment most efficiently. Genetic recombination was not mandatory for the formation of resting cysts, but supported higher numbers of resistant cysts and enhanced germination capacity after a resting period. Findings from this study confirm that A. ostenfeldii follows a generalist germination and cyst formation strategy, driven by strong seasonality, which may support its persistence and possibly expansion in marginal environments in the future, if higher temperatures facilitate a longer growth season.     

Dinoflagellate cyst distribution along a shelf to slope transect of an oligotrophic tropical sea (Sunda Shelf, South China Sea)

From 51 surface samples collected along a shelf to slope transect of the Sunda Shelf, South China Sea, 36 taxa of organic‐walled dinoflagellate cysts are identified. Oligotrophic tropical shelf assemblages on the Sunda Shelf are dominated by gonyaulacoids such as Spiniferites species, Operculodinium centmcarpum and Operculodinium israelianum. Concentrations of dinoflagellate cysts in the shelf sediments are generally low and correlate well with the content of fine‐grained (clay and silt fraction) sediments. Detailed comparisons of sediment grain‐size distributions to concentrations of dominant dinoflagellate taxa (Spiniferites species, 0. centmcarpum and 0. israelianum) in the shelf sediments indicate that these taxa behave in water like sediment particles with size range φ 5.75–6.25 (13–18 μm). In contrast, slope assemblages in fine‐grained sediments are dominated by protoperidinioids. This may reflect higher nutrient availability as a result of weak winter upwelling. The concentrations of dinoflagellate cysts in the shelf sediments are mainly controlled by transport and winnowing processes and are probably not representative of surface water conditions.