Omnivory in the calanoid copepod Temora longicornis: feeding, egg production and egg hatching rates

We measured in laboratory experiments the ingestion, egg production and egg hatching rates of female Temora longicornis as a function of diet. The diets consisted of a diatom (Thalassiosira weissflogii), an autotrophic dinoflagellate (Heterocapsa triquetra), and a bacterivorous ciliate (Uronema sp.) given as sole foods, or combinations of these single-food items: diatom+dinoflagellate, diatom+ciliate, dinoflagellate+ciliate, and diatom+ciliate+dinoflagellate. For the three single-item diets, the functional response was similar; i.e., ingestion rate increased linearly with food concentration (food range: ∼25 to ∼600 μg C l⁻¹). When all diets were considered, maximum daily carbon ration (∼70% of body weight) was independent of food type. However, the maximum daily egg production rate (12% of body carbon) was obtained with the diatom diet. For all diets, both ingestion and egg production rates increased with food concentration. Ingestion and egg production rates were affected differently by the interaction of food concentration and food type: at low food concentrations, ingestion rates were highest on diets containing the diatom. At high food concentrations, egg production rates were highest on the two phytoplankter diets and their combination. The presence of the ciliate in the diet did not enhance ingestion rate or egg production. Mixed-food diets did not enhance egg production relative to single-food diets. Hence, dietary diversity did not appear to be particularly advantageous for reproduction. Carbon-specific egg production efficiency (EPE; egg production/ingestion) was independent of food concentration and type, and equaled 9%. Egg hatching success was low (mean<30%) and independent of food concentration and type, and egg production rates. Our results are consistent with previous observations that egg production in T. longicornis is enhanced during diatom blooms. However, the relatively low EPE and egg hatching success suggest that reproduction and recruitment in this study were severely constrained by the biochemical composition of the diet, or the physiological condition of the females towards the end of their season of growth in Long Island Sound.     

Collapse of Calanus chilensis reproduction in a marine environment with high diatom concentration

Variations of egg production rate (EPR), hatching success (HS), production of abnormal larvae (AL) and histology of gonads have been investigated with Calanus chilensis females sampled weekly, from late November to December 2004, at a station located in the coastal zone off Dichato (Chile), at time diatom concentration in phytoplankton bloom was high. Weekly EPR estimate in nature did not change significantly during this period. It remained close to normal values (25-40 eggs/female/day), whereas HS was constantly low and high proportions of AL were observed. In parallel, bioassays revealed that EPR was strongly depressed by artificially enriched diets, corresponding to natural diatom assemblages (NDA) occurring in the field, while abnormal HS and AL values could not be improved. Ingestion of diatoms by females was estimated by faecal pellet production rates and SEM examination of diatom remains in pellet samples. Low HS and the high amounts of abnormal larvae were not reversible when females were offered a favourable food, the dinoflagellate P. minimum (PM). Minor cell degradations were observed in gonads of females fed NDA diets. In comparison with other environments, present results show that impairment of Calanoid copepod reproductive factors can occur at both high and low diatom concentrations, depending on maternal diets and diatom species in blooms.     

A bacterium that inhibits the growth of Pfiesteria piscicida and other dinoflagellates

Toxic dinoflagellate blooms have increased in estuaries of the east coast of the United States in recent years, and the discovery of Pfiesteria piscicida has brought renewed attention to the problem of harmful algal blooms (HAB) in general. Many bacteria and viruses have been isolated that have algicidal or algistatic effects on phytoplankton, including HAB species. Twenty-two bacterial isolates from the Delaware Inland Bays were screened for algicidal activity. One isolate (Shewanella IRI-160) had a growth-inhibiting effect on all three dinoflagellate species tested, including P. piscicida (potentially toxic zoospores), Prorocentrum minimum, and Gyrodinium uncatenum. This bacterium did not have a negative effect on the growth of any of the other four common estuarine non-dinoflagellate species tested, and in fact had a slight stimulatory effect on a diatom, a prasinophyte, a cryptophyte, and a raphidophyte. Shewanella IRI-160 is the first non-microzooplankton example of a microbe with the ability to control and inhibit the growth of P. piscicida, suggesting that bacteria in the natural environment could play a role in controlling the growth and abundance of P. piscicida and other dinoflagellates. Such bacteria could also potentially be used as management tools to prevent the proliferation of potentially harmful dinoflagellates in estuaries and coastal waters.     

Correlation between the presence of Gonyaulax fragilis (Dinophyceae) and the mucilage phenomena of the Emilia-Romagna coast (northern Adriatic Sea)

The formation of massive amounts of suspended mucilaginous organic matter which periodically affects the Adriatic Sea, has been regarded as a complex physico-chemical phenomenon resulting from the production of extracellular material by phytoplankton. Although the exact cause has remained obscure, the mechanism of its formation has usually been considered to be a long-term process, starting after the late winter-early spring blooms, and involving the participation of various algal species, mainly within the diatom group. In this paper we report on the results of a phytoplankton monitoring programme in northern Adriatic seawaters off the Emilia-Romagna coast of Italy which revealed the constant concomitant presence of the dinoflagellate Gonyaulax fragilis (Schütt) Kofoid and mucilaginous formations. In the early stages of the phenomenon the dinoflagellate was clearly observable by microscopic examination in the mucilage, but as the mucilage aged this alga almost completely decomposed and diatom cells increased in number and became predominant. Although characterized by a slow growth rate in culture, in natural seawater G. fragilis was observed to reach cell densities of up to 7.0×10⁶ cells l⁻¹. The results of this study lead us to propose the hypothesis that the appearance of mucilage in the water column of the Adriatic Sea is the consequence of a seasonal growth of this dinoflagellate favoured by specific environmental circumstances.     

Mixotrophy in the newly described phototrophic dinoflagellate Woloszynskia cincta from western Korean waters: feeding mechanism, prey species and effect of prey concentration

Woloszynskia species are dinoflagellates in the order Suessiales inhabiting marine or freshwater environments; their ecophysiology has not been well investigated, in particular, their trophic modes have yet to be elucidated. Previous studies have reported that all Woloszynskia species are photosynthetic, although their mixotrophic abilities have not been explored. We isolated a dinoflagellate from coastal waters in western Korea and established clonal cultures of this dinoflagellate. On the basis of morphology and analyses of the small/large subunit rRNA gene (GenBank accession number=FR690459), we identified this dinoflagellate as Woloszynskia cincta. We further established that this dinoflagellate is a mixotrophic species. We found that W. cincta fed on algal prey using a peduncle. Among the diverse prey provided, W. cincta ingested those algal species that had equivalent spherical diameters (ESDs) ≤12.6 μm, exceptions being the diatom Skeletonema costatum and the dinoflagellate Prorocentrum minimum. However, W. cincta did not feed on larger algal species that had ESDs≥15 μm. The specific growth rates for W. cincta increased continuously with increasing mean prey concentration before saturating at a concentration of ca. 134 ng C/ml (1,340 cells/ml) when Heterosigma akashiwo was used as food. The maximum specific growth rate (i.e. mixotrophic growth) of W. cincta feeding on H. akashiwo was 0.499 d(-1) at 20 °C under illumination of 20 μE/m(2) /s on a 14:10 h light-dark cycle, whereas its growth rate (i.e. phototrophic growth) under the same light conditions without added prey was 0.040 d(-1). The maximum ingestion and clearance rates of W. cincta feeding on H. akashiwo were 0.49 ng C/grazer/d (4.9 cells/grazer/d) and 1.9 μl/grazer/h, respectively. The calculated grazing coefficients for W. cincta on co-occurring H. akashiwo were up to 1.1 d(-1). The results of the present study suggest that grazing by W. cincta can have a potentially considerable impact on prey algal populations.     

围隔藻类水华演替过程中二甲基硫化物的含量动态

于2005年6月至7月,研究了海洋围隔不同藻类水华演替过程中二甲基硫化物的含量动态,并考察了相关环境参数对二甲基硫化物含量的影响。2个围隔实验组均出现未知藻水华-硅藻水华-甲藻水华的演替过程,这3次不同藻类水华分别对应了二甲基硫化物含量的3次高峰,表明藻类水华对二甲基硫化物含量有重要贡献。不同藻类水华的贡献有较大差异,甲藻水华的贡献最大,硅藻次之,未知藻类水华的贡献最小。实验结果还表明PO4^3-、NO2^-和NH4^＋主要通过影响藻类生长状态,进而影响DMSP和DMSO的含量;NO2^-和NH4^＋亦可能通过调节DMSP和DMSO在藻细胞内的生理功能,影响DMSP和DMSO的含量;PO4^3-、NO2^-和NH4^＋与DMS含量无显著相关。     

Importance of copepod faecal pellets to the fate of the DSP toxins produced by Dinophysis spp.

An ingestion experiment was carried out in Rı́a de Pontevedra (Spain) with the copepod Temora longicornis in order to determine ingestion rates of the DSP toxin-producers, Dinophysis spp. (Dinophyceae), and the excretion rate of Dinophysis spp. cells within the faecal pellets. Ingestion rate was a function of dinoflagellate abundance and did not vary with either the amount, or the composition of the co-occurring phytoplankton species in the food suspension. Faecal pellet production increased at higher food concentrations. Intact Dinophysis spp. cells representing 34.4% of the total Dinophysis cells ingested by the copepods were found within the pellets. T. longicornis was the only dominant copepod species in the area that fed on Dinophysis spp., thus the pellets produced by T. longicornis were the main source of copepod “toxic” pellets in the media during blooms of Dinophysis spp. These “toxic” pellets might contribute to the maintenance of the toxic algal blooms, if the cells inside the pellets remain viable, can spread the potential toxicity of the toxic dinoflagellates throughout the pelagic food web due to coprophagy, and/or be an important toxic vector into the benthic food web. However, during a Dinophysis spp. bloom, the percentage of cells excreted daily within the pellets was lower than 1% of the total dinoflagellate population and, moreover, copepod faecal pellets represent a small fraction of the sinking material in this area. Although it was not possible to measure the amount of toxins in the pellets, we concluded that copepod faecal pellets do not have an important role in the transport of DSP toxins through the food web in this area.     

Growth rates and elemental composition of Alexandrium monilatum, a red-tide dinoflagellate

The combined effects of temperature and salinity on growth of Alexandrium monilatum were studied in laboratory cultures. This toxic, red-tide dinoflagellate grew faster with higher temperatures, up to a maximum of approximately 1 division per day at 31 °C. Salinities above 15 psu had a lesser effect on growth rate, as might be expected for an estuarine species. Growth rates of cultures exposed to natural light and temperature fluctuations were comparable to laboratory cultures. The minimum N cell quota suggested that high N flux would be required to support bloom development. A literature survey of documented A. monilatum blooms indicated that within US waters, blooms occur in July–September in nearshore or estuarine regions of the Gulf of Mexico and the Florida Atlantic coast. Temperature and salinity measured during blooms correspond to the optimal growth conditions of the laboratory cultures. Nevertheless, the occurrence of A. monilatum blooms is sporadic compared to the occurrence of seemingly optimal growth conditions. Laboratory growth experiments predict when blooms of this species are unlikely due to low growth rates, but so far cannot predict individual blooms.     

有害甲藻孢囊的分类鉴定研究进展

有害甲藻孢囊主要是指能产生毒素和(或)能引起有害藻华发生并对水生态系统产生各种危害效应的甲藻孢囊。我国沿海共记录了10属18种,占全球有害甲藻孢囊的3/4。这些有害甲藻孢囊广泛分布于我国沿海,会对水产养殖业造成严重的经济损失,甚至会威胁人类的身体健康。因此,有害甲藻孢囊的多样性及分布越来越受到人们的关注。对有害甲藻孢囊的准确判断不仅对研究其多样性及分布至关重要,而且有助于水产品的安全检验和有害藻华的早期预警。对有害甲藻孢囊的分类主要存在鉴定困难、鉴定不准确等问题。本文综述了有害甲藻孢囊的危害、中国沿海有害甲藻孢囊的种类和分布,以及有害甲藻孢囊的鉴定等3个方面的研究进展,并提出利用孢囊及营养细胞的形态学特征、分子生物学、毒理学等多学科研究手段准确鉴定有害甲藻孢囊的建议。     

The use of diatoms in monitoring the development of created wetlands at a sandmining site in Western Australia

Former sand-mining pits at Capel, 200 km south of Perth in Western Australia, have been rehabilitated into artificial wetlands since 1975–1979. A chain of fresh water lakes was created as a potential waterbird refuge and an area for passive recreation. Initially, the lakes had low pH, high ammonium, iron and manganese levels and low phosphorus concentration. The lakes were characterised by low diversity of diatoms dominated by acidophilous species. Following an increase in pH in the effluent water discharged into the lake from the mining process plant and landscaping of the lakes since 1988, the diversity of diatoms gradually increased. The system is now dominated by periphytic diatom communities, preferring high conductivity. There has been a marked transition in the diatom community from acidophilous to alkaliphilous species. Planktonic diatom blooms replaced dinoflagellate blooms. Concomitantly, there has been a dramatic increase in the diversity of invertebrates and waterbirds in these lakes. The value of diatoms in assessing the progressive development of created wetlands as self-sustaining ecosystems at sand mines in Australia is discussed.     

The role of particulate matter in the productivity of surface waters

Except for the special cases of upwelled water and the spring blooms in temperate and boreal waters, the productivity of the oceans is largely governed by the rate of nutrient regeneration in surface waters. This rate of regeneration is a function of the number of actively metabolizing bacteria present, which in turn appears to be a function of the particle content of the water. Thus, particle content may be the basic control on the productivity of the open oceans. The possibility of increasing productivity by artificially increasing the particle content of the water should be considered for regions characterized by low particle count, such as the Sargasso Sea. Since silicate can only be resupplied by re-solution of diatom tests, a process taking place at depth, plankton blooms following regeneration kinetics will typically be dinoflagellate rather than diatom blooms.     

南麂列岛海洋自然保护区浮游植物的种类多样性及其生态分布

于2006年5月至2007年2月之间,对南麂列岛海域的浮游植物类群进行了4个季节的调查,分析了该海域浮游植物的种类组成、优势种类、群落结构以及水平分布等特征参数的季节变化。共鉴定浮游植物80种,隶属于4个门,硅藻种类最多,甲藻其次。浮游植物可划分为3个生态类群,以广温类群为主。春季和夏季分别以三角棘原甲藻和中肋骨条藻为绝对优势种,秋冬季的优势种类组成多样化。共鉴定57种赤潮生物,占浮游植物种类数的71.25%。调查期间,三角棘原甲藻和中肋骨条藻分别于春季和夏季形成赤潮。浮游植物的物种丰富度呈现春、夏、秋、冬递减的趋势。浮游植物细胞丰度的年平均值为1.03×106cells/L,春夏季显著高于秋冬季。春季和夏季时,浮游植物高值区集中在南麂岛西北近岸海域;秋季和冬季时,浮游植物高值区相对集中在南麂岛东南近岸海域。浮游植物群落的多样性指数(H')以秋季最高,冬季最低。春季的三角棘原甲藻赤潮期间,水体中N/P值显著升高;夏季的中肋骨条藻赤潮期间,水体中N/P值显著降低。     

The relationship between size,budding rate,and growth efficiency in three species of hydra

Three species of the fresh water carnivore hydra, H. littoralis, H. pseudoligactis, and C. viridissima present a graduation in size with the first species the largest and albino Chlorohydra the smallest. When presented with a daily overabundance of food (artemia), considerable variation in food intake and gross efficiency of growth (proportion of food energy consumed that is turned into new protoplasm or buds) existed among the species. The degree of association between size of species and food intake was highly significant. However, budding efficiency among the species was found to be independent of food intake (when the effects of species size were eliminated) and of species size (when the effects of food intake were removed). However, species with high (low) efficiencies have significantly higher (lower) reproductive rates. A lowering of the temperature from 25° to 15° C. increased the size of the species, increased food intake, but decreased reproductive rate. In all species except H. pseudoligactis a corresponding increase in the production of bud energy with no change in efficiency also occurred. On the other hand, lowering of the temperature for H. pseudoligactis significantly lowered reproductive efficiency but had no effect on the total calorific output of buds. This species, in constrast to the others, appears to have a compensatory ability to adjust its efficiency to maintain a high calorific output when temperature increases. It was also found that albino Chlorohydra have budding efficiencies of around 35 percent which are not influenced by changes in food intake or light. Normal green hydras, however, have efficiencies which range from 40 to 62 percent above their albino counterparts when fed once a day and once every two days in light respectively. It it concluded first, that the symbiotic algae in the gastrodermals cells of green hydra contribute quantitatively in the order of the above amounts to the growth process in this species, and second, that green hydras have the ablity to increase their growth efficiency when food intake is reduced thus reducing the drop in calorific but output that normally occurs in the albino (control) form.     

Comparing diatom and Alexandrium catenella/tamarense blooms in Thau lagoon: Importance of dissolved organic nitrogen in seasonally N-limited systems

Diatom blooms in Thau lagoon are always related to rain events leading to inputs of inorganic nutrients such as phosphate, ammonium and nitrate through the watershed with time lags of about 1 week. In contrast, blooms of Alexandrium catenella/tamarense can occur following periods of 3 weeks without precipitation and no significant input of conventional nutrients such as nitrate and phosphate. Field results also indicate a significant drop (from 22–25 to 15–16 μM over 3 days) in dissolved organic nitrogen (DON) at the bloom peak, as well as a significant inverse relationship between A. catenella/tamarense cell density and DON concentrations that is not apparent for diatom blooms. Such dinoflagellate blooms are also associated with elevated (6–9 μM) ammonium concentrations, a curious feature also observed by other investigators, possibly the results of ammonium excretion by this organism during urea or other organic nitrogen assimilation.The potential use of DON by this organism represents short cuts in the nitrogen cycle between plants and nutrients and requires a new model for phytoplankton growth that is different from the classical diatom bloom model. In contrast to such diatom blooms that are due to conventional (nitrate, phosphate) nutrient pulses, Alexandrium catenella/tamarense blooms on the monthly time scale are due to organic nutrient enrichment, a feature that allows net growth rates of about 1.3 d⁻¹, a value higher than that generally attributed to such organisms.     

Description, host-specificity, and strain selectivity of the dinoflagellate parasite Parvilucifera sinerae sp. nov. (Perkinsozoa)

A new species of parasite, Parvilucifera sinerae sp. nov., isolated from a bloom of the toxic dinoflagellate Alexandrium minutum in the harbor of Arenys de Mar (Mediterranean Sea, Spain), is described. This species is morphologically, behaviourally, and genetically (18S rDNA sequence) different from Parvilucifera infectans, until now the only species of the genus Parvilucifera to be genetically analyzed. Sequence analysis of the 18S ribosomal DNA supported P. sinerae as a new species placed within the Perkinsozoa and close to P. infectans. Data on the seasonal occurrence of P. sinerae, its infective rates in natural and laboratory cultures, and intra-species strain-specific resistance are presented. Life-cycle studies in field samples showed that the dinoflagellate resting zygote (resting cyst) was resistant to infection, but the mobile zygote (planozygote) or pellicle stage (temporary cyst) became infected. The effects of light and salinity levels on the growth of P. sinerae were examined, and the results showed that low salinity levels promote both sporangial germination and higher rates of infection. Our findings on this newly described parasite point to a complex host-parasite interaction and provide valuable information that leads to a reconsideration of the biological strategy to control dinoflagellate blooms by means of intentional parasitic infections.     

Meta‐analysis reveals enhanced growth of marine harmful algae from temperate regions with warming and elevated CO2 levels

Elevated pCO₂ and warming may promote algal growth and toxin production, and thereby possibly support the proliferation and toxicity of harmful algal blooms (HABs). Here, we tested whether empirical data support this hypothesis using a meta‐analytic approach and investigated the responses of growth rate and toxin content or toxicity of numerous marine and estuarine HAB species to elevated pCO₂ and warming. Most of the available data on HAB responses towards the two tested climate change variables concern dinoflagellates, as many members of this phytoplankton group are known to cause HAB outbreaks. Toxin content and toxicity did not reveal a consistent response towards both tested climate change variables, while growth rate increased consistently with elevated pCO₂. Warming also led to higher growth rates, but only for species isolated at higher latitudes. The observed gradient in temperature growth responses shows the potential for enhanced development of HABs at higher latitudes. Increases in growth rates with more CO₂ may present an additional competitive advantage for HAB species, particularly as CO₂ was not shown to enhance growth rate of other non‐HAB phytoplankton species. However, this may also be related to the difference in representation of dinoflagellate and diatom species in the respective HAB and non‐HAB phytoplankton groups. Since the proliferation of HAB species may strongly depend on their growth rates, our results warn for a greater potential of dinoflagellate HAB development in future coastal waters, particularly in temperate regions.     

Effects of natural and field-simulated blooms of the dinoflagellate Prorocentrum minimum upon hemocytes of eastern oysters, Crassostrea virginica, from two different populations

Oysters, Crassostrea virginica, from two populations, one from a coastal pond experiencing repeated dinoflagellate blooms (native), and the other from another site where blooms have not been observed (non-native), were analyzed for cellular immune system profiles before and during natural and simulated (by adding cultured algae to natural plankton) blooms of the dinoflagellate Prorocentrum minimum. Significant differences in hemocytes between the two oyster populations, before and after the blooms, were found with ANOVA, principal components analysis (PCA) and ANOVA applied to PCA components. Stress associated with blooms of P. minimum included an increase in hemocyte number, especially granulocytes and small granulocytes, and an increase in phagocytosis associated with a decrease in aggregation and mortality of the hemocytes, as compared with oysters in pre-bloom analyses. Non-native oysters constitutively had a hemocyte profile more similar to that induced by P. minimum than that of native oysters, but this profile did not impart increased resistance. The effect of P. minimum on respiratory burst was different according to the origin of the oysters, with the dinoflagellate causing a 35% increase in the respiratory burst of the native oysters but having no effect on that of the non-native oysters. Increased respiratory burst in hemocytes of native oysters exposed to P. minimum in both simulated and natural blooms may represent an adaptation to annual blooms whereby surviving native oysters protect themselves against tissue damage from ingested P. minimum.     

Effects of a bacterial algicide,IRI-160AA,on dinoflagellates and the microbial community in microcosm experiments

Filtrates from the bacterium Shewanella sp. IRI-160 (termed IRI-160AA) have been shown to inhibit population growth and kill a variety of dinoflagellates grown in culture. Here we test the immediate efficacy of IRI-160AA in laboratory microcosms initiated from three natural dinoflagellate blooms (Prorocentrum minimum, Karlodinium veneficum and Gyrodinium instriatum). We measured target dinoflagellate abundance, total chlorophyll-a, photosystem II (PSII) photochemistry, and changes to the prokaryotic and eukaryotic community composition over 2–3 days of IRI-160AA incubation. Naked dinoflagellates were impacted more, while abundance of the thecate P. minimum was not affected. However, dinoflagellate growth inhibition was generally lower than that observed in uni-algal cultures, and took longer to occur. Eukaryotic community composition in IRI-160AA treated microcosms was significantly different from control incubations, and was driven predominantly by increases in heterotrophic protists (e.g. Euplotes sp. and Paraphysomonas sp.). Similarly, significant changes to the prokaryotic community structure were evident. Microcosms of G. instriatum with higher algicide concentrations indicated that algicidal activity was enhanced in a dose dependent manner. Furthermore, total ciliate abundance as well as a bactivorous chyrsophyte (Paraphysomonas sp.) increased in a dose dependent manner. Total diatom abundance increased at lower IRI-160AA concentrations, but increased less with increasing dose. Overall, the bio-activity of IRI-160AA on naturally occurring dinoflagellates in mixed natural microbial communities is encouraging from the applied perspective of using the active compound(s) in IRI-160AA as natural agent(s) to manage harmful dinoflagellate blooms.     

Prorocentrum minimum (Pavillard) Schiller: A review of a harmful algal bloom species of growing worldwide importance

Prorocentrum minimum (Pavillard) Schiller, a common, neritic, bloom-forming dinoflagellate, is the cause of harmful blooms in many estuarine and coastal environments. Among harmful algal bloom species, P. minimum is important for the following reasons: it is widely distributed geographically in temperate and subtropical waters; it is potentially harmful to humans via shellfish poisoning; it has detrimental effects at both the organismal and environmental levels; blooms appear to be undergoing a geographical expansion over the past several decades; and, a relationship appears to exist between blooms of this species and increasing coastal eutrophication. Although shellfish toxicity with associated human impacts has been attributed to P. minimum blooms from a variety of coastal environments (Japan; France; Norway; Netherlands; New York, USA), only clones isolated from the Mediterranean coast of France, and shellfish exposed to P. minimum blooms in this area, have been shown to contain a water soluble neurotoxic component which killed mice. Detrimental ecosystem effects associated with blooms range from fish and zoobenthic mortalities to shellfish aquaculture mortalities, attributable to both indirect biomass effects (e.g., low dissolved oxygen) and toxic effects. P. minimum blooms generally occur under conditions of high temperatures and incident irradiances and low to moderate salinities in coastal and estuarine environments often characterized as eutrophic, although they have been found under widely varying salinities and temperatures if other factors are conducive for growth. The physiological flexibility of P. minimum in response to changing environmental parameters (e.g., light, temperature, salinity) as well as its ability to utilize both inorganic and organic nitrogen, phosphorus, and carbon nutrient sources, suggest that increasing blooms of this species are a response to increasing coastal eutrophication.     

MIXOTROPHY AND NITROGEN UPTAKE BY PFIESTERIA PISCICIDA (DINOPHYCEAE)

The nutritional versatility of dinoflagellates is a complicating factor in identifying potential links between nutrient enrichment and the proliferation of harmful algal blooms. For example, although dinoflagellates associated with harmful algal blooms (e.g. red tides) are generally considered to be phototrophic and use inorganic nutrients such as nitrate or phosphate, many of these species also have pronounced heterotrophic capabilities either as osmotrophs or phagotrophs. Recently, the widespread occurrence of the heterotrophic toxic dinoflagellate, Pfiesteria piscicida Steidinger et Burkholder, has been documented in turbid estuarine waters. Pfiesteria piscicida has a relatively proficient grazing ability, but also has an ability to function as a phototroph by acquiring chloroplasts from algal prey, a process termed kleptoplastidy. We tested the ability of kleptoplastidic P. piscicida to take up ¹⁵N-labeled NH     , NO     , urea, or glutamate. The photosynthetic activity of these cultures was verified, in part, by use of the fluorochrome, primulin, which indicated a positive relationship between photosynthetic starch production and growth irradiance. All four N substrates were taken up by P. piscicida , and the highest uptake rates were in the range cited for phytoplankton and were similar to N uptake estimates for phagotrophic P. piscicida . The demonstration of direct nutrient acquisition by kleptoplastidic P. piscicida suggests that the response of the dinoflagellate to nutrient enrichment is complex, and that the specific pathway of nutrient stimulation (e.g. indirect stimulation through enhancement of phytoplankton prey abundance vs. direct stimulation by saprotrophic nutrient uptake) may depend on P. piscicida 's nutritional state (phagotrophy vs. phototrophy).